Publications
2024
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(2024) Crystal Growth and Design. 24, 20, p. 8468-8475 Abstract
Straight facets and sharp edges are among the most distinctive indicators of well-defined crystals and often reflect the polyhedral geometry and symmetry of the underlying close-packed, molecular structure. Curved morphologies are sometimes observed in biogenic crystals where templating or nonclassical crystallization processes (e.g., crystallization via amorphous states) occur. Here we report the formation and growth of copper-based metal-organic frameworks (MOFs) that crystallize in the hexagonal space group P622. The individual MOFs are homochiral and are formed from achiral compounds. The crystals begin as hexagonal-like structures, developing over time into complex flower-like structures having two decks joined in the center. Each deck layer has six well-defined petals with curved lateral surfaces. The growth mechanism shows initial straight petals that become progressively more curved with increased faceting. Remarkably, despite its multidomain appearance, the entire entity is a single crystal. The curved morphology is correlated to the crystallographic structure and the arrangement of nanosized channels within this structure. Crystal habits are typically considered to be inconsistent with curved morphologies. This work suggests that crystallographic explanations can support the development of such surfaces for low-density structures.
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(2024) Chem. 10, 8, p. 2408-2417 Abstract
The search for new materials to fabricate electronic devices mainly targets composites of metals, metal oxides, organic molecules, and polymers. We demonstrate here the fabrication and operation of a multilevel-per-bit metallo-organic memory cell (MOMC). This conceptually new memory cell has dual functionality. Information can be written and stored electrochemically and read out both electrochemically and optically. An electrochemical readout will reset the device to its ground state (read-while-write), whereas an optical readout can be continuous. The use of a nanoscale trilayer of electrochromically active ruthenium, iron, and osmium polypyridyl complexes resulted in up to four distinct states that can be addressed by applying different potentials. The information stored in the MOMC was used as input to an integrated circuit (IC), and it was visualized using light-emitting diodes (LEDs). These findings show the potential of metallo-organic materials to design hybrid ICs.
2023
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(2023) Journal of the American Chemical Society. 145, 32, p. 18075-18083 Abstract
We demonstrate here the assembly of a nanolayer of electrochromic iron complexes on the top of composite layers of cobalt and ruthenium complexes. Depending on the ratio of the latter two complexes, we can tailor materials that show different electron transport pathways, redox activities, and color transitions. No redox activity of the top layer, consisting of iron complexes, is observable when the relative amount of the ruthenium complexes is low in the underlying composite layer because of the insulating properties of the isostructural cobalt complexes. Increasing the amount of ruthenium complexes opens an electron transport channel, resulting in charge storage in both the cobalt and iron complexes. The trapped charges can be chemically released by redox-active ferrocyanide complexes at the film-water interface.
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(2023) Angewandte Chemie (International ed.). 62, 4, e202214041. Abstract
In this study, the precise positioning and alignment of arrays of two different guest molecules in a crystalline host matrix has been engineered and resulted in new optically active materials. Sub-nm differences in the diameters of two types of 1D channels are sufficient for size-selective inclusion of dyes. Energy transport occurs between the arrays of different dyes that are included in parallel-positioned nanochannels by Förster resonance energy transfer (FRET). The color of individual micro-sized crystals are dependent on their relative position under polarized light. This angular-dependent behavior is a result of the geometrically constrained orientation of the dyes by the crystallographic packing of the host matrix and is concentration dependent.
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(2023) Chemistry - A European Journal. e202301825. Abstract
We demonstrate here how nitrate salts of bivalent copper, nickel, cobalt, and manganese, along with an achiral organic ligand, assemble into various structures such as symmetrical double-decker flowers, smooth elongated hexagonal bipyramids, and hexagonal prisms. Large morphological changes occur in these structures because of different metal cations, although they maintain isomorphous hexagonal crystallographic structures. Metal cations with stronger coordination to ligands (Cu and Ni) tend to form uniform crystals with unusual shapes, whereas weaker coordinating metal cations (Mn and Co) produce crystals with more regular hexagonal morphologies. The unusual flower-like crystals formed with copper nitrate have two pairs of six symmetrical petals with hexagonal convex centers. The texture of the petals indicates dendritic growth. Two different types of morphologies were formed by using different copper nitrate-to-ligand ratios. An excess of the metal salt results in uniform and hexagonal crystals having a narrow size distribution, whereas the use of an excess of ligand results in double-decker morphologies. Mechanistically, an intermediate structure was observed with slightly concave facets and a domed center. Such structures most likely play a key role in the formation of double-decker crystals that can be formed by fusion processes. The coordination chemistry results in isostructural chiral frameworks consisting of two types of continuous helical channels. Four pyridine units from four separate ligands are coordinated to the metal center in a plane having a chiral (propeller-type) arrangement. The individual double-decker flower crystals are homochiral and a batch consists of crystals having both handedness.
2022
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(2022) Journal of the American Chemical Society. 144, 50, p. 22838-22843 Abstract
The symmetry of a crystal's morphology usually reflects the symmetry of the crystallographic packing. For single crystals, the space and point groups allow only a limited number of mathematical descriptions of the morphology (forms), all of which are convex polyhedrons. In contrast, concave polyhedrons are a hallmark of twinning and polycrystallinity and are typically inconsistent with single crystallinity. Here we report a new type of structure: a concave polyhedron shape single crystal having a multidomain appearance and a rare space group (P622). Despite these unusual structural features, the hexagonal symmetry is revealed at the morphological levels.
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(2022) Angewandte Chemie (International ed.). 61, 34, e202205238. Abstract
We show that metal-organic frameworks, based on tetrahedral pyridyl ligands, can be used as a morphological and structural template to form a series of isostructural crystals having different metal ions and properties. An iterative crystal-to-crystal conversion has been demonstrated by consecutive cation exchanges. The primary manganese-based crystals are characterized by an uncommon space group ( P622 ). The packing includes chiral channels that can mediate the cation exchange, as indicated by energy-dispersive X-ray spectroscopy on microtome-sectioned crystals. The observed cation exchange is in excellent agreement with the Irving-Williams series (Mn Zn) associated with the relative stability of the resulting coordination nodes. Furthermore, we demonstrate how the metal cation controls the optical and magnetic properties. The crystals maintain their morphology, allowing a quantitative comparison of their properties at both the ensemble and single-crystal level.
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(2022) Chemistry : a European journal. 28, 54, e202201108. Abstract
We demonstrate the formation of highly interpenetrated frameworks. An interesting observation is the presence of very large adamantane-shaped cages in a single network, making these crystals new entries in the collection of diamondoid-type metal-organic frameworks (MOFs). The frameworks were constructed by assembling tetrahedral pyridine ligands and copper dichloride. Currently, the networks degree of interpenetration is among the highest reported and increases when the size of the ligand is increased. Highly interpenetrated frameworks typically have low surface contact areas. In contrast, in our systems, the voids take up to 63% of the unit cell volume. The frameworks are chiral but formed from achiral components. The chirality is manifested by the coordination chemistry frameworks around the metal center, the structure of the helicoidal channels and the motifs of the individual networks. Channels of both handedness are present within the unit cells. This phenomenon shapes the walls of the channels, which are composed of 10, 16, or 32 chains correlated to the degree of interpenetration 10-, 16- and 32-fold. By changing the distance between the center of the ligand and the coordination moieties, we succeeded in tuning the diameter of the channels. Relatively large channels were formed, having diameters up to 31.0 Å × 14.8 Å.
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(2022) Energy technology (Weinheim, Germany). 10, 2, 2100769. Abstract
Herein, two different functionalities of one metallo-organic film assembled on the surface of a transparent metal-oxide electrode are described. Two redox-active elements, electrochromic ironpolypyridyl complexes and catalytically active palladium centers, operate by applying different potentials in aqueous solutions. The color of the material can be cycled 1500 times from dark purple to colorless by electrochemically addressing the Fe2+/3+ centers at 0.01.0V (vs Ag/Ag+). The differences between the transmittance of these two states is high: ΔT=52%. Catalytic water oxidation can occur by palladium oxide particles that form in situ by applying a higher potential (1.222.0V vs Ag/Ag+), resulting in the formation of hydrogen and oxygen. The catalyst is active for at least 7h in an aqueous electrolyte at pH=6.9, with a Faradaic efficiency of ≈70%.
2021
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(2021) Journal of the American Chemical Society. 143, 41, p. 16913-16918 Abstract
We demonstrate the formation of both metallo-organic crystals and nanoscale films that have entirely different compositions and structures despite using the same set of starting materials. This difference is the result of an unexpected cation exchange process. The reaction of an iron polypyridyl complex with a copper salt by diffusion of one solution into another resulted in iron-to-copper exchange, concurrent ligand rearrangement, and the formation of metal-organic frameworks (MOFs). This observation shows that polypyridyl complexes can be used as expendable precursors for the growth of MOFs. In contrast, alternative depositions of the iron polypyridyl complex with a copper salt by automated spin coating on conductive metal oxides resulted in the formation of electrochromic coatings, and the structure and redox properties of the iron complex were retained. The possibility to form such different networks from the same set of molecular building blocks by "in solution"versus "on surface"coordination chemistry broadens the synthetic space to design functional materials.
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(2021) ACS Nano. 15, 9, p. 14643-14652 Abstract
We demonstrate the solvent-free amorphous-to-cocrystalline transformations of nanoscale molecular films. Exposing amorphous films to vapors of a haloarene results in the formation of a cocrystalline coating. This transformation proceeds by gradual strengthening of halogen-bonding interactions as a result of the crystallization process. The gassolid diffusion mechanism involves formation of an amorphous metastable phase prior to crystallization of the films. In situ optical microscopy shows mass transport during this process, which is confirmed by cross-section analysis of the final structures using focused ion beam milling combined with scanning electron microscopy. Nanomechanical measurements show that the rigidity of the amorphous films influences the crystallization process. This surface transformation results in molecular arrangements that are not readily obtained through other means. Cocrystals grown in solution crystallize in a monoclinic centrosymmetric space group, whereas the on-surface halogen-bonded assembly crystallizes into a noncentrosymmetric material with a bulk second-order nonlinear optical response.
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(2021) Angewandte Chemie (International ed.). 60, 33, p. 18256-18264 Abstract
We demonstrate here a unique metalloorganic material where the appearance and the internal crystal structure are in contradiction. The eggshaped (ovoid) crystals have a brainlike texture. Although these microsized crystals are monodispersed; like fingerprints their grainy surfaces are never exactly alike. Remarkably, our Xray and electron diffraction studies unexpectedly revealed that these structures are singlecrystals comprising a continuous coordination network of two differently shaped homochiral channels. By using the same building blocks under different reaction conditions, a rare series of crystals have been obtained that are uniquely rounded in their shape. In stark contrast to the brainlike crystals, these isostructural and monodispersed crystals have a comparatively smooth appearance. The sizes of these crystals vary by several orders of magnitude.The eggshaped homochiral crystals: A brainlike texture combined with single crystallinity. A series of isomorphous crystals found in a rare space group have been formed with varied morphologies. These crystals contain chiral channels and are made from achiral components.
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(2021) Nature Communications. 12, 1, 957. Abstract
The coexistence of single-crystallinity with a multidomain morphology is a paradoxical phenomenon occurring in biomineralization. Translating such feature to synthetic materials is a highly challenging process in crystal engineering. We demonstrate the formation of metallo-organic single-crystals with a unique appearance: six-connected half-rods forming a hexagonal-like tube. These uniform objects are formed from unstable, monodomain crystals. The monodomain crystals dissolve from the inner regions, while material is anisotropically added to their shell, resulting in hollow, single-crystals. Regardless of the different morphologies and growth mechanism, the crystallographic structures of the mono- and multidomain crystals are nearly identical. The chiral crystals are formed from achiral components, and belong to a rare space group (P622). Sonication of the solvents generating radical species is essential for forming the multidomain single-crystals. This process reduces the concentration of the active metal salt. Our approach offers opportunities to generate a new class of crystals.
2020
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(2020) Advanced Electronic Materials. 10, 2000407. Abstract
In this work, the performance of surface-confined molecular assemblies, combined with a solid-state electrolyte, as multicolor displays is reported. An ultraviolet (UV) crosslinked polymer is used as a matrix for the electrolyte. No dedicated ion storage layer is required to operate the laminated devices; up to 4500 redox cycles are demonstrated at a low voltage. The intense coloration and switching properties (t = 0.4-2.8 s) are retained even after thermally treating these devices at 100 degrees C for 24 h in air. The versatility of this approach is demonstrated by the use of spray-coated bimolecular assemblies for multicolor display fabrication. The molecular assemblies have coloration efficiencies as high as 338 cm(2)C(-1).
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(2020) Journal of the American Chemical Society. 142, 33, p. 14210-14221 Abstract
We demonstrate the formation of uniform and oriented metal-organic frameworks using a combination of anion-effects and surface-chemistry. Subtle but significant morphological changes result from the nature of the coordinative counter-anion of the following metal salts: NiX2 with (X = Br-, Cl-, NO3-, and OAc-). Crystals could be obtained in solution or by template surface growth. The latter resulting in truncated crystals that resemble a half-structure of the solution-grown ones. The oriented surface-bound metal-organic frameworks (sMOFs) are obtained via a one-step solvothermal approach, rather than in a layer-by-layer approach. The MOFs are grown on Si/SiOx substrates modified with an organic monolayer or on glass substrates covered with a transparent conductive oxide (TCO). Regardless of the different morphologies, the crystallographic packing is nearly identical and is not affected by the type of anion, nor by solution versus the surface chemistry. A propeller-type arrangement of the non-chiral ligands around the metal center affords a chiral structure with two geometrically different helical channels in a 2:1 ratio with the same handedness. To demonstrate the accessibility and porosity of the macroscopically-oriented channels, a chromophore (resorufin sodium salt) was successfully embedded into the channels of the crystals by diffusion from solution, resulting in fluorescent crystals. These "colored" crystals displayed polarized emission (red) with a high polarization ratio because of the alignment of these dyes imposed by the crystallographic structure. A second-harmonic generation (SHG) study revealed Kleinman-symmetry forbidden non-linear optical properties. These surface-bound and oriented SHG-active MOFs have the potential for use as single non-linear optical (NLO) devices.
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(2020) Advanced Materials Interfaces. 7, 16, 2000718. Abstract
An integrated electrochromic-hybrid supercapacitor (EHSC) is demonstrated; the device's operation (charging-discharging) is indicated by optical changes (from colored to transparent). The heart of the device is an electrochromic metallo-organic layer that functions as both the battery-type electrode and the charge indicator. The capacitive electrode is a layered composite of multiwalled carbon nanotubes (MWCNTs) and a conductive polymer (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, PEDOT:PSS). The device operates under low potentials (-0.6 to 2 V), displays high energy and power densities (approximate to 2.2 Wh kg(-1)and approximate to 2529 W kg(-1)), a high coulomb efficiency (99%), a short charging time (approximate to 2 s), and a charge retention (V-1/2) of approximate to 60 min. Stability, both in color and energy, for more than 1000 consecutive charging-discharging cycles is demonstrated. No significant changes in device temperature are indicated under the operating conditions. The EHSC is wired with a conventional circuit board to be charged and subsequently to operate a diode. The results demonstrate the potential of metallo-organic assemblies for usage in these types of supercapacitors.
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(2020) Nature Communications. 11, 1, 380. Abstract
Naturally occurring single crystals having a multidomain morphology are a counterintuitive phenonomon: the macroscopic appearance is expected to follow the symmetry of the unit cell. Growing such crystals in the lab is a great challenge, especially from organic molecules. We achieve here uniform metallo-organic crystals that exhibit single crystallinity with apparently distinct domains and chirality. The chirality is present at both the molecular and macroscopic levels, although only achiral elements are used. \u201cYo-yo\u201d-like structures having opposite helical handedness evolve from initially formed seemingly achiral cylinders. This non-polyhedral morphology coexists with a continuous coordination network forming homochiral channels. This work sheds light on the enigmatic aspects of fascinating crystallization processes occurring in biological mineralization. Our findings open up opportunities to generate new porous and hierarchical chiral materials.
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(2020) Angewandte Chemie - International Edition. 59, 7, p. 2612-2617 Abstract
We demonstrate controlled charge trapping and release, accompanied by multiple color changes in a metalloorganic bilayer. The dual functionality of the metallo-organic materials provides fundamental insight into the metal-mediated electron transport pathways. The electrochemical processes are visualized by distinct, four color-to-color transitions: red, transparent, orange, and brown. The bilayer is composed of two elements: 1) a nanoscale gate consisting of a layer of well-defined polypyridyl ruthenium complexes bound to a flexible transparent electrode, and 2) a charge storage layer consisting of isostructural iron complexes attached to the surface of the gate. This gate mediates or blocks electron transport in response to an applied voltage. The charge storage and release depend on the oxidation state of the layer of ruthenium complexes (= gate). Combining electrochemistry with optical data revealed mechanistic information: the brown coloration of the bilayer directly relates to the formation of intermediate ruthenium species, providing evidence for catalytic positive charge release mediated through the gate.
2019
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(2019) ChemistrySelect. 4, 41, p. 12104-12110 Abstract
Flower-like gold microstructures are directly formed from solution on the surface of nanometric molecular assemblies. We show that the size and morphology of these microstructures are controlled by the nanoscale thickness of the assemblies, which consist of ruthenium polypyridyl complexes crosslinked with a palladium salt. Gold electrodeposition on these ultrathin (3-15 nm) molecular assemblies, bound to a conductive substrate, follows an instantaneous nucleation regime that results in multiple small clusters. On thicker assemblies (15-55 nm) a progressive nucleation mode is dominant, which leads to the formation of larger (up to 50 times) and highly branched microstructures. The ability to control the characteristics of these microstructures by nanoscale assemblies is based on the mechanistic insights of the nucleation and growth processes obtained by electrochemical means and scanning electron microscopy (SEM) measurement.
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(2019) ChemPhysChem. 19, p. 2403-2407 Abstract
Herein, we present an approach that integrates molecular logic functions using surface-confined metallo-organic assemblies. These assemblies are electrochromic and mimic the behaviour of logic elements. The logic elements are addressed individually by electrochemical methods, and their outputs are simultaneously read-out optically by UV/Vis absorption spectroscopy. The versatility of our setup is demonstrated by the integration of two multi-component assemblies; each acting as ternary logic elements. We used also a laminated cell configuration to demonstrate color-to-color and color-to-transparent transitions. This concept offers a route for the future development of devices with multiple logic states.
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(2019) ACS Applied Materials and Interfaces. 11, 25, p. 22858-22868 Abstract
We demonstrate the on-surface formation of homogeneous and uniform electrochromic films via ultrasonic spray coating. This fully automated process is capable of fabricating metallo-organic films on transparent conducting oxides (TCOs) on glass or flexible poly(ethylene terephthalate) (PET) with surface areas of up to 36 cm2 and film thicknesses of half a micron. The assembly process involves alternatingly spray-coating dilute solutions of structurally well-defined iron polypyridyl ([Fe(mbpy-py)3]2+) complexes and bis(benzonitrile)palladium dichloride (Pd(PhCN)2Cl2) onto conductive substrates, where the latter palladium salt was used as the inorganic cross-linker. The on-surface self-assembled three-dimensional networks are intensely colored and were subsequently integrated into laminated electrochromic devices (ECDs) containing a lithium-based gel electrolyte. The ECDs retain their intense color in the ground state, having a ΔTmax of 4049% at λmax ≈ 600 nm, and can be operated for up to 1500 redox cycles. The fluorine-doped tin oxide counter electrode coated with poly(3,4-ethylene-dioxythiophene)polystyrene sulfonate (PEDOT:PSS) as a charge-storage layer resulted in these stable devices. A significant decrease in the potential window of ΔE ≈ 2.5 V was achieved by using a metal grid on PET as the counter electrode. The operation of the electrochromic films is diffusion-controlled, and the diffusion coefficients (Df) reflect their molecular densities. During these studies, we found that ClO4 is a suitable counterion of the lithium-based electrolytes for optimal ECD performance.
2018
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(2018) Angewandte Chemie - International Edition. 57, 41, p. 13459-13464 Abstract
We demonstrate the mediation of charge transport and release in thin films and devices by shifting the redox properties of layers of metal complexes by light. The nanoscale surface arrangement of both photo- and electrochemically-active components is essential for the function of the thin films. Layers of well-defined ruthenium complexes on indium-tin-oxide electrodes provide electron-transport channels that allow the electrochemical addressing of layers of isostructural cobalt complexes. These cobalt complexes are electrochemically inactive when assembled directly on transparent metal-oxide electrodes. The interlayer of ruthenium complexes on such electrodes allows irreversible oxidation of the cobalt complexes. However, shifting the redox properties of the ruthenium complexes by excitation with light opens up an electron-transport channel to reduce the cobalt complexes; hence releasing the trapped positive charges.
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(2018) Advanced Materials. 30, 41, 1706641. Abstract
Electrochromic films undergo optical changes in response to a redox stimulus. This intriguing phenomenon can be used for a wide range of applications, including smart windows, sensors, color displays, and memory elements. Despite the rapid progress of late, designing suitable electrochromic materials that offer low-cost production, appealing colors, and pronounced optical contrast with high efficiency, as well as long-term stability remains an engineering challenge. Solid-state metal oxides, liquid crystals, and organic polymers have been for many years the leading candidates, successfully making their way into commercial products. An alternative class of materials relies on metal complexes that can be processed from solution, offer a variety of colors, and have metal-centered stable and reversible redox chemistry. These metallo-organic materials possess a full range of electrochromic properties, including ultrahigh coloration efficiencies, and cyclic stability. Here, some of the recent scientific developments in this field are highlighted.
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(2018) Journal of the American Chemical Society. 140, 29, p. 9132-9139 Abstract
In this work, we report the formation of superstructures assembled from organic tubular crystals mediated by metal-coordination chemistry. This template-free process involves the crystallization of molecules into crystals having a rectangular and uniform morphology, which then go on to fuse together into multibranched superstructures. The initially hollow and organic crystals are obtained under solvothermal conditions in the presence of a copper salt, whereas the superstructures are subsequently formed by aging the reaction mixture at room temperature. The mild thermodynamic conditions and the favorable kinetics of this unique self-assembly process allowed us to ex-situ monitor the superstructure formation by electron microscopy, highlighting a pivotal and unusual role for copper ions in their formation and stabilization.
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(2018) Journal of the American Chemical Society. 140, 26, p. 8162-8171 Abstract
We demonstrate that molecular gradients on an organic monolayer is formed by preferential binding of ruthenium complexes from solutions also containing equimolar amounts of isostructural osmium complexes. The monolayer consists of a nanometer-thick assembly of 1,3,5-tris(4-pyridylethenyl)benzene (TPEB) covalently attached to a silicon or metal-oxide surface. The molecular gradient of ruthenium and osmium complexes is orthogonal to the surface plane. This gradient propagates throughout the molecular assembly with thicknesses over 30 nm. Using other monolayers consisting of closely related organic molecules or metal complexes results in the formation of molecular assemblies having an homogeneous and equimolar distribution of ruthenium and osmium complexes. Spectroscopic and computational studies revealed that the geometry of the complexes and the electronic properties of their ligands are nearly identical. These subtle differences cause the isostructural osmium and ruthenium complexes to pack differently on modified surfaces as also demonstrated in crystals grown from solution. The different packing behavior, combined with the organic monolayer significantly contributes to the observed differences in chemical composition on the surface.
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(2018) ACS Omega. 3, 6, p. 6533-6539 Abstract
In this paper, we demonstrate the formation of hybrid nanostructures consisting of two distinctive components mainly in a one-to-one ratio. Thermolysis of inorganic nanotubes (INT) and closed-cage, inorganic fullerene-like (IF) nanoparticles decorated with a dense coating of metallic nanoparticles (M = Au, Ag, Pd) results in migration of relatively small NPs or surface-enhanced diffusion of atoms or clusters, generating larger particles (ripening). AuNP growth on the surface of INTs has been captured in real time using in situ electron microscopy measurements. Reaction of the AuNP-decorated INTs with an alkylthiol results in a chemically induced NP fusion process at room temperature. The NPs do not dissociate from the surfaces of the INTs and IFs, but for proximate IFs we observed fusion between AuNPs originating from different IFs.
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(2018) Crystal Growth and Design. 18, 4, p. 1967-1977 Abstract
In this study we have used a tetrahedral oligopyridine and four different fluoroiodides to obtain three-dimensional (3D) nanoporous halogen-bonded cocrystals. Many of the halogen-bonded cocrystals reported to date are one-dimensional chains or two-dimensional sheet-like structures; these new cocrystals possess multiple channels of 300-800 Å
3 volume per unit cell. The extended 3D channels can be designed by varying the molecular structure of the halogen bond donor and were found to occupy 20-40% of the unit cell volume. The N···I distances in our cocrystals are ∼80% of the sum of the van der Waals radii of the nitrogen and iodine atoms, and the N···I-C angles are nearly linear. Noncovalent stacking (π-π) interactions as well as H-bonding to solvents were also observed in some of the cocrystals. The supramolecular structures obtained in this study are effectively derived out of different donor-acceptor XB interactions, solvent and other noncovalent interactions. The weak nature of halogen bonds as well as the existence of multiple interactions make these cocrystal structures and their supramolecular organization difficult to predict. Even though this work does not attempt to single out the individual contributions of different factors affecting the supramolecular assemblies, we show here how the structure and hence the potential porosity of the halogen-bonded organic frameworks can be varied via careful design and combination of structurally different donors and acceptors. -
(2018) Journal of Physical Chemistry C. 122, 12, p. 6748-6759 Abstract
We report here a unique and efficient methodology for the surface functionalization of closed-cage inorganic fullerene-like (IF) nanoparticles and inorganic nanotubes (INTs) composed of two-dimensional nanomaterials of transition-metal chalcogenides (MS
2; M = W or Mo). The first step is the physical coverage of these robust inorganic materials with monodispersed and dense monolayers of gold, silver, and palladium nanoparticles. The structural continuity at the interface between the IF/INT and the metallic nanoparticles is investigated. Lattice matching between these nanocrystalline materials and strong chemical affinity lead to efficient binding of the metallic nanoparticles onto the outer sulfide layer of the MS
2-based structures. It is shown that this functionalization results in narrowing of the IF/INT optical band gap, increased work function, and improved surface-enhanced Raman scattering. In the second step, functionalization of the surface-bound nanoparticles is carried out by a ligand-exchange reaction. This ligand exchange involving the tetraoctylammonium bromide capping layer and an alkyl thiol enhances the solubility (∼10×) of the otherwise nearly insoluble materials in organic solvents. The scope of this method is further demonstrated by introducing a ruthenium(II) polypyridyl complex on the surface of the surface-bound AuNPs to generate fluorescent multicomponent materials. -
(2018) Langmuir. 34, 7, p. 2464-2470 Abstract
We report here a new methodology for the formation of freestanding nanotubes composed of individual gold nanoparticles (NPs) cross-linked by coordination complexes or porphyrin molecules using WS
2 nanotubes (INT-WS
2) as a template. Our method consists of three steps: (i) coverage of these robust inorganic materials with monodispersed and dense monolayers of gold NPs, (ii) formation of a molecular AuNP network by exposing these decorated tubes to solutions containing a ruthenium polypyridyl complex or meso-tetra(4-pyridyl)porphyrin, and (iii) removal of the INT-WS
2 template with a hydrogen peroxide solution. Nanoindentation of the template-free AuNP tubes with atomic force microscopy indicates a radial elastic modulus of 4 GPa. The template-free molecular AuNP tubes are characterized using scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, and micro-Raman spectroscopy. The methodology provides a convenient and scalable strategy for the realization of molecular AuNP tubes with a defined length and diameter, depending on the dimensions of the template.
2017
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(2017) Journal of the American Chemical Society. 139, 33, p. 11471-11481 Abstract
In this study, we demonstrate a versatile approach for the formation of electrochromic nanoscale assemblies on transparent conductive oxides on both rigid and flexible substrates. Our method is based on the application of alternating spin-coated layers of well-defined metal polypyridyl complexes and a palladium(II) salt to form electrochemically addressable films with a high chromophore density. By varying the central metal ion of the polypyridyl complexes (Os, Ru, and" Fe) and their ligands and by mixing these complexes, coatings with a wide range of colors can be achieved. These coatings cover a large area of RGB color space. The coloration intensities of these nanoscale films can be tuned by "the number of deposition steps. The materials have very attractive ON/OFF ratios, electrochemical stabilities, and coloration efficiencies. Reversible color-to-colorless and color-to-color transitions were demonstrated, and the films were further integrated into sandwich cells.
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(2017) Journal Of Physical Chemistry B. 121, 29, p. 6981-6988 Abstract
Redox reactions play key roles in fundamental biological processes. The related spatial organization of donors and acceptors is assumed to undergo evolutionary optimization facilitating charge mobilization within the relevant biological context. Experimental information from submolecular functional sites is needed to understand the organization strategies and driving forces involved in the self-development of structure-function relationships. Here we exploit chemically resolved electrical measurements (CREM) to probe the atom-specific electrostatic potentials (ESPs) in artificial arrays of bacteriochlorophyll (BChl) derivatives that provide model systems for photoexcited (hot) electron donation and withdrawal. On the basis of computations we show that native BChl's in the photosynthetic reaction center (RC) self-assemble at their ground-state as aligned gates for functional charge transfer. The combined computational and experimental results further reveal how site-specific polarizability perpendicular to the molecular plane enhances the hot-electron transport. Maximal transport efficiency is predicted for a specific, ∼5 Å, distance above the center of the metalized BChl, which is in remarkably close agreement with the distance and mutual orientation of corresponding native cofactors. These findings provide new metrics and guidelines for analysis of biological redox centers and for designing charge mobilizing machines such as artificial photosynthesis.
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(2017) ChemistrySelect. 2, 2, p. 577-582 Abstract
We demonstrated a heterogeneous and efficient photocatalytic method to reduce the bacterial content in water with reusable metallo-organic thin films. These functionalized assemblies constructed of a network of polypyridyl complexes generate reactive singlet oxygen upon visible light irradiation that induces lysis of bacterial cells. The singlet oxygen photosensitizers are robust and were successfully applied to surface water samples contaminated with both Gram-positive and -negative bacteria. The catalytic formation of singlet oxygen by our antibacterial coatings was confirmed chemically by reactions with both inorganic and organic compounds.
2016
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(2016) Journal of the American Chemical Society. 138, 50, p. 16398-16406 Abstract
In this paper, we demonstrate how photo chemically enhancing the permeability of metal-organic assemblies results in a significant enhancement of the electrochemical activity of metal complexes located within the assembly. The molecular assemblies consist of different layers of redox-active metal complexes ([M(mbpy-py)(3)][PF6](2); M = Ru or Os) that are separated by redox-inactive spacers consisting of 1,4-bis[2-(4-pyridyl)ethenyl]benzene (BPEB) and PdCl2 of variable thicknesses (0-13.4 nm). UV-irradiation (lambda = 254 nm) of our assemblies induces a photochemical reaction in the redox-inactive spacer increasing the permeability of the assembly. The observed increase was evident by trapping organic ((Bu4NBF4)-Bu-n) and inorganic (NiCl2) salts inside the assemblies, and by evaluating the electrochemical response of quinones absorbed inside the molecular assemblies before and after UV irradiation. The increase in permeability is reflected by higher currents and a change in the directionality of electron transfer, i.e., from mono- to bidirectional, between the redox-active metal complexes and the electrode surface. The supramolecular structure of the assemblies dominates the overall electron transfer properties and overrules possible electron transfer mediated by the extensive pi-conjugation of its individual organic components.
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(2016) Chemical Communications. 52, 52, p. 8079-8082 Abstract
We report here how the crystallinity of AuNPs and the choice of binding sites of molecular cross-linkers control their aggregation. The combination of different binding moieties (N-oxides, ArF-I) and the reactivity of the particles' facets allow control over the organization and crystallinity of the AuNP assemblies.
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(2016) Russian Physics Journal. 58, 11, p. 1569-1572 Abstract
Conditions are described under which films of [(aminophenyl)azo]pyridine are formed by molecular layer epitaxy, and their optical absorption and x-ray photoelectron spectra are investigated. The nonlinear properties of such structures are described with the help of measurements of the intensity of second harmonic generation as a function of the angle of incidence.
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(2016) Chemical Communications. 52, 13, p. 2683-2686 Abstract
We demonstrate a process that results in the formation of palladium nanoparticles during the assembly of molecular thin films. These nanoparticles are embedded in the films and are generated by a chemical reaction of the counter anions of the molecular components with the metal salt that is used for cross-linking these components.
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(2016) Chemistry - A European Journal. 22, 5, p. 1728-1734 Abstract
The immobilization of metal nanoparticles (NPs) with molecular control over their organization is challenging. Herein, we report the formation of molecularly cross-linked AuNP assemblies using a layer-by-layer approach. We observed four types of assemblies: 1) small aggregates of individual AuNPs, 2) large aggregates of individual AuNPs, 3) networks of fused AuNPs, and 4) gold islands. Interestingly, these assemblies with the different cross-linkers and capping layers represent different stages in the complete fusion of AuNPs to afford islands of continuous gold. We demonstrate that the stability toward fusion of the nanoparticles of the on-surface structures can be controlled by the reactivity of the cross-linkers and the hydrophilicity/hydrophobicity of the nanoparticles.
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(2016) Journal of Materials Chemistry C. 4, 21, p. 4634-4639 Abstract
The fundamental science behind the design of organic photovoltaic (OPV) cells lies in the formation of energy level gradients for efficient charge separation and collection. Tuning the energy levels at the device electrodes by the right choice of the components is a key requirement for achieving enhanced characteristics. Here we demonstrate control and optimization of OPV cell performance by using a set of polypyridyl complexes based on iron, ruthenium, and osmium centers with tunable frontier orbital energies as interlayers for inverted bulk heterojunction solar cells. We found that changing the metal center of isostructural transition-metal complexes results in evident shifts of the HOMO and LUMO energy levels and the work functions of the corresponding interlayers, which has a prominent effect on the device performance. We generalize our approach by combining the interlayers with different sets of photoactive materials to test the electron transporting as well as the hole blocking characteristics of the interlayers.
2015
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(2015) Chemistry (Weinheim an der Bergstrasse, Germany). 21, 45, p. 16113-16125 Abstract
This contribution describes the reactivity of a zero-valent palladium phosphine complex with substrates that contain both an aryl halide moiety and an unsaturated carbon-carbon bond. Although η2-coordination of the metal center to a C=C or C≡C unit is kinetically favored, aryl halide bond activation is favored thermodynamically. These quantitative transformations proceed under mild reaction conditions in solution or in the solid state. Kinetic measurements indicate that formation of η2-coordination complexes are not nonproductive side-equilibria, but observable (and in several cases even isolated) intermediates en route to aryl halide bond cleavage. At the same time, DFT calculations show that the reaction with palladium may proceed through a dissociation-oxidative addition mechanism rather than through a haptotropic intramolecular process (i.e., ring walking). Furthermore, the transition state involves coordination of a third phosphine to the palladium center, which is lost during the oxidative addition as the C-halide bond is being broken. Interestingly, selective activation of aryl halides has been demonstrated by adding reactive aryl halides to the η2-coordination complexes. The product distribution can be controlled by the concentration of the reactants and/or the presence of excess phosphine.
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(2015) Crystal Growth and Design. 15, 10, p. 4756-4759 Abstract
The prediction of supramolecular structures involving different weak interactions is challenging. In this study, single-atom modifications to the molecular structure allow us to address their hierarchy. The resulting series of unimolecular assemblies are mainly based on halogen bonding (XB), hydrogen bonding (HB), or a combination of both. By varying the XB donor (F, Cl, Br, and I) and the XB and HB acceptors (pyridine vs pyridine-N-oxide) we can control the primary motifs directing the structure.
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(2015) Angewandte Chemie (International ed. in English). 54, 42, p. 12457-12462 Abstract
We demonstrate how the distance over which electron transfer occurs through organic materials can be controlled and extended. Coating of conductive surfaces with nanoscale layers of redox-active metal complexes allows the electrochemical addressing of distant layers that are otherwise electrochemically silent. Our materials can pass electrons selectively in directions that are determined by positioning of layers of metal complexes and the distances between them. These electron-transfer processes can be made dominantly uni- or bidirectional. The design involves 1) a set of isostructural metal complexes with different electron affinities, 2) a scalable metal-organic spacer, and 3) a versatile assembly approach that allows systematic variation of composition, structure, and electron-transfer properties. We control the electrochemical communication between interfaces by the deposition sequence and the spacer length, therefore we are able to program the bulk properties of the assemblies.
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(2015) Journal of the American Chemical Society. 137, 12, p. 4050-4053 Abstract
We demonstrate high-performance electrochromic assemblies that exhibit a practical combination of low-voltage operation and efficient electrochromic switching as well as long-term thermal and redox stability (1.12 × 105 cycles). Our molecular assemblies can be integrated into a solid-state configuration. Furthermore, we also show how the molecular structure of the chromophores correlates with the materials' growth and function. The coloration efficiencies of our assemblies are higher than those of inorganic materials and many conducting polymers, in addition to offering an alternative fabrication approach.
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(2015) Organometallics. 34, 6, p. 1098-1106 Abstract
This combined experimental and computational study demonstrates how a metal center can drastically influence the reactivity of a coordinated ligand. We found that the consecutive formation of zerovalent bimetallic complexes proceeds by fast n2-C - C coordination of only one Pd(PEt3)2 moiety, which then significantly slows down the subsequent reaction. Electronic effects induced by complexation of the first metal center have a major effect on the subsequent formation of the bimetallic complexes. These effects are reduced by partial fluorination of the bis(vinylpyridine)-arene ligand. The monometallic complexes display migration of the Pd(PEt3)2 moiety between the two olefinic bonds of the corresponding ligand, as indicated by various solution NMR experiments, including variable-temperature NMR spectroscopy, 2D 1H-1H exchange spectroscopy, and spin saturation transfer. Density functional theory studies were performed at the SMD(toluene)-PBE0+d(v3)/B2//B97D/B1 level of theory.
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(2015) Journal of the American Chemical Society. 137, 1, p. 226-231 Abstract
Despite the tremendous progress made in the design of supramolecular and inorganic materials, it still remains a great challenge to obtain uniform structures with tailored size and shape. Metal-organic frameworks and infinite coordination polymers are examples of rapidly emerging materials with useful properties, yet limited morphological control. In this paper, we report the solvothermal synthesis of diverse metal-organic (sub)-microstructures with a high degree of uniformity. The porous and thermally robust monodisperse crystalline solids consist of tetrahedral polypyridyl ligands and nickel or copper ions. Our bottom-up approach demonstrates the direct assembly of these materials without the addition of any surfactants or modulators. Reaction parameters in combination with molecular structure encoding are the keys to size-shape control and structural uniformity of our metal-organic materials.
2014
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(2014) Accounts of Chemical Research. 47, 12, p. 3407-3416 Abstract
(Figure Presented) CONSPECTUS: Since the first description of coordination complexes, many types of metal-ligand interactions have creatively been used in the chemical sciences. The rich coordination chemistry of pyridine-type ligands has contributed significantly to the incorporation of diverse metal ions into functional materials. Here we discuss molecular assemblies (MAs) formed with a variety of pyridine-type compounds and a metal containing cross-linker (e.g., PdCl2(PhCN2)). These MAs are formed using Layer-by-Layer (LbL) deposition from solution that allows for precise fitting of the assembly properties through molecular programming. The position of each component can be controlled by altering the assembly sequence, while the degree of intermolecular interactions can be varied by the level of π-conjugation and the availability of metal coordination sites. By setting the structural parameters (e.g., bond angles, number of coordination sites, geometry) of the ligand, control over MA structure was achieved, resulting in surface-confined metal-organic networks and oligomers. Unlike MAs that are constructed with organic ligands, MAs with polypyridyl complexes of ruthenium, osmium, and cobalt are active participants in their own formation and amplify the growth of the incoming molecular layer. Such a self-propagating behavior for molecular systems is rare, and the mechanism of their formation will be discussed. These exponentially growing MAs are capable of storing metal salts that can be used during the buildup of additional molecular layers. Various parameters influencing the film growth mechanism will be presented, including (i) the number of binding sites and geometry of the organic ligands, (ii) the metal and the structure of the polypyridyl complexes, (iii) the influence of the metal cross-linker (e.g., second or third row transition metals), and (iv) the deposition conditions. By systematic variation of these parameters, switching between linear and exponential growth could be demonstrated for MAs containing structurally well-defined polypyridyl complexes. The porosity of the MAs has been estimated by using electrochemically active probes. Incorporating multiple polypyridyl complexes of osmium and ruthenium into a single assembly give rise to composite materials that exhibit interesting electrochemical and electrochromic properties. These functional composites are especially attractive as they exhibit properties that neither of each metal complex possesses individually. Some of our MAs have very high coloration efficiencies, redox stability, fast responsive times and operate at voltages
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(2014) Journal of the American Chemical Society. 136, 34, p. 11926-11929 Abstract
Enabling and understanding new methodologies to fabricate molecular assemblies driven by intermolecular interactions is fundamental in chemistry. Such forces can be used to control crystal growth and enable surface-confinement of these materials, which remains challenging. Here we demonstrate for the first time, a solvent-free on-surface crystal-to-co-crystal conversion process driven by halogen bonding (XB). By exposing a polycrystalline organic material, consisting of a XB-acceptor moiety, to the vapors of a complementary XB-donor compound, the corresponding halogen-bonded co-crystals were formed. Furthermore, we demonstrate that this approach can also be utilized for non-crystalline materials to afford surface-confined organic composites. Our stepwise vapor-based approach offers a new strategy for the formation of hybrid supramolecular materials.
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(2014) Chemical Communications. 50, 60, p. 8154-8156 Abstract
We demonstrated the one-pot disassembly of self-propagating molecular assemblies (SPMAs) by ligand exchange and the subsequent covalent binding of the molecular components to other surfaces. These functionalized surfaces are suitable for regenerating the SPMAs.
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(2014) Chemical Communications. 50, 35, p. 4635-4638 Abstract
Here we show a step-wise approach for the formation of continuous shell-structures on surface-confined gold nanoparticles. The nanoparticle-cores induce order in the shell-structure, which consists of metal-organic networks. Communication between the organic and inorganic parts is reflected in their optical properties.
2013
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(2013) Journal of the American Chemical Society. 135, 44, p. 16533-16544 Abstract
The controlled deposition of metal complexes from solution on inorganic surfaces offers access to functional materials that otherwise would be elusive. For such surface-confined interfaces to form, specific assembly sequences are often used. We show here that varying the assembly sequence of two well-defined and iso-structural osmium and ruthenium polypyridyl complexes results in interfaces with strikingly different spectroelectrochemical properties. Successive deposition of redox-active layers of osmium and ruthenium polypyridyl complexes, leads to self-propagating molecular assemblies (SPMAs) with distinct internal interfaces and individually addressable components. In contrast, the clear separation of these interfaces upon sequential deposition of these two complexes, results in charge trapping or electrochemical communication between the metal centers, as a function of layer thickness and applied assembly sequence. The SPMAs were characterized using a variety of techniques, including: UV-vis spectroscopy, spectroscopic ellipsometry, electrochemistry, synchrotron X-ray reflectivity, angle-resolved X-ray photoelectron spectroscopy, and spectroelectrochemistry. The combined data demonstrate that the sequence-dependent assembly is a decisive factor that influences and provides the material properties that are difficult to obtain otherwise.
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(2013) Journal of the American Chemical Society. 135, 45, p. 17052-17059 Abstract
Surface-confined double-helical polymers are generated by dynamic covalent assembly with preservation of chirality, metal coordination environment, and oxidation state of the precursor complexes. This one-step procedure involves both in solution and solution-to-surface assembly and resulted in chiral interfaces where pairs of ligands are wrapped around arrays of metal ions. In-plane XRD experiments revealed the formation of a highly ordered structure along the substrate surface. The chirality of the surfaces is expressed by the selective recognition of 3,4-dihydroxyphenylalanine (DOPA). The CD measurements show a response of the Δ-polymer-modified quartz substrates toward d-DOPA, whereas no change was observed after treatment with l-DOPA. These coordination-based interfaces assembled on metal-oxide substrates in combination with a redox-probe, [Os(bpy)3](PF6)2, in solution can resemble the behavior of a rectifier.
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(2013) Chemistry-A European Journal. 19, 27, p. 8821-8831 Abstract
Molecular assemblies (MAs) of oligofurans and oligothiophenes were formed from solutions on various substrates. These films were obtained by alternating deposition of organic chromophores (oligofurans or oligothiophenes) and a palladium salt. These coordination-based MAs were characterized by UV/Vis spectroscopy, spectroscopic ellipsometry, atomic force microscopy (AFM), X-ray reflectivity (XRR), X-ray photoelectron spectroscopy (XPS), and electrochemistry. The MAs exhibit similar electrochemical behavior and their growth and structure are apparently not affected when different organic template layers are used. The density of the MAs is a function of the structure of the molecular component. The oligothiophene density is approximately 50 % higher than that observed for the oligofuran-based assemblies. The optical and electrochemical properties of the MAs scale linearly with their thickness. The UV/Vis data indicate that upon increasing the film thickness, there is no significant conjugation between the metal-separated organic chromophores. DFT calculations confirmed that the HOMO-LUMO gap of the surface-bound oligofuran and oligothiophene metal oligomers do not change significantly upon increasing their chain length. However, electrochemical measurements indicate that the susceptibility of the MAs towards oxidation is dependent on the number of chromophore units. Coming to the surface: Surface-bound molecular assemblies (MA-1 to MA-3) were constructed by an iterative solution deposition approach. The formed MAs consist of either oligofuran or oligothiophene chromophores coordinated to a palladium salt.
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(2013) Organometallics. 32, 10, p. 3074-3082 Abstract
We present a mechanistic study demonstrating selective Arf-Cl bond activation preceded by η2 coordination of Pd(PEt 3)2 to a C=C moiety of a partially fluorinated substrate. An intramolecular ring-walking process to activate the Arf-Cl bond is plausible, but an intermolecular reaction becomes dominant in the presence of PEt3. The latter pathway is significantly enhanced, since PEt 3 promotes dissociation of Pd(PEt3)3 from the C=C moiety followed by activation of the Arf-Cl bond. Our observations also show that PEt3 can be used to control reaction selectivity. The experimental observations are supported by density functional theory (DFT) calculations (at the SMD(toluene)-DSD-PBEP86/cc-pV(D+d)Z-PP//DF- PBE+dv2/SDD(d) level of theory).
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(2013) Micro- And Nanotechnology Sensors, Systems, And Applications V. Abstract
Since the development of molecule-based sensors and the introduction of molecules mimicking the behavior of the AND gate in solution by de Silva in 1993, molecular (Boolean) Logic and Computing (MBLC) has become increasingly popular. The molecular approach toward Boolean logic resulted in intriguing proofs of concepts in solution including logic gates, half-adders, multiplexers, and flip-flop logic circuits. Molecular assemblies can perform diverse logic tasks by reconfiguring their inputs. Our recent research activities focus on MBLC with electrochromic polymers and immobilized polypyridyl complexes on solid support. We have designed a series of coordination-based thin films that are formed linearly by stepwise wet-chemical deposition or by self-propagating molecular assembly. The electrochromic properties of these films can be used for (i) detecting various analytes in solution and in the air, (ii) MBLC, (iii) electron-transfer studies, and (iv) interlayers for efficient inverted bulk-heterojunction solar cells. Our concept toward MBLC with functionalized surfaces is applicable to electrochemical and chemical inputs coupled with optical readout. Using this approach, we demonstrated various logic architectures with redox-active functionalized surfaces. Electrochemically operated sequential logic systems (e. g., flip-flops), multi-valued logic, and multi-state memory have been designed, which can improve computational power without increasing spatial requirements. Applying multi-valued digits in data storage and information processing could exponentially increase memory capacity. Our approach is applicable to highly diverse electrochromic thin films that operate at practical voltages (
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(2013) Chemical Communications. 49, 34, p. 3531-3533 Abstract
Halogen bonding between complementary organic monolayers was directly observed in an organic environment using force spectroscopy. This non-covalent interaction is significantly affected by the nature of the organic media. We also demonstrated the effect of lateral packing interactions on the optical properties of the monolayers.x
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(2013) Angewandte Chemie - International Edition. 52, 2, p. 704-709 Abstract
Variation's what you need: variation of the assembly sequence in which layers of two isostructural metal complexes are built up leads to molecular materials with electrochemical properties that depend on the assembly sequence. These properties vary from reversible electron transfer to unidirectional current flows and even charge trapping. The sequence-dependent assembly strategy has implications for various disciplines that involve self-assembly.
2012
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(2012) Angewandte Chemie - International Edition. 51, 49, p. 12268-12271 Abstract
To mix or not to mix: Integrated (left) and segregated (right) assemblies were obtained upon treating functionalized γ-Fe2O3 nanoparticles (NPs) with AuNPs. Their binary nature is controlled by the capping layer of the γ-Fe2O3 NPs and the AuNP initial aggregation state. The segregated assembly formation is induced by AuNP aggregates which act as nucleation sites for growth of the γ-Fe 2O3 NP domains.
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(2012) Angewandte Chemie - International Edition. 51, 34, p. 8598-8601 Abstract
Plug and play: The mimicking of integrated circuits by using two individual monolayers (molecular chips) is shown. These monolayers can be individually addressed using identical inputs. Upon combination of their optical outputs, the input/output characteristics of a molecular encoder is obtained. Since the encoder functionality is only displayed when both chips are active, the device behaves according to a plug-and-play principle.
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(2012) Angewandte Chemie - International Edition. 51, 29, p. 7142-7145 Abstract
Nanoparticles (NPs) may be exploited to make practical materials that are capable of the selective detection of (bio)molecules.[17] Sensing with NPs often depends on the ability to selectively form aggregates. For instance, Mirkin et al. introduced a bio-barcode amplification method for ultrasensitive protein detection.[8] Another important study involves the detection of copper ions by hybrid AuNP assemblies in click chemistry.[9, 10] The structures of AuNPbased assemblies can also be controlled electrochemically or by light.[1113] However, despite these successes, controlling the properties and structure of NP-based assemblies with organic cross-linkers (CLs) still remains a challenge.[14] We have previously shown that the molecular geometry of CLs and the number of possible NP binding sites are related to the formation of hybrid AuNP assemblies and their associated optical properties.[15]
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(2012) Organometallics. 31, 4, p. 1271-1274 Abstract
We report here that the undesired hydrodehalogenation in cross-coupling reactions with fluorinated substrates involves water as a possible hydrogen source. Moreover, the product distribution (hydrodehalogenation vs carbon-carbon coupling) can be controlled by varying the phosphine substituents. Significant hydrodehalogenation occurs prior to the formation of Ar F-Pd(II)-Br complexes. DFT calculations were used to evaluate a direct hydrodehalogenation route with a phosphine and water. These findings provide new mechanistic insight into aryl-Br bond activation with fluorinated substrates and selective arene functionalization.
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(2012) Chemical Science. 3, 1, p. 66-71 Abstract
A series of six self-propagating molecular assemblies (SPMAs) were generated by alternative solution-deposition of ruthenium polypyridyl complexes and d8 palladium and platinum salts on glass and silicon substrates. The d6 polypyridyl complexes have three pyridine units available for forming networks by coordination to the metal salts. This two-step film growth process is fast (15 min/step) and can be carried out conveniently under ambient conditions in air. The reactivity of the common metal salts (ML 2X2: M = Pd, X = Cl, L = PhCN, 1,5-cyclooctadiene (COD), SMe2 and M = Pt, X = Cl, Br, I, L = PhCN) is a dominant factor in the film growth. Although the assembly structures are comparable, their exponential growth can be controlled by varying the metals salts. The co-ligands, halides, and metal centers can be used to control the film thicknesses and light absorption intensities of the metal-to-ligand charge transfer (MLCT) bands by a factor of ∼3.5 for 13 deposition steps, whereas the surface morphologies and molecular densities of the SPMAs are similar. The surface-confined assemblies have been characterized using a combination of optical (UV/Vis, ellipsometry) spectroscopy, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and synchrotron X-ray reflectivity (XRR).
2011
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(2011) Angewandte Chemie - International Edition. 50, 50, p. 11846-11848 Abstract
Engineering crystals: Molecular single crystals of well-defined metal complexes selectively and reversibly undergo ligand-exchange reactions. These results are a prelude to sensing and new catalytic reactions within the confined space of a crystal lattice.
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(2011) Journal of Materials Chemistry. 21, 44, p. 17575-17581 Abstract
Since it was first suggested that molecules could be used for information processing, there has been a significant effort to generate systems that behave according to various logic schemes. Here we will summarize and discuss various approaches that serve one common goal: constructing a (bio)-molecular flip-flop. This logic circuit is at the core of random access memory (RAM) and is one of the pillars of sequential logic. We will highlight the concept underlying various approaches towards flip-flops and we will discuss how these constitute and expand the field of molecular logic. This multi-disciplinary approach results in various schemes that range from all photonic systems to transition metal complexes and hybrid nanoparticle/protein-based systems on solid supports.
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(2011) Journal of the American Chemical Society. 133, 36, p. 14264-14266 Abstract
This study provides insight into the internal structure of surface-confined molecular assemblies. The permeability of the layer-by-layer grown thin films can be controlled systematically by varying their composition and the structure of their molecular components. Moreover, the thickness can be used to control molecule permeation versus electron transfer.
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(2011) Accounts of Chemical Research. 44, 8, p. 563-573 Abstract
Stimuli responsive materials are capable of mimicking the operation characteristics of logic gates such as AND, OR, NOR, and even flip-flops. Since the development of molecular sensors and the introduction of the first AND gate in solution by de Silva in 1993, Molecular (Boolean) Logic and Computing (MBLC) has become increasingly popular. In this Account, we present recent research activities that focus on MBLC with electrochromic polymers and metal polypyridyl complexes on a solid support. Metal polypyridyl complexes act as useful sensors to a variety of analytes in solution (i.e., H(2)O, Fe(2+/3+), Cr(6+), NO(+)) and in the gas phase (NO in air). This information transfer, whether the analyte is present, is based on the reversible redox chemistry of the metal complexes, which are stable up to 200 degrees C in air. The concurrent changes in the optical properties are nondestructive and fast. In such a setup, the input is directly related to the output and, therefore, can be represented by one-input logic gates. These input-output relationships are extendable for mimicking the diverse functions of essential molecular logic gates and circuits within a set of Boolean algebraic operations. Such a molecular approach towards Boolean logic has yielded a series of proof-of-concept devices: logic gates, multiplexers, half-adders, and flip-flop logic circuits. MBLC is a versatile and, potentially, a parallel approach to silicon circuits: assemblies of these molecular gates can perform a wide variety of logic tasks through reconfiguration of their inputs. Although these developments do not require a semiconductor blueprint, similar guidelines such as signal propagation, gate-to-gate communication, propagation delay, and combinatorial and sequential logic will play a critical role in allowing this field to mature. For instance, gate-to-gate communication by chemical wiring of the gates with metal ions as electron carriers results in the integration of stand-alone systems: the output of one gate is used as the input for another gate. Using the same setup, we were able to display both combinatorial and sequential logic.We have demonstrated MBLC by coupling electrochemical inputs with optical readout, which resulted in various logic architectures built on a redox-active, functionalized surface. Electrochemically operated sequential logic systems such as flip-flops, multivalued logic, and multistate memory could enhance computational power without increasing spatial requirements. Applying multivalued digits in data storage could exponentially increase memory capacity. Furthermore, we evaluate the pros and cons of MBLC and identify targets for future research in this Account.
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(2011) ACS Nano. 5, 8, p. 6553-6563 Abstract
In this study halogen bonding (XB) is used as the driving force for the noncovalent assembly of gold nanoparticles (AuNPs) on silicon and quartz substrates functionalized with organic monolayers. The AuNPs are functionalized with XB-donor ligands, whereas the monolayers have pyridine groups as XB-acceptors. The surface-confined systems are formed by iteratively exposing the monolayers to solutions of organic cross-linkers having 2-4 pyridine groups and functionalized AuNPs. UV-vis spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM) reveal how the structure of the resulting surface-bound assemblies are controlled by (i) the properties of the monolayers, (ii) the molecular structure and the number of XB binding sites of the organic cross-linker, and (iii) the number of functionalized AuNP and cross-linker deposition steps. Moreover, these structures exhibit surface-enhanced Raman scattering and significant changes are observed in the morphology of some of the surface-bound assemblies upon aging.
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(2011) Angewandte Chemie - International Edition. 50, 14, p. 3224-3226 Abstract
Ball or tube: Flexible and amorphous nanotubes were generated with a palladium salt and a multidentate ligand having a tetrahedral structure (right). In contrast, regardless of the number of metal coordination sites, ligands with a two-dimensional geometry lead to the formation of spheres and their aggregates (left).
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(2011) Langmuir. 27, 4, p. 1319-1325 Abstract
Multicomponent self-propagating molecular assemblies (SPMAs) have been generated from an organic chromo-phore, a redox-active polypyridyl complex, and PdCl2. The structure of the multicomponent SPMA is not a linear combination of two assemblies generated with a single molecular constituent. Surface-confined assemblies formed from only the organic chromophore and PdCl2 are known to follow linear growth, whereas the combination of polypyridyl complexes and PdCl2 results in exponential growth. The present study demonstrates that an iterative deposition of both molecular building blocks with PdCl2 results in an exponentially growing assembly. The nature of the assembly mechanism is dictated by the polypyridyl complex and overrides the linear growth process of the organic component. Relatively smooth, multicomponent SPMAs have been obtained with a thickness of ∼20 nm on silicon, glass, and indium-tin oxide (ITO) coated glass. Detailed information of the structure and of the surface-assembly chemistry were obtained using transmission optical (UV/Vis) spectroscopy, ellipsometry, atomic force microscopy (AFM), synchrotron X-ray reflectivity (XRR), and electrochemistry.
2010
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(2010) ACS Applied Materials and Interfaces. 2, 12, p. 3578-3585 Abstract
The ever-increasing flow of information requires new approaches for high-density data storage (HDDS). Here, we present a novel solution that incorporates the easily accessible polymer poly(3,4-ethylenedioxythiophene) (PEDOT) with multistate memory. The electrical addressable polymer is able to store up to five different memory states which are stable up to 20 min. The observed memory states are generated by the optical output signature of the PEDOT deposited on indium oxide (ITO) coated glass upon applying specific electrical inputs. Moreover, the demonstrated platforms can be represented by a general logic circuit which allows the construction of multistate memory, such as flip-flops and flip-flap-flop logic circuits.
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(2010) Journal Of Physical Chemistry B. 114, 45, p. 14283-14286 Abstract
The electrochemical properties of a metallosupramolecular network that undergoes reversible redox chemistry on indium-tin oxide (ITO)-coated glass substrates have been investigated. The redox-active osmium complexes are electrochemically accessible even for films with a thickness > 15 nm. The electrochemical data correlates well with our previously observed self-propagating growth process, for which the electron density for the assemblies remains constant during film growth. Electron-transfer rate constants obtained by potential step chronoamperometry experiments suggest an exceptionally low attenuation factor, β, of 0.013 ± 0.001 Å-1. However, the intrinsically porous nature of the assembly could be to a large extent or even entirely responsible for such a low value.
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(2010) Journal of the American Chemical Society. 132, 41, p. 14554-14561 Abstract
Metal-organic networks (MONs) were created by a stepwise solution deposition approach from vinylpyridine-based building blocks and PdCl 2. The combined experimental and computational study demonstrates the formation of saturated, structurally organized systems on solid supports. The rigid nature and geometry of the components are well-suited to form honeycomb and parallelogram structures, as predicted by a computational study. Detailed structural information of the new MONs was obtained by optical (UV/vis) spectroscopy, ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and synchrotron X-ray reflectivity (XRR). Notably, the XPS elemental composition indicates the formation of a palladium coordination-based network.
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(2010) Journal of the American Chemical Society. 132, 36, p. 12528-12530 Abstract
Here we report the first use of self-propagating molecule-based assemblies (SPMAs) as efficient electron-transporting layers for inverted organic photovoltaic (OPV) cells. P3HT-PCBM cells functionalized with optimized SPMAs exhibit power conversion efficiencies approaching 3.6% (open circuit voltage = 0.6 V) vs 1.5% and 2.4% for the bare ITO and Cs2CO3-coated devices, respectively. The dependence of cell response parameters on interlayer thickness is investigated, providing insight into how to further optimize device performance.
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(2010) Journal of the American Chemical Society. 132, 27, p. 9295-9297 Abstract
Here we present the critical role of the molecular structure and reaction parameters on the nature of thin-film growth, using a versatile two-step assembly method with organic and metal organic chromophores cross-linked with palladium. It was found that the polypyridyl complexes exhibit exponential growth, whereas, under identical conditions, the organic systems exhibit linear behavior. The internal film morphology plays a pivotal role in the storage and usage of the palladium, where a more porous structure results in exponential growth. Interestingly, through proper tuning of the reaction conditions, the growth of the molecular assemblies can be controlled, resulting in a changeover from exponential to linear growth. These findings unequivocally demonstrate the importance of both the internal film structure and deposition conditions on the assembly of molecular-based films.
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(2010) Angewandte Chemie - International Edition. 49, 28, p. 4780-4783 Abstract
Flip-flopping away: Multivalue random access memory can be achieved using electrically addressable Os2+-based multilayers. The controllable optical properties of the multilayers allow the construction of memory devices that are able to store up to five different states that depend on the given electrical inputs (see picture). The functions of the devices can be represented by sequential logic circuits that are equivalent to flip-flop and flip-flap-flop devices.
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(2010) Chemistry-A European Journal. 16, 23, p. 6744-6747 Abstract
(Figure Presented) Designer materials: HOMO-LUMO engineering of coordination-based oligomers covalently bound to silicon or glass has been achieved by the use of a partially fluorinated chromophore (see graphic). The experimental and computationally derived physical chemical properties of these assemblies are compared to their non-fluorinated analogues.
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(2010) Angewandte Chemie - International Edition. 49, 7, p. 1218-1221 Abstract
"Figure Presented" Stolen identity: The molecular geometries of a series of cross-linkers that bear between one and four pyridyl moieties are expressed in the optical properties of AuNP assemblies (see picture). TEM analysis indicates that the molecular-level structural differences of the cross-linkers are also transferred at the submicrometer level in the formation of the AuNP assemblies.
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(2010) Angewandte Chemie - International Edition. 49, 1, p. 169-172 Abstract
Having a selective memory: Osmium(II)-based monolayers on glass substrates are versatile platforms for the generation of several sequential logic circuits with multiple inputs which are able to display random access memory (RAM) functionality in the form of a set/reset latch. Additionally, the type of logic displayed, for example, sequential or combinatorial, can be controlled by keeping the current state static or dynamic.
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(2010) Angewandte Chemie - International Edition. 49, 5, p. 926-929 Abstract
A new glue: Supramolecular assembly of gold nanoparticles (AuNPs, see picture) mediated by halogen bonding interactions is demonstrated. The primary time-dependent assembly of functionalized AuNPs controls the inner structure, whereas the appearance of the overall hybrid structures can be engineered by varying the concentration of the organic linker.
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(2010) ACS Applied Materials and Interfaces. 2, 1, p. 7-10 Abstract
Signal amplification has been demonstrated with surface-bound electrochromic complexes that can exist in one of two oxidation states (M 2+/3+). Reaction of FeCl3 with covalently immobilized Os2+ complexes on glass substrates converts the metal centers from one oxidation state to the other. The formed Fe2+ reduces a series of Ru3+-based monolayers. The absorption of light is coupled with the oxidation state of the complexes and provides the output for the monolayer-based device. The gain of the setup can be controlled by the addition of a Fe 2+ chelating ligand.
2009
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(2009) Chemistry of Materials. 21, 19, p. 4676-4684 Abstract
Assemblies with molecular-level organization based on organic chromophores and a bimetallic palladium complex are presented. A layer-by-layer strategy is employed by alternately coordinating vinylpyridine-terminated chromophores to the metal centers to form cationic oligomers. These new structures are formed from solution on quartz and silicon substrates functionalized with a covalently bound template layer. Twelve consecutive deposition steps result in structurally regular assemblies as demonstrated by linear increases in the ellipsometrically determined thickness and UV-vis optical absorption. The increase in thickness for each additional layer shows that the long-range order of the system is determined by the structure of the chromophores and by the square-planar geometry of the metal centers. Furthermore, the optical properties indicate that the conjugation length of the assembly component does not increase in the surface-bound oligomers with each additional deposition cycle. Structural communication is transferred via the system components, but they remain electronically isolated. This is supported by density functional theory (DFT) calculations.
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(2009) Chemistry-A European Journal. 15, 39, p. 10025-10028 Abstract
Aryl-halide bond activation and coordination of unsaturated substrates are key steps in many metal-mediated carbon-carbon bond-forming reactions. A series of reactions with stillbenes 1-7 and Pt(PEt3)4 were used to explore the role of substrate coordination on aryl-halide activation. The complexes 8-14 were characterized by NMR spectroscopy, and complexes 9-12 and 14 were isolated and characterized by elemental analysis. Nearly identical spectroscopic properties were reported for structurally related stillbazole complexes. Prolonged reaction times and elevated temperatures resulted in the quantitative formation of complexes 15-21. The X-ray structure of complex 12, confirms the n2-coordination of the central carbon-carbon double bond of the stillbene to the metal center. A good linear correlation was observed between the rates of these transformations and the electronic properties of the substituent of the stillbenes.
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(2009) Inorganic Chemistry. 48, 9, p. 4021-4030 Abstract
Four analogous platinum stilbene- and stilbazole-based complexes exhibit unusual long-range heteronuclear spin-spin coupling in solution. Single crystal analysis and NMR experiments show that the 19F, 31P, and 195Pt nuclei communicate over large distances (0.9-1.3 nm) through bond rather than through space. Spin-spin couplings between 195Pt and 19F over seven bonds and between 31P and 19F over eight bonds are observed with 7JPtF = 2.9 Hz and 8JPF = 11.8 Hz. Remarkably, a very large spin coupling between 195Pt and 19F over six bonds (6J PtF = 40.1 Hz) is also observed in a structurally related pyridinium complex. Experimental and gNMR (version 5.0) simulated 19F{ 1H}, 31P{1H}, and 195Pt{ 1H} spectra of the complexes reveal a three-spin AMY system (A = 31P, M = 31P, Y = 19F) or a five-spin AMY3 flanked by a four-spin AMXY or a six-spin AMXY3 system (X = 195Pt), respectively. Density functional theory calculations at the PBE0/SDD level of theory show a π-conjugated metal-ligand network, which may contribute to the experimentally observed spin-spin interactions.
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(2009) Journal of the American Chemical Society. 131, 10, p. 3468-3469 Abstract
A metallo-supramolecular network undergoes reversible redox chemistry on indium-tin oxide (ITO) coated glass substrates with concurrent color change. The switching time, long-term stability, and coloration efficiency are competitive with polymeric materials such as the industrially important PEDOT.
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(2009) Angewandte Chemie - International Edition. 48, 1, p. 28-30 Abstract
(Chemical Equation Presented) A quick fix: The recently reported process of "oxygen-atom metathesis", which is akin to olefin metathesis, may be involved in the activation of carbon dioxide. Nucleophilic attack by the d8 metal center of an in situ generated Fischer carbene complex on carbon dioxide affords an unstable metallacycle. Elimination of an organic component, tert-butyl formate, results in the concurrent formation of an iridium-carbonyl complex.
2008
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(2008) Crystal Growth & Design. 8, 8, p. 3066-3072 Abstract
A series of phenylethynyl pyridine derivatives 1-4 possessing both perfluorocarbon (PFC) and hydrocarbon (HC) moieties have been synthesized and used for the formation of halogen bonding (XB)-based networks. X-ray crystal structure analyses indicate the dominance of XB synthons, which represent the one-dimensional (ID) structure directing interaction, leading to the formation of supramolecular chains. The influence of structural/electronic factors (e.g., electron donor/acceptor strength, sterically demanding substituents) on XB formation of compounds 1-4 have been compared with structurally related stilbazole systems (I, II). The XB-bonded networks are formed in collaboration with other noncovalent interactions such as π-π stacking, hydrogen bonding, C-H⋯F and F⋯F. Molecular electrostatic potentials and atomic polar tensor (APT) charges of the donor and acceptor sites have been determined by density functional theory (DFT) calculations.
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(2008) Journal of the American Chemical Society. 130, 26, p. 8162-8163 Abstract
Polycrystalline halogen-bonded assemblies fabricated by physical vapor deposition (PVD) exhibit controllable morphologies and microstructures. Although the solid-state packing may vary going from a solution crystal growth process (used for chromophore single-crystal determination) to a vapor-phase deposition process (used for PVD film fabrication), the corresponding film microstructures are independent of the substrate surface chemistry.
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(2008) Journal of the American Chemical Society. 130, 28, p. 8913-8915 Abstract
Accelerated growth of a molecular-based material that is an active participant in its continuing self-propagated assembly has been demonstrated. This nonlinear growth process involves diffusion of palladium into a network consisting of metal-based chromophores linked via palladium.
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(2008) Angewandte Chemie - International Edition. 47, 29, p. 5322-5326 Abstract
(Chemical Equation Presented) A layer of logic: A series of Boolean operations has been demonstrated with redox-active monolayers of osmium and ruthenium complexes on glass substrates. High stability, selective reactivity, and reversible redox-chemistry, coupled with significant optical changes, make these monolayers versatile logic gates. The electron-transfer-based systems are suitable as standalone systems or as operational parts in logic circuits.
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(2008) Inorganic Chemistry. 47, 12, p. 5114-5121 Abstract
Activation of a strong aryl-Br bond of a halogenated vinylarene by nickel(0) is demonstrated in the presence of aryl-I containing substrates. η2-Coordination of Ni(PEt3)2 to the C=C moiety of halogenated vinylarenes is kinetically preferable and is followed by an intramolecular aryl-halide bond activation process. This "ring- walking" process is quantitative and proceeds under mild reaction conditions in solution. Mechanistic studies indicate that the metal insertion into the aryl-halide bond is not the rate-determining step. The reaction obeys first-order kinetics in the η2-coordination complexes with almost identical activation parameters for Br and I derivatives. The ring-walking process is kinetically accessible as shown by density functional theory (DFT) calculations at the PBE0/SDB-cc-pVDZ//PBE0/SDD level of theory.
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(2008) Inorganic Chemistry. 47, 9, p. 3815-3822 Abstract
The fluxional behavior of two analogous platinum complexes has been studied in solution by NMR spectroscopy to elucidate the reaction mechanism and to determine the activation parameters. This includes variable temperature NMR spectroscopy, 2D 1H-1H exchange spectroscopy, and spin saturation transfer measurements. A platinum moiety, Pt(PEt3) 2, translocates between two carbon-carbon double bonds of two vinylpyridine moieties bridged by an arene (i.e., phenyl, anthracene) at elevated temperatures. Magnetization transfer NMR experiments in the presence of free ligands unambiguously revealed an intramolecular pathway for the "phenyl" system. An intermolecular pathway is proposed for the "anthracene" complex.
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(2008) Journal of the American Chemical Society. 130, 15, p. 5040-5041 Abstract
Combining strong metal-ligand coordination and π-π interactions affords a 3D-ordered molecular-based multilayer. The organization of the assembly is apparent from the optical properties and X-ray reflectivity.
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(2008) Journal of the American Chemical Society. 130, 9, p. 2744-2745 Abstract
Selective optical sensing of parts per million levels of Cr6+ in water under acidic conditions with robust, osmium-chromophore-based monolayers is demonstrated. The sensor system can be reset by washing with water at neutral pH and can be readily monitored by UV/vis spectroscopy,
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(2008) Crystal Growth & Design. 8, 3, p. 786-790 Abstract
Two cocrystals (4, 5) have been obtained with 1,3,5-tris[4-pyridyl(ethenyl) ]benzene (1), sym-triiodo-trifluorobenzene (2), and diiodo-tetrafluorobenzene (3), respectively. Cocrystal 4 contains both compounds 1 and 2 in a molecular ratio of 1:2, whereas cocrystal 5 contains both compounds 1 and 3 in a molecular ratio of 1:0.5 and the solvent used for crystallization, namely, chloroform. Both cocrystals (4, 5) contain N⋯I halogen bonds with distances of ∼80% of the sum of the van der Waals radii. Compound 1 forms four halogen bonding interactions in cocrystal 4, resulting in an infinite halogen-bonded network with two types of N⋯I halogen bonds, whereas in cocrystal 5, compound 1 forms only one type of halogen bonding interaction with compound 3.
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(2008) Angewandte Chemie - International Edition. 47, 12, p. 2260-2262 Abstract
Keywords: CHROMOPHORE-BASED MONOLAYERS; SELF-ASSEMBLED MONOLAYERS; PER-MILLION LEVELS; RESPONSIVE MATERIALS; OPTICAL-PROPERTIES; ELECTRON-TRANSFER; ORGANIC-SOLVENTS; SURFACES; RECOGNITION; MODEL
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(2008) Chemical Communications. 25, p. 2900-2902 Abstract
Optical detection of parts-per-million (ppm) levels of CO by a structurally well-defined monolayer consisting of bimetallic rhodium complexes on glass substrates has been demonstrated.
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Mono- and multilayer assemblies for the formation of chemical sensors(2008) American Chemical Society - 235th National Meeting, Abstracts of Scientific Papers. Abstract
Siloxane-based metal-organic monolayers for parts-per-million (ppm) level sensing of water, CO, NOx and various metal ions as well as multilayer assemblies composed of individual wire-like molecules are described. The monolayer-based sensors are thermally robust and can be monitored optically. Validation of the analytical performance characteristics of the sensors were performed including reversibility, reproducibility, stability, and the detection range. Furthermore, a bottom-up patterning technique is demonstrated that is compatible with the multilayer assemblies and may facilitate the construction of nanoscale precise sensing devices.
2007
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(2007) Journal of the American Chemical Society. 129, 40, p. 12296-12303 Abstract
Reagentless optical recognition and parts-per-million (ppm) quantification of FeCl3 in CH3CN was demonstrated using a redox-active Os(II)-chromophore-based monolayer on glass. The Fe3+-induced oxidation of the monolayer is fully reversible and can be monitored optically with a conventional UV/vis spectrophotometer (260-800 nm). The system can be reset with water within
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(2007) Journal of Physical Chemistry C. 111, 37, p. 13652-13654 Abstract
The study of molecular electric properties is an intriguing, rapidly developing field in which technological and basic scientific challenges and developments are evolving. Nevertheless, understanding of the interplay of intermolecular interactions, substrate effects, and electrode contacts remains challenging. Here, we present noncontact chemically resolved electrical measurements (CREM) of halide-terminated molecular layers and a straightforward model for quantitative analysis of submolecular chemical site capacitance. We demonstrate that under low current densities, the main electronic effects can be accounted for by considering the (sub)molecular properties of the monolayers, whereas the excess potential due to charge injection can be described as site capacitance corresponding to chemically identifiable molecular sites.
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(2007) Organometallics. 26, 18, p. 4528-4534 Abstract
This contribution describes the reactivity of Pt(PEt3) 4 with (4-bromo-phenyl)-pyridin-4-yl-diazene. η2- Coordination of Pt(PEt3)2 to the N=N moiety is kinetically preferable and followed by an aryl-halide bond activation process. This quantitative transformation proceeds under mild reaction conditions in solution and in the solid state. Mechanistic studies in solution indicate that the metal insertion into the aryl-halide bond is the rate-determining step. The reaction obeys first-order kinetics in the η2-coordination complex with ΔG298K = 24.6 ± 1.6 kcal/mol, ΔH = 26.5 ± 1.6 kcal/mol, and ΔS = 6.6 ± 5.0 eu. No effect on the reaction progress and NMR line shape has been observed in the presence of excess PEt3. However, competition experiments with the η-coordination complex and PhBr reveal that the product ratio can be altered by the presence of PEt3, indicating that the two aryl-halide bond activation processes proceed via different mechanistic pathways. Numerical analysis of a series of competition experiments fits a reaction scheme involving a unimolecular transformation from the η2-coordination complex to the product of aryl-halide oxidative addition. This "ring-walking" process is kinetically accessible as shown by density functional theory (DFT) calculations at the PCM:PBEO/SDB-cc-pVDZ/PBE0/SDD level of theory.
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(2007) Journal of Physical Chemistry C. 111, 12, p. 4655-4660 Abstract
Exposure of an osmium(II)-based monolayer on glass to organic solvents containing 0.36-116 ppm of NOBF4 results in one-electron transfer from the covalently immobilized complexes to the inorganic analyte with concurrent optical changes. The NO+ induced oxidation of the monolayer can be detected optically with an of-the-self-UV/vis spectrophotometer (260-800 nm). The redox-based NO+ detection and quantification system can be reset with water within ∼20 s for at least 40 times. The reaction of the monolayer with a THF solution containing 5 ppm of NOBF 4 follows pseudo first-order kinetics in the monolayer with ΔG298K = 21.5 ±0.7 kcal/mol, ΔH = 9.5 ± 0.3 kcal/mol, and ΔS = -40.6 ±1.1 eu. The monolayer structure and properties have been resolved by electrochemical measurements and synchrotron X-ray reflectivity measurements in combination with density functional theory calculations (B3LYP/SDD level of theory).
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(2007) Crystal Growth & Design. 7, 2, p. 386-392 Abstract
The potential triple-halogen-bond acceptor, sym-triiodo-trifluorobenzene IFB (1), has been co-crystallized with a series of bipyridyl derivatives (2-4) to gain insight to the factors controlling formation of multiple halogen bonds with a single aromatic system. Co-crystals 5-7 were obtained that consistently contained two N⋯I halogen bonds. The reluctance to the formation of a supramolecular assembly having a third N⋯I halogen bond does not depend on the size of the bispyridine donor systems (2-4). Apparently, there are limitations to the number of halogen bonds that can be formed with a single aromatic halogen donor. The solid-state structure of co-crystal (5) contains short I⋯F contacts of 2.96 and 3.05 A. DFT calculations were performed at the PBE0/(apc1-aSDBDZ)//PBEO/(pc1-SDBDZ) level of theory to investigate the nature of the interaction between the pyridine nitrogen and IFB (1). These calculations reveal a weakening of N⋯I interactions as more pyridine moieties coordinate to the IFB (1), which might be a contributing factor to the consistent formation of two rather than three N⋯I halogen bonds.
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(2007) CrystEngComm. 9, 7, p. 538-540 Abstract
A new partially fluorinated stilbazole represents the first example of a halogen bonding based donor-acceptor system which exhibits an intriguing solid-state structure consisting of infinite parallel helices.
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(2007) Chemical Communications. 2007, 46, p. 4878-4880 Abstract
Optical detection of parts-per-million (ppm) levels of NO2 (1-10 ppm) and NOx (800-2550 ppm) by a structurally well-defined monolayer consisting of osmium polypyridyl complexes on glass substrates has been demonstrated.
2006
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(2006) Nano Letters. 6, 12, p. 2848-2851 Abstract
Potential profiles across molecular layers are constructed by means of noncontact electrically stimulated photoelectron spectroscopy, probing for the first time the molecule-substrate interface potential and resolving local screening effects across inner phenyl groups.
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(2006) Journal Of Physical Chemistry B. 110, 33, p. 16781-16786 Abstract
The optical O2 recognition capability of a covalently assembled monolayer (CAM) of 5,10,15-tri-{p-dodecanoxyphenyl}-20-(p-hydroxyphenyl) porphyrin on silica-based substrates was studied at room temperature by both UV-vis and photoluminescence (PL) measurements. The optical properties of this robust monolayer setup appear to be highly sensitive to the O2 concentration in N2. Both UV-vis and PL measurements were used to study the porphyrin-oxygen interactions. The monolayer-based sensor exhibits a short response time and can be restored within seconds. The oxygen-induced luminescence quenching of the monolayer involves both ground and excited states. The proposed mechanism responsible for the luminescence quenching involves different kinds of interactions between the monolayer and O2.
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(2006) Journal of the American Chemical Society. 128, 26, p. 8400-8401 Abstract
Optical sensing of parts per million (ppm)-levels of H2O (10-300 ppm) in an organic solvent with robust, osmium chromophore-based monolayers is demonstrated. The H2O-induced reduction of the covalently bound osmium(III) complex is fully reversible and can be readily monitored by UV/vis absorption spectroscopy. The thermally robust sensor can be reactivated chemically within 3 min.
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(2006) Organometallics. 25, 14, p. 3308-3310 Abstract
Reaction of a zerovalent platinum complex with a partially fluorinated stilbazole ligand results in direct Arf-Br oxidation addition. The exclusive formation of the Arf-PtII-Br complex does not proceed via η2-C=C coordination on the reaction coordinate or as a side-equilibrium prior to the observed C-Br bond activation. Arf-Br activation by Pt0 is the kinetically and most probably also the thermodynamically favorable process independent of the reaction temperature and solvent polarity. This is in stark contrast with the reactivity of isostructural nonfluorinated stilbazole systems, where there is a lower barrier for η2-C=C coordination than for Ar-Br oxidative addition with Pt0.
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(2006) Journal of the American Chemical Society. 128, 22, p. 7374-7382 Abstract
Layer-by-layer assembly of two palladium coordination-based multilayers on silicon and glass substrates is presented. The new assemblies consist of rigid-rod chromophores connected by terminal pyridine moieties to palladium centers. Both colloidal palladium and PdCl2(PhCN)2 were used in order to determine the effect of the metal complex precursor on multilayer structure and optical properties. The multilayers were formed by an iterative wet-chemical deposition process at room temperature in air on a siloxane-based template layer. Twelve consecutive deposition steps have been demostrated resulting in structurally regular assemblies with an equal amount of chromophore and palladium added in each molecular bilayer. The optical intensity characteristics of the metal-organic films are clearly a function of the palladium precursor employed. The colloid-based system has a UV-vis absorption maximum an order of magnitude stronger than that of the PdCl 2-based multilayer. The absorption maximum of the PdCl 2-based film exhibits a significant red shift of 23 nm with the addition of 12 layers. Remarkably, the structure and physiochemical properties of the submicron scale PdCl2-based structures are determined by the configuration of the ∼15 A thick template layer. The refractive index of the PdCl2-based film was determined by spectroscopic ellipsometry. Well-defined three-dimensional structures, with a dimension of 5 μm, were obtained using photopatterned template monolayers. The properties and microstructure of the films were studied by UV-vis spectroscopy, spectroscopic ellipsometry, atomic force microscopy (AFM), X-ray reflectivity (XRR), scanning electron microscopy (SEM), and aqueous contact angle measurements (CA).
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(2006) Chemistry of Materials. 18, 6, p. 1379-1382 Abstract
A study that illustrates the straightforward chemical modulation of the optical absorbance of a thermally monolayer in air by exploitation of redox properties of metal complex, is presented. The monolayers have been demonstrated as thermally robust, and 25 Os2+/Os3+ redox cycles. the solution-surface electron transfer process induced changes in the molecular properties of the monolayer have been monitored by UV/vis transmission spectroscopy. This siloxane-basaed monolayers and surface chemistry could open up new opportunities in interfacial engineering and the formation of monolayer-based memory devices.
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(2006) Journal Of Physical Chemistry B. 110, 4, p. 1506-1508 Abstract
Submolecular electrical information is successfully derived by applying element-specific, chemically resolved electrical measurements to a covalently bound stilbazole-based monolayer on a silicon substrate. Pronounced affinity effects are found in the response of adjacent atomic sites to external charge injection, accompanied by intramolecular polarization variations. These noncontact electrical read-out capabilities may provide a first entry toward the realization of organic devices based on submolecular electrical units.
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(2006) Journal of the American Chemical Society. 128, 6, p. 2142-2153 Abstract
The new dibranched, heterocyclic "push-pull" chromophores bis{1-(pyridin-4-yl)-2-[2-(N-methyl-pyrrol-5-yl)]ethane}methane (1), 1-(pyrid-4-yl)-2-(N-methyl-5-formylpyrrol-2-yl)ethylene (2), {1-(N-methylpyridinium-4-yl)-2-[2-(N-methylpyrrol-5-yl)]ethane} {(1-(pyridin-4-yl)-2-[2-(N-methylpyrrol-5-yl)]ethane}-methane (3), N-methyl-2-[1-(N-methylpyrid-4-yl)ethen-2-yl]-5-[pyrid-4-yl]ethen-2-yl]pyrrole iodide (4), bis-{1-(N-methyl-4-pyridinio)-2-[2-(N-methylpyrrol-5-yl)]ethane} methane iodide (5), and N-methyl-2,5-[1-(N-methylpyrid-4-yl)ethen-2-yl]pyrrole iodide (6) have been synthesized and characterized. The neutral (1 and 2) and monomethyl salts (3 and 4) undergo chemisorptive reaction with iodobenzyl-functionalized surfaces to afford chromophore monolayers SA-1/SA-2 and SA-3/SA-4, respectively. Molecular structures and other physicochemical properties have been defined by 1H NMR, optical spectroscopy, and XRD. Thin-film characterization by a variety of techniques (optical spectroscopy, specular X-ray reflectivity, atomic force microscopy, X-ray photoelectron spectroscopy, and angle-dependent polarized second harmonic generation) underscore the importance of the chromophore molecular architecture as well as film growth method on film microstructure and optical/electrooptic response.
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(2006) Molecular Crystals And Liquid Crystals. 446, p. 141-150 Abstract
A competitive approach to obtain functionalized polymers is described in this presentation. It consists on the use functionalized sol-gel systems, known to form excellent optical quality thin films. Usually these systems suffer of the low density of active chromophores. In this approach the chromophore concentration is significantly higher. The active NLO chromophores are oriented by corona poling technique. The kinetics of poling was studied by the spectrophotometric method.
2005
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(2005) Journal of Materials Chemistry. 15, 42, p. 4480-4487 Abstract
"Smart" patternable polymer-based materials that can be designed from various molecular building blocks show great potential, as they may be used in many fields, including nanotechnology, biochemistry, organic and physical chemistry, and materials science. The focus of this highlight will be on the basic design characteristics of practical Stimuli Responsive Materials (SRMs), the wide range of potential applications and the challenges to be accomplished in this rapidly expanding area. In particular, recent developments are described which are related to two of the many fundamental aspects of stimuli triggered responses: those that are photo-triggered and those that are solvent triggered. These selected state-of-the-art examples demonstrate the large scope and diversity in terms of activation mechanism, response time and property control.
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(2005) Crystal Growth & Design. 5, 5, p. 1671-1673 Abstract
The title compound, a new fluorinated stilbazole-based chromophore combines hydrocarbon (HC) donor and perfluorocarbon (PFC) acceptor sites in one rigid-rod-type conjugated molecule. The bifunctional stilbazole derivative combines several possibilities of weak, medium, and strong intermolecular interactions leading to an attractive crystal packing. The chromophore shows strong pi-pi stacking behavior in solution, while its crystal structure consists of an infinite unimolecular network involving both interchain pi-pi stacking and (NBr)-Br-... halogen bonding. The latter Lewis acid-base interactions do not play a dominant role in controlling the solution optical properties. In the solid state, the molecules are aligned in one-dimensional infinite head-to-tail chains as a result of attractive halogen bonding. Adjacent chains are oriented in opposite directions.
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(2005) Journal of the American Chemical Society. 127, 26, p. 9322-9323 Abstract
The reaction between tetrakis(triethylphosphine)platinum(0) and 4-[trans-2-(4-bromophenyl)vinyl]pyridine (1) is examined. Initially, the metal center coordinates to the bridging double bond of 1. Complexes 2 and 3 were fully characterized, and their X-ray crystallography structures are presented. Upon heating, either in solution or in the solid state, complex 2 undergoes C-Br oxidative addition to give complex 3. Kinetic studies revealed that this conversion is unimolecular and does not involve dissociation of the metal center from the double bond. Density functional studies show that a plausible mechanism involves the metal center "walking" around the pi-system from the bridging C=C double bond to the C-Br bond.
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(2005) Journal Of Physical Chemistry B. 109, 29, p. 14144-14153 Abstract
Trimethoxy-[11-(2-nitrobenzyloxy)undecyl]silane (1) and trimethoxy-[17-(2-nitrobenzyloxy)heptadecyl]silane (2) have been used for the covalent assembly of siloxane-based photopatternable monolayers. Exposing the monolayers to UV light (312 ± 10 nm) results in the generation of reactive hydroxyl-terminated monolayers without affecting the film quality. The new monolayers, deprotection chemistry, and the effect of photoinduced headgroup lift-off on the monolayer microstructure have been studied in detail by a full complement of physicochemical techniques, including optical (UV-vis) spectroscopy, ellipsometry, aqueous contact angle (CA) measurements, X-ray photoelectron spectroscopy (XPS), synchrotron X-ray reflectivity (XRR), and atomic force microscopy (AFM and AFM-force spectroscopy). AFM-force spectroscopy was used to analyze hydrogen-bond interactions as a function of the nature of the solid-liquid interface. AFM-force spectroscopy indicates a hydrogen-bond energy for photodeprotected monolayers of 8.2 kJ mol -1 (∼2 kcal mol -1). Scanning electron microscopy (SEM) revealed that treatment of photopatterned monolayers with ZnEt 2 solutions resulted in well-defined ∼2 μm × 2 μm features of 10 Å thick ZnO layers.
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(2005) Journal of Physical Chemistry A. 109, 24, p. 5454-5462 Abstract
The static and dynamic first hyperpolarizabilities for a series of substituted metallabenzene-based nonlinear optical (NLO) chromophores were determined by time-dependent density functional theory (TDDFT). The electronic excitation contributions to the first hyperpolarizability are rationalized in terms of the two-level model. The effects on the hyperpolarizabilities of (a) the metal center (Os, Ir, Pt); (b) the ligand environment (PH3, CO, Cl); (c) various donor and acceptor substituents (NH2, OH, Me, H, Cl, Br, I, COOMe, COOH, CN, NO2); and (d) the length of π-conjugation were studied. Our calculations predict that metallabenzenes have significant second-order NLO susceptibilities, ranging from βtot0, = 1.0 × 10-29 to 5.6 × 10-28 esu and from μβtot0 = 3.0 × 10-47 to 1.1 × 10-44 esu, that can be tuned by changing the metal center and/or ligand environment.
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(2005) Angewandte Chemie (International Edition in English). 44, 21, p. 3237-3240 Abstract
(Chemical Equation Presented) Don't forget to write! Electrochemical charge storage in a ruthenium-based monolayer on a hydrophilic substrate (for example, indium tin oxide coated glass) produces redox switching of the optical properties of the system (see picture). (Graph Presented) This read/write process can be carried out at low voltage in air and monitored by UV/Vis spectrophotometry. This makes the monolayer system a suitable candidate for nonvolatile memory devices.
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(2005) Journal Of Physical Chemistry B. 109, 15, p. 6933-6935 Abstract
A general approach is demonstrated for the formation of monolayers comprised of free-base and metalated Bacteriochlorophyll-based derivatives providing a new vehicle for studying photosynthetic motifs and chromophore thin-film interactions. Accessibility to covalent and self-assembled systems on conducting, semiconducting, and insulating substrates is realized utilizing identical molecular building blocks. The monolayers retain the optical features typical for the new systems in solution. Molecular organization of chromophore interaction motifs can be sequentially designed using preassembled building blocks in solution and expressed in the thin film optical properties. For instance, intramolecular π-π stacking is conserved for the dimeric Ni-based chromophores as deduced from the spectroscopic measurements of the monolayers and in solution.
2004
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(2004) Optical Materials in Defence Systems Technology. Vol. 5621. p. 105-116 Abstract
Self-assembled superlattices (SASs) are intrinsically acentric and highly cross-linked structures. For organic electro-optics, they offer great advantages such as not requiring electric field poling for creating an acentric, EO-active microstructure and having excellent chemical, thermal, and orientational stabilities. In this paper, a greatly improved two-step all "wet-chemical" self-assembly (SA) approach is reported. Excellent radiation hardness of the SAS films is demonstrated by high-energy proton irradiation experiments. By introducing metal oxide nanolayers during SA, we show that the refractive indices of SAS films can be tuned over a wide range. Through special chromophore design, the optical absorption maxima of SAS films can also be greatly blue-shifted. Prototype waveguiding electro-optic modulators have been fabricated using the SAS films integrated with low-loss polymeric materials functioning as partial guiding and cladding layers. EO parameters such as the half-wave voltage and the effective electro-optic coefficient are reported.
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(2004) Angewandte Chemie - International Edition. 43, 44, p. 5961-5963 Abstract
Do-it-yourself oxidation: The RhIII complex 1 undergoes a self-oxidative coupling process, in which the phenolate oxygen atom serves as the oxidant, to give 2 and the hydride complex 3 (2:3 = 1:3). This reaction involves cleavage of a strong aryl-oxygen bond. X-ray analysis of 2 reveals that the two quinonoid C=O bonds are n2-coordinated to the metal centers.
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(2004) Journal Of Physical Chemistry B. 108, 45, p. 17505-17511 Abstract
A new series of stilbene-based chromophores have been used to prepare structurally related siloxane-based monolayers in order to determine which factors control the intermolecular chromophore-chromophore interactions in the solid state. The reaction of chromophore precursors 4-styrylpyridine (1), 4-[2-(4-bromophenyl)-vinyl]-pyridine (2), 4-(2-naphthalen-1-ylvinyl)-pyridine (3), 4-(2-anthracen-9-ylvinyl)-pyridine (4), and 4- (2-pyren-2-ylvinyl)-pyridine (5) with excess 3-iodo-n-propyl-1-trimethoxysilane resulted in the corresponding salts 6-10 in quantitative yield. The assembly of chromophores 6-10 on hydrophilic substrates from solution resulted in the formation of densely packed monolayers with a film thickness of similar to1 nm. The average chromophore density (similar to1 chromophore/50 Angstrom(2)) is well within the range that allows pi-pi stacking to occur. Transmission UV-vis spectroscopy of the siloxane-based films shows that the intermolecular interactions are a function of the aryl groups (e.g., phenyl, bromophenyl, naphthalene, anthracene, and pyrene). Relatively weak electronic interactions occur between the surface-bound chromophores 6, 7, and 10, whereas strong electronic interactions occur between surface-bound chromophores 8 and 9. The series of monolayers on sodium lime glass and polished silicon is characterized by a combination of physicochemical methods including X-ray photoelectron spectroscopy, advancing aqueous contact angle measurements, optical spectroscopy, atomic force microscopy, and synchrotron X-ray reflectivity.
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(2004) Inorganica Chimica Acta. 357, 13, p. 4015-4023 Abstract
Reaction of Nil(2) with the PCP-ligand {1-Et-2,6-((CH2PPr2)-Pr-/)(2)-C6H3} (1) results in selective activation of the strong sp(2)-sp(3) arylethyl bond to afford the aryl-nickel complex [Ni{2,6-((CH2PPr2)-Pr-i)(2)-C6H3}I] (2), whereas reaction of Nil(2) with {1,3,5-(CH3)(3)-2,6((CH2PPr2)-Pr-i)(2)-C6H} (4) leads to the formation of the benzylic complex [Ni{1-CH2-2,6-((CH2PPr2)-Pr-i)(2)-3,5-(CH3)(2)-C6H}I] (5) by selective C-H bond activation. Thermolysis of 5 results in formation of [Ni{2,6-((CH2PPr2)-Pr-i)(2)-3,5-(CH3)(2)-C6H}I] (6) by activation of the sp(2)-sp(3) C-C bond. The identity of the new 16-electron complexes 2 and 6 was confirmed by reaction of NiI2 with {1,3-((CH2PPr2)-Pr-i)(2)-C6H4 (3) and 11,3-(CH3)(2)-4,6-((CH2PPr2)-Pr-i)(2)-C6H2) (7), respectively, lacking the aryl-alkyl groups between the "phosphines arms" (alkyl = ethyl, methyl). Complexes 2 and 5 have been fully characterized by X-ray analysis. Nickel-based activation of an unstrained C-O single bond was observed as well. Reaction of the aryl-methoxy bisphosphine {1-OMe-2,6(CH2-(PPr2)-Pr-i)-C6H3 (8) with NiI2 results in the formation of the phenoxy complex [Ni{1-O-2,6-((CH2PPr2)-Pr-i)(2)-C6H3}I] (9) by selective sp(3)sp(3) C-O bond activation.
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(2004) Journal of the American Chemical Society. 126, 37, p. 11699-11710 Abstract
Various metallabenzene complexes, analogues of benzene where one CH unit has been replaced by an organometallic fragment, have been reported in the literature. A detailed theoretical investigation on the chemistry of these complexes is presented here. This includes an evaluation of their aromaticity, the mechanisms of formation of osmium, iridium, and platinum metallabenzene complexes, and one intriguing aspect of their chemistry, the formation of cyclopentadienyl (Cp) complexes. X-ray photoelectron spectroscopy (XPS) measurements on two osmabenzene examples are also presented. In addition, diffuse functions for use with the SDD and SDB-cc-pVDZ basis set-RECP combinations are presented for the transition metals.
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(2004) Inorganica Chimica Acta. 357, 6, p. 1854-1864 Abstract
Halide abstraction from the 18 electron Ru(II) complex RuCl(CO)(2)[2,6-(CH2P'Bu-2)(2)C6H3] (2) with AgPF6 results in the exclusive formation of the cationic complex {Ru(CO)(2)[2,6-(CH2P'Bu)(2)C6H3]}+PF6-(3). The molecular structures of 2 and 3 were determined by complete single-crystal diffraction studies. X-ray crystallographic analysis of 3 reveals that the "open" coordination site is occupied by an agostic interaction between the metal center and an sp(3) C-H bond of a tert-butyl substituent. DFT gas phase calculations (B97-1/SDD) show the necessity of two sterically demanding tert-butyl substituents on one P donor atom for the agostic interaction to occur. The reaction of 3 with H-2 results in the quantitative conversion to {Ru(H)(CO)(2)[2,6-((CH2PBu2)-Bu-t)(2)C6H4]}+PF6- (4) where the aromatic C-ipso-H bond is eta(2)-coordinated to the metal center. Treatment of the agostic complex 4 with Et3N results in the formation of the neutral complex Ru(H)(CO)(2)[2,6-((CH2PBu2)-Bu-t)(2)C6H3] (5). The mechanistic details of 3 + H-2 --> 4 were investigated by DFT calculations at the B97-1/SDB-cc-pVDZ//B97-1/SDD level of theory.
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(2004) Journal of the American Chemical Society. 126, 9, p. 2700-2701 Abstract
We demonstrate the design and formation of thin films having divergent physicochemical properties by using two porphyrin building blocks with high chemical and optical resemblance. A predetermined variation in the molecular design is efficiently transferred and enhanced when constituting a two-dimensional film via control of molecular orientation. Variations of the peripheral substituents on the porphyrin ring resulted in control of the molecular orientation at the surface.
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Layer-by-layer assembly of molecular materials for electrooptical applications(2004) Polymers For Microelectronics And Nanoelectronics. Hedrick J., Lin Q. & Pearson R.(eds.). p. 30-43 Abstract
Intrinsically acentric organic films are becoming a valuable alternative to poled-polymers and inorganic materials such a lithium niobate (LiNbO3) for the formation of electro-optic devices. The ability to fabricate noncentrosymmetric structures in a molecular layer-by-layer approach offers nanoscale control over film dimensions and allows tailoring of microstructural and optical properties at the molecular level.
2003
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(2003) Langmuir. 19, 25, p. 10531-10537 Abstract
The evolution of an intrinsically acentric monolayer formed by self-assembly of a trimethoxysilane-functionalized, high-β azobenzene-based chromophore from solution onto hydrophilic silicon oxide surfaces shows two distinctly different adsorption steps. The kinetics of the monolayer formation on sodium lime glass and polished silicon were monitored ex situ by a combination of aqueous contact angle measurements, optical spectroscopy, transmission second-harmonic generation (at λ0 = 1064 nm), and synchrotron X-ray reflectivity. The initial adsorption step from toluene onto the substrate surface at 80°C is completed within minutes, k1 ≈ 1 × 10-2 s-1, and results in an acentric material having ∼65% of the properties of a fully formed, dense monolayer (i.e., thickness, coverage, optical absorption, wettability, and nonlinear optical response). The second assembly stage is about 2 orders of magnitude slower, k2 ≈ 1.3 × 10-4 s-1, and reaches completion after about 6 h.
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(2003) Journal of the American Chemical Society. 125, 43, p. 13020-13021 Abstract
One common reaction that metallabenzene complexes undergo is the formation of cyclopentadienyl (Cp) complexes. Density functional theory (DFT) was used here to investigate the reaction mechanism. It was found that the reaction can proceed via a carbene migratory insertion class of C−C coupling. Cp complexes are found to be thermodynamically favored, except for the case of (C5H5Ir)(PH3)2Cl2 (1j) where the metallabenzene was favored. Isolation a rhodiabenzene of the type (C5H5Rh)(PH3)2Cl2 (1m) and a palladiabenzene, such as (C5H5Pd)Cp (1p), may be possible.
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(2003) Journal of the American Chemical Society. 125, 38, p. 11702-11709 Abstract
The mechanistic details of 1,2- and 1,4-cycloaddition reactions of acetone, CO2, and CS2 to isostructural iridiabenzene, iridiapyrylium, and iridiathiabenzene complexes, as well as their rhodium analogues, were elucidated by density functional theory (DFT) at the PCM/mPW1K/SDB-cc-pVDZ//mPW1K/ SDD level of theory. The calculated reaction profiles concur with reported experimental observations. It was found that the first complex reacts via a concerted reaction mechanism, while the latter two react by a stepwise mechanism. Several factors affecting the reaction mechanisms and outcome were identified. They include the composition and size of the metal-aromatic ring, the length of the substrate C=X (X = O, S) bond, the geometry of the product, the symmetry of the frontier molecular orbitals, and the type of reaction mechanism involved.
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(2003) Chemical Reviews. 103, 5, p. 1759-1792 Abstract
Bernard Shaw's pioneering research1-15 on cyclometalated phosphine-based pincer complexes has inspired many others.16-208 Although the first metal complexes with ligands 1 and 2 were reported about 30 years ago, it is still a hot and emerging topic covering all aspects of modern organometallic chemistry and touching other fields as well.
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(2003) Chemistry of Materials. 15, 5, p. 1064-1072 Abstract
The synthesis of the heterocycle-based diethanolaminomethyl-functionalized derivative 1-(pyridin-4-yl)-2-[(N-methylpyrrol-2-yl)-5-methylenediethanolamine]ethene (2) from (1-pyridin-4-yl)-2-(N-methyl-pyrrol-2-yl)ethene (1) and their methylpyridinium dyes 1-(N-methylpyridinio)-2-[(N-methylpyrrol-2-yl)-5- methylenediethanolamine]ethene iodide (3) and 1-(N-methylpyridinio)-2-[(N-methylpyrrol-2-yl)-5- methylenediethanolamine]ethene iodide (4) is described. NLO-active chromophore monolayers SA-1 and SA-2 were obtained by reaction of p-iodomethylphenyldiiodochlorosilane (5)-functionalized substrates and dye precursors 1 and 2, respectively. A χ(2) value of ∼120 pm/V is observed for SA-2. The new diethanolaminomethyl-functionalized chromophore 2 is also a suitable building block for the layer-bylayer formation of intrinsically acentric, highly transparent nonlinear optical/electro-optic multilayers (SAS = self-assembled superlattice). The organic SAS films are characterized by a combination of physicochemical methods including synchrotron specular X-ray reflectivity, angle-dependent polarized second-harmonic generation, optical (absorption and photoluminescence) spectroscopy, X-ray photospectroscopy, atomic force microscopy, and advancing contact angle measurements.
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(2003) Polymer (Guilford). 44, 4, p. 1051-1056 Abstract
Nanometer-scale thick liquid films of poly(methylhydro-dimethyl)siloxane copolymer (PMDMS) deposited on hydrophilic and hydrophobic solid organic films have been studied using synchrotron X-ray specular reflectivity (XRR). The physico-chemical properties of liquid PMDMS at the interfacial level are controlled by the nature of the solid surface. Detailed analysis of the XRR-data revealed the formation of a low-density region in the liquid PMDMS film in the vicinity of the hydrophobic surface, whereas a densely packed molecular layer is formed at the liquid PMDMS-hydrophilic substrate interface. Non-covalent polymer chains are 'frozen' at the solid-liquid interfaces in the confined liquid films and interactions with the substrate surfaces (i.e. hydrogen bonding) are responsible for distinctly different density profiles.
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Mechanistic aspects of acetone addition to metalloaromatic complexes of iridium: A DFT investigation(2003) Chemical Communications. 1, p. 132-133 Abstract
DFT calculations were used to reveal the unexpected reactivity and mechanism of the addition of acetone to metallabenzene, metallapyrylium and metallathiabenzene complexes of iridium.
2002
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(2002) Chemistry of Materials. 14, 12, p. 4996-5005 Abstract
Novel heterocycle-based azine/azinium-(π-bridge)-pyrrole systems, [(1-(pyrid-4-yl)-2-(N-methylpyrrol-2-yl)]ethene (1), 5-[(N-methylpyrrol-2-yl)azo]quinoline (2), 5-[(N-methylpyrrol-2-yl)azo] isoquinoline (3), and the corresponding N-methotriflates 4-6 were synthesized and characterized. Chromophore precursors 1-3 react with iodobenzyl-functionalized surfaces (10) affording polar-ordered σ-bonded thin films 7-9, respectively. All systems were studied by optical (UV-visible, photoluminescence) spectroscopies, electrochemical (CV), and thermal (TGA, DSC) techniques. Self-assembled chromophore monolayers 7-9 on glass, quartz, and silicon substrates have been characterized by a full complement of physicochemical techniques: optical spectroscopy, aqueous advancing contact angle measurements, specular X-ray reflectivity, atomic force microscopy, and angle-dependent polarized second harmonic generation. Film second harmonic generation responses χ(2)zzz vary more than 1 order of magnitude ranging from 1.3 and 1.6, to 34 × 10-8 esu for 9-, 8-, and 7-based monolayers, respectively. This study demonstrates the following: (i) SA monolayers can be prepared using various types of azine-containing precursors; (ii) chromophore and SA film properties are influenced predominantly by the nature of the π-deficient azinium acceptor; (iii) unsubstituted, π-excessive pyrrol-2-yl rings can act as primary donor groups in push-pull conjugated systems.
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(2002) Angewandte Chemie (International Edition in English). 41, 18, p. 3363-3366 Abstract
Moieties that can be cross-linked appear to be the necessary feature of dendrimers that can be used to make poled polymers displaying high chromophore density, excellent electrooptic properties, and good thermal and temporal stability. Such materials could find use in optical telecommunications and public networking.
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(2002) Langmuir. 18, 9, p. 3704-3707 Abstract
A broadly applicable approach to formation of self-assembled organic electrooptic superlattices for high-speed switching is reported. This two-step \u201cone-pot\u201d method involves (i) layer-by-layer covalent self-assembly of intrinsically acentric monolayers of a new high-hyperpolarizability donor−acceptor aminophenylbenzothiazolpyridinium chromophore (β(0.65 eV)calcd = 1620 × 10-30 cm5 esu-1) on hydrophilic substrates and (ii) in-situ chromophore deprotection concurrent with self-limiting \u201ccapping\u201d/planarization of each chromophore layer with octachlorotrisiloxane. The resulting organic films are characterized using a combination of physicochemical methodolgies including synchrotron X-ray specular reflectivity, angle-dependent polarized second harmonic generation spectroscopy, optical spectrometry, X-ray photoelectron spectroscopy, and advancing contact angle measurements. The superlattices exhibit very large second-order responses, χ(2) ≈ 370 pm/V, and a large macroscopic electrooptic coefficient, r33 ≈ 120 pm/V, is estimated at λ0 = 1064 nm.
2001
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(2001) Journal of Chemical Physics. 115, 14, p. 6722-6727 Abstract
The microstructures of siloxane-based self-assembled electro-optic superlattices composed of high-hyperpolarizable organic chromophore arrays intercalated with Ga and In oxide sheets were analyzed. The film thickness increased linearly as a function of the number of layers, underscoring the high structural regularity and efficiency of the synthetic approach. The dependence of the relative surface roughness on the number of layers was found to be identical for self-assembled organic and organic-inorganic hybrid film structures. The solution-based deposition of Ga and In metal oxide layers was found to be suitable for nanometer scale film construction, and enables tunable, metal-dependent modification of feasible superlattice physicochemical properties.
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(2001) Advanced Functional Materials. 11, 5, p. 393-397 Abstract
The refractive indices of self-assembled organic electro-optic superlattices can be tuned by intercalating high-Z optically transparent group 13 metal oxide sheets into the structures during the self-assembly process. Microstructurally regular acentricity and sizable electro-optic responses are retained in this straightforward synthetic procedure. This "one-pot" all wet-chemistry approach involves: i) layer-by-layer covalent self-assembly of intrinsically acentric multilayers of high-hyperpolarizability chromophores on inorganic oxide substrates, ii) protecting group cleavage to generate a large density of reactive surface hydroxyl sites, iii) self-limiting capping of each chromophore layer with octachlorotrisiloxane, iv) deposition of metal oxide sheets derived from THF solutions of Ga(OiC3H7)3 or In(OiC3H7)3, v) covalent capping of the resulting superlattices.
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(2001) Langmuir. 17, 19, p. 5939-5942 Abstract
Regioselective alkylation of 1-(4-pyridyl)-2-[5-(dicyanomethanide)thien-2-yl]ethylene sodium salt (2) by a covalently surface-bound benzylhalide (4) produces a polar-ordered zwitterionic thin film (5). Chemisorption of 2 on the coupling layer 4 was monitored by UV-vis spectroscopy, contact-angle measurements, X-ray photoelectron spectroscopy, and second harmonic generation (SHG). Relative to carbanion 2, the film UV-vis spectrum exhibits a red-shifted (Δλ ∼ 170 nm) charge-transfer band, consistent with quaternization of the pyridyl group of 2 by 4. The nonlinear optical properties of the blue films of 5 were measured by SHG. The average molecular tilt angle of the chromophore was determined to be ∼46°, and χZZZ(2) ∼ 5.0 × 10-8 esu (∼20 pm/V) was estimated at a fundamental wavelength of 1064 nm.
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(2001) Applied Physics Letters. 79, 5, p. 587-589 Abstract
Traveling-wave electro-optic modulators based on chromophoric self-assembled superlattices (SASs) possessing intrinsically polar microstructures have been designed and fabricated. Although the thickness of the SAS layer is only ∼ 150 nm, a π-phase shift is clearly observed. From the measured Vπ value, the effective electro-optic coefficient of the SAS film is determined to be ∼ 21.8 pm/V at an input wavelength of 1064 nm.
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(2001) Chemistry of Materials. 13, 1, p. 15-17 Abstract
Protection-deprotection of silyl-derivatized surface hydroxyl groups represents a new, "one-pot" strategy for the layer-by-layer construction of polar, covalently interlinked electrooptic molecular superlattices. These, siloxane-based multilayer structures are shown to be smooth, adherent, and structurally regular and to exhibit excellent electrooptic/nonlinear optic responses with χ(2)zzz∼220 pm/V and r33∼80 pm/V without electric field poling. This all "wet-chemical self-assembly approach should be readily amenable to automation.
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(2001) Israel Journal of Chemistry. 41, 3, p. 163-172 Abstract
The reaction of the PCP-type complex Pd(Me) {2,6-(1Pr2PCH2)2C6H 3} (3) with phenyl iodide results in the formation of Pd(1) {2,6-(1Pr2PCH2)2C6H 3} (5). methyl iodide, toluene, and biphenyl. Formation of Pd(Ph) {2,6-(1Pr2PCH2)2C6H 3} (4) is observed during the reaction by 31P NMR. Reaction of 4 with aryl iodides results in the formation of 5 and Ph-Ph. Ph-Ar, and Ar-Ar, products indicative of a radical reaction. Under pseudo-first-order conditions, the rates of the reactions follow the order p-OMe > p-Me > H > p-NO2 > m-Cl. The reaction is likely to involve electron transfer from 4 to the aryl iodide followed by fast decomposition of a postulated radical cation [Pd(Ph) {2,6-(1Pr2PCH2)2C6H 3}]+. (4+.) to give a phenyl radical and [Pd {2,6-(1Pr2PCH2)2C6H 3}]+ (6+). Facile decomposition of the aryl iodide radical union generates an aryl radical and I . Recombination of aryl radicals gives rise to mixed biaryls, and 6+ combines with 1 to give 5.
2000
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(2000) Journal of the American Chemical Society. 122, 32, p. 7723-7734 Abstract
Oxidative addition of aryl iodides ArI (Ar = (a) C6H5, (b) C6H4CF3, (c) C6H3(CF3)2, (d) C6H4CH3, (e) C6H4OCH3), to the PCP-type complex Rh(PPh3)[CH2C6H(CH3)2(CH2PPh2)2] (1), yields the complexes Rh(Ar)[CH2C6H(CH3)2(CH2PPh2)2](I) (2a-e). Compounds 2a-e undergo intramolecular methylene transfer from the bis-chelating ligand to the incoming aryl under mild conditions (room temperature) giving Rh(CH2-Ar)[C6H(CH3)2(CH2PPh2)2](I) (3a-e). The methylene transfer, which is a unique sequence of sp2-sp3 C-C bond reductive elimination and sp2-sp3 C-C bond activation, was investigated kinetically (reaction 2a → 3a), yielding the activation parameters ΔH(+) = 17 ± 3 kcal/mol, ΔS(+) = -23 ± 4 eu. The rate-determining step of this reaction is the C-C reductive elimination rather than the C-C activation step. X-ray structural analysis of 2a and 3b demonstrates that the Rh atom is located in the center of a square pyramid with the aryl (2a) and the benzyl (3b) trans to the vacant coordination site. Reaction of the complex Rh(CH2C6H4CF3)[C6H3(CH2PPh2)2](Br) (7c) with carbon nucleophiles (MeLi, PhLi, BzMgCl) leads to a competitive sp2-sp3 and sp3-sp3 C-C coupling, resulting in migration of a methylene or benzylidene into the bis-chelating ring and formation of the corresponding organic products, sp2-sp3 C-C coupling was shown to be kinetically preferred over the sp3-sp3 one, and the more electron-rich the benzyl ligand, the better the migratory aptitude observed. X-ray structural analysis of two benzyl migration products, complexes Rh(PPh3)[CH(C6H4CF3)C6H3(CH2PPh2)2] (11) and Rh(PPh3)[CH(C6H5)C6H(CH3)2(CH2PPh2)2] (16), demonstrates that the rhodium atom is located in the center of a square planar arrangement where the PPh3 ligand occupies the position trans to the methyne carbon of the benzylidene bridge. The methylene and benzylidene migration reaction is an important transformation for the regeneration of the methylene-donating moiety in the methylene-transfer process.
1999
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(1999) Organometallics. 18, 19, p. 3873-3884 Abstract
Reaction of RuCl2(PPh3)3 with the bisphosphine {1,3,5-(CH3)3-2,6-(iPr2PCH 2)2C6H} (1) under 30 psi H2 results in quantitative C-C activation of an Ar-CH3 bond to afford Ru(Cl)-(PPh3){2,6-(iPr2PCH2) 2-3,5-(CH3)2C6H} (2) and CH4, whereas reaction of RuCl2(PPh3)3 with 1 in the presence of NaOtBu results in selective ArCH2-H bond activation to afford the benzylic complex Ru(Cl)(PPh3H{1-CH2-2,6-(iPr2PCH 2)2-3,5-(CH3)2C6H} (7). The identity of the 16-electron complex 2 was confirmed by reaction of the bisphosphine {2,6-(iPr2PCH2)2-3,5-(CH 3)2C6H2} (3), lacking the Ar-CH3 group between the phosphine arms, with RuCl2(PPh3)3. Metal insertion into an Ar-Et bond was observed as well. Follow-up of the reaction of RuHCl-(PPh3)3 with 1 by NMR and deuterium labeling studies reveal that the kinetic products of ArCH2-H bond activation (7 and H2) are irreversibly converted into the thermodynamically more stable products of Ar-C bond activation (2 and CH4) via reversal of the C-H activation process. Reaction of (COD)PtCl2 (COD = cycloocta-1,5-diene) with a stoichiometric amount of 1 at room temperature results in the exclusive formation of the benzylic Pt(II) complex Pt(Cl){1-CH2-2,6-(iPr2PCH2) 2-3,5-(CH3)2C6H} (8) and HCl. The iodide analogue of 8 has been characterized by X-ray analysis. Reaction of 8 with a 10-fold excess of HCl results in selective C-C bond activation to afford Pt(Cl){2,6-(iPr2PCH2)2-3,5-(CH 3)2C6H} (10) and MeCl. The activation parameters for the overall process are ΔH = 10.6 kcal/mol, ΔS = -40.1 eu, and ΔG(298) = 23.1 kcal/mol in a benzene/dioxane solution (5.5:1 v/v) and ΔH = 2.1 kcal/mol, ΔS = -65.4 eu, and ΔG(298) = 21.6 kcal/mol in dioxane.
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(1999) Journal of the American Chemical Society. 121, 28, p. 6652-6656 Abstract
Oxidative addition of one of the strongest C-C bonds, Aryl-CF3, to Rh(I) takes place upon treating 1-CF3-2,6-(CH2P(t)Bu2)2-C6H3 (1, PCP) with [RhClL2]2 (L = C2H4 or C8H14) in dioxane or toluene at elevated temperatures leading to quantitative formation of Rh(CF3)(2,6- (CH2P(t)Bu2)2-C6H3)Cl (2-Cl). The iodide analogue 2-I was prepared by reacting Rh(η1-N2)(2,6-(CH2P(t)Bu2)2-C6H3) (3) with CF3I at room temperature. ArCF2-F bond cleavage was not observed in parallel to the C-C bond activation. Treating a dioxane solution of the thermally stable Rh(III)- CF3 complexes 2-Cl,I with excess trifluoromethanesulfonic acid (HOTf) at room temperature resulted in CF bond cleavage and selective formation of the unique difluoromethylene-arenium complexes [Rh(1-CF2-2,6-(CH2P(t)Bu2)2- C6H3)X][OTf] (4; X = Cl, I) which were characterized spectroscopically by NMR, UV/vis, and FD-MS. No reaction was observed with HCl. Reaction of 2-Cl with BF3 or Ph3CBF4 (trityl cation) also resulted in C-F bond cleavage to give [Rh(1-CF2-2,6-(CH2P(t)Bu2)2-C6H3)Cl]BF4 (10).
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(1999) Organometallics. 18, 13, p. 2413-2419 Abstract
The unsaturated PCP-type complexes Rh(L){2,6-(CH2PtBu2)2C 6H3}X (L = Et, nPr; X = Cl, I) complexes convert upon heating to the corresponding Rh(III)-hydride complexes Rh(H)-{2,6-(CH2PtBu2)2C 6H3}X (X = Cl, I) and ethylene or propylene, products indicative of a β-H elimination process. The iPr analogue is observed upon reaction of Rh(η1-N2){2,6-(CH2Pt-Bu 2)2C6H3} with iPrI at -10°C and decomposes readily at room temperature to give Rh-(H){2,6-(CH2PtBu2)2C 6H3}I and propylene. Analogous alkyl complexes - lacking β-hydrogens - are stable under the applied reaction conditions. The mechanism of this process has been studied by NMR, using 13C and deuterium labeling of the alkyl ligand (L = Et-d5, 13CH2-CH3). 13C labeling shows that the β-H elimination is irreversible. A deuterium isotope effect of kEt/kEt-d5 = 1.4 and a rate order of Et nPr ≪ iPr were observed. The overall process follows first-order kinetics in the Rh(III)-alkyl complexes. The activation parameters for the thermolysis of Rh(Et)(2,6-(CH2PtBu2)2C 6H3)I in toluene were determined: ΔH = 21.2 kcal/mol, ΔS = -21.1 eu, and ΔG298K = 27.5 kcal/mol.
1998
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(1998) Journal of the American Chemical Society. 120, 51, p. 13415-13421 Abstract
Reaction of [RhCIL2]2 (L = cyclooctene or ethylene) with 2 equiv of the phosphine {1-Et-2,6(CH2P(t)Bu2)2C6H3} (1) in toluene results in a selective metal insertion into the strong Ar-Et bond. This reaction proceeds with no intermediacy of activation of the weaker sp3-sp3 ArCH2-CH3 bond. The identity of complex Rh(Et){2,6-(CH2P(t)Bu2)2C6H3}Cl (3) was confirmed by preparation of the iodide analogue 6 by reaction of the new Rh(η(I)-N2){2,6-(CH2P(t)Bu2)2C6H3} (7) with EtI. It is possible to direct the bond activation process toward the benzylic C-H bonds of the aryl- alkyl group by choice of the Rh(I) precursor, of the substituents on the phosphorus atoms ((t)Bu vs Ph), and of the alkyl moiety (Me vs Et). A Rh(III) complex which is analogous to the product of insertion into the ArCH2-CH3 bond (had it taken place) was prepared and shown not to be an intermediate in the Ar-CH2CH3 bond activation process. Thus, aryl-C activation by Rh(I) is kinetically preferred over activation of the alkyl-C bond in this system. Moreover, cleavage of an Ar-CH2CH3 bond, followed by β-H elimination, may be preferred over sp2-sp3 C-C activation of an Ar-CH3 group.
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(1998) Organometallics. 17, 19, p. 4263-4266 Abstract
An unusual, highly selective C-Si coupling reaction takes place upon treating the platinum(II) alkyl complex cis-(DIPPIDH)(DIPPID)PtMe (2) (DIPPIDH = α2-(diisopropyl-phosphino)isodurene, 1) with HSiR3 (R3 = Et3, EtMe2, Me2Ph), leading to formation of MeSiR3 and the hydrido complex trans-(DIPPIDH)(DIPPID)PtH (5). The overall process involves activation of a Si-H bond, reverse-cyclometalation of the phosphine ligand DIPPIDH (1), and C-Si elimination. The resulting thermally stable platinum(II) hydride complex 5 was independently prepared by thermolysis of a platinum(II) dihydride, trans-(DIPPIDH)2PtH2 (6), which was obtained by reaction of 2 with H2. In the absence of silane, the cis complex 2 isomerizes thermally to trans-(DIPPIDH)(DIPPID)PtMe (3), which does not react productively with silanes. Our results indicate that opening of the metallacycle of 2 by benzylic C-H formation is kinetically preferred over formation of CH4 or CH3SiR3.
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(1998) Journal of the American Chemical Society. 120, 26, p. 6531-6541 Abstract
Reaction of [RhCl(C8H14)2]2 (C8H14 = cyclooctene) with 2 equiv of the aryl methyl ether phosphine 1 in C6D6 results in an unprecedented metal insertion into the strong sp2-sp3 aryl-O bond. This remarkable reaction proceeds even at room temperature and occurs directly, with no intermediacy of C-H activation or insertion into the adjacent weaker ArO-CH3 bond. Two new phenoxy complexes (8 and 9), which are analogous to the product of insertion into the ArO-CH3 bond (had it taken place) were prepared and shown not to be intermediates in the Ar-OCH3 bond cleavage process. Thus, aryl-O bond activation by the nucleophilic Rh(I) is kinetically preferred over activation of the alkyl-O bond. The phenoxy Rh(I)-η1-N2 complex (8) is in equilibrium with the crystallographically characterized Rh(I)-μ-N2-Rh(I) dimer(12). Reaction of [RhClC8H14)2]2 with 2 equiv of the aryl methyl ether phosphine 2, PPh3, and excess HSiR3 (R = OCH2CH3, CH2CH3) results also in selective metal insertion into the aryl-O bond and formation of (CH3O)SiR3. Thus, transfer of a OCH3 group from carbon to silicon was accomplished, showing that hydrosilation of an unstrained aryl-O single bond by a primary silane is possible. The selectivity of C-O bond activation is markedly dependent on the transition-metal complex and the alkyl group involved, allowing direction of the C-O bond activation process at either the aryl-O or alkyl-O bond. Thus, contrary to the reactivity of the rhodium complex, reaction of NiI2 or Pd(CF3CO2)2 with 1 equiv of 1 in ethanol or C6D6 at elevated temperatures results in exclusive activation of the sp3-sp3 ArO-CH3 bond, while reaction of the analogous aryl ethyl ether 4 and Pd(CF3CO2)2 results in both sp3-sp3 and sp2-sp3 C-O bond activation. The resulting phenoxy Pd(II) complex (18) is fully characterized by X-ray analysis. Heating the latter under mild dihydrogen pressure results in hydrodeoxygenation to afford an aryl-Pd(II) complex (19).
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(1998) Chemical Communications. 8, p. 917-918 Abstract
Unprecedented oxidative addition of a strong, unstrained Ar-CF3 bond to a metal complex in solution yields the new aryl-RhIII-CF3 complex 2.
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(1998) Chemical Communications. 6, p. 687-688 Abstract
Reaction of [RhCl(C8H14)2]2 with an excess of the diphosphine 1,3-bis(diisopropylphosphinomethylene)mesitylene 1 in dioxane under mild H2 pressure (25 psi) or with an excess of HSi(OEt)3 results in catalytic selective cleavage of a strong C-C single bond.
1997
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(1997) Angewandte Chemie-International Edition In English. 36, 6, p. 625-626 Abstract
An unprecedented metal insertion into the strong sp2-sp3 aryl-O bond to form 2 is observed for the reaction of [{RhCl(C8H14)2}2] (C8H14 = cyclooctene) with two equivalents of the aryl ether phosphane 1 . This reaction proceeds directly, even at room temperature, with no rhodium insertion into the adjacent weaker ArO-CH3 bond. However, the alkyl-O bond is activated by the reaction of 1 with [Pd(CF3CO2)2] to give 3 . Selectivity of CO activation can therefore be directed by choice of metal complex.
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(1997) Journal of Organometallic Chemistry. 527, 1, p. 263-276 Abstract
Reactions of bis(iminophosphoranyl)methane, CH2(PPh2 - N-aryl)2, (BIPM; 1a,b) or 1,1-bis(iminophosphoranyl)ethane (1,1-BIPE; 1e) with Pd- and Pt-dichlorides containing weakly coordinating ligands (L) such as nitriles or cyclo-octadiene, afforded several products depending on the reaction time, type of ligand (1a-c) or nature of the metal. The first reaction observed is a metal-assisted tautomerization of BIPM to aryl-N-PPh2-CH-PPh2-NH-aryl. When BIPM reacts with PdCl2(L)2, exclusive formation of the C,N-chelate PdCl2{CH(PPh2 - N-aryl)(PPh2-NH-aryl)} (2a,b) is observed, whereas with 1,1-BIPE (1c) a product mixture consisting of C, N-chelate (2c) and an N,N-chelate (3) is found. Orthometallation of the four-membered palladacycle (2) took place upon heating. giving the five-membered palladacycle [PdCl2{2-C6H4-PPh(NH[-pTol)-CH-PPh2(NH-pTol)})-C,C] (4). The molecular structure of 4 has been determined by X-ray crystallography. Reactions of BIPM (1a,b) with PtCl2(RCN)2 (Rphenyl, p-tolyl) afforded entirely different products: the six-membered platinacycles [PtCl(RC-N) [aryl-N-(R)-N-PPl2-CH-PPb2-NH-aryl)-C,N]Cl (5) and [PtCl2(aryl-N-C(R)-N-PPh2-NH-aryl]-C,N] (6), due to an unexpected 2 + 2 cycloaddition of a nitrile with a P-N group.
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(1997) Journal of the American Chemical Society. 119, 48, p. 11687-11688 Abstract
The palladium-catalyzed vinylation of aryl halides, also known as the \u201cHeck reaction\u201d (eq 1) is a very useful synthetic method for the generation of carbon-carbon bonds1 that has found many applications.2 We report here a new type ofpalladium catalyst for this reaction which shows outstanding activities and yields. These catalysts are exceedingly thermally stable and are not sensitive to oxygen. Our evidence suggests that, contrary to the classical mechanism, in this case Pd(0) may not be involved as the active species. The issue of a possible Pd(II)/Pd(IV) cycle in Heck catalysis is currently under debate.3,4 Recently, highly efficient catalysis using cyclopalladated tri-otolylphosphine complexes3,5 and palladium carbene complexes6 has been reported
1996
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(1996) Organometallics. 15, 10, p. 2562-2568 Abstract
The mechanism of the selective formation of the new benzylic mono- and bicyclometalated platinum(II) complexes 2a,b and 3b has been studied. Monitoring the initial sp3 C-H activation of the ligand DIPPIDH (α2-(diisopropylphosphino)isodurene; 1) and (1,5-cyclooctadiene)dimethylplatinum(II) ((COD)PtMe2) at room temperature by 31P{1H} NMR reveals that, after displacement of 1,5-cyclooctadiene, the complex cis-(DIPPIDH)(DIPPID)-PtMe (2a) is formed selectively by oxidative addition of a benzylic C-H bond and subsequent reductive elimination of CH4. This reaction is controlled by electronic factors. Heating of 2a results in isomerization to the thermodynamically more stable isomer trans-(DIPPIDH)-(DIPPID)PtMe (2b) as well as the formation of the double-C-H activated complex trans-(DIPPID)2Pt (3b) with liberation of CH4 in parallel pathways. Continuous heating results in the quantitative formation of the thermally stable 3b. Mechanistically, the second C-H activation proceeds analogously to the first one. The molecular structure of 3b, possessing two six-membered metallacycles, two methylene bridges, and two phosphines in mutually trans positions, was determined by complete single-crystal diffraction studies.
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(1996) Inorganic Chemistry. 35, 24, p. 7068-7073 Abstract
The coordination behavior prior to C-M bond formation of the chelating aromatic PCP substrate DPPMH (3; DPPMH = l,3-bis((diphenylphosphino)methylene)mesitylene) has been studied in order to determine the factors which control the complex formation of such ligands. Reacting 3 with (RCN)2MCl2 (R = Me, Ph; M = Pd, Pt) and (COD)PtX2 (X = Cl, Me; COD = 1,5-cyclooctadiene) resulted in the formation of several 8- and 16-membered mono- and binuclear palladium(II) and platinum(II) macrocycles: trans-[(DPPMH)PdCl2]2 (5), trans-[(DPPMH)-PtCl2]2 (6), cis-(DPPMH)PtCl2 (7), cis-(DPPMH)PtMe2 (8), and cis-[(DPPMH)PtMe2]2 (9). Compounds 5-9 were fully characterized using NMR, FAB-MS, FD-MS, elemental analysis, and X-ray crystallography. Thermolysis of the bimetallic trans-[(DPPMH)PtCl2]2 (6) results in the formation of the monomeric cis-(DPPMH)PtCl2 (7). The product formation depends on the neutral- (nitriles or COD) and anionic ligands (Cl and CH3) of the metal precursor. The molecular structures of trans-[(DPPMH)PdCl2]2 (5) and cis-[(DPPMH)PtMe2]2 (9) have been determined by complete single-crystal diffraction studies. Crystal data for 5: monoclinic, space group P21 /n with a = 14.547(3) Å, b = 17.431(4) Å, c = 27.839 (5) Å, β= 99.56(2)°, V = 6961(3) Å3, and Z = 4. The structure converged to R = 0.048 and Rw = 0.049. Crystal data for 9: monoclinic, space group P21/n with a = 19.187(4) Å, b = 19.189(4) Å c = 20.705(2) Å, β= 103.41(3)°, V = 7415(3) Å3, and Z = 4. The structure refinement converged to R = 0.0977 and Rw = 0.2212.
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(1996) Chemical Communications. 1996, 18, p. 2167-2168 Abstract
The aromatic phosphine 1 reacts with (cod)PtCl2 to yield the C-H activation product 2; treatment of 2 with HCl results in overall selective functionalization of the strong Ar-Me bond to generate MeCl and the Ar-Pt complex 3.