Publications
2024
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(2024) Journal of Physical Chemistry C. 128, 14, p. 5988-5995 Abstract
The crystallinity of polymeric materials defines their properties, in particular, the mechanical ones. High-resolution transmission electron microscopy (TEM) imaging of polymers would be critical to address intricate polymer crystallinity, yet it is challenging due to polymer sensitivity to the electron beam. We performed high-resolution TEM imaging of polycaprolactone (PCL) thin films employing low-dose focal series reconstruction (LDFSR). LDFSR enabled submolecular resolution imaging of polymer crystals. The direct imaging study was augmented by scanning nanobeam electron diffraction (NBED) using the 4D STEM technique to map micro- and nanoscale crystalline domains. Employing LDFSR combined with 4D STEM, we directly observed interacting polymer chains in the crystal lattice, elucidating the crystal structure with a high degree of precision including lattice deformations. We also imaged PCL lamella using conventional TEM. Our methodology enables long-sought insights into the polymer structure, introducing a new tool for high resolution studies of polymer crystallinity that fills a critical gap in the structural science of polymer materials.
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(2024) Advanced Functional Materials. 34, 7, 2309742. Abstract
While individual single-wall carbon nanotubes (SWCNTs) have remarkable strength and electrical conductivity, SWCNT networks fabricated from dispersions have inferior properties due to nanotube bundling, limiting the potential applications of SWCNT materials. Herein, a common dye molecule (purpurin) is used to exfoliate SWCNTs via noncovalent functionalization and to fabricate SWCNT materials by a simple solution-based process. The advantageous noncovalent interactions result in efficient exfoliation and metallic SWCNT enrichment, affording SWCNT materials with high mechanical robustness and electrical conductivity. This method is used to prepare mechanically robust SWCNT films and flexible transparent conductive electrodes.Purpurin, a common dye molecule, is utilized to efficiently exfoliate single-wall carbon nanotubes (SWCNTs) in an aqueous solution. The advantageous noncovalent interactions result in metallic SWCNT enrichment, affording the fabrication of flexible, robust, and highly electrical conductive SWCNT networks. Transparent conductive electrodes based on those networks show excellent optoelectronic performance suitable for application in touch screens and LEDs.image
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(2024) Crystal Growth and Design. 24, 3, p. 1284-1292 Abstract[All authors]
Four crystalline polymorphs of the proinsecticide chlorfenapyr [4-bromo-2-(4-chlorophenyl)-1-ethoxymethyl-5-trifluoromethyl-1H-pyrrole-3-carbonitrile] have been identified and characterized by polarized light optical microscopy, differential scanning calorimetry, Raman spectroscopy, X-ray diffraction, and electron diffraction. Three of the four structures were considered polytypic. Chlorfenapyr polymorphs show similar lethality against fruit flies (Drosophila melanogaster) and mosquitoes (Anopheles quadrimaculatus) with the least stable polymorph showing slightly higher lethality. Similar activities may be expected to be consistent with structural similarities. Knockdown kinetics, however, depend on an internal metabolic activating step, which further complicates polymorph-dependent bioavailability.
2023
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(2023) ACS Nano. 17, 21, p. 20962-20967 Abstract
Development of biodegradable plastic materials is of primary importance in view of acute environmental and health problems associated with the accumulation of plastic waste. We fabricated a biodegradable composite material based on hydroxyethyl cellulose polymer and tyrosine nanocrystals, which demonstrates enhanced strength and ductility (typically mutually excluding properties), superior to most biodegradable plastics. This emergent behavior results from an assembly pattern that leads to a uniform nanoscale morphology and strong interactions between the components. Water-resistant biodegradable composites encapsulated with hydrophobic polycaprolactone as a protection layer were also fabricated. Self-assembly of robust sustainable plastics with emergent properties by using readily available building blocks provides a valuable toolbox for creating sustainable materials.
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(2023) ACS Applied Energy Materials. 6, 9, p. 4511-4519 Abstract
Lithiumsulfur (LiS) batteries (LSBs) have high energy densities and employ inexpensive materials. However, the poor sulfur conductivity and rapid capacity fading hamper their applications. We developed a free-standing composite cathode based on multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs), whose fabrication follows a solution-based, scalable method. The two CNT types create a synergic effect: SWCNTs result in high conductivity, high surface area, and mechanical strength/flexibility; MWCNTs larger pores ensure facile ionic diffusion and trapping of lithium polysulfides. The composite cathode exhibits a peak discharge capacity of 1221 mAh/g, maintaining 876 mAh/g after 100 cycles at a 0.1C rate.
2022
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(2022) Crystal Growth & Design. 22, 11, p. 6647-6655 Abstract
Organic crystal nucleation and growth are complex processes that often do not fit into the framework of the existing crystallization theories. We investigated a crystal growth mechanism of an organic dye, perylene diimide, using high -resolution cryogenic transmission electron microscopy and optical spectroscopy. The elucidated mechanism involves classical (monomer attachments) and nonclassical pathways, exhibiting a self-assembly sequence where all steps are interconnected. It starts from the assembly of molecular pi-stacks that are initially disordered. They gradually optimize their structure, rigidify, and interact to form crystalline domains. The latter further evolve via the addition of individual molecules, and crystal fusion (via oriented attachment). All the observed supramolecular trans-formations are connected and follow a clear hierarchy starting from the molecular-scale interactions. The elucidation of the complex pathway of organic crystallization as a series of coordinated supramolecular transformations at multiple scales conceptually advances the understanding of order evolution in organic matter.
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(2022) Advanced materials (Weinheim). 34, 26, 2202088. Abstract
Structural analysis of beam-sensitive materials by transmission electron microscopy (TEM) represents a significant challenge, as high resolution TEM (HRTEM) requires high electron doses that limits its applicability to stable inorganic materials. Beam sensitive materials, such as organic crystals (of key importance in pharmaceuticals, organic electronics, and biology) must be imaged under low dose conditions, leading to problematic contrast interpretation and the loss of fine structural details. Here, we describe HRTEM imaging of organic crystalline materials with near-atomic resolution of up to 1.6 Å that enabled the real-space study of crystal structures, as well as observation of co-existing polymorphs, crystal defects, and atoms. This is made possible by a low-dose focal series reconstruction (LD-FSR) methodology developed by us, which provides HRTEM images where contrast reflects true object structure and can be performed on contemporary cryo-EM instruments available to many research institutions. We imaged copper phthalocyanine (CuPc), perchlorinated analogue of CuPc, and indigo crystalline films. In the case of indigo crystals, we were able to observe co-existing polymorphs and individual atoms (carbonyl oxygen). In the case of CuPc, we observed several polymorphs, including a new one, for which we elucidated the crystal structure based on direct in-focus imaging, accomplishing real-space crystal structure elucidation. Direct structural analysis of beam sensitive materials with high resolution that enables the real-space study of crystals can be transformative for structural science of organic materials.
2021
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(2021) Nano Letters. 21, 23, p. 9916-9921 Abstract
Colloidal inorganic nanofluorides have aroused great interest for various applications with their development greatly accelerated thanks to advanced synthetic approaches. Nevertheless, understanding their colloidal evolution and the factors that affect their dispersion could improve the ability to rationally design them. Here, using a multimodal in situ approach that combines DLS, NMR, and cryogenic-TEM, we elucidate the formation dynamics of nanofluorides in water through a transient aggregative phase. Specifically, we demonstrate that ligand-cation interactions mediate a transient aggregation of as-formed CaF2 nanocrystals (NCs) which governs the kinetics of the colloids' evolution. These observations shed light on key stages through which CaF2 NCs are dispersed in water, highlighting fundamental aspects of nanofluorides formation mechanisms. Our findings emphasize the roles of ligands in NCs' synthesis beyond their function as surfactants, including their ability to mediate colloidal evolution by complexing cationic precursors, and should be considered in the design of other types of NCs.
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(2021) ChemPhysChem. 22, 21, p. 2182-2189 Abstract
The mechanism by which safranine O (SFO), an ice growth inhibitor, halts the growth of single crystal tetrahydrofuran (THF) clathrate hydrates was explored using microfluidics coupled with cold stages and fluorescence microscopy. THF hydrates grown in SFO solutions exhibited morphology changes and were shaped as truncated octahedrons or hexagons. Fluorescence microscopy and microfluidics demonstrated that SFO binds to the surface of THF hydrates on specific crystal planes. Cryo-TEM experiments of aqueous solutions containing millimolar concentrations of SFO exhibited the formation of bilayered lamellae with an average thickness of 4.2±0.2 nm covering several μm2. Altogether, these results indicate that SFO forms supramolecular lamellae in solution, which might bind to the surface of the hydrate and inhibit further growth. As an ice and hydrate inhibitor, SFO may bind to the surface of these crystals via ordered water molecules near its amine and methyl groups, similar to some antifreeze proteins.
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(2021) Crystal Growth and Design. 21, 7, p. 4064-4072 Abstract
The rational design of organic crystalline materials is exceedingly challenging owing to the insufficient understanding of crystallization mechanisms. Here, we used a polymer (polystyrene) as a crystallization medium for an organic semiconductor (rubrene). This enabled a slow crystallization process whose mechanism was elucidated via direct transmission electron microscopy imaging and further employed to control crystallization and fabricate devices based on rubrene crystals. The elucidated mechanism involved (1) the initial formation of amorphous aggregates; (2) nucleation within the aggregates; (3) crystal growth; (4) break up into single nanocrystals; and (5) oriented attachment of nanocrystals to form platelet-like crystals. This mechanistic insight indicated that uniform nonclassical nucleation can be realized using a precise temperature control, leading to high-quality rubrene crystals. By employing this methodology, we fabricated solution-processed organic field-effect transistors (OFETs) and organic phototransistors (OPTs) that exhibited high mobility, reproducibility, and environmental stability. The devices showed an average mobility of 1 ± 0.8 cm2 V-1 s-1, a threshold voltage of -10 ± 6 V, and an on/off ratio of up to 106. Under white light irradiation, rubrene OPTs exhibited strong photoresponse with a photo/dark current ratio of P ≈ 105. Our work demonstrated that mechanistic information can be employed to fabricate high-quality OFETs, having implications for the rational design of crystalline organic electronic materials.
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(2021) ACS Central Science. 7, 5, p. 900-908 Abstract
The crystallization mechanisms of organic molecules in solution are not well-understood. The mechanistic scenarios where crystalline order evolves directly from the molecularly dissolved state ("classical") and from initially formed amorphous intermediates ("nonclassical") are suggested and debated. Here, we studied crystallization mechanisms of two widely used analgesics, ibuprofen (IbuH) and etoricoxib (ETO), using direct cryogenic transmission electron microscopy (cryo-TEM) imaging. In the IbuH case, parallel crystallization pathways involved diverse phases of high and low density, in which the instantaneous formation of final crystalline order was observed. ETO crystallization started from well-defined round-shaped amorphous intermediates that gradually evolved into crystals. This mechanistic diversity is rationalized by introducing a continuum crystallization paradigm: order evolution depends on ordering in the initially formed intermediates and efficiency of molecular rearrangements within them, and there is a continuum of states related to the initial order and rearrangement rates. This model provides a unified view of crystallization mechanisms, encompassing classical and nonclassical pictures.
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(2021) Nature Communications. 12, 1, 229. Abstract[All authors]
Understanding inorganic nanocrystal (NC) growth dynamic pathways under their native fabrication environment remains a central goal of science, as it is crucial for rationalizing novel nanoformulations with desired architectures and functionalities. We here present an in-situ method for quantifying, in real time, NCs size evolution at sub-nm resolution, their concentration, and reactants consumption rate for studying NC growth mechanisms. Analyzing sequential high-resolution liquid-state 19F-NMR spectra obtained in-situ and validating by ex-situ cryoTEM, we explore the growth evolution of fluoride-based NCs (CaF2 and SrF2) in water, without disturbing the synthesis conditions. We find that the same nanomaterial (CaF2) can grow by either a particle-coalescence or classical-growth mechanism, as regulated by the capping ligand, resulting in different crystallographic properties and functional features of the fabricated NC. The ability to reveal, in real time, mechanistic pathways at which NCs grow open unique opportunities for tunning the properties of functional materials.
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(2021) Beilstein Journal of Organic Chemistry. 17, p. 42-51 Abstract
The facile fabrication of free-floating organic nanocrystals (ONCs) was achieved via the kinetically controlled self-assembly of simple perylene diimide building blocks in aqueous medium. The ONCs have a thin rectangular shape, with an aspect ratio that is controlled by the content of the organic cosolvent (THF). The nanocrystals were characterized in solution by cryogenic transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering. The ONCs retain their structure upon drying, as was evidenced by TEM and atom force microscopy. Photophysical studies, including femtosecond transient absorption spectroscopy, revealed a distinct influence of the ONC morphology on their photonic properties (excitation energy transfer was observed only in the high-aspect ONCs). Convenient control over the structure and function of organic nanocrystals can enhance their utility in new and developed technologies.
2020
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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) Nanoscale. 12, 16, p. 8909-8914 Abstract
Sustainable energy storage devices are required in view of the current demand for environmentally friendly technology. We fabricated a fully recyclable electrochemical double-layer supercapacitor (EDLC), based on multiwalled carbon nanotube (MWCNT) electrodes, an organic nanocrystalline (ONC) dielectric membrane, and an aqueous electrolyte. The entire EDLC device was fabricated and recycled using simple solution processing. The pristine and recycled EDLC devices maintained high stability after 18,000 cycles in cyclic voltammetry testing. Our results advance a concept of sustainable energy storage devices that are easy to fabricate and recycle.
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(2020) Nature. 579, 7800, p. 540-543 Abstract
Protein crystallization is important in structural biology, disease research and pharmaceuticals. It has recently been recognized that nonclassical crystallizationinvolving initial formation of an amorphous precursor phaseoccurs often in protein, organic and inorganic crystallization processes15. A two-step nucleation theory has thus been proposed, in which initial low-density, solvated amorphous aggregates subsequently densify, leading to nucleation4,6,7. This view differs from classical nucleation theory, which implies that crystalline nuclei forming in solution have the same density and structure as does the final crystalline state1. A protein crystallization mechanism involving this classical pathway has recently been observed directly8. However, a molecular mechanism of nonclassical protein crystallization915 has not been established9,11,14. To determine the nature of the amorphous precursors and whether crystallization takes place within them (and if so, how order develops at the molecular level), three-dimensional (3D) molecular-level imaging of a crystallization process is required. Here we report cryogenic scanning transmission microscopy tomography of ferritin aggregates at various stages of crystallization, followed by 3D reconstruction using simultaneous iterative reconstruction techniques to provide a 3D picture of crystallization with molecular resolution. As crystalline order gradually increased in the studied aggregates, they exhibited an increase in both order and density from their surface towards their interior. We observed no highly ordered small structures typical of a classical nucleation process, and occasionally we observed several ordered domains emerging within one amorphous aggregate, a phenomenon not predicted by either classical or two-step nucleation theories. Our molecular-level analysis hints at desolvation as the driver of the continuous order-evolution mechanism, a view that goes beyond current nucleation models, yet is consistent with a broad spectrum of protein crystallization mechanisms.
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(2020) ACS Applied Electronic Materials. 2, 3, p. 790-795 Abstract
We envisaged that rapid thermal processing (RTP) widely used in semiconductor device fabrication can be employed for fabricating organic crystalline devices since heating and mass transfer are localized within a small area in RTP. This may result in crystal growth at the location relevant to organic device fabrication in situ, for which RTP has not been used so far. We utilized the RTP technique for the growth of high quality organic crystals from thin films of copper phthalocyanine (CuPc) and rubrene. The crystals were grown in situ on silicon surfaces, which were directly used for device fabrication (organic field effect transistors, OFETs, and organic phototransistors, OPTs). For CuPc devices, the mobility was 0.12 ± 0.11 cm2 V1 s1, on/off ratio of up to 106, and photo/dark current ratio of P > 105 (for OPT devices). The mobility of rubrene-based OFETs was 0.31 ± 0.15 cm2 V1 s1, on/off ratio of up to 105, and photo/dark current ratio of P ≈ 105. The mobilities are similar to those of previously reported single-crystalline CuPc and rubrene OFETs fabricated on untreated surfaces, and the photoresponses are stronger than those of the reported CuPc and rubrene OPTs. RTP is a general and efficient method to grow high quality organic crystals in situ, significantly advancing fabrication methodology for organic electronic and optoelectronic devices.
2019
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(2019) Polymers for Advanced Technologies. 30, 10, p. 2549-2557 Abstract
We describe a new family of composite materials, polymer/organic nanocrystal (ONC) hybrids. These were prepared from soluble ONCs based on perylene diimides (PDI) and water-soluble polymers (sodium alginate and polyvinyl alcohol). Polymer/ONC films were characterized by optical spectroscopy, electron microscopy, and tensile strength studies. The films show enhanced chemical and mechanical stability due to synergy between the constituents. The hybrid films are stable in both water and organic solvents, unlike the individual components. The ONCs we employed possess nonlinear optical activity (second harmonic generation, SHG); they showed improved photostability (stable SHG under laser light) in the hybrids. Tensile strength enhancement (as high as twofold in the film having just 2.4% ONCs by weight) was observed as revealed by mechanical measurements. Hybrids with aligned ONCs were also prepared using simple extrusion via syringe needle followed by gelation. Employing ONCs in polymeric hybrid materials enables facile fabrication in aqueous media, synergy, chemical, mechanical, and photostability as well as useful photofunction (SHG), introducing a versatile class of composite materials.
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(2019) Journal of Physical Chemistry C. 123, 41, p. 25031-25041 Abstract
Organic photovoltaics enable cost-efficient, tunable, and flexible platforms for solar energy conversion, yet their performance and stability are still far from optimal. Here, we present a study of photoinduced charge transfer processes between electron donor and acceptor organic nanocrystals as part of a pathfinding effort to develop robust and efficient organic nanocrystalline materials for photovoltaic applications. For this purpose, we utilized nanocrystals of perylenediimides as the electron acceptors and nanocrystalline copper phthalocyanine as the electron donor. Three different configurations of donor-acceptor heterojunctions were prepared. Charge transfer in the heterojunctions was studied with Kelvin probe force microscopy under laser or white light excitation. Moreover, detailed morphology characterizations and time-resolved photoluminescence measurements were conducted to understand the differences in the photovoltaic processes of these organic nanocrystals. Our work demonstrates that excitonic properties can be tuned by controlling the crystal and interface structures in the nanocrystalline heterojunctions, leading to a minimization of photovoltaic losses.
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(2019) ACS Nano. 13, 10, p. 11097-11106 Abstract
In view of their facile fabrication and recycling, functional materials that are built from small molecules ("molecular plastics") may represent a cost-efficient and sustainable alternative to conventional covalent materials. We show how molecular plastics can be made robust and how their (nano)structure can be tuned via modular construction. For this purpose, we employed binary composites of organic nanocrystals based on a perylene diimide derivative, with graphene oxide (GO), bentonite nanoclay (NC), or hydroxyethyl cellulose (HEC), that both reinforce and enable tailoring the properties of the membranes. The hybrids are prepared via a simple aqueous deposition method, exhibit enhanced mechanical robustness, and can be recycled. We utilized these properties to create separation membranes with tunable porosity that are easy to fabricate and recycle. Hybrids 1/HEC and 1/NC are capable of ultrafiltration, and 1/NC removes heavy metals from water with high efficiency. Hybrid 1/GO shows mechanical properties akin to covalent materials with just 2-10% (by weight) of GO. This hybrid was used as a membrane for immobilizing β-galactosidase that demonstrated long and stable biocatalytic activity. Our findings demonstrate the utility of modular molecular nanoplastics as robust and sustainable materials that enable efficient tuning of structure and function and are based on self-assembly of readily available inexpensive components.
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(2019) Small. 15, 38, 1902936. Abstract
The widely employed crystallization of organic molecules in solution is not well understood and is difficult to control. Employing polymers as crystallization media may allow enhanced control via temperature-induced regulation of polymer dynamics. Crystallization of a small organic molecule (perylene diimide) is investigated in polymer matrices (polystyrene) that enable the mechanistic study and control over order evolution. The crystallization is induced by heating above the glass transition temperature of the polymer, and quenched by cooling, leading to stabilization of crystallization intermediates. The mechanistic studies include direct imaging by electron microscopy, revealing a complex self-assembly process starting from amorphous aggregates that densify and transform into an unstable crystalline phase of N,N '-bis(2,6-dimethylphenyl)perylene-3,4,9,10-tetracarboxylic diimide (DMP-PDI), followed by a conversion into a more stable crystalline form. Stabilization of crystallization intermediates at room temperature provides diverse structures based on a single molecular component. These findings have implications for the rational design of organic crystalline materials.
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(2019) Accounts of Chemical Research. 52, 9, p. 2634-2646 Abstract
CONSPECTUS: Most robust functional organic materials are currently based on polymers. These materials exhibit high stability, but once formed they are difficult to modify, adapt to their environment, and recycle. Materials based on small molecules that are held together by noncovalent interactions can offer an alternative to conventional polymer materials for applications that require adaptive and stimuli-responsive features. However, it is challenging to engineer macroscopic noncovalent materials that are sufficiently robust for practical applications.This Account summarizes progress made by our group towards the development of noncovalent "aqua materials" based on well-defined organic molecules. These materials are uniquely assembled in aqueous media, where they harness the strength of hydrophobic and is pi-pi interactions between large aromatic groups to achieve robustness. Despite their high stability, these supramolecular systems can dynamically respond to external stimuli. We discuss design principles, fundamental properties, and applications of two classes of aqua materials: (1) supramolecular gels and (2) nanocrystalline arrays. The materials were characterized by a combination of steady-state and time-resolved spectroscopic techniques, electrical measurements, molecular modeling, and high-resolution microscopic imaging, in particular cryogenic transmission electron microscopy (cryo-TEM) and cryogenic scanning electron microscopy (cryo-SEM).All investigated aqua materials are based on one key building block, perylene diimide (PDI). PDI exhibits remarkably stable intermolecular bonds, together with useful chemical and optoelectronic properties. PDI-based amphiphiles carrying poly(ethylene glycol) (PEG) were designed to form linear supramolecular polymers in aqueous media. These one-dimensional arrays of noncovalently linked molecules can entangle and form three-dimensional supramolecular networks, leading to soft gel-like materials. Tuning the strength of interactions between fibers enables dynamic adjustment of viscoelastic properties and functional characteristics. Besides supramolecular gels, we show that simple PDI-based molecules can self-assemble in aqueous medium to form robust organic nanocrystals (ONCs). The mechanical and optoelectronic properties of ONCs are distinctly different from gel-phase materials. ONCs are excellent building blocks for macroscopic free-standing materials that can be used in dry state, unlike hydrogels. Being constructed from small molecules, ONC materials are easy to fabricate and recycle. High thermal robustness, good mechanical properties, and modular design render ONC materials versatile and suitable for a variety of applications.In the future, noncovalent aqua materials can become a sustainable alternative to conventional polymer materials. Examples from our research include stimuli-responsive and recyclable filtration membranes for preparative nanoparticle separation, water purification and catalysis, light-harvesting hydrogels for solar energy conversion, and nanocrystalline films for switchable surface coatings and electronic devices.
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(2019) Nanoscale. 11, 8, p. 3733-3740 Abstract
We report on utilizing free-standing hybrid perylenediimide/carbon nanotube (PDI/CNT) films fabricated in air as back contacts for fully inorganic perovskite solar cells (glass/FTO/dense TiO2/mesoporous TiO2/ CsPbBr3/back electrode). The back contact electrode connection is performed by film transfer rather than by vacuum deposition or by wet processing, allowing the formation of highly homogeneous contacts under ambient conditions. The use of this novel electrode in solar cells based on CsPbBr3 resulted in efficiency of 5.8% without a hole transporting layer; it is significantly improved in comparison to the reference cells with standard gold electrodes. Overall device fabrication can be performed on air, using inexpensive processing methods. The hybrid film electrodes dramatically improve the cell photo-stability under ambient conditions and under real-life operating conditions outdoors. The champion unencapsulated device demonstrated less than 30% efficiency loss over 6 weeks of outdoor aging in Negev desert conditions. The CNT/PDI electrodes offer the combination of fabrication simplicity, unique contacting approach, high efficiency and good operational stability for perovskite photovoltaics.
2018
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(2018) Nanoscale. 10, 43, p. 20147-20154 Abstract
Upon photoexcitation, self-assembled PDI nanocrystals (S1S0) in the form of rods of 70 nm width and 1 mu m length are subject to a symmetry breaking charge separation (SBCS) as the first step in the singlet fission (SF) sequence. Hereby, the correlated pair of triplet excited states (1)(T1T1) is formed with a quantum yield of 122%. Decoherence and triplet diffusion within the nanocrystals affords a long-lived, uncorrelated pair of triplet excited states (T-1 + T-1) with a quantum yield of 24%.
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(2018) Journal of Materials Chemistry C. 6, 39, p. 10597-10602 Abstract
We report on the fabrication and characterization of organic phototransistors (OPTs) based on fluorescent nanocrystals assembLed from a simple organic dye molecule (N,N' -bis(2,4-dimethylpent-3-yl)perylene-3,4:9,10-tetracarboxylic diimide, DMP-PDI). The OPT active Layer is based on DMP-PDI nanocrystals assembLed in aqueous solution or within polymer films. Despite the absence of any pi-overlap, the nanocrystals show mobilities as high as (5 +/- 1) x 10(-3) cm(2) V-1 s(-1) in polymer films, which is due to imide/ pi-core noncovalent interactions Leading to substantial electronic coupling as revealed by computational studies. The OPTs strongly respond to white Light irradiation, resulting in a decrease in threshoLd voltage by as much as 40 V. OPTs based on nanocrystals assembLed within poLymer fiLms have threshoLd voltages dose to 0 V upon illumination and a high photo/dark current ratio (P = 4 x 10(3)). We show that the organic crystals Lacking pi-overlap mediate charge mobility and are advantageous as active Layers for OPTs due to diminished nonradiative decay.
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(2018) ACS Central Science. 4, 8, p. 1031-1036 Abstract
Organic crystals are of primary importance in pharmaceuticals, functional materials, and biological systems; however, organic crystallization mechanisms are not well-understood. It has been recognized that "nonclassical" organic crystallization from solution involving transient amorphous precursors is ubiquitous. Understanding how these precursors evolve into crystals is a key challenge. Here, we uncover the crystallization mechanisms of two simple aromatic compounds (perylene diimides), employing direct structural imaging by cryogenic electron microscopy. We reveal the continuous evolution of density, morphology, and order during the crystallization of very different amorphous precursors (well-defined aggregates and diffuse dense liquid phase). Crystallization starts from initial densification of the precursors. Subsequent evolution of crystalline order is gradual, involving further densification concurrent with optimization of molecular ordering and morphology. These findings may have implications for the rational design of organic crystals.
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(2018) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 57, 29, p. 8871-8874 Abstract
An amphiphile based on polyethylene glycol (PEG) polymer and two molecular moieties (perylene diimide and C-7 fluoroalkyl, PDI and C7F) attached to its termini assembles into crystalline films with long-range order. The films reversibly switch from crystalline to amorphous above the PEG melting temperature. The adaptive behavior stems from the responsiveness of the PEG domain and the robustness of the PDI and C7F assemblies. The hydrophobicity of the film can be controlled by heating, resulting in switching from highly hydrophobic to superhydrophilic. The long-range order, reversible crystallinity switching, and the temperature-controlled wettability demonstrate the potential of block copolymer analogues based on simple polymeric/molecular hybrids.
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(2018) Journal of the American Chemical Society. 140, 14, p. 4761-4764 Abstract
We demonstrate a solution-based fabrication of centimeter-size free-standing films assembled from organic nanocrystals based on common organic dyes (perylene diimides, PDIs). These nanostructured films exhibit good mechanical stability, and thermal robustness superior to most plastics, retaining the crystalline microstructure and macroscopic shape upon heating up to 250-300 degrees C. The films show nonlinear optical response and can be used as ultrafiltration membranes. The macroscopic functional materials based on small molecules can be alternative or complementary to materials based on macromolecules.
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(2018) Chemistry-A European Journal. 24, 12, p. 2898-2912 Abstract
Several transition metal ions, like Fe2+, Co2+, Ni2+, and Zn2+ complex to the ditopic ligand 1,4-bis(2,2': 6', 2 ''-terpyridin-4'-yl) benzene (L). Due to the high association constant, metal-ion induced self-assembly of Fe2+, Co2+, and Ni2+ leads to extended, rigid-rod like metallo-supramolecular coordination polyelectrolytes (MEPEs) even in aqueous solution. Here, we present the kinetics of growth of MEPEs. The species in solutions are analyzed by light scattering, viscometry and cryogenic transmission electron microscopy (cryo-TEM). At near-stoichiometric amounts of the reactants, we obtained high molar masses, which follow the order NiMEPE & Co-MEPE
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(2018) ACS Nano. 12, 1, p. 317-326 Abstract
Designing supramolecular nanotubes (SNTs) with distinct dimensions and properties is highly desirable, yet challenging, since structural control strategies are lacking. Furthermore, relatively complex building blocks are often employed in SNT self-assembly. Here, we demonstrate that symmetric bolaamphiphiles having a hydrophobic core comprised of two perylene diimide moieties connected via a bipyridine linker and bearing polyethylene glycol (PEG) side chains can self-assemble into diverse molecular nanotubes. The structure of the nanotubes can be controlled by assembly conditions (solvent composition and temperature) and a PEG chain length. The resulting nanotubes differ both in diameter and cross section geometry, having widths of 3 nm (triangular-like cross-section), 4 nm (rectangular), and 5 nm (hexagonal). Molecular dynamics simulations provide insights into the stability of the tubular superstructures and their initial stages of self-assembly, revealing a key role of oligomerization via side-by-side aromatic interactions between bis-aromatic cores. Probing electronic and photonic properties of the nanotubes revealed extended electron delocalization and photoinduced charge separation that proceeds via symmetry breaking, a photofunction distinctly different from that of the fibers assembled from the same molecules. A high degree of structural control and insights into SNT self-assembly advance design approaches toward functional organic nanomaterials.
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(2018) Advanced Materials. 30, 2, 1705027. Abstract
Organic crystalline materials are used as dyes/pigments, pharmaceuticals, and active components of photonic and electronic devices. There is great interest in integrating organic crystals with inorganic and carbon nanomaterials to create nanocomposites with enhanced properties. Such efforts are hampered by the difficulties in interfacing organic crystals with dissimilar materials. Here, an approach that employs organic nanocrystallization is presented to fabricate solution-processed organic nanocrystal/carbon nanotube (ONC/CNT) hybrid materials based on readily available organic dyes (perylene diimides (PDIs)) and carbon nanotubes. The hybrids are prepared by self-assembly in aqueous media to afford free-standing films with tunable CNT content. These exhibit excellent conductivities (as high as 5.78 ± 0.56 S m−1), and high thermal stability that are superior to common polymer/CNT hybrids. The color of the hybrids can be tuned by adding various PDI derivatives. ONC/CNT hybrids represent a novel class of nanocomposites, applicable as optoelectronic and conductive colorant materials.
2017
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(2017) Chemistry-A European Journal. 23, 43, p. 10328-10337 Abstract
The self-assembly behavior of DNA conjugates possessing a perylenediimide (PDI) head group and an N-oligonucleotide tail has been investigated using a combination of optical spectroscopy and cryogenic transmission electron microscopy (cryo-TEM) imaging. To obtain insight into the interplay between PDI hydrophobic interactions and DNA base-pairing we employed systematic variation in the length and composition of the oligo tails. Conjugates with short (TA)n or (CG)n oligo tails (n≤3) form helical or nonhelical fibers constructed from π-stacked PDI head groups with pendent oligo tails in aqueous solution. Conjugates with longer (TA)n oligo tails also form stacks of PDI head groups, which are further aggregated by base-pairing between their oligo tails, leading to fiber bundling and formation of bilayers. The longer (CG)n conjugates form PDI end-capped duplexes, which further assemble into PDI-stacked arrays of duplexes leading to large scale ordered assemblies. Cryo-TEM imaging reveals that (CG)3 gives rise to both fibers and large assemblies, whereas (CG)5 assembles preferentially into large ordered structures.
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(2017) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 56, 8, p. 2203-2207 Abstract
Aqua materials that contain water as their major component and are as robust as conventional plastics are highly desirable. Yet, the ability of such systems to withstand harsh conditions, for example, high pressures typical of industrial applications has not been demonstrated. We show that a hydrogel- like membrane self-assembled from an aromatic amphiphile and colloidal Nafion is capable of purifying water from organic molecules, including pharmaceuticals, and heavy metals in a very wide range of concentrations. Remarkably, the membrane can sustain high pressures, retaining its function. The robustness and functionality of the water-based self-assembled array advances the idea that aqua materials can be very strong and suitable for demanding industrial applications.
2016
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Interface Modification by Simple Organic Salts Improves Performance of Planar Perovskite Solar Cells(2016) Advanced Materials Interfaces. 3, 23, 1600506. Abstract
Simple organic salts are used as a cheap alternative for hole-conducting materials in methylammonium lead bromide perovskite solar cells and obtaining power conversion efficiency of 4.4%. The findings suggest that the polar organic salts interact with the perovskite surface, leading to formation of a surface dipole or change of an existing one on the perovskites that changes its effective work function.
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(2016) Chemical Reviews. 116, 4, p. 2414-2477 Abstract
This review discusses one-dimensional supramolecular polymers that form in aqueous media. First, naturally occurring supramolecular polymers are described, in particular, amyloid fibrils, actin filaments, and microtubules. Their structural, thermodynamic, kinetic, and nanomechanical properties are highlighted, as well as their importance for the advancement of biologically inspired supramolecular polymer materials. Second, five classes of synthetic supramolecular polymers are described: systems based on (1) hydrogen-bond motifs, (2) large pi-conjugated surfaces, (3) host guest interactions, (4) peptides, and (5) DNA. We focus on recent studies that address key challenges in the field, providing mechanistic understanding, rational polymer design, important functionality, robustness, or unusual thermodynamic and kinetic properties.
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(2016) Angewandte Chemie (International ed. in English). 55, 1, p. 179-182 Abstract
Understanding and controlling organic crystallization in solution is a long-standing challenge. Herein, we show that crystallization of an aromatic amphiphile based on perylene diimide in aqueous media involves initially formed amorphous spherical aggregates that evolve into the crystalline phase. The initial appearance of the crystalline order is always confined to the spherical aggregates that are precursors for crystalline evolution. The change in the solvation of the prenucleation phase drives the crystallization process towards crystals that exhibit very different structure and photofunction. The initial molecular structure and subsequent crystal evolution can be regulated by tuning the hydrophobicity at various stages of crystallization, affording dissimilar crystalline products or hindering crystallization. Thus, the key role of the precrystalline states in organic crystal evolution enables a new strategy to control crystallization by precrystalline state manipulation.
2015
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(2015) Nano Letters. 15, 11, p. 7232-7237 Abstract[All authors]
Facile molecular self-assembly affords a new family of organic nanocrystals that, unintuitively, exhibit a significant nonlinear optical response (second harmonic generation, SHG) despite the relatively small molecular dipole moment of the constituent molecules. The nanocrystals are self-assembled in aqueous media from simple monosubstituted perylenediimide (PDI) molecular building blocks. Control over the crystal dimensions can be achieved via modification of the assembly conditions. The combination of a simple fabrication process with the ability to generate soluble SHG nanocrystals with tunable sizes may open new avenues in the area of organic SHG materials.
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(2015) Advanced Materials. 27, 35, p. 5102-5112 Abstract[All authors]
The conclusions reached by a diverse group of scientists who attended an intense 2-day workshop on hybrid organic-inorganic perovskites are presented, including their thoughts on the most burning fundamental and practical questions regarding this unique class of materials, and their suggestions on various approaches to resolve these issues.
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(2015) Journal of the American Chemical Society. 137, 23, p. 7429-7440 Abstract
The unique properties of carbon nanotubes (CNT) are advantageous for emerging applications. Yet, the CNT insolubility hampers their potential. Approaches based on covalent and noncovalent methodologies have been tested to realize stable dispersions of CNTs. Noncovalent approaches are of particular interest as they preserve the CNTs structures and properties. We report on hybrids, in which perylene diimide (PDI) amphiphiles are noncovalently immobilized onto single wall carbon nanotubes (SWCNT). The resulting hybrids were dispersed and exfoliated both in water and organic solvents in the presence of two different PDI derivatives, PP2b and PP3a. The dispersions were investigated using cryogenic transmission electron microscopy (cryo-TEM), providing unique structural insights into the exfoliation. A helical arrangement of PP2b assemblies on SWCNTs dominates in aqueous dispersions, while a single layer of PP2b and PP3a was found on SWCNTs in organic dispersions. The dispersions were probed by steady-state and time-resolved spectroscopies, revealing appreciable charge redistribution in the ground state, and an efficient electron transfer from SWCNTs to PDIs in the excited state. We also fabricated hybrid materials from the PP2b/SWCNT dispersions. A supramolecular membrane was prepared from aqueous dispersions and used for size-selective separation of gold nanoparticles. Hybrid buckypaper films were prepared from the organic dispersions. In the latter, high conductivity results from enhanced electronic communication and favorable morphology within the hybrid material. Our findings shed light onto SWCNT/dispersant molecular interactions, and introduce a versatile approach toward universal solution processing of SWCNT-based materials.
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(2015) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 54, 19, p. 5636-5640 Abstract
Metal-organic self-assembly has proven to be of great use in constructing structures of increasing size and intricacy, but the largest assemblies lack the functions associated with the ability to bind guests. Here we demonstrate the self-assembly of two simple organic molecules with CdII and PtII into a giant heterometallic supramolecular cube which is capable of binding a variety of mono- and dianionic guests within an enclosed cavity greater than 4200 Å3. Its structure was established by X-ray crystallography and cryogenic transmission electron microscopy. This cube is the largest discrete abiological assembly that has been observed to bind guests in solution; cavity enclosure and coulombic effects appear to be crucial drivers of host-guest chemistry at this scale. The degree of cavity occupancy, however, appears less important: the largest guest studied, bound the most weakly, occupying only 11-% of the host cavity. Brobdingnagian: A giant, heterometallic cube with host-guest properties was prepared by successful application of a rational strategy to increase the dimensions whilst maintaining an enclosed cavity (see X-ray crystal structure). A variety of mono- and dianionic guests was bound in the cavity in solution. Hierarchical aggregation of the cubes into a rigid monolayer was visualized by cryogenic transmission electron microscopy.
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(2015) Chemistry-A European Journal. 21, 1, p. 166-176 Abstract
In the current work, we demonstrate how coordination chemistry can be employed to direct self-assembly based on strong hydrophobic interactions. To investigate the influence of coordination sphere geometry on aqueous self-assembly, we synthesized complexes of the amphiphilic perylene diimide terpyridine ligand with the first-row transition-metal centers (zinc, cobalt, and nickel). In aqueous medium, aggregation of these complexes is induced by hydrophobic interactions between the ligands. However, the final shapes of the resulting assemblies depend on the preferred geometry of the coordination spheres typical for the particular metal center. The self-assembly process was characterized by UV/Vis spectroscopy, zeta potential measurements, and cryogenic transmission electron microscopy (cryo-TEM). Coordination of zinc(II) and cobalt(II) leads to the formation of unique nanospiral assemblies, whereas complexation of nickel( II) leads to the formation of straight nanofibers. Notably, coordination bonds are utilized not as connectors between elementary building blocks, but as directing interactions, enabling control over supramolecular geometry.
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(2015) Journal of Materials Chemistry A. 3, 40, p. 20305-20312 Abstract
A small molecule based on N,N-dialkyl perylenediimide (PDI) as core derivatized with thiophene moieties (Th-PDI) was synthesized. Its HOMO (highest occupied molecular orbital) level was measured to be between 5.7 and 6.3 eV vs. local vacuum level depending on doping and measurement method. Th-PDI was successfully applied as hole-transporting material (HTM) in CH3NH3PbBr3 hybrid perovskite solar cells. Three different cell architectures, each with a different mode of operation, were tested: (1) using a mesoporous (mp) TiO2 substrate; (2) mp-Al2O3 substrate; (3) planar dense TiO2 substrate. The first gave the best overall efficiency of 5.6% while the mp-Al2O3 gave higher open-circuit photovoltage (VOC) but lower efficiency (2.2%). The cells exhibited good reproducibility with very little J-V hysteresis (the mp-Al2O3 showed a more appreciable hysteresis of individual photovoltaic parameters but little dependence of efficiency on scan direction). Storage of unencapsulated cells in 25-30% relative humidity demonstrated fairly good stability with
2014
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(2014) Journal Of Physical Chemistry B. 118, 41, p. 12068-12073 Abstract
Investigation of supramolecular kinetics is essential for elucidating self-assembly mechanisms. Recently, we reported on a noncovalent system involving a bolaamphiphilic perylene diimide dimer that is kinetically trapped in water but can rearrange into a different, more ordered assembly in water/THF mixtures (Angew. Chem. Int. Ed. 2014, 53, 4123). Here we present a kinetic mechanistic study of this process by employing UV-vis spectroscopy. The transformation exhibits a rapid decrease in the red-shifted absorption band, which is monitored in order to track the kinetics at different temperatures (15-50 °C) and concentrations. Fitting the data with the 1D KJMA (Kolmogorov-Johnson-Mehl-Avrami) model affords the activation parameters. The latter as well as seeding experiments indicates that the transformation occurs without the detachment of covalent units, and that hydration dynamics plays a significant role in nucleation, with entropic factors being dominant. Switching off the transformation, and the formation of off-pathway intermediates were observed upon heating to temperatures above 55 °C. These insights into kinetically controlled supramolecular polymer transformations provide mechanistic information that is needed for a fundamental understanding of noncovalent processes, and the rational design of noncovalent materials.
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(2014) Journal of the American Chemical Society. 136, 38, p. 13249-13256 Abstract
Hybrid organic/lead halide perovskites are promising materials for solar cell fabrication, resulting in efficiencies up to 18%. The most commonly studied perovskites are CH3NH3PbI3 and CH3NH3PbI3-x Clx where x is small. Importantly, in the latter system, the presence of chloride ion source in the starting solutions used for the perovskite deposition results in a strong increase in the overall charge diff usion length. In this work we investigate the crystallization parameters relevant to fabrication of perovskite materials based on CH3NH3PbI3 and CH3NH3PbBr3. We find that the addition of PbCl2 to the solutions used in the perovskite synthesis has a remarkable eff ect on the end product, because PbCl2 nanocrystals are present during the fabrication process, acting as heterogeneous nucleation sites for the formation of perovskite crystals in solution. We base this conclusion on SEM studies, synthesis of perovskite single crystals, and on cryo-TEM imaging of the frozen mother liquid. Our studies also included the effect of different substrates and substrate temperatures on the perovskite nucleation efficiency. In view of our findings, we optimized the procedures for solar cells based on lead bromide perovskite, resulting in 5.4% efficiency and Voc of 1.24 V, improving the performance in this class of devices. Insights gained from understanding the hybrid perovskite crystallization process can aid in rational design of the polycrystalline absorber films, leading to their enhanced performance.
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(2014) Chemistry-A European Journal. 20, 33, p. 10332-10342 Abstract
Understanding the crystallization of organic molecules is a long-standing challenge. Herein, a mechanistic study on the self-assembly of crystalline arrays in aqueous solution is presented. The crystalline arrays are assembled from perylene diimide (PDI) amphiphiles bearing a chiral N-acetyltyrosine side group connected to the PDI aromatic core. A kinetic study of the crystallization process was performed using circular dichroism spectroscopy combined with time-resolved cryogenic transmission electron microscopy (cryo-TEM) imaging of key points along the reaction coordinate, and molecular dynamics simulation of the initial stages of the assembly. The study reveals a complex self-assembly process starting from the formation of amorphous aggregates that are transformed into crystalline material through a nucleation-growth process. Activation parameters indicate the key role of desolvation along the assembly pathway. The insights from the kinetic study correlate well with the structural data from cryo-TEM imaging. Overall, the study reveals four stages of crystalline self-assembly: 1)collapse into amorphous aggregates; 2)nucleation as partial ordering; 3)crystal growth; and 4)fusion of smaller crystalline aggregates into large crystals. These studies indicate that the assembly process proceeds according to a two-step crystallization model, whereby initially formed amorphous material is reorganized into an ordered system. This process follows Ostwald's rule of stages, evolving through a series of intermediate phases prior to forming the final structure, thus providing an insight into the crystalline self-assembly process in aqueous medium.
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(2014) Journal of Physical Chemistry B. 118, 29, p. 8642-8651 Abstract[All authors]
Synthetic peptides offer enormous potential to encode the assembly of molecular electronic components, provided that the complex range of interactions is distilled into simple design rules. Here, we report a spectroscopic investigation of aggregation in an extensive series of peptide-perylene diiimide conjugates designed to interrogate the effect of structural variations. By fitting different contributions to temperature dependent optical absorption spectra, we quantify both the thermodynamics and the nature of aggregation for peptides by incrementally varying hydrophobicity, charge density, length, as well as asymmetric substitution with a hexyl chain, and stereocenter inversion. We find that coarse effects like hydrophobicity and hexyl substitution have the greatest impact on aggregation thermodynamics, which are separated into enthalpic and entropic contributions. Moreover, significant peptide packing effects are resolved via stereocenter inversion studies, particularly when examining the nature of aggregates formed and the coupling between it electronic orbitals. Our results develop a quantitative framework for establishing structure function relationships that will underpin the design of self-assembling peptide electronic materials.
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(2014) Journal of the American Chemical Society. 136, 26, p. 9443-9452 Abstract
Achieving supramolecular polymerization based on strong yet reversible bonds represents a significant challenge. A solution may be offered by perfluoroalkyl groups, which have remarkable hydrophobicity. We tested the idea that a perfluorooctyl chain attached to a perylene diimide amphiphile can dramatically enhance the strength of supramolecular bonding in aqueous environments. Supramolecular structures and polymerization thermodynamics of this fluorinated compound (1-F) were studied in comparison to its non-fluorinated analogue (1-H). Depending on the amount of organic cosolvent, 1-F undergoes cooperative or isodesmic aggregation. The switching between two polymerization mechanisms results from a change in polymer structure, as observed by cryogenic electron microscopy. 1-F showed exceptionally strong noncovalent binding, with the largest directly measured association constant of 1.7 X 10(9) M-1 in 75:25 water/THF mixture (v/v). In pure water, the association constant of 1-F is estimated to be at least in the order of 10(15) M-1 (based on extrapolation), 3 orders of magnitude larger than that of 1-H. The difference in aggregation strength between 1-F and 1-H can be explained solely on the basis of the larger surface area of the fluorocarbon group, rather than a unique nature of fluorocarbon hydrophobicity. However, differences in aggregation mechanism and cooperativity exhibited by 1-F appear to result from specific fluorocarbon conformational rigidity.
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(2014) Israel Journal of Chemistry. 54, 5-6, p. 748-758 Abstract
We used a helical polymer backbone (polyacrylamide) as a scaffold to organize perylene diimide chromophores into well-defined foldamers, which further undergo self-assembly into supramolecular tube-like arrays in aqueous media, as revealed by cryo-TEM imaging. The arrays are supramolecular polymers, whose structure is templated by folded primary building blocks, representing a useful tool for directing self-assembly. Exciton migration in the supramolecular arrays was studied by transient absorption and revealed a moderate exciton diffusion propensity.
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(2014) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 53, 16, p. 4123-4126 Abstract
In covalent polymerization, a single monomer can result in different polymer structures due to positional, geometric, or stereoisomerism. We demonstrate that strong hydrophobic interactions result in stable noncovalent polymer isomers that are based on the same covalent unit (amphiphilic perylene diimide). These isomers have different structures and electronic/photonic properties, and are stable in water, even upon prolonged heating at 100 °C. Such combination of covalent-like stability together with structural/functional variation is unique for noncovalent polymers, substantially advancing their potential as functional materials. A strong hold: Strong hydrophobic interactions result in stable noncovalent polymer isomers derived from a single covalent unit. These isomers have different electronic and photonic properties and are stable in water, even upon prolonged heating to 100 °C.
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(2014) Chemistry-A European Journal. 20, 49, p. 16070-16073 Abstract
A family of shape-persistent alleno-acetylenic macrocycles (SPAAMs), peripherally decorated with structurally diverse pendant groups, has been synthesized and characterized in enantiomerically pure form. Their electronic circular dichroism (ECD) spectra feature a strong chiroptical response, which is more than two times higher than for open-chain tetrameric analogues. A water-soluble oligo(ethylene glycol)-appended SPAAM undergoes selfassembly in aqueous solution. Morphology studies by cryogenic transmission electron microscopy (cryo-TEM) revealed the formation of aggregates with fibrous fine structures that correspond to tubular, macrocyclic stacks.
2013
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(2013) Journal Of Physical Chemistry B. 117, 47, p. 14649-14654 Abstract
We report the self-assembly and thermal dissociation of DNA dumbbell conjugates having a perylenediimide (PDI) linker on each end separated by 6-16 A-T base pairs. In the presence of NaCl these dumbbells form one-dimensional supramolecular assemblies as a consequence of the hydrophobic association of their PDI sticky ends. The dependence of assembly formation on dumbbell concentration, salt concentration, and temperature can be conveniently monitored by UV-vis spectroscopy. The melting of these linear assemblies follows two limiting mechanisms, depending on the length of the dumbbells. Upon heating in the presence of salt, the assemblies formed by the longer dumbbells undergo a sequential transition from assembly to base-paired monomer to random coiled monomer, whereas the assemblies formed by the shorter dumbbells undergo disassembly and base-pair melting cooperatively. In all cases, the intramolecular hydrophobic association of the PDI chromophores is observed at elevated temperature. The thermal behavior of these one-dimensional assemblies is compared to that of other sticky-ended assemblies.
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(2013) Hierarchical Macromolecular Structures. Percec V.(eds.). p. 363-388 Abstract
Self-assembled polymeric nanoscale systems that are robust yet adaptive are of primary importance for fabricating multifunctional stimuli-responsive nanomaterials. Noncovalent interactions in water can be strong, and biological systems exhibit excellent robustness and adaptivity. Synthetic amphiphiles can also result in robust assemblies in water. Can we rationally design water-based noncovalent polymers? Can we program them to perform useful functions that rival covalent materials? We review here advancements related to these questions, focusing on aromatic selfassembly in aqueous media. Regarding functional materials, we present examples from our work on water-based recyclable noncovalent membranes, which can be used for size-selective separations of nanoparticles and biomolecules. These systems introduce the paradigm of noncovalent nanomaterials as a versatile and environmentally friendly alternative to covalent materials. We also address emerging rational design principles for creating 1D, 2D, and 3D functional nanoarrays hierarchically assembled from welldefined molecular units in aqueous media, enabling new synthetic strategies for fabricating complex water-based materials.
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(2013) PLoS ONE. 8, 5, e63188. Abstract
Membrane separation of biomolecules and their application in biocatalysis is becoming increasingly important for biotechnology, demanding the development of new biocompatible materials with novel properties. In the present study, an entirely noncovalent water-based material is used as a membrane for size-selective separation, immobilization, and biocatalytic utilization of proteins. The membrane shows stable performance under physiological conditions, allowing filtration of protein mixtures with a 150 kDa molecular weight cutoff (∼8 nm hydrodynamic diameter cutoff). Due to the biocompatibility of the membrane, filtered proteins stay functionally active and retained proteins can be partially recovered. Upon filtration, large enzymes become immobilized within the membrane. They exhibit stable activity when subjected to a constant flux of substrates for prolonged periods of time, which can be used to carry out heterogeneous biocatalysis. The noncovalent membrane material can be easily disassembled, purified, reassembled, and reused, showing reproducible performance after recycling. The robustness, recyclability, versatility, and biocompatibility of the supramolecular membrane may open new avenues for manipulating biological systems.
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(2013) ACS Nano. 7, 4, p. 3547-3556 Abstract
A methodology leading to facile self-assembly of crystalline aromatic arrays in dilute aqueous solutions would enable efficient fabrication and processing of organic photonic and electronic materials in water. In particular, soluble 2D crystalline nanosheets may mimic the properties of photoactive thin films and self-assembled monolayers, covering large areas with ordered nanometer-thick material. We designed such solution-phase arrays using hierarchical self-assembly of amphiphilic perylene diimides in aqueous media. The assemblies were characterized by cryogenic transmission electron microscopy (cryo-TEM), revealing crystalline order and 2D morphology (confirmed by AFM studies). The order and morphology are preserved upon drying as evidenced by TEM and AFM. The 2D crystalline-like structures exhibit broadening and red-shifted absorption bands in UV-vis spectra, typical for PDI crystals and liquid crystals. Photophysical studies including femtosecond transient absorption spectroscopy reveal that two of the assemblies are superior light-harvesters due to excellent solar spectrum coverage and fast exciton transfer, in one case showing exciton diffusion comparable to solid-state crystalline systems based on perylene tetracarboxylic dianhidride (PTCDA).
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(2013) Micro- And Nanotechnology Sensors, Systems, And Applications V. 8725, 87250E. Abstract
Self-assembled nanoscale systems that are robust yet adaptive and prone to facile fabrication and reversible disassembly are of primary importance for creating multifunctional adaptive nanomaterials. We introduce the emergent field of robust noncovalent nanomaterials and, using this context, present our work on water-based noncovalent materials, including membranes that can be used for size-selective separations of nanoparticles and biomolecules. We will also describe emerging rational design principles for creating highly ordered functional nanoarrays assembled from well-defined molecular units, enabling a general approach to photonic nanomaterials. These findings advance a paradigm of noncovalent nanomaterials as a versatile and environmentally friendly alternative to covalent systems.
2012
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(2012) Current Opinion In Colloid & Interface Science. 17, 6, p. 330-342 Abstract
Supramolecular systems based on noncovalent bonds are adaptive due to the reversible nature of the noncovalent interactions, enabling stimuli responsiveness, self-healing, facile fabrication, and recyclability. There is much effort devoted to developing new synthetic tools in supramolecular chemistry. Progress in mechanistic understanding is of crucial importance for rational design targeting functional noncovalent nanoscale assemblies. So far, insufficient insight into evolution of noncovalent assemblies hindered our ability to make progress in the field. The typical paradigm in the case of non-covalent self-assembling systems involves the concept of rapid equilibration at ambient conditions. However, when strong noncovalent interactions are involved, kinetic control may dominate the outcome of the self-assembly processes. The ability of water to impose very strong hydrophobic interactions leads to slow transformations between different structural motifs, amenable to structural mechanistic studies. Cryo-TEM emerges as a method that enables direct structural analysis via imaging of "frozen" evolving assemblies. In this review we focus on cryo-TEM imaging of intermediate structures that evolve along a supramolecular transformation pathway. The structures investigated were trapped and directly visualized, in some cases with subnanometer resolution. Direct structural information obtained by time-resolved cryo-TEM proves to be critical for mechanistic understanding of complex multistep self-assembly processes. Such knowledge is necessary to address the challenge related to rational design of novel functional self-assembled materials.
2011
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(2011) Journal of the American Chemical Society. 133, 40, p. 16201-16211 Abstract
Self-assembly in aqueous medium is of primary importance and widely employs hydrophobic interactions. Yet, unlike directional hydrogen bonds, hydrophobic interactions lack directionality, making difficult rational self-assembly design. Directional hydrophobic motif would significantly enhance rational design in aqueous self-assembly, yet general approaches to such interactions are currently lacking. Here, we show that pairwise directional hydrophobic/π- stacking interactions can be designed using well-defined sterics and supramolecular multivalency. Our system utilizes a hexasubstituted benzene scaffold decorated with 3 (compound 1) or 6 (compound 2) amphiphilc perylene diimides. It imposes a pairwise self-assembly mode, leading to well-defined supramolecular polymers in aqueous medium. the assemblies were characterized using cryogenic electron microscopy, small-angle X-ray scattering, optical spectroscopy, and EPR. Supramolecular polymerization studies in the case of 2 revealed association constants in 10 8 M -1 range, and significant enthalpic contribution to the polymerization free energy. The pairwise PDI motif enables exciton confinement and localized emission in the polymers based on 1 and 2's unique photonic behavior, untypical of the extended π-stacked systems. Directional pairwise hydrophobic interactions introduce a novel strategy for rational design of noncovalent assemblies in aqueous medium, and bring about a unique photofunction.
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(2011) ACS Nano. 5, 9, p. 6791-6818 Abstract
Noncovalent systems are adaptive and allow facile processing and recycling. Can they be at the same time robust? How can one rationally design such systems? Can they compete with high-performance covalent materials? The recent literature reveals that noncovalent systems can be robust yet adaptive, self-healing, and recyclable, featuring complex nanoscale structures and unique functions. We review such systems, focusing on the rational design of strong noncovalent interactions, kinetically controlled pathway-dependent processes, complexity, and function. The overview of the recent examples points at the emergent field of noncovalent nanomaterials that can represent a versatile, multifunctional, and environmentally friendly alternative to conventional covalent systems.
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(2011) Chemistry-A European Journal. 17, 33, p. 9016-9026 Abstract
The adaptive properties of noncovalent materials allow easy processing, facile recycling, self-healing, and stimuli responsiveness. However, the poor robustness of noncovalent systems has hampered their use in real-life applications. In this Concept Article we discuss the possibility of creating robust noncovalent arrays by utilizing strong hydrophobic interactions. We describe examples from our work on aqueous assemblies based on aromatic amphiphiles with extended hydrophobic cores. These arrays exhibit fascinating properties, including robustness, multiple stimuli-responsiveness, and pathway-dependent self-assembly. We have shown that this can lead to functional materials (filtration membranes) rivaling covalent systems. We anticipate that water-based noncovalent materials have the potential to replace or complement conventional polymer materials in various fields, and to promote novel applications that require the combination of robustness and adaptivity.
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Photoinduced singlet charge transfer in a ruthenium(II) perylene-3,4:9,10-bis(dicarboximide) complex(2011) Journal Of Physical Chemistry B. 115, 23, p. 7533-7540 Abstract
Elucidation of photoinduced charge transfer behavior in organic dye/metal hybrids is important for developing photocatalytic systems for solar energy conversion. We report the synthesis and photophysical characterization of a perylene-3,4:9,10-bis(dicarboximide) (PDI)-ruthenium(II) complex, bis-PDI-2,2-bipyridineRu(II)Cl2(CNtbutyl) 2, which has favorable energetics, ΔGCS ≈ -1.0 eV, for singlet electron transfer from the Ru complex to PDI. Time-resolved optical spectroscopy reveals that upon selective photoexcitation of PDI, ultrafast charge transfer (
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(2011) Fundamentals of Materials for Energy and Environmental Sustainability. p. 349-364 Abstract
Focus In natural photosynthesis, organisms optimize solar energy conversion through organized assemblies of photofunctional chromophores and catalysts within proteins that provide specifically tailored environments for chemical reactions. As with their natural counterparts, artificial photosynthetic systems for practical production of solar fuels must collect light energy, separate charge, and transport charge to catalytic sites where multielectron redox processes occur. Although encouraging progress has been made on each aspect of this complex problem, researchers have not yet developed self-ordering components and the tailored environments necessary to realize a fully functional artificial photosynthetic system. Synopsis Previously, researchers used complex, covalent molecular systems comprising chromophores, electron donors, and electron acceptors to mimic both the light-harvesting (antenna) and charge-separation functions of natural photosynthetic arrays. These systems allow one to derive fundamental insights into the dependences of electron-transfer rate constants on donoracceptor distance and orientation, electronic interaction, and the free energy of the reaction. However, significantly more complex systems are required in order to achieve functions comparable to natural photosynthesis. Self-assembly provides a facile means for organizing large numbers of molecules into supramolecular structures that can bridge length scales from nanometers to macroscopic dimensions. To achieve an artificial photosynthetic system, the resulting structures must provide pathways for the migration of light excitation energy among antenna chromophores, and from antennas to reaction centers. They also must incorporate charge conduits, that is, molecular \u201cwires\u201d that can efficiently move electrons and holes between reaction centers and catalytic sites. The central challenge is to develop small, functional building blocks that have the appropriate molecular-recognition properties to facilitate self-assembly of complete, functional artificial photosynthetic systems.
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(2011) Chemistry-A European Journal. 17, 22, p. 6068-6075 Abstract
Most molecular self-assembly strategies involve equilibrium systems, leading to a single thermodynamic product as a result of weak, reversible non-covalent interactions. Yet, strong non-covalent interactions may result in non-equilibrium self-assembly, in which structural diversity is achieved by forming several kinetic products based on a single covalent building block. We demonstrate that well-defined amphiphilic molecular systems based on perylene diimide/peptide conjugates exhibit kinetically controlled self-assembly in aqueous medium, enabling pathway-dependent assembly sequences, in which different organic nanostructures are evolved in a stepwise manner. The self-assembly process was characterized using UV/Vis circular dichroism (CD) spectroscopy, and cryogenic transmission electron microscopy (cryo-TEM). Our findings show that pathway-controlled self-assembly may significantly broaden the methodology of non-covalent synthesis.
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(2011) Journal of Physical Chemistry A. 115, 10, p. 2047-2056 Abstract
It was recently reported (Shirman, J. Phys. Chem. B, 2008, 112, 8855) that the stable dianion of perylene diimide can be prepared in water. Herein, a computational study (using DFT at the M06-2X/6-31++G* level of theory) of this species is presented. It is shown that this dianion is aromatic and that its reaction with water is highly endergonic. The primary cause for this is the stabilization provided by the enhanced aromaticity of the dianion relative to its neutral counterpart. Comparison with other aromatic dianions is also presented.
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(2011) Nature Nanotechnology. 6, 3, p. 141-146 Abstract
Most practical materials are held together by covalent bonds, which are irreversible. Materials based on noncovalent interactions can undergo reversible self-assembly, which offers advantages in terms of fabrication, processing and recyclability, but the majority of noncovalent systems are too fragile to be competitive with covalent materials for practical applications, despite significant attempts to develop robust noncovalent arrays. Here, we report nanostructured supramolecular membranes prepared from fibrous assemblies in water. The membranes are robust due to strong hydrophobic interactions, allowing their application in the size-selective separation of both metal and semiconductor nanoparticles. A thin (12 μm) membrane is used for filtration (∼5 nm cutoff), and a thicker (45 μm) membrane allows for size-selective chromatography in the sub-5 nm domain. Unlike conventional membranes, our supramolecular membranes can be disassembled using organic solvent, cleaned, reassembled and reused multiple times.
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(2011) Polymers for Advanced Technologies. 22, 1, p. 133-138 Abstract
Two-dimensional porous networks self-assembled in solution are rare, while maintaining the solution-phase network structure upon casting on solid supports presents a major challenge. We report on oligoarylacetylene bearing amphiphilic perylene diimide moieties that self-assemble into a two-dimensional porous network in aqueous solution that can be cast on surfaces, while maintaining the porous structure. The networks were characterized by cryogenic electron microscopy (cryo-TEM and cryo-SEM), and by atomic force microscopy, revealing formation of thin porous films (4-nm thick). The network can be cast on various solid surfaces, preserving its solution-phase structure. The design motif utilizing an oligoarylacetylene backbone with interacting amphiphilic pendants appears to be of wide utility for formation of novel assembly patterns. Copyright (C) 2010 John Wiley & Sons, Ltd.
2010
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(2010) Journal Of Physical Chemistry B. 114, 45, p. 14389-14396 Abstract
We report on the synthesis of organic dye-metal nanoparticle hybrids from two thiol-derivatized perylenediimide (PDI) ligands and 1.5 nm gold nanoparticles. The hybrids form spherical nanostructures when cast from 40% methanol/chloroform solution and toluene. The spherical aggregates are in the size range 50-230 nm in 40% MeOH/CHCl3 mixture and 100-400 nm in toluene solution, as evidenced by transmission electron microscopy (TEM). Scanning electron microscopy (SEM) measurements show that these spherical aggregates are vesicles with a hollow interior. The π-π interactions of the perylenediimides are the predominant driving force leading to the aggregation of the hybrids, whereby the sizes of the nanospheres can be regulated via the PDI linker moiety and solvent choice. Femtosecond transient absorption studies of the hybrids reveal complex photophysical behavior involving electron transfer from the gold nanoparticles to the PDI moieties. This study shows that the formation of well-defined hybrid nanostructures as well as tuning their sizes can be achieved through employing a combination of the capping ligand choice and regulating the solvophobic interactions between the ligands.
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(2010) Journal of the American Chemical Society. 132, 44, p. 15808-15813 Abstract
The self-assembly of DNA dumbbell conjugates possessing hydrophobic perylenediimide (PDI) linkers separated by an eight-base pair A-tract has been investigated. Cryo-TEM images obtained from dilute solutions of the dumbbell in aqueous buffer containing 100 mM NaCl show the presence of structures corresponding to linear end-to-end assemblies of 10-30 dumbbell monomers. The formation of assemblies of this size is consistent with analysis of the UV-vis and fluorescence spectra of these solutions for the content of PDI monomer and dimer chromophores. Assembly size is dependent upon the concentration of dumbbell and salt as well as the temperature. Kinetic analysis of the assembly process by means of salt-jump stopped-flow measurements shows that it occurs by a salt-triggered isodesmic mechanism in which the rate constants for association and dissociation in 100 mM NaCl are 3.2 × 107 M -1s-1 and 1.0 s-1, respectively, faster than the typical rate constants for DNA hybridization. TEM and AFM images of samples deposited from solutions having higher concentrations of dumbbell and NaCl display branched assemblies with linear regions >1 μm in length and diameters indicative of the formation of small bundles of dumbbell end-to-end assemblies. These observations provide the first example of the use of hydrophobic association for the assembly of small DNA duplex conjugates into supramolecular polymers and larger branched aggregates.
2009
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(2009) Journal of the American Chemical Society. 131, 40, p. 14365-14373 Abstract
Design of an extensive supramolecular three-dimensional network that is both robust and adaptive represents a significant challenge. The molecular system PP2b based on a perylene diimide chromophore (PDI) decorated with polyethylene glycol groups self-assembles in aqueous media into extended supramolecular fibers that form a robust three-dimensional network resulting in gelation. The self-assembled systems were characterized by cryo-TEM, cryo-SEM, and rheological measurements. The gel possesses exceptional robustness and multiple stimuli-responsiveness. Reversible charging of PP2b allows for switching between the gel state and fluid solution that is accompanied by switching on and off the material's birefringence. Temperature triggered deswelling of the gel leads to the (reversible) expulsion of a large fraction of the aqueous solvent. The dual sensibility toward chemical reduction and temperature with a distinct and interrelated response to each of these stimuli is pertinent to applications in the area of adaptive functional materials. The gel also shows strong absorption of visible light and good exciton mobility (elucidated using femtosecond transient absorption), representing an advantageous light harvesting system.
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(2009) Organometallics. 28, 2, p. 523-533 Abstract
Rhodium complexes based on the electron-withdrawing PCP-type pincer ligand dipyrrolylphoshi-noxylene (DPyPX, PyrPCP) were synthesized and their reactivity was studied. Reaction of RhI(pyrPCP)PR 3 (2) (R = Et (a). Ph (b); Pyr (pyrrolyl, NC4H 4) (c); Pyd (pyrrolydinyl, NC4H8) (d)) with MeI was strongly dependent on the sterics and nucleophilicity of PR3. Complex 2a (PEt3 cone angle, Θo, 132°) reacted with MeI to give isomers of RhΠI(PyrPCP)Me(I)PEt3, 3. Reaction of 2b (ΘoPR3 = 145°, R = Pyd (2d), Ph (2b), Pyr (2c)) with MeI was accompanied by release of PPh3 and is thought to proceed via the 14e intermediate RhI(PyrPCP). While the PPyd3 complex 2d reacted with MeI to give [Rh ΠI(PyrPCP)Me(I)2][MePPyd3], 4a, the PPyr3 complex 2c did not react, owing to Steric hindrance around Rh1 and the low nucleophilicity of PPyr3. The aptitude of complexes 2 toward activation of H2 was also examined. Our results support the involvement of 14e intermediates in the olefin hydrogenation process. The ancillary ligand substitution at the RhI center of 2 was found to proceed by an associative mechanism. ML5 d8 intermediates were clearly detected by 31P{ 1H} NMR at 213 K during equilibrium between 2a and 2c.
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(2009) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 48, 5, p. 926-930 Abstract
(Chemical Equation Presented) Four from one: Nanoscale ribbons, tubes, vesicles, and platelets can be formed from the self-assembly of a single covalent unit, which is based on an amphiphilic perylene diimide functionalized with a terpyridine ligand (see picture). The assembly diversity arises from the encoding of multiple inputs through hydrophobic interactions and metal coordination.
2008
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(2008) Journal of the American Chemical Society. 130, 45, p. 14966-14967 Abstract
Self-assembling systems, whose structure and function can be reversibly controlled in situ are of primary importance for creating multifunctional supramolecular arrays and mimicking the complexity of natural systems. Herein we report on photofunctional fibers self-assembled from perylene diimide cromophores, in which interactions between aromatic monomers can be attenuated through their reduction to anionic species that causes fiber fission. Oxidation with air restores the fibers. The sequence represents reversible supramolecular depolymerization-polymerization in situ and is accompanied by a reversible switching of photofunction.
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(2008) Journal of Physical Chemistry B. 112, 30, p. 8855-8858 Abstract
Perylene diimide (PDI) bearing polyethylene glycol substituents at the imide positions was reduced in water with sodium dithionite to produce an aromatic dianion. The latter is stable for months in deoxygenated aqueous solutions, in contrast to all known aromatic dianions which readily react with water. Such stability is due to extensive electron delocalization and the aromatic character of the dianion, as evidenced by spectroscopic and theoretical studies. The dianion reacts with oxygen to restore the parent neutral compound, which can be reduced again in an inert atmosphere with sodium dithionite to give the dianion. Such reversible charging renders PDIs useful for controlled electron storage and release in aqueous media. Simple preparation of the dianion, reversible charging, high photoredox power, and stability in water can lead to development of new photofunctional and electron transfer systems in the aqueous phase.
2007
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(2007) The Chemistry of Pincer Compounds. Morales-Morales D. & Jensen J. M.(eds.). p. 87-105 Abstract
Extremely facile metal insertion into strong chemical bonds that are normally inert can take place in pincer systems, resulting in unequivocal demonstration and mechanistic evaluation of metal insertion into unstrained C-C and C-O bonds in a solution. This chapter presents an overview of intramolecular C-C and C-O bond activation in pincer systems. C-H and O-H bond activation processes that occur concurrently with C-C and C-O cleavages in most of the studied pincer complexes are discussed in the chapter. PCX-metal systems prove to be excellent models for strong bond activation studies. They provide mechanistic insight into the activation of strong C-C bonds. In most PCX-metal systems, C-C oxidative addition is thermodynamically more favorable than other competing C-H activation processes. C-C activation can proceed directly and does not require prior C-H activation.
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(2007) Journal of Organic Chemistry. 72, 16, p. 5973-5979 Abstract
(Chemical Equation Presented) A novel method for the bromination of perylene diimides, PDI (1), under mild conditions is reported. Variation of the reaction conditions allows mono- and dibromination of PDIs to afford 2 and 3 (these can be separated through standard procedures) or exclusive dibromination to afford 3. Pure 1,7 regioisomers are obtained through repetitive crystallization. The structure of 1,7-3b was elucidated by a single-crystal X-ray analysis. The facility of the bromination reaction, which decreases in the order 1a > 1b > 1c, depends on PDI aggregation propensities. Monobrominated PDIs were utilized for the syntheses of novel unsymmetrical piperidinyl (4a and 4b) and trimethylsilylethynyl derivatives (5a and 5b). Computational studies (DFT) on imide substituent rotation in PDIs reveal that in the case of bulky groups there is a restricted rotation leading to isomers, in agreement with our experimental results. An aromatic core twist in PDIs bearing one and two bromine substituents was also investigated by DFT.
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(2007) Inorganic Chemistry. 46, 12, p. 4790-4792 Abstract
We prepared the first σ-bonded metal complexes of widely utilized organic dyes, perylene tetracarboxylic acid diimides (PDIs). These 1,7-dipalladium PDI complexes were synthesized by C-Br oxidative addition of 1,7-dibromo-N,N-dicyclohexyl PDI (Br2PDI) to Pd(0) phosphine complexes bearing triphenylphosphine and bischelating 1,2-bis(diphenylphosphino) ethane (dppe). The structures of Pd-PDI complexes were elucidated by single-crystal X-ray analysis. Surprisingly, despite direct attachement of two late transition metal centers, Pd-PDI systems are highly fluorescent (Φ = 0.65 and 0.22 for triphenylphosphine and dppe systems, respectively). This is rationalized in terms of weak electronic interactions between the metal centers and PDI π-system, as revealed by TD-DFT calculations.
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(2007) Journal of the American Chemical Society. 129, 14, p. 4114-4115 Abstract
Foldamers provide important insights into the fundamentals of noncovalent folding, which is of primary importance for understanding biological systems and developing novel self-assembling materials. The structure of artificial foldamers in solution has been studied using a variety of indirect spectroscopic techniques and theoretical methods. X-ray scattering using a high flux synchrotron source has been recently shown to provide a powerful means of studying small organic and biological arrays that are formed because of noncovalent self-assembly in solution but has not been applied to foldamer structures. We present small angle X-ray scattering (SAXS) studies on a m-phenylene ethynylene oligomer (mPE) in acetonitrile, providing for the first time-direct structural data on a mPE foldamer in solution.
2005
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(2005) Journal of the American Chemical Society. 127, 43, p. 15265-15272 Abstract
A new general, synthetically simple, and safe method for the preparation of metal carbene complexes, which is based on diphenyl sulfonium salts as carbenoid precursors, has been developed, and its scope and applications were studied. In general, deprotonation of a sulfonium salt with a base results in a sulfur ylide, which, in turn, reacts with an appropriate metal precursor to give the corresponding metal carbene complex. Thus, starting from benzyldiphenylsulfonium salt, the complexes (PCX)Rh=CHPh (X = P, N) were prepared in quantitative yield. Syntheses of Grubbs' catalyst, (PCy 3)2Cl2Ru=CHPh, and of Werner's carbene, [Os(=CHPh)HCl(CO)(PiPr3)2], were achieved by this method. Novel trans-bisphosphine Rh and Ir carbenes, (iPr 3P)2(Cl)M=CHPh, which could not be prepared by other known methods, were synthesized by the sulfur ylide approach. The method is not limited to metal benzylidenes, as demonstrated by the preparation of the Ru vinyl-alkylidene, (PCy3)2Cl2Ru=CH-CH=CH 2, methoxycarbonyl-alkylidene, (PCy3)2Cl 2Ru=CH(CO2Me), and alkylidene (PCy3) 2Cl2Ru=CH(CH3), (PCy3) 2Cl2Ru=CH2 compounds. The problem of recycling of starting materials as well as the issue of facile purification of the product metal carbene complex were addressed by the synthesis of a polymer-supported diarylsulfide, the carrier of the carbenoid unit in the process. Based on the sulfur ylide route, a methodology for the synthesis of metallocarbenes anchored to a polymer via the carbene ligand, using a commercial Merrifield resin, was developed.
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(2005) Chemistry - A European Journal. 11, 8, p. 2319-2326 Abstract
The novel pi-accepting, pincer-type ligand, dipyrrolylphoshinoxylene (DPyPX), is introduced. This ligand has the strongest pi-accepting phosphines used so far in the PCP family of ligands and this results in some unusual coordination chemistry. The rhodium(i) complex, [(DPyPX)Rh(CO)(PR3)] (4, R=Ph, Et, pyrrolyl) is prepared by treating the relevant [(DPyPX)Rh(PR3)] (3) complex with CO and is remarkably resistant to loss of either ligand. X-ray crystallographic analysis of complex 4b (R = Et) reveals an unusual cisoid coordination of the PCP phosphine ligands. These observations are supported by density functional theory (DFT) calculations.
2004
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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) Activation and Functionalization of CH Bonds. Goldman A. S. & Goldberg K. I.(eds.). p. 70-85 Abstract
An overview of recent results regarding the activation of strong C-C and C-H bonds by Rh(I) and Ir(I) in PCX (X=P,N,O) type ligand systems is presented. Whereas both C-C and C-H oxidative addition involve non-polar 3-centered transition states and 14 electron intermediates, steric requirements differ markedly and chelation plays a much more important role in C-C than in C-H activation. Control over C-C vs C-H activation can be achieved by choice of ligand and solvent. Under optimal conditions, C-C activation can be thermodynamically and kinetically more favorable than C-H activation and proceed even at 70°C. Activation parameters for an apparent single-step metal insertion into a C-C bond were obtained. A combination of C-H and C-C activation in conjunction with oxidative addition of other bonds has led to unique methylene transfer chemistry.
2003
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(2003) Journal of the American Chemical Society. 125, 51, p. 15692-15693 Abstract
An unprecedented metal-stabilized phenoxonium cation was prepared by a process involving dearomatization of a phenoxy complex. The unique η2 CO−metal (iridium) coordination mode leaves the positive charge delocalized within the former aromatic ring. The X-ray structure and conversion into η2 CO-coordinated metal−quinone complex are described.
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(2003) Journal of the American Chemical Society. 125, 36, p. 11041-11050 Abstract
The aryl-PC type ligand 3, benzyl(di-tert-butyl)phosphane, reacts with [Rh(coe)2(solv)n]BF4 (coe = cyclooctene, solv = solvent), producing the C-H activated complexes 4a-c (solv = (a) acetone, (b) THF, (c) methanol). Complexes 4a-c undergo reversible arene C-H activation (observed by NMR spin saturation transfer experiments, SST) and H/D exchange into the hydride and aryl ortho-H with ROD (R = D, Me). They also promote catalytic H/D exchange into the vinylic C-H bond of olefins, with deuterated methanol or water utilized as D-donors. Unexpectedly, complex 2, based on the benzyl-PC type ligand 1 (analogous to 3), di-tert-butyl(2,4,6-trimethylbenzyl)phosphane, shows a very different reversible C-H activation pattern as observed by SST. It is not active in H/D exchange with ROD and in catalytic H/D exchange with olefins. To clarify our observations regarding C-H activation/reductive elimination in both PC-Rh systems, density functional theory (DFT) calculations were performed. Both nucleophilic (oxidative addition) and electrophilic (H/D exchange) C-H activation proceed through η2-C,H agostic intermediates. In the aryl-PC system the agostic interaction causes C-H bond acidity sufficient for the H/D exchange with water or methanol, which is not the case in the benzyl PC-Rh system. In the latter system the C-H coordination pattern of the methyl controls the reversible C-H oxidative addition leading to energetically different C-H activation processes, in accordance with the experimental observations.
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Metallacarbenes from diazoalkanes: An experimental and computational study of the reaction mechanism(2003) Journal of the American Chemical Society. 125, 21, p. 6532-6546 Abstract
PCP ligand (1,3-bis-[(diisopropyl-phosphanyl)-methyl]-benzene), and PCN ligand ({3-[(di-tert-butyl-phosphanyl)-methyl]-benzyl}-diethyl-amine) based rhodium dinitrogen complexes (1 and 2, respectively) react with phenyl diazomethane at room temperature to give PCP and PCN-Rh carbene complexes (3 and 5, respectively). At low temperature (-70 °C), PCP and PCN phenyl diazomethane complexes (4 and 6, respectively) are formed upon addition of phenyl diazomethane to 1 and 2. In these complexes, the diazo moiety is η1 coordinated through the terminal nitrogen atom. Decomposition of complexes 4 and 6 at low temperatures leads only to a relatively small amount of the corresponding carbene complexes, the major products of decomposition being the dinitrogen complexes 1 and 2 and stilbene. This and competition experiments (decomposition of 6 in the presence of 1) suggests that phenyl diazomethane can dissociate under the reaction conditions and attack the metal center through the diazo carbon producing a η1-C bound diazo complex. Computational studies based on a two-layer ONIOM model, using the mPW1 K exchange-correlation functional and a variety of basis sets for PCP based systems, provide mechanistic insight. In the case of less bulky PCP ligand bearing H-substituents on the phosphines, a variety of mechanisms are possible, including both dissociative and nondissociative pathways. On the other hand, in the case of i-Pr substituents, the η1-C bound diazo complex appears to be a critical intermediate for carbene complex formation, in good agreement with the experimental results. Our results and the analysis of reported data suggest that the outcome of the reaction between a diazoalkane and a late transition metal complex can be anticipated considering steric requirements relevant to η1-C diazo complex formation.
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(2003) Angewandte Chemie - International Edition. 42, 17, p. 1949-1952 Abstract
An unprecedented reaction : Dinitrogen pincer Rh complexes react with azines in a moderately catalytic fashion by \u201cnonsymmetrical\u201d N-N bond cleavage to the corresponding nitriles and imines (see scheme).
2002
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C-C vs C-H activation: Models based on hemilabile pincer-type PCO and PCN systems.(2002) p. A33-A33 Abstract
2001
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(2001) Journal of the American Chemical Society. 123, 37, p. 9064-9077 Abstract
C-H bond activation was observed in a novel PCO ligand 1 (C6H(CH3)3(CH2OCH3) (CH2P(t-Bu)2)) at room temperature in THF, acetone, and methanol upon reaction with the cationic rhodium precursor, [Rh(coe)2(solv)n]BF4 (solv = solvent; coe = cyclooctene). The products in acetone (complexes 3a and 3b) and methanol (complexes 4a and 4b) were fully characterized spectroscopically. Two products were formed in each case, namely those containing uncoordinated (3a and 4a) and coordinated (3b and 4b) methoxy arms, respectively. Upon heating of the C-H activation products in methanol at 70 °C, C-C bond activation takes place. Solvent evaporation under vacuum at room temperature for 3-4 days also results in C-C activation. The C-C activation product, ((CH3)Rh(C6H(CH3)2 (CH2OCH3)(CH2P(t-Bu)2) BF4), was characterized by X-ray crystallography, which revealed a square pyramidal geometry with the BF4- anion coordinated to the metal. Comparison to the structurally similar and isoelectronic nonchelating Rh-PC complex system and computational studies provide insight into the reaction mechanism. The reaction mechanism was studied computationally by means of a two-layer ON1OM model, using both the B3LYP and mPW1K exchange-correlation functionals and a variety of basis sets. Polarization functions significantly affect relative energetics, and the mPW1K profile appears to be more reliable than its B3LYP counterpart. The calculations reveal that the electronic requirements for both C-C and C-H activation are essentially the same (14e intermediates are the key ones). On the other hand, the steric requirements differ significantly, and chelation appears to play an important role in C-C bond activation.
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(2001) Journal of the American Chemical Society. 123, 22, p. 5372-5373 Abstract
The chemistry of late-transition-metal carbene complexes has recently received much attention, primarily due to the high catalytic activity of phosphine ruthenium carbene complexes in olefin metathesis.1,2 The most useful Ru carbene in these series is Grubbs' catalyst, (PCy3)2Cl2RuCHPh, bearing a benzylidene unit.1 Being highly active and remarkably tolerant to common functional groups, this compound found broad applications in both organic2b,c and polymer chemistry.2d,e Therefore, synthesis and investigation of late-metal benzylidene complexes (MCHPh) is a topic of great interest.1-4a There are several synthetic approaches toward alkylidene complexes,4 with the ones utilizing the corresponding diazoalkane being the most popular.1,2,5 However, the instability of diazo compounds and the safety issues involved in handling them seriously limit this method. Another recent approach, involving the reaction of precursors to unstable Ru(0) complexes with alkyl dihalides,6 is limited by the difficult synthesis of the unstable Ru(COD)(COT) precursor.7 New approaches toward Ru−alkylidenes starting from Ru−hydride complexes and utilizing alkyne or alkene functions were reported recently.8 Here, we present a new, synthetically simple and safe method toward carbene preparation by using sulfur ylides as carbenoid precursors.9 Such ylides are extensively used in organic chemistry.10 To our knowledge, there are no examples for the synthesis of metal carbene complexes by using these compounds, although metal complexes of sulfur ylides are reported.11,12 Also, the \u201ctransylidation\u201d reaction, transfer of a carbenoid unit between heteroatom centers, is well-known among main group elements.13 The new synthetic route is general and can be applied to different metals. Moreover, it can be used for the synthesis of carbene complexes which could not be prepared by known methods.
2000
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(2000) Chemistry-A European Journal. 6, 17, p. 3287-3292 Abstract
Reaction of the complex [Rh(coe)2(solv)n]BF4 (coe = cyclooctene) with the phosphane 1-di-tert-butylphosphinomethyl-2,4,6-trimethylbenzene (1) results in selective C-H bond activation, yielding the spectroscopically characterized solvento complexes [(solv)nRhH{CH2C6H2(CH 3)2-[CH2P(tBu)2]}]BF4 (solv = acetone, 2a; THF, 2b; methanol, 2c). The stability of these complexes is solvent dependent, alcohols providing significant stabilization. Although cis-alkylrhodium hydride complexes containing labile ligands are generally unstable, 2a-c are stable at room temperature. Complex [(acetone)-(ketol)RhH{CH2C6H2(CH 3)2[CH2P(t-Bu)2]}]BF4 (2d, ketol = 4-hydroxy-4-methyl-2-pentanone, the product of acetone aldol condensation), crystallized from a solution of 2a in acetone and was structurally characterized. Unusual solvent- and temperature-dependent selectivity in reversible C-H bond elimination of these complexes, most probably controlled by a special mode of strong agostic interactions, is observed by spin saturation transfer experiments.
1999
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(1999) Journal of the American Chemical Society. 121, 18, p. 4528-4529 Abstract
Transition metal insertion into C−C bonds in solution is a topic of much current interest.1-5 Since C−H bond activation appears to be generally kinetically and thermodynamically more favorable than C−C bond activation,1,2 special design of systems which can provide thermodynamic and kinetic driving forces for C−C bond activation is required.1,3 The facile C−C bond oxidative addition of strong unactivated Caryl−Calkyl bonds in pincer PCP (1,3-bis(phosphinomethyl)arene)1,4 and PCN ((1-phosphinomethyl-3-aminomethyl)arene)5 systems has been studied in our group, providing insight into this uncommon process. It was found that in the case of neutral metal centers and bulky PCP ligands C−C activation was thermodynamically and slightly kinetically more favorable than C−H activation.4d Here we report a cationic PCP−rhodium system, in which the reaction can be driven toward the exclusive activation of a C−C or a C−H bond at room temperature by solvent choice. This unique selectivity, together with the reversible interconversion of C−C and C−H activation products by solely varying the reaction solvent, allows the achievement of a remarkable degree of control over metal insertion into strong C−H vs C−C bonds.
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(1999) Angewandte Chemie - International Edition. 38, 7, p. 870-883 Abstract
What kind of ligated metal center is necessary for insertion into the \u201chidden\u201d C−C bond? How can one tune the metal center for C−C bond activation by variation of the steric and electronic properties of ligands? What are the possible mechanisms of C−C bond activation in various reaction systems? A systematic look at the available data on C−C bond activation in solution provides some answers to these questions.
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(1999) Organometallics. 18, 5, p. 895-905 Abstract
Protonation of rhodium and iridium complexes of formula ClM(CH3)[C6H(CH3)(2)(CH2P(t-Bu)(2))(2)] (M = Rh (1), Ir (3)) with a strong acid (HOTf, trifluoromethanesulfonic acid) results in clean formation of the new methylene arenium metal complexes ClM[CH2=C6H(CH3)(2)(CH2P(t-Bu)(2))(2)]+OTf- (M = Rh (2a), Ir (4), respectively), which have been fully characterized including an X-ray single-crystal analysis. This new method can be applied to complexes having both electron-donating and electron-withdrawing substituents in the aromatic ring. The methylene arenium complexes bear most of the positive charge in the ring, resulting in their high CH acidity. Deprotonation of these complexes with NEt3 gives the new metal xylylene complexes 12 (M = Rh) and 13 (M = Ir), which can be converted back to the methylene arenium complexes by reaction with HOTf. Reaction of the cationic complexes -OTf+Rh(R)[C6H(CH3)(2)(CH2P(t Bu)(2))(2)] (R = CH3, PhCH2) with CO gives the new alkyl sigma-arenium rhodium complexes Rh(CO)[R-C6H(CH3)(2)(CH2P(t-Bu)(2))(2)]+OTf- (R = CH3 (9a), PhCH2 (9b)). On the basis of the NMR data, the X-ray crystal structure analysis, and the reactivity, these Rh complexes were shown to have much less positive charge in the ring as compared to the methylene arenium complexes, most probably due to stabilization of an arene form by an agostic interaction of the arene-alkyl C-ipso-C bond with the metal center. The nature of the reported phenomena is discussed. The interconversion of the methylene arenium and alkyl sigma-arenium metal complexes is also presented.
1997
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(1997) Organometallics. 16, 17, p. 3786-3793 Abstract
Reaction of HIr(PPh3)3(CO) (1) with 1,3-bis[(diisopropylphosphino)methyl]benzene (2) in THF or benzene at 60 °C affords trans-H2Ir(CO)L (3; L = C6H3(CH2P(i-Pr2)2). The cationic complex [HIr(PPh3)(CO)L]Br (5) was obtained in the reaction of 1 with 1,3-bis[(diisopropylphosphino)methyl]-2-bromobenzene (4). Reaction of 5 with KO-t-Bu leads to the Ir(I) complex Ir(CO)L (6), which adds methyl iodide to form IIr(CO)(CH3)L (7). Addition of H2 to 6 results in formation of cis-(L)Ir(H)2CO) (8). This reaction is reversible, due to the rigid squareplanar geometry of 6. Under H2 pressure 8 isomerizes into 3, demonstrating cis- into trans-dihydride isomerization, which is unprecedented for octahedral complexes. The trans-dihydride six-coordinate complex 3 exhibits hydridic reactivity which is unusual for a neutral late transition metal complex and is due to the strong mutual irons influence of the hydride ligands. Various electrophiles ([Ph3C]BF4, PhC(O)Cl, CS2, MeI) directly attack the hydride ligand of 3, forming products of selective hydride transfer. Abstraction of the hydride ligand with the electrophiles affords the iridium complexes [HIr(PPh3)(CO)L]Br (9), HIr(Cl)(CO)L (10), HIr(CO)(SC(S)H)L (11), and HIr(I)(CO)L (12).
1996
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(1996) Journal of the American Chemical Society. 118, 49, p. 12406-12415 Abstract
The diphosphine 1,3-bis[(di-tert-butylphosphino)methyl]-2,4,6-trimethylbenzene (1a) upon reacting with the rhodium and iridium olefin complexes M2(olefin)4Cl2 (M = Rh, Ir) undergoes rapid, selective metal insertion into the strong unstrained aryl-methyl bond under very mild conditions (room temperature), yielding CIM(CH3)[C6H(CH3)2(CH2P(t-Bu)2)2] (M = Rh (4a), Ir (7a)). The carbon-carbon bond activation is competitive with a parallel C-H activation process, which results in formation of complexes CIMH(L)[CH2C6H(CH3)2(CH2P(t-Bu)2)2] (M = Rh (3a), Ir (6a); L = cyclooctene in the case of 6a and is absent in 3a). Complexes 3a and 6a undergo facile C-H reductive elimination (at room temperature (3a) or upon moderate heating (6a)), followed by C-C oxidative addition, resulting in clean formation of 4a and 7a, respectively. The C-C bond activation products are stable under the reaction conditions, demonstrating that this process is the thermo dynamically favorable one. X-ray single-crystal analysis of 4a demonstrates that the rhodium atom is located in the center of a square pyramid, with the methyl group occupying the position trans to the vacant coordination site. Direct kinetic comparison of the C-C and C-H activation processes shows that-in contrast to theoretical calculations-metal insertion into the carbon-carbon bond in this system is not only thermodynamically but also kinetically preferred over the competing insertion into the carbon-hydrogen bond. When the ligand 1,3-bis[(di-tert-butylphosphino)methyl]-2,4,6-trimethyl-5-methoxybenze ne (1b), bearing the strong electron-donating methoxy group in the position trans to the Ar-CH3 bond to be cleaved, was used instead of 1a, no effect on the reaction rate or on the ratio between the C-H and C-C activation products was observed. Our observations indicate that the C-C oxidative addition proceeds via a three-centered mechanism involving a nonpolar transition state, similar to the one proposed for C-H activation of hydrocarbons. A η2-arene complex is not involved in the C-C activation process.