Suhrit Ghosh

Control of H‐ and J‐Type π Stacking by Peripheral Alkyl Chains and Self‐Sorting Phenomena in Perylene Bisimide Homo‐ and Heteroaggregates

The synthesis, self-assembly, and gelation ability of a series of organogelators based on perylene bisimide (PBI) dyes containing amide groups at imide positions are reported. The synergetic effect of intermolecular hydrogen bonding among the amide functionalities and pi-pi stacking between the PBI units directs the formation of the self-assembled structure in solution, which beyond a certain concentration results in gelation. Effects of different peripheral alkyl substituents on the self-assembly were studied by solvent- and temperature-dependent UV-visible and circular dichroism (CD) spectroscopy. PBI derivatives containing linear alkyl side chains in the periphery formed H-type pi stacks and red gels, whereas by introducing branched alkyl chains the formation of J-type pi stacks and green gels could be achieved. Sterically demanding substituents, in particular, the 2-ethylhexyl group completely suppressed the pi stacking. Coaggregation studies with H- and J-aggregating chromophores revealed the formation of solely H-type pi stacks containing both precursor molecules at a lower mole fraction of J-aggregating chromophore. Beyond a critical composition of the two chromophores, mixed H-aggregate and J-aggregate were formed simultaneously, which points to a self-sorting process. The versatility of the gelators is strongly dependent on the length and nature of the peripheral alkyl substituents. CD spectroscopic studies revealed a preferential helicity of the aggregates of PBI building blocks bearing chiral side chains. Even for achiral PBI derivatives, the utilization of chiral solvents such as (R)- or (S)-limonene was effective in preferential population of one-handed helical fibers. AFM studies revealed the formation of helical fibers from all the present PBI gelators, irrespective of the presence of chiral or achiral side chains. Furthermore, vortex flow was found to be effective in macroscopic orientation of the aggregates as evidenced from the origin of CD signals from aggregates of achiral PBI molecules.

Supramolecular Assemblies by Charge-Transfer Interactions between Donor and Acceptor Chromophores

We have collated various supramolecular designs that utilize organic donor-acceptor CT complexation to generate noncovalently co-assembled structures including fibrillar gels, micelles, vesicles, nanotubes, foldamers, conformationally restricted macromolecules, and liquid crystalline phases. Possibly inspired by nature, chemists have extensively used hydrogen bonding as a tool for supramolecular assemblies of a diverse range of abiotic building blocks. As a structural motif, CT complexes can be compared to hydrogen-bonded complexes in its directional nature and complementarities. Additional advantages of CT interactions include wider solvent tolerance and easy spectroscopic probing. Nevertheless the major limitation is their low association constant. This article shows different strategies have evolved over the years to overcome this drawback by reinforcing the CT interactions with auxiliary noncovalent forces without hampering the alternate stacking mode. Emerging reports on promising CT complexes in organic electronics are intimately related to various supramolecular designs that one can postulate based on donor-acceptor CT interactions.

Self-Assembly of Semiconductor Organogelator Nanowires for Photoinduced Charge Separation

We investigated an innovative concept of general validity based on an organogel/polymer system to generate donor-acceptor nanostructures suitable for charge generation and charge transport. An electron conducting (acceptor) perylene bisimide organogelator forms nanowires in suitable solvents during gelation process. This phenomenon was utilized for its self-assembly in an amorphous hole conducting (donor) polymer matrix to realize an interpenetrating donor-acceptor interface with inherent morphological stability. The self-assembly and interface generation were carried out either stepwise or in a single-step. Morphology of the donor-acceptor network in thin films obtained via both routes were studied by a combination of scanning electron microscopy and atomic force microscopy. Additionally, photoinduced charge separation and charge transport in these systems were tested in organic solar cells. Fabrication steps of multilayer organogel/polymer photovoltaic devices were optimized with respect to morphology and surface roughness by introducing additional smoothening layers and charge injection/blocking layers. An inverted cell geometry was used here in which electrons are collected at the bottom electrode and holes at the top electrode. The simultaneous preparation of the interface exhibits almost 3-fold improvement in device characteristics compared to the successive method. The device characteristics under AM1.5 spectral conditions and 100 mW/cm(2) for the simultaneous preparation route are short circuit current J(sc) = 0.28 mA cm(-2), open circuit voltage V(OC) = 390 mV, fill factor FF = 38%, and a power conversion efficiency eta = 0.041%.

Disassembly of Noncovalent Amphiphilic Polymers with Proteins and Utility in Pattern Sensing

A simple strategy for pattern recognition of proteins through micellar disassembly is introduced. Five different noncovalently assembled receptors have been generated, and the disassembly was studied by monitoring the encapsulated dye release in response to five different proteins. The disassembly induced fluorescence change of the guest molecule produces protein-specific patterns.

Hydrogen-bonding-mediated J-aggregation and white-light emission from a remarkably simple, single-component, naphthalenediimide chromophore.

Supramolecular assembly of various functional p systems has been investigated extensively in recent decades with the aim to achieve precise control over the mode of their spatial organization, which governs the photophysical properties of the resulting self-assembled material. Such self-assembled materials with hierarchical structure and defined photophysical properties are considered as promising candidates for applications ranging from biology to materials science. Hydrogen-bonding interactions have played a pivotal role in generating various supramolecular nanostructures from suitably designed small molecular building blocks. While many structurally complex hydrogen-bonding motifs have been utilized to fulfill this objective, surprisingly not much is known regarding the utility of simple selfcomplementary hydrogen-bonding functionalities, such as carboxylic acid, in the context of chromophore assembly. Considering structural simplicity and versatile hydrogenbonding motifs of carboxylic acid group as described in the literature related to crystal engineering, we envisaged it would be worth exploring its utility as a structure-directing functionality in self-assembly of organic functional p-systems. In the recent past, we have been engaged in understanding effect of structural variations on hydrogen-bonding mediated self-assembly of 1,4,5,8-naphthalenetetracarboxdiACHTUNGTRENNUNGimide (NDI) derivatives due to the prospect of using this particular chromophore as an n-type semiconductor. NDI derivatives have also been extensively utilized as building blocks for various elegant supramolecular materials, such as synthetic ion channels, organogels, hydrogels, nanotubes, catenanes, rotaxanes, foldamers, supramolecular photosystems, nanoparticles, and other amphiphilic nanostructures. As a part of our continuing interest in self-assembly of NDI chromophore, we synthesized NDI-1 (Figure 1), which contains a carboxylic acid group that is capable of forming a self-complementary hydrogen-bonding network and a dodecyl chain to provide enhanced solubility in relatively nonpolar solvents. Herein we revealed hydrogen-bonding-mediated, spontaneous J-aggregation of NDI-1 and unprecedented aggregation-induced white-light emission. Self-assembly of NDI-1 was primarily investigated by solvent-dependent UV/Vis studies (Figure 1). In CHCl3, a good solvent for solvating extended p systems, NDI-1 shows wellresolved sharp absorption bands in the range of 300–400 nm due to a p–p* transition along the long axis of the chromophore in the monomeric state. Going from CHCl3 to a less polar solvent, methylcyclohexane (MCH), the absorption bands exhibited drastic reduction in intensity with concomitant redshift of 14 nm, which can be considered as strong evidence for J-type p stacking among the NDI-chromophores. 14,21] Note that for the control molecule NDI-2 (Figure 1), in which the carboxylic acid group was replaced by its methyl ester, no such solvent-dependent changes in the absorption spectra were observed (Figure 1), except for a slight blueshift due to a trivial solvatochromic effect. This ascertains that hydrogen-bonding among the free carboxylic acid functionalities is indeed responsible for the observed self-assembly. Existence of hydrogen bonding was further [a] M. R. Molla, Dr. S. Ghosh Polymer Science Unit Indian Association for the Cultivation of Science 2A & 2B Raja S. C. Mullick Rd. Kolkata, 700032 (India) Fax: (+91) 33-2473-2805 E-mail : psusg2@iacs.res.in Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201103600. Figure 1. Top: Structure of various NDI chromophores. Bottom: Solventdependent (dashed line-CHCl3, solid line-MCH) UV/Vis absorption spectra of NDI-1 (black line) and NDI-2 (gray line); concentration =0.1 mm, T= 25 8C, path length =1 cm. Inset: Variable-temperature UV/Vis absorption spectra of NDI-1 (concentration=0.1 mm) in MCH/CHCl3 (95:5).

Hydrogen‐Bonding‐Mediated Vesicular Assembly of Functionalized Naphthalene–Diimide‐Based Bolaamphiphile and Guest‐Induced Gelation in Water

This paper describes the spontaneous vesicular assembly of a naphthalene-diimide (NDI)-based non-ionic bolaamphiphile in aqueous medium by using the synergistic effects of π-stacking and hydrogen bonding. Site isolation of the hydrogen-bonding functionality (hydrazide), a strategy that has been adopted so elegantly in nature, has been executed in this system to protect these moieties from the bulk water so that the distinct role of hydrogen bonding in the self-assembly of hydrazide-functionalized NDI building blocks could be realized, even in aqueous solution. Furthermore, the electron-deficient NDI-based bolaamphiphile could engage in donor-acceptor (D-A) charge-transfer (CT) interactions with a water-insoluble electron-rich pyrene donor by virtue of intercalation of the latter chromophore in between two NDI building blocks. Remarkably, even when pyrene was located between two NDI blocks, intermolecular hydrogen-bonding networks between the NDI-linked hydrazide groups could be retained. However, time-dependent AFM studies revealed that the radius of curvature of the alternately stacked D-A assembly increased significantly, thereby leading to intervesicular fusion, which eventually resulted in rupturing of the membrane to form 1D fibers. Such 2D-to-1D morphological transition produced CT-mediated hydrogels at relatively higher concentrations. Instead of pyrene, when a water-soluble carboxylate-functionalized pyrene derivative was used as the intercalator, non-covalent tunable in-situ surface-functionalization could be achieved, as evidenced by the zeta-potential measurements.

One‐Pot Synthesis of an Acid‐Labile Amphiphilic Triblock Copolymer and its pH‐Responsive Vesicular Assembly

An amphiphilic triblock copolymer segmented by an acid-labile b-thiopropionate linker was synthesized from a dithiol, a diacrylate, and an acrylate-terminated hydrophilic polymer in two steps in one pot. It showed spontaneous vesicular assembly, and the stimuli-responsive disassembly at mild acidic pH values resulted in sustained release of noncovalently encapsulated guest molecules. Angewandte Chemie

Self‐Sorted Assembly in a Mixture of Donor and Acceptor Chromophores

A simple and novel supramolecular approach for orthogonal self-assembly of donor and acceptor chromophores has been demonstrated. Suitably designed 1,5-dialkoxynaphthalene (DAN) and naphthalene tetracarboxylic acid diimide (NDI) derivatives were used as the donor and acceptor systems, respectively. The molecular design for self-sorting relies upon the precise control over the placement of the self-complementary hydrogen-bonding units (amide functionality) with respect to the individual chromophore. By design, the distances between the two amide groups in the donor and acceptor chromophores are not identical, and consequently the effect of the hydrogen-bonding interaction cannot be maximised in the case of alternate donor-acceptor-type pi-stacking. Thus a relatively weak charge-transfer interaction is expected to be sacrificed, and segregated assembly among the individual chromophores should be enforced by virtue of the much stronger effects of hydrogen bonding and pi-pi stacking. Detailed spectroscopic studies were carried out to probe the mode of self-assembly in various derivatives of the DAN-NDI donor-acceptor pairs to establish the utility of the molecular design as a generalised one for orthogonal self-assembly.

Hydrogen‐Bonding Induced Alternate Stacking of Donor (D) and Acceptor (A) Chromophores and their Supramolecular Switching to Segregated States

This paper reports comprehensive studies on the mixed assembly of bis-(trialkoxybenzamide)-functionalized dialkoxynaphthalene (DAN) donors and naphthalene-diimide (NDI) acceptors due the cooperative effects of hydrogen bonding, charge-transfer (CT) interactions, and solvophobic effects. A series of DAN as well as NDI building blocks have been examined (wherein the relative distance between the two amide groups in a particular chromophore is the variable structural parameter) to understand the structure-dependent variation in mode of supramolecular assembly and morphology (organogel, reverse vesicle, etc.) of the self-assembled material. Interestingly, it was observed that when the amide functionalities are introduced to enhance the self-assembly propensity, the mode of co-assembly among the DAN and NDI chromophores no longer remained trivial and was dictated by a relatively stronger hydrogen-bonding interaction instead of a weak CT interaction. Consequently, in a highly non-polar solvent like methylcyclohexane (MCH), although kinetically controlled CT-gelation was initially noticed, within a few hours the system sacrificed the CT-interaction and switched over to the more stable self-sorted gel to maximize the gain in enthalpy from the hydrogen-bonding interaction. In contrast, in a relatively less non-polar solvent such as tetrachloroethylene (TCE), in which the strength of hydrogen bonding is inherently weak, the contribution of the CT interaction also had to be accounted for along with hydrogen bonding leading to a stable CT-state in the gel or solution phase. The stability and morphology of the CT complex and rate of supramolecular switching (from CT to segregated state) were found to be greatly influenced by subtle structural variation of the building blocks, solvent polarity, and the DAN/NDI ratio. For example, in a given D-A pair, by introducing just one methylene unit in the spacer segment of either of the building blocks a complete change in the mode of co-assembly (CT state or segregated state) and the morphology (1D fiber to 2D reverse vesicle) was observed. The role of solvent polarity, structural variation, and D/A ratio on the nature of co-assembly, morphology, and the unprecedented supramolecular-switching phenomenon have been studied by detail spectroscopic and microscopic experiments in a gel as well as in the solution state and are well supported by DFT calculations.

Aggregation and pH responsive disassembly of a new acid-labile surfactant synthesized by thiol-acrylate Michael addition reaction.

Nucleophilic thiol-acrylate Michael reaction between a hydrophobic thiol and hydrophilic acrylate derivative generated a nonionic surfactant with acid-labile β-thiopropionate linker. Micellization of the surfactant, its ability to encapsulate hydrophobic dye, acid-induced disruption of the aggregate and pH-selective dye release profile have been revealed in this report. The micellar aggregates were found to be stable under neutral conditions, but they could be disrupted in acidic pH (5.3), and thus the encapsulated hydrophobic dye molecules could be selectively released. Appropriate control experiments revealed that the sulfur atom in the β-position is essential for acidic hydrolysis of the ester functionality of the surfactant.