Subi J. George

Pathway complexity in supramolecular polymerization

Self-assembly provides an attractive route to functional organic materials, with properties and hence performance depending sensitively on the organization of the molecular building blocks. Molecular organization is a direct consequence of the pathways involved in the supramolecular assembly process, which is more amenable to detailed study when using one-dimensional systems. In the case of protein fibrils, formation and growth have been attributed to complex aggregation pathways that go beyond traditional concepts of homogeneous and secondary nucleation events. The self-assembly of synthetic supramolecular polymers has also been studied and even modulated, but our quantitative understanding of the processes involved remains limited. Here we report time-resolved observations of the formation of supramolecular polymers from π-conjugated oligomers. Our kinetic experiments show the presence of a kinetically favoured metastable assembly that forms quickly but then transforms into the thermodynamically favoured form. Quantitative insight into the kinetic experiments was obtained from kinetic model calculations, which revealed two parallel and competing pathways leading to assemblies with opposite helicity. These insights prompt us to use a chiral tartaric acid as an auxiliary to change the thermodynamic preference of the assembly process. We find that we can force aggregation completely down the kinetically favoured pathway so that, on removal of the auxiliary, we obtain only metastable assemblies.

Supramolecular Hydrogels and High‐Aspect‐Ratio Nanofibers through Charge‐Transfer‐Induced Alternate Coassembly

Weak charge-transfer interactions between electron-rich and electron-deficient aromatic molecules have been extensively used for the design of various supramolecular assemblies in solution, such as rotaxanes, catenanes, and foldamers. The conformation of various donor–acceptor (D–A) polymers in solution has been tuned by chargetransfer interactions in synergy with either solvophobic or ion-binding interactions. In addition, extended solid-phase assemblies of alternate donor and acceptor molecules for device applications were attained by cocrystallization and liquid-crystalline mesophase coassembly. Columnar mesophases of alternate donor and acceptor molecules have enhanced columnar organization and higher charge-carrier mobilities compared to their individual components. 1D self-assembled nanowires based on organic p-conjugated systems have attracted a great deal of attention in recent years in the field of organic and supramolecular electronics. 10] Several approaches, which make use of hydrogenbonding and p-stacking interactions, have been used to design self-assembled fibers of donor–acceptor arrays, and create supramolecular p–n heterojunctions for photovoltaics. However, use of charge-transfer interactions for the design of two-component, alternate 1D supramolecular fibers of donor and acceptor molecules in solution is difficult to achieve, as the orthogonal self-assembly (phase separation) of individual components may prevent their efficient coassembly, and hence a careful design of monomers is required. 1D charge-transfer nanofibers are expected to exhibit better conductivity caused by the directional movement of their high-density charge carriers and a well-ordered, stronger p-stacked architecture as a result of better p overlapping of face-to-face-packed aromatic molecules. 13] These observations prompted us to investigate whether nanofibers of alternate donor and acceptor molecules coassembled through charge-transfer interactions can be constructed using supramolecular chemistry design principles. Herein, we show that high-aspect-ratio cylindrical micelles and hydrogels of a donor–acceptor charge-transfer complex can be constructed by self-assembly in water. Although a variety of peptide and rod–coil amphiphiles, as well as sugar derivatives, have been reported to form hydrogels, this is the first report of hydrogel formation that exploits charge-transfer interactions between chromophores. We synthesized a donor–acceptor pair, a coronene tetracarboxylate tetrapotassium salt (CS), and a dodecylfunctionalized methyl viologen derivative (DMV) for the design of coassembled nanostructures (Scheme 1). After

Noncovalent functionalization, exfoliation, and solubilization of graphene in water by employing a fluorescent coronene carboxylate.

Single-layer graphene stabilization: An efficient methodology to make stable aqueous solutions of single-layer graphene has been demonstrated by exploiting charge-transfer interactions with a coronene tetracarboxylate acceptor molecule (see figure). Microscopic studies reveal exfoliation of few-layer graphene and selective stabilization of single-layer graphene in large quantities.

Light-Harvesting Hybrid Assemblies

Light-harvesting hybrids have gained much importance as they are considered as potential mimics for photosynthetic systems. In this Concept article we introduce the design concepts involved in the building up of light-harvesting hybrids; these resemble the well-studied organic-based assemblies for energy transfer. We have structured this article into three parts based on the strategies adopted in the synthesis of hybrid assemblies, as covalent, semicovalent, and noncovalent procedures. Furthermore, the properties and structural features of the hybrids and analogous organic assemblies are compared. We also emphasize the challenges involved in the processability of these hybrid materials for device applications and present our views and results to address this issue through the design of soft-hybrids by a solution-state, noncovalent, self-assembly process.

Oligo(p-phenylenevinylene) peptide conjugates: Synthesis and self-assembly in solution and at the solid-liquid interface

Two oligo(p-phenylenevinylene)-peptide hybrid amphiphiles have been synthesized using solid- and liquid-phase strategies. The amphiliphiles are composed of a pi-conjugated oligo(p-phenylenevinylene) trimer (OPV) which is coupled at either a glycinyl-alanyl-glycinyl-alanyl-glycine (GAGAG) silk-inspired beta-sheet or a glycinyl-alanyl-asparagyl-prolyl-asparagy-alanyl-alanyl-glycine (GANPNAAG) beta-turn forming oligopeptide sequence. The solid-phase strategy enables one to use longer peptides if strong acidic conditions are avoided, whereas the solution-phase coupling gives better yields. The study of the two-dimensional (2D) self-assembly of OPV-GAGAG by scanning tunneling microscopy (STM) at the submolecular level demonstrated the formation of bilayers in which the molecules are lying antiparallel in a beta-sheet conformation. In the case of OPV-GANPNAAG self-assembled monolayers could not be observed. Absorption, fluorescence, and circular dichroism studies showed that OPV-GAGAG and OPV-GANPNAAG are aggregated in a variety of organic solvents. In water cryogenic temperature transmission electron microscopy (cryo-TEM), atomic force microscopy (AFM), light scattering, and optical studies reveal that self-assembled nanofibers are formed in which the helical organization of the OPV segments is dictated by the peptide sequence.

Highly pure solid-state white-light emission from solution-processable soft-hybrids.

Highly pure and solution processable white-light-emitting hybrids are presented. These soft-hybrids are designed by an organic-inorganic supramolecular co-assembly in water. White-light emission is achieved by partial energy transfer (ET) between donor and acceptor molecules anchored on the inorganic component. The unique and remarkable processability feature of these hybrids is demonstrated by painting/writing onto large glass and flexible plastic substrates.

Ultrafast response humidity sensor using supramolecular nanofibre and its application in monitoring breath humidity and flow

Measuring humidity in dynamic situations calls for highly sensitive fast response sensors. Here we report, a humidity sensor fabricated using solution processed supramolecular nanofibres as active resistive sensing material. The nanofibres are built via self- assembly of donor and acceptor molecules (coronene tetracarboxylate and dodecyl methyl viologen respectively) involved in charge transfer interactions. The conductivity of the nanofibre varied sensitively over a wide range of relative humidity (RH) with unprecedented fast response and recovery times. Based on UV-vis, XRD and AFM measurements, it is found that the stacking distance in the nanofibre decreases slightly while the charge transfer band intensity increases, all observations implying enhanced charge transfer interaction and hence the conductivity. It is demonstrated to be as a novel breath sensor which can monitor the respiration rate. Using two humidity sensors, a breath flow sensor was made which could simultaneously measure RH and flow rate of exhaled nasal breath. The integrated device was used for monitoring RH in the exhaled breath from volunteers undergoing exercise and alcohol induced dehydration.

Amine‐Responsive Adaptable Nanospaces: Fluorescent Porous Coordination Polymer for Molecular Recognition

Flexible and dynamic porous coordination polymers (PCPs) with well-defined nanospaces composed of chromophoric organic linkers provide a scaffold for encapsulation of versatile guest molecules through noncovalent interactions. PCPs thus provide a potential platform for molecular recognition. Herein, we report a flexible 3D supramolecular framework {[Zn(ndc)(o-phen)]⋅DMF}n (o-phen = 1,10-phenanthroline, ndc = 2,6-napthalenedicarboxylate) with confined nanospaces that can accommodate different electron-donating aromatic amine guests with selective turn-on emission signaling. This system serves as a molecular recognition platform through an emission-readout process. Such unprecedented tunable emission with different amines is attributed to its emissive charge-transfer (CT) complexation with o-phen linkers. In certain cases this CT emission is further amplified by energy transfer from the chromophoric linker unit ndc, as evidenced by single-crystal X-ray structural characterization.

What Molecular Features Govern the Mechanism of Supramolecular Polymerization

An understanding of the mechanisms of supramolecular polymerization from a molecular point of view is lacking. Several reports in the literature on the mechanism exhibited by different classes of molecules are examined in an attempt to correlate the molecular features to the aggregation pathway followed. It is proposed that long-range interactions between oligomers could lead to their cooperative growth. The lack thereof leads to isodesmicity.

Emerging Solvent‐Induced Homochirality by the Confinement of Achiral Molecules Against a Solid Surface

The unique handedness of chiral molecules affects chemical, physical, and biological phenomena. While observed in solution for helical polymers and self-assembled stacks of molecules, transmission of chiral information is particularly selective at ordered interfaces as a result of geometrical restrictions introduced by two-dimensional (2D) confinement. Chiral amplification of enantiomerically enriched mixtures has been demonstrated either by chemical reactions at the air–water interface, or upon self-assembly on solid surfaces. Homochirality in achiral enantiomorphous monolayers can be realized by merging chiral modifiers in the monolayer or by exposing monolayers to magnetic fields. Alternatively, the potential role of solvents in amplification of chirality and emergence of homochirality at surfaces remains unexplored to date. Herein we show how solvent-induced macroscopic chirality emerges within self-assemblies of achiral molecules on achiral surfaces. It is an exclusive surface-confined process, and as such it differs from “chiral-solvent-” or “chiral-guestinduced” chirality of supramolecular systems in solution. To demonstrate that homochirality emerges at the interface between a chiral liquid and the surface of highly oriented pyrolytic graphite (HOPG), we selected a hydrogen-bonding achiral diamino triazine oligo-(p-phenylenevinylene) oligomer (A-OPV4T, Figure 1). The chiral analogue, ((S)-OPV4T, Figure 1), was recently shown to assemble exclusively in a counter-clockwise (CCW) hydrogen-bonded rosette motif at the liquid–solid interface, with 1-phenyloctane as the achiral solvent. Molecular homochirality is expressed at the supramolecular level as a result of the 2D packing of the chiral rosette. The chiral solvent in the current study,