Chidambar Kulkarni

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.

Extended phenylene based microporous organic polymers with selective carbon dioxide adsorption

Two microporous conjugated polymers with extended phenylene backbones have been synthesized through Suzuki cross-coupling reactions. The structure and the pores of the polymers have been controlled by the use of para- and meta-structure directing 1,3,5-triphenyl tribromide monomers. Gas sorption studies revealed an unprecedented CO2 selectivity over N2 for these conjugated polymer networks. The networks have furthermore been tested as hydrogen storage materials and showed significant hydrogen uptake at high pressures.

Dipole-Moment-Driven Cooperative Supramolecular Polymerization

While the mechanism of self-assembly of π-conjugated molecules has been well studied to gain control over the structure and functionality of supramolecular polymers, the intermolecular interactions underpinning it are poorly understood. Here, we study the mechanism of self-assembly of perylene bisimide derivatives possessing dipolar carbonate groups as linkers. It was observed that the combination of carbonate linkers and cholesterol/dihydrocholesterol self-assembling moieties led to a cooperative mechanism of self-assembly. Atomistic molecular dynamics simulations of an assembly in explicit solvent strongly suggest that the dipole-dipole interaction between the carbonate groups imparts a macro-dipolar character to the assembly. This is confirmed experimentally through the observation of a significant polarization in the bulk phase for molecules following a cooperative mechanism. The cooperativity is attributed to the presence of dipole-dipole interaction in the assembly. Thus, anisotropic long-range intermolecular interactions such as dipole-dipole interaction can serve as a way to obtain cooperative self-assembly and aid in rationalizing and predicting the mechanisms in various synthetic supramolecular polymers.

Carbonate Linkage Bearing Naphthalenediimides: Self‐Assembly and Photophysical Properties

Self-assembly of carbonate linkage bearing naphthalene diimides (NDI) showed unusually red-shifted excimer emission at approximately 560 nm. On the other hand, the ether linkers showed usual excimers at around 520 nm, highlighting the role of the carbonate group in tuning the molecular organization and the resultant photophysical properties of NDI.

Synthesis and self-assembly of a C3-symmetric benzene-1,3,5-tricarboxamide (BTA) anchored naphthalene diimide disc

A novel naphthalene diimide disc shaped molecule, anchored onto a C3-symmetric BTA hydrogen-bonded core, is synthesized, which self-assembles in nonpolar solvents to form helically coiled nanofibers through an isodesmic mechanism.

Self-Assembly of Coronene Bisimides: Mechanistic Insight and Chiral Amplification

The study of the organization of small π-conjugated molecules is imperative to understanding and controlling its properties for various applications. Coronene bisimides (CBIs) are potential candidates for novel liquid-crystalline materials and active n-type semiconductor molecules in organic electronics. To understand the self-assembly of this seldom-studied chromophore, we have designed two derivatives of CBIs bearing chiral and achiral 3,4,5-trialkoxyphenyl groups at the imide position, named as CBI-GCH and CBI-GACH, respectively. CBI-GCH self-assembles mainly through π-stacking and van der Waals interactions in nonpolar methylcyclohexane to result in long 1D fibrillar stacks. The mechanism of supramolecular polymerization was probed by using chiroptical studies, which showed an isodesmic pathway for CBI-GCH. The thermodynamic parameters that govern the self-assembly are detailed. CBI-GACH also shows similar self-assembly behavior as its chiral counterpart. X-ray diffraction studies of both molecules reveals a 2D hexagonal columnar arrangement. The coassembly of CBI-GCH and CBI-GACH shows chiral amplification (sergeant and soldiers experiment) with saturation at 30-50 % of the chiral derivative, which was further used to study the dynamics of the assembly. Thus, this study presents a rare report of chiral amplification in an isodesmic system.

Charge-transfer complexation between naphthalene diimides and aromatic solvents.

Naphthalene diimides (NDIs) form emissive ground-state charge-transfer (CT) complexes with various electron rich aromatic solvents like benzene, o-xylene and mesitylene. TD-DFT calculation of the complexes suggests CT interaction and accounts for the observed ground-state changes.

Observation of Pore‐Switching Behavior in Porous Layered Carbon through a Mesoscale Order–Disorder Transformation

Structural flexibility in biomacromolecules is a well-known phenomenon in nature. For example, enzymes efficiently change their tertiary structures, the channels and cavities in which are reversibly modified to accommodate a guest molecule. The high specificity of enzymes in biologically important reactions is primarily attributed to their ability to change their structures in response to external stimuli. Taking a cue from nature, researchers are now looking for ways to make structurally flexible porous materials, as such materials could find wide-ranging application in catalysis, separation, sensors, fuel cells, and gas storage owing to their unique properties and functions. Recently, metal–organic frameworks (MOFs) were shown to modify their framework structure in response to chemical or physical stimuli. 6] However, such a rearrangement of structure is not possible in rigid inorganic porous solids, such as zeolites, activated charcoal, and mesoporous silica. Considering their industrial significance, structural flexibility in these porous materials would be very advantageous for the size-selective separation of molecules or switching between different properties of the material itself. Herein, we report the first observation of a reversible, mesoscale order–disorder transformation of a porous layered carbon (PLC) as a response to an applied mechanical force. The PLC containing crystallographically oriented graphene nanocrystallites on its layers was synthesized by graphitizing glucose within an aminoclay template. Although clay galleries have been used previously to make structurally rigid, porous carbon materials containing nanographene domains, these materials failed to show any mesoscale, long-range order replicating the stacked clay layer structure. 8] On the other hand, carbon materials obtained by using mesoporous silica as the template are often amorphous and exhibit ordered but rigid pores replicating the template. In contrast to these rigid carbon materials, the PLC that we obtained showed a flexible pore size associated with a mesoscale order–disorder transformation, which we exploited for the size-selective separation (sorption) of dye molecules. We used an aminoclay template to make layered carbon. 12] Aminoclay is an organophyllosilicate of approximate composition R8Si8Mg6O16(OH)4 (in which R = CH2CH2NH2) consisting of octahedrally coordinated MgO/ OH sheets (brucite) overlaid on both sides with a tetrahedrally coordinated aminopropyl-functionalized silicate network. Since the amine groups get protonated in water, the clay layers can readily be exfoliated owing to charge repulsion between the layers. 14] The exfoliated clay in water consists of a single layer or bundles containing a few layers (nanobundles). The dispersed layers can be restacked by the addition of ethanol (appearance of a white precipitate). In the present study, we first mixed a solution of glucose with a transparent aminoclay dispersion and then induced the stacking of clay layers/nanobundles by adding ethanol (see Scheme S1 in the Supporting Information). During the precipitation, glucose molecules get trapped in between the layers as well as in the space between the nanobundles. Thermogravimetric analysis (TGA) of the resulting composite after the excess glucose had been washed out with ethanol showed nearly 60% weight loss, mainly as a result of the removal of glucose (results not shown). Carbonization of the composite and subsequent etching of the clay, followed by filtration, left porous layered carbon (PLC). The absence of Si and Mg peaks in the energy-dispersive X-ray spectrum of the resulting PLC confirmed the complete removal of the clay template (see Figure S1 in the Supporting Information). The powder X-ray diffraction pattern of the resulting PLC is shown in Figure 1a. The low-angle diffraction peak at 2q = 1.278 corresponding to a basal distance of 6 nm confirms the existence of mesostructural order in the carbon after the removal of the clay. Considering that the basal distance of the clay–glucose composite is 2.5 nm (and that this peak is lost on carbonization), it is unlikely that the individual clay layers would have acted as the template to generate this large d spacing of 6 nm. The PLC with a large d spacing (6 nm) therefore originates from the carbonization of glucose mostly trapped between the clay nanobundles, which are 2–3 layers thick, during the precipitation. The broad peak at 2q = 258 (the characteristic d002 graphitic peak in the higher-angle XRD pattern) indicates that the PLC is composed of nano[*] K. K. R. Datta, Dr. D. Jagadeesan, C. Kulkarni, A. Kamath, Prof. M. Eswaramoorthy Chemistry and Physics of Materials Unit and DST Unit on Nanoscience, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) Jakkur P.O., Bangalore 560064 (India) Fax: (+ 91)80-2208-2766 E-mail: eswar@jncasr.ac.in Homepage: http://www.jncasr.ac.in/eswar/

Vibrational Spectra of Linear Oligomers of Carbonic Acid: A Quantum Chemical Study

Gas phase quantum chemical calculations of linear, hydrogen bonded oligomers of carbonic acid have been carried out to examine the feasibility for such species to be the building blocks of crystalline carbonic acid. Infrared and Raman vibrational spectra have been calculated and are compared against experimentally known spectra for two polymorphs of carbonic acid. The calculated anharmonic frequencies of the linear oligomer agree well with the experimental data for the centrosymmetric β-carbonic acid, rather than with that for the α polymorph. These calculations strongly suggest that β-carbonic acid should consist of one-dimensional hydrogen bonded carbonic acid molecules in the anti-anti conformation.

Cooperativity Scale: A Structure–Mechanism Correlation in the Self-Assembly of Benzene-1,3,5-tricarboxamides

Conspectus The self-assembly of small and well-defined molecules using noncovalent interactions to generate various nano- and microarchitectures has been extensively studied. Among various architectures, one-dimensional (1-D) nano-objects have garnered significant attention. It has become increasingly evident that a cooperative or nucleation–elongation mechanism of polymerization leads to highly ordered 1-D supramolecular polymers, analogous to shape-persistent biopolymers such as actin. With this in mind, achieving cooperativity in self-assembled structures has been actively pursued with significant success. Only recently, researchers are focusing on the origin of the mechanism at the molecular level in different synthetic systems. Taking a step further, a thorough quantitative structure–mechanism correlation is crucial to control the size, shape, and functions of supramolecular polymers, and this is currently lacking in the literature. Among a plethora of molecules, benzene-1,3,5-tricarboxamides (BTAs) provide a unique combination of important noncovalent interactions such as hydrogen bonding, π-stacking, and hydrophobic interactions, for self-assembly and synthetic ease. Due to the latter, a diverse range of BTA derivatives with all possible structural mutations have been synthesized and studied during the past decade, mainly from our group. With such a large body of experimental results on BTA self-assembly, it is time to embark on a structure–mechanism correlation in this family of molecules, and a first step toward this will form the main focus of this Account. The origin of the cooperative mechanism of self-assembly in BTAs has been ascribed to 3-fold intermolecular hydrogen bonding (HB) between monomers based on density-functional theory (DFT) calculations. The intermolecular hydrogen-bonding interaction forms the central premise of this work, in which we evaluate the effect of different moieties such as alkyl chains, and amino acids, attached to the core amides on the strength of intermolecular HB, which consequently governs the extent of cooperativity (quantified by the cooperativity factor, σ). In addition to this, we evaluate the effect of amide connectivity (C- vs N-centered), the role of solvents, amides vs thioamides, and finally the influence of the benzene vs cyclohexane core on the σ. Remarkably, every subtle structural change in the BTA monomer seems to affect the cooperativity factor in a systematic and rationalizable way. The take home message will be that the cooperativity factor (σ) in the BTA family forms a continuous spectrum from 1 (isodesmic) to <10–6 (highly cooperative) and it can be tuned based on the appropriate modification of the BTA monomer. We anticipate that these correlations drawn from the BTA series will be applicable to other systems in which HB is the main driving force for cooperativity. Thus, the understanding gained from such correlations on a prototypical self-assembling motif such as BTA will aid in designing more complex systems with distinct functions.