The Team
WHAT EXCITES US !
Research in our group focusses on the self-assembly of molecular chromophores. There are two interesting aspects to this quest, one complementing the other. Molecular chromophores are π-conjugated molecules, and are potential building blocks for organic semiconductors. Close packing of such molecules in the assembled state paves the way for moderate to strong electronic coupling, which often leads to interesting and unforeseen consequences. While emergence of new materials with interesting optical and electronic properties remains the primary motivation, there is another equally rewarding direction. Molecular chromophores exhibit intense and structured spectral signatures in optical absorption and emission. The electronic spectrum is strongly influenced by the nature and the strength of interchromophoric coupling, which in turn depends on the relative arrangement of chromophores in the assembled state. This allows one to reliably and conveniently monitor the nature and the degree of self-assembly from steady-state optical spectroscopy in solution. One can extract relevant information about the thermodynamics and kinetics of a molecular self-assembly from such measurements. This offers a unique perspective into the complex nature of the assembly process, and a better understanding of the experimental conditions that govern its fate.
Our research interests can be broadly classified into these, often strongly overlapping themes:
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Interplay of weak interactions: the whole is "different" than the sum of the parts
The molecular structure is the biggest determinant of the molecular assembly. Weak noncovalent interactions built into a molecular structure drive the self-assembly process and largely dictate its outcome. But, how does a molecule with multiple noncovalent interactions behave? Do different noncovalent interactions influence each other? Is their combined effect additive? Can they counteract each other? These are some of the fundamental questions that are likely to inform molecular design, and are therefore fascinating to us.
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Representative Publications:
ACS Cent. Sci. 2021, 7, 1391−1399
Chem. Commun. 2018, 54, 12186
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Pathway Complexity and Supramolecular polymorphism: in the molecular Legoland
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Having multiple noncovalent interaction motifs gives rise to the possibility of more than one aggregation pathways, leading to different assembled structures. Our initial efforts were towards understanding the effect of molecular conformation on its self-assembly pathways of flexible bichromophoric molecules. More recently, we have invested significantly in understanding the importance of solvent-solute interactions in molecular self-assembly in the context of supramolecular polymorphism and chirality.
Some of the ongoing projects in this sub-area focus on achieving control over the stability of a kinetic species using an external stimulus. The objective is to use the kinetic aggregates as a monomer reservoir that can be used on-demand for the controlled supramolecular growth. In another ongoing work, we are investigating two-dimensional polymorphism in molecular chromophores.
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Representative Publications:
J. Am. Chem. Soc. 2023, DOI: 10.1021/jacs.2c12253
Angew. Chem. Int. Ed. 2022, 134, e202201956
J. Phys. Chem. Lett. 2017, 8, 3427-3432
Chem. Commun. 2017, 53, 3994-3997
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Emergent Properties
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Optical
Self-assembled structures can act as highways for a facile and long-range transfer of excitation energy, making them useful candidates for light harvesting applications. In H-aggregated assemblies of perylene bisimide (PBI), exciton migration is defeated by a very efficient self-trapping process that leads to the formation of excimers. We developed several excimer free H-aggregated PBI that emit a circularly polarized luminescence with a very high anisotropy factor. In collaboration with Dr. C. Ramanan’s group (Vrije Univ., Amsterdam) we are investigating singlet fission and symmetry breaking charge transfer in these systems.
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Representative Publications:
Org. Mater. 2021, 3, 455-468
Chem. Sci. 2020, 11, 5710-5715
Electrical
The same highway that allows excitons to migrate can aid charge transport too! We have developed molecules for n-type OFETs. One in particular shows an exceptional ambient stability and a very low threshold voltage, making it suitable for low-cost printable electronic sensor chips. In a related project, we developed resistive nonvolatile memristors that show robust bipolar switching with exceptionally high ON/OFF ratios. These projects are carried out in close collaboration with Dr. R. Vijayaraghavan’s group at IISER-Kolkata.
Mechanical
The inherent reversibility of noncovalent interactions also makes these useful for mechanical applications. Some of our works in this direction have been on demonstrating electrochemical actuation in a flexible PBI dimer, and more recently on photoresponsive aggregates of azobenzene-substituted PBI.
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In a one-off fun project, we developed a small molecule oranogelator that could congeal heavy crude oil from water. An exceptional mechanical strength of the resultant gel, easy recoverability of oil, ability to congeal oil at 0 °C, and the negligible aqueous solubility and biotoxicity makes our compound on of the best reported phase-selective organogelator for tackling marine oil spills.
Representative Publications: