Investigation of Structural and Electronic Interactions in Multichromophoric Self-assemblies

Sao, Soumik (2019) Investigation of Structural and Electronic Interactions in Multichromophoric Self-assemblies. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Molecular self-assembly refers to the spontaneous organization of molecules to form ordered aggregates under suitable conditions. An important subclass of this interesting material system is multichromophoric assemblies. In recent past, mutlichromophoric self-assemblies have gained a lot of attention due to its relevance in the area of organic semiconductors and energy harvesting. The work presented in this thesis is focussed on one of the most widely studied molecular chromophore, namely naphthalene diimide (NDI), which is well known for n-type semiconducting properties and an exceptional ability to self-assemble into functional materials. The thesis divided into five chapters. Chapter 1 gives a brief introduction to some of the key concepts that are often encountered in the area of multichromophoric assemblies. In Chapter 2, an unusual donor–acceptor (D–A) state is reported in a pair of regioisomeric xylylene bridged naphthalenediimide (NDI) dimers. The D–A state is characterized by a weak, red-shifted absorbance and a photoluminescence (PL) with very high quantum efficiency. PL and PL excitation (PLE) spectroscopy reveals the partial charge-transfer character of the D–A state. Disruption of this D–A interaction in the presence of a competing donor, reveals the unique onedonor- two acceptors stoichiometry. Finally, the effect of intramolecular pi-stacking on the D–A interaction is discussed. A pair of weak non-covalent interactions that strengthen each other synergistically can be an important tool in supramolecular chemistry, because of its ability to predispose a molecular selfassembly towards a specific pathway. Chapter 3 presents assisted pi-stacking, a synergistic association between regular aromatic pi-stacking and a much weaker non-covalent interaction that is characterized by an exceptional strength and thermal stability, unprecedented among the strongest known pi-stacking interactions. The nature of assisted pi-stacking was investigated in a pair of covalently bridged naphthalene diimide (NDI) and perylene bisimide (PBI) folda-dimers, and its role in the unusual stabilization of the folded conformation. A selective termination of the weaker secondary interaction causes a spontaneous disruption of the assisted pi-stacked state, validating the synergistic nature of the interaction. A similar assisted pi-stacking interaction motif, when incorporated in a monomeric NDI, was shown to drive its self-assembly in a nonaggregating solvent medium. Chapter 4 presents the investigation of an unusual self-assembly behaviour of a flexible NDI trimer. For a molecular self-assembly in solution to be spontaneous, enthalpy gain from intermolecular interactions must prevail over the combined loss of solvation enthalpy and entropy. By incorporating three NDI units in one trimer molecule significantly lowers the entropic penalty, as a result of which the trimer exhibits an unusually high tendency to aggregate even in a good solvent. The observed equilibrium constant is three orders of magnitude higher than reported results. Our results show that a traditionally rigid descriptions of solvents as good or bad cannot be a strict one, and that the enthalpic and entropic factors associated are often very comparable. Further, the conformational flexibility allows the trimer to chart a completely different aggregation pathway in a traditionally bad solvent, such as hexane. Chapter 5 is an investigation of molecular assembly of weakly pi-stacking core-substituted naphthalene diimides (cNDIs) in spatially confined environment. Aggregation of cNDIs typically requires a participation of strong side-group interactions. We however demonstrated that spatial confinement within the core of a polymeric micelle leads to locally elevated concentrations and reduced entropy that drives a rapid aggregation in cNDIs. A detailed kinetic study further reveals that the rapid aggregation is followed by a slower aggregate reorganization process. Fast aggregation kinetics also leads to a self-sorting of aggregates.

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