Emergence of chirality in phenyleneethynylene based molecular assemblies: experimental and theoretical study

Abstract

Noncovalent interactions play a major role in a biological system to determine various functions: this strategy is energetically economical, and many efforts are driven towards the understanding of various noncovalent interactions in order to modulate the associated functionalities. Supramolecular chemistry studies the arrangement of molecules into assemblies, using weak noncovalent interactions. Supramolecular chirality includes two categories: (i) induced chirality and (ii) self-assembly. The induced chirality includes the host-guest inclusion complexes where chirality is transferred from one component to the other. In this case, either one or both the interacting molecule(s) should be chiral. Another category is self-assembly, where chirality arises due to the helical arrangement of molecules. In this case, it is not always necessary that the molecule should possess a chiral centre. Emergence of chiroptical properties in organic assembled systems, especially the role of transition dipoles of chromophores in dictating the chiral response, is not well-explored. The over-all objective of the thesis is to carry out a joint experimental and theoretical investigation on the nature of chiral response observed in supramolecular chiral assembly. Phenyleneethynylene based molecular assemblies were chosen as model systems due to their ease of synthesis, photostability, and tendency to aggregate. Chiral aggregates of phenyleneethynylene derivatives are obtained exploiting the functionalization with chiral side chains. The (chiro) optical response of aggregates is deeply investigated using experimental techniques, and the exciton model, supported by DFT/TD-DFT calculations, which in turn allowed us to rationalize the observed spectroscopic behaviour. Attention is paid to the role of intermolecular interaction in determining the optical properties of aggregates.