Chiroptical Phenomena in Conjugated Systems

Abstract

There has been an expanding interest in the photophysical properties of π-conjugated materials owing to their proven potential in the field of organic electronics. Within π-conjugated systems, the electronic interactions between molecules control important functional properties such as photon emission, spectral band shape, lifetime and energy transfer. These properties emanate from different length scales and are intrinsically linked to the nature of molecular arrangement within an aggregate, held by weak non-covalent intermolecular interactions. Strong electronic interactions between molecules promote efficient energy diffusion of optical excitation, while weak interactions make energy hop from site to site. However, in organic electronic devices, molecular interactions that govern energy and charge transport lie somewhere in between the two regimes, which is not well explored. This can be addressed with molecular chirality, a novel spectroscopic marker to examine sensitively the structure-property correlations in molecular aggregates; this is accomplished by measuring the asymmetric optical response of left and right circularly polarized light, often referred to as chiroptical phenomena. In this thesis, I discuss my PhD research work on chiroptical properties of nanoscale organic semiconductor materials. By employing a combination of complementary chiroptical techniques, we demonstrate that subtle variations in the local molecular arrangement can give rise to unique photophysical properties at different time and length scales, across a wide range of conjugated systems. Bringing together elements from theory and spectroscopy, this thesis contributes to understand better the molecular parameters that underpin contemporary optoelectronic phenomena including solar energy harvesting via singlet fission, polarized emission in OLEDs and ultrafast polarization switching for data communication.