Photophysical and Theoretical Studies of Aza-heterocycles: Mechanistic Pathways to Excited State Proton Transfer Phenomenon

Abstract

Excited state intramolecular proton transfer (ESIPT) has been attracting considerable interest for the past few years in the field of optoelectronics because of their desirable unique photophysical properties. The photophysical properties of ESIPT fluorophores determine their possible applicability in functional materials, molecular fluorescence probes, luminescent materials, UV stabilizers, OLEDs, and molecular logic gates. Further, integration of ESIPT process with aggregation-induced emission (AIE) phenomenon (where the fluorescence quantum yield increases with the formation of aggregates) has the advantage of large Stokes’ shift and do not require hydrophilic modification, hence, act as potential candidates for applications in chemical and biological sensing, imaging, optics, etc. The research investigations in the present thesis revealed that the presence of functional units highly affects the ESIPT process. The key finding revealed that the presence of electron-withdrawing units make the ESIPT process more facile, while the electron-donating units induced the excited state intramolecular charge transfer (ESICT) along with the proton transfer mechanism. Findings also suggest that the electron- donating units sometimes results in the supremacy of the ESICT process over the ESIPT phenomenon. The integration of the AIE process along with the ESIPT phenomenon results in high quantum yields due to the restriction of intramolecular rotations in the aggregated state, which prohibits energy dissipation via non-radiative channels in the absence and presence of the different analyte. The dual or multiple mechanisms were observed in azine and naphthalimide based Schiff bases. The excited intramolecular charge-coupled double proton was observed in the asymmetrical quinoline-benzimidazole compound. The research finding also includes the structural elucidation of imidazo-[1,2-a] pyrazine-based synthesized rotamers and the presence of the ESIPT process. All experimental outcomes were validated with theoretical calculations. Therefore, it is believed that dual or multiple mechanism-based compounds will be potential candidates in diverse fields such as bio-imaging, biotic and abiotic sensing, and developing optoelectronic devices.