Ultrafast Photophysics of Aminoboranes by Transient Absorption and Ultrafast Raman Loss Spectroscopy

Abstract

B–N based aminoboranes systems have recently emerged as promising materials for organic optoelectronic and biomedical applications. They exhibit a wide range of functionalities such as delayed fluorescence, room-temperature phosphorescence, stimuli-responsive luminescence and aggregation-induced emission, which are directly governed by unique electronic structures and tunable photophysical properties. Despite their growing importance, however, the fundamental excited state photophysics of such systems remain poorly understood. In my thesis work, I will present our investigation into the real-time tracking of photoinduced electronic relaxation and structural dynamics in aminoboranes using femtosecond broadband transient absorption and ultrafast Raman loss spectroscopy. Following optical excitation of carbazole-based aminoboranes, transient absorption measurements reveal an ultrafast appearance of a red-shifted stimulated emission signal, indicating the formation of an intramolecular charge transfer (ICT) character in polar solvents. Such excited-state structural evolution is accompanied by the gradual blue-shifting of the B–C stretching frequency in transient Raman loss signals, reporting electron density localization around B–C bonds in the acceptor moiety following the charge transfer. It brings the singlet state energetically closer to the triplet states, thus impacting intersystem and reverse intersystem-crossing events. We also demonstrate structural control of ICT dynamics through substituent effects, thus tuning the delayed fluorescence and room-temperature phosphorescence. We find that, unlike carbazole, diphenylamine-based aminoboranes exhibit a multi-relaxation pathway and display nuclear dynamics similar to isoelectronic C=C-based materials, underscoring the role of structural relaxation in aggregation-induced emission. In phenoxazine- and phenothiazine-based aminoboranes, we find that excited-state aromaticity, coupled with coherently driven vibrational modes, plays a crucial role in electron transfer and charge separation processes. Altogether, this thesis work illustrates how femtosecond-resolved electronic and vibrational spectroscopy provides detailed insight into the interplay of molecular geometry and charge transfer in aminoboranes.