Investigating the Excited State Dynamics of 1,6-Diphenyl-1,3,5-Hexatriene Using Time Correlated Single Photon Counting

Abstract

Time correlated single photon counting techniques have been combined with absorption and fluorescence measurements of 1,6-diphenyl-1,3,5-hexatriene (DPH) in 13 different room temperature solutions and in a cryogenic glass, to study the effect of environment on its excited state dynamics.
Absorption and fluorescence spectra of DPH in different solvents reveal different states involved in absorption and fluorescence, with fluorescence from a dark state justified with Herzberg-Teller coupling. Correlations are made between the absorption origin of 1,6-diphenyl-1,3,5-hexatriene and various properties of the solvents. The absorption wavelength for the electronic and first vibronic transition were extracted and used as excitation wavelengths for time correlated single photon counting (TCSPC) measurements. The excited state lifetime data of DPH in numerous solvents, at room and cryogenic temperatures, explored the effects of different environments on excited state dynamics.
TCSPC measurements monitored the excited state fluorescence lifetimes with higher time-resolution than hitherto reported, and an instrument response (IRF) of 140 ps. These measurements revealed that the majority of fluorescence does not originate from the initially photoexcited 1Bu electronic state, but instead a lower-lying 2Ag state. The solvent environment modulated the 1Bu–2Ag energy gap, the barrier, and thus associated rate, with 1Bu2Ag interconversion. Data were divided into two categories: where the internal conversion was slower than the IRF, and thus a delayed rise in 2Ag fluorescence resolved by the TCSPC measurements, or to those occurring on a faster timescale. The timescale for the 2Ag state also varied significantly with solvent and was found to be correlated with viscosity. The dependence on solvent viscosity was rationalised by the presence of a barrier to a non-radiative 2Ag deactivation pathway that involves out-of-plane motions, and is competitive with fluorescence, and thus the latter process is the most competitive in more rigid environments.

Date of Award	2 Dec 2021
Original language	English
Awarding Institution	University of Bristol
Supervisor	Tom Oliver (Supervisor) & Chris A Russell (Supervisor)