Vibronic effects in pathways of photochemistry and vibrational relaxation

Abstract

A photochemical and photophysical investigation has been carried out on 2,2-dimethyl-7,8-benzo-2H-chromene and 2,2-dimethyl-5,6-benzo-2H-chromene. Absolute quantum yields of fluorescence [ Φ F( n)] and photochemistry [ Φ PC( n)] have been determined and are functions of the individual states, modes, and vibrational levels ( n) excited and thus, there are many quantum yields of each of the foregoing processes. We have found that photochemistry and vibrational relaxation (non-radiative decay) are competitive processes at each vibrational level of each mode of all states. Using photochemistry as a probe, we have determined: (1) the pathways of vibrational relaxation and photochemical processes and have quantitatively evaluated the competitive nature of these; (2) that vibrational relaxation only occurs within the mode excited, there is no crossing between different modes during relaxation; (3) significant radiationless relaxation occurs directly between any pure excited state, S m (0), and the ground state; (4) the photochemical yield is largely determined by the mode excited, not the energy of excitation.