Vibronic effects in pathways of photochemistry and vibrational relaxation

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 [ΦF(n)] and photochemistry [ΦPC(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, Sm(0), and the ground state; (4) the photochemical yield is largely determined by the mode excited, not the energy of excitation.