ELECTRONIC RELAXATION IN ISOLATED FLUOROBENZENES

Fluorescence quantum yields and decay times for excitation of selected vibrational levels in isolated o-, m- and p-difluorobenzene in the vapour phase are reported, and compared with values from the literature. For excitation in the 1on (ring breating mode) progression, the rate constant for radiative decay was found to decrease, with vibrational quantum number in qualitative but not quantitative agreement with expectation. Fluorination increases the rate constant for non-radiative (S1 - T1)decay by an order of magnitude except in the case of p-difluorobenzene. It is suggested that a different mechnism of intersystem crossing from that in benzene occurs in substituted benzenes of lower symmetry. The relative change in non-radiative rate constant as a function of vibrational quantum number for all compounds studied upon excitation of the 1on progression was explicable in terms of the number of CH stretching frequencies available to the molecule as acceptor modes in the triplet state. © 1979.