The excess-energy dependence of the photophysics of a number of meso-substituted anthracene derivatives has been studied in the isolated vapour phase. By use of a model involving singlet/ triplet energy-level inversion, and by comparison with results in the relaxed vapour phase, an explanation of these widely varying properties is proposed in terms of differing rates of vibrational redistribution and non-radiative decay processes in the triplet manifold. Fluorescence quantum yields and lifetimes have been measured in both phases and non-exponential decays for anthracene, 9-cyanoanthracene and 9,lO-diphenylanthracene in the relaxed vapour phase are reported.The photophysics of anthracene and its meso-substituted derivatives is similar to that of other polyacenes giving rise to a strongly fluorescent singlet ( m r * ) state upon optical excitation. The molecules are usually photochemically stable and in most cases it is found that within experimental error +f+ +isc = 1 .O,' demonstrating that in solution at least, the dominant n.r. deactivation process is intersystem crossing, i.s.c. The temperature and excess-energy dependences of the rates of these processes are of some interest and have been investigated in this work.In polyacenes of three or more rings, the ordering of the energy levels is ' L a < ' L h < ' B h with values of of ca. lo4, lo2 and lo5 dm3 cm-' mol-', respectively.' Consequently, the ' A -+ ' L h transition is often hidden under the ' A --+ ' L, band and, since the fluorescence rate constant, k,, is related approximately to the absorption-band intensity by the Strickler-Berg equation,2 the radiative properties of ' L b are not expected to be observed. In these molecules both temperature-dependent internal conversion, i.c., and i.s.c. processes have been and it is clear that two states must be involved, the highest being in a region that is thermally acccessible at the temperatures of interest. Activation energies of 500-2000cm-' have often been observed by measurement of changes in the fluorescence lifetime, Tf, with temperature, and in some cases this has been shown to lead to dual fluorescence owing to the thermal repopulation of Sz.
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