The energy gap dependence of the radiationless transition rates in azulene and its derivatives

Abstract

The decay processes of the first and second excited singlet ??? S 1 and S 2, of azulene and some simple derivatives have been examined. The fluorescence S 2 → S 1, observed for the first time in azulene derivatives, allows a precise measurement of the substituent-induced variation of the S 2S 1 energy gap. The internal conversion (IC) rates are interpreted using the Englman-Jortner description of radiationless transitions in the weak coupling limit. The IC S 2-S 1 is the most important decay mechanism. The very small deuterium effect on the S 2 lifetime indicates that most of the energy is accepted by the CC skeletal stretching modes rather than the high-frequency CH stretching modes. This interpretation leads to parameters in the rate constant expression which are of the same order of magnitude as those for other aromatic hydrocarbons. A similar energy gap dependence is observed in the S 1 lifetimes of three carbonyl derivatives of azulene. The experimental data, however, cannot be satisfactorily interpreted with the Englman-Jortner model, because the matrix element connecting S 1 and S 2 is too large for the weak coupling limit. It is suggested that this is due to the coupling strength approaching the intermediate region, a conclusion which is supported by the strong temperature dependence of the emission quantum yield and by the very short S 1 lifetime previously reported.