Influence of weak and strong donor groups on the fluorescence parameters and the intersystem crossing rate constant

The absorption and fluorescence properties of 35 specially selected methyl and stronger donor substituted benzene, naphthalene, biphenyl, anthracene and 2-azaanthracene compounds are studied experimentally (at 293 K) and quantum chemically. The fluorescence quantum yields, gamma, and decay times, tauf, for deaerated and non-deaerated solutions are measured. The oscillator strength, fe, natural lifetime, tauf 0 and fluorescence and intersystem crossing rate constants, kf and kST, are calculated for each compound. The orbital nature of the lowest excited singlet state, S1, is determined. The investigation shows that the introduction of methyl groups onto aromatic compounds may produce different effects. The symmetry and hence kST and kf may change. As a result, gamma will also change. Steric hindrance, possibly due to the CH3 group, will decrease kf while increasing kST. In cases where the introduction of the methyl group leaves the symmetry unchanged, there is a slight increase in kST and a slight decrease in kf. This effect is cumulative (more CH3 groups lead to a greater decrease in gamma) and can be explained by the torsional vibrations of the methyl groups. The introduction of strong donor groups usually produces dramatic changes. kST always increases, as does kf and the increase in kf is usually greater. Hence, gamma usually increases, sometimes dramatically. The nature of the S1 state changes from pipi* (for an aromatic molecule) to pil,pi*. There are three reasons for the observed increase in kST: (i) a decrease in symmetry; (ii) the internal heavy atom effect; and (iii) an improved mixing of the S(1pil,pi*) state with Ti states. It is also found that, in many cases, the effect of methyl and stronger donor groups on the fluorescence parameters and kST depends on the position of substitution, as well as changes in the molecular symmetry. The substituent groups have different effects on the p- and alpha-bands. The fluorescence parameters obtained and trends observed may be useful for different theoretical and practical purposes.