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, tau(f), for deaerated and non-deaerated solutions are measured. The oscillator strength, f(e), natural lifetime, tau(f)(0) and fluorescence and intersystem crossing rate constants, k(f) and k(ST), are calculated for each compound. The orbital nature of the lowest excited singlet state, S-1, is determined. The investigation shows that the introduction of methyl groups onto aromatic compounds may produce different effects. The symmetry and hence k(ST) and k(f) may change. As a result, gamma will also change. Steric hindrance, possibly due to the CH3 group, will decrease k(f) while increasing k(ST). In cases where the introduction of the methyl group leaves the symmetry unchanged, there is a slight increase in k(ST) and a slight decrease in k(f). 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. k(ST) always increases, as does kf and the increase in k(f) is usually greater. Hence, gamma usually increases, sometimes dramatically. The nature of the S-1 state changes from pipi* (for an aromatic molecule) to pi/pi*. There are three reasons for the observed increase in k(ST): (i) a decrease in symmetry; (ii) the internal heavy atom effect; and (iii) an improved mixing of the S1(pi/pi*) state with T-i states. It is also found that, in many cases, the effect of methyl and stronger donor groups on the fluorescence parameters and k(ST) 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. (C) 2002 Elsevier Science B.V. All rights reserved.