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

Abstract

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, γ, and decay times, τ f, for deaerated and non-deaerated solutions are measured. The oscillator strength, f e, natural lifetime, τ 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, γ will also change. Steric hindrance, possibly due to the CH 3 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 CH 3 groups lead to a greater decrease in γ) 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 k f and the increase in k f is usually greater. Hence, γ usually increases, sometimes dramatically. The nature of the S 1 state changes from ππ * (for an aromatic molecule) to πl, π *. 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 S 1πl, π* 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 α-bands. The fluorescence parameters obtained and trends observed may be useful for different theoretical and practical purposes.