Systematic investigation of the influence of methyl groups upon fluorescence parameters and the intersystem crossing rate constant of aromatic molecules.

The absorption and fluorescence properties of 29 specially selected methyl substituted benzene, naphthalene, anthracene, biphenyl, fluorene and p-oligophenylenes compounds are studied experimentally (at 293K) and quantum-chemically. The fluorescence quantum yield, gamma, and decay times, tau(f), for deaerated and non-deaerated solutions were measured. The oscillator strength, f(e), natural lifetime, tau(f)(0) and fluorescence and intersystem crossing rate constants, k(f) and k(ST), respectively, 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. For example, the symmetry and hence k(ST) and k(f) may change. As a result, gamma will also change. Steric hindrance, caused by the CH(3) group(s) 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) but a slight decrease in k(f). This effect is cumulative (more CH(3) groups lead to a greater decrease in gamma) and is attributable to the torsional vibrations of the methyl groups. Cases where introduction of CH(3) group(s) causes the inversion of (1)L(a) and T(beta) levels are observed and explained. Such inversion is accompanied by the increase in k(f) and sometimes, by the significant decrease in k(ST), which leads to a dramatic increase in the fluorescence quantum yield. Such changes improve laser properties or can convert a non-lasing compound to a lasing one. It is predicted that the CH(3) group(s) can cause the inversion of (1)L(a) and (1)L(a) or S(1)pipi) and T(1)pipi) levels, which can change the nature of fluorescence or can change a nonfluorescent compound to a fluorescent one. The trends observed may be used to change fluorescence properties of an aromatic compound in the necessary direction without changing its pi-system.