Abstract

The absorption and fluorescence properties of 29 specially selected methyl substituted benzene, naphthalene, anthracene, biphenyl, fluorene and p-oligophenylenes compounds are studied experimentally (at 293 K) and quantum-chemically. The fluorescence quantum yield, γ, and decay times, τ f , for deaerated and non-deaerated solutions were measured. The oscillator strength, f e , natural lifetime, τ 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, γ 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 γ) 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 β 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 π π * 1 and T n π * 1 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 π-system.

Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call