The influence of symmetrical substitution on fluorescence parameters and the intersystem crossing rate constant in aromatic molecules

Fluorescence properties of 36 specially chosen aromatic compounds are experimentally studied at a temperature of 293 K. The compounds are arranged in pairs so that even numbered compounds, because of symmetrical substitutions, are designated a higher symmetry group than those of odd numbered compounds. The pairs are, however, family-related in pi-structure. The quantum yield of fluorescence gamma, and decay times tau(f), of deaerated and nondeaerated cyclohexane solutions are measured. The oscillator strength f(e), the fluorescence rate constants K-f, natural lifetimes tau(0)(T) and intersystem crossing rate constants K-st, are calculated. Investigations showed that transition from a lower to higher symmetry, but family-related in pi-structure molecule depending on the nature of the S-1 excited state, has different effects on parameters such as gamma, tau(f), f(e), K-f and K-st. If the S-1 excited state has the nature of the S-P(La-1) state, then the value of nu(00) decreases. This is accompanied by an increase in the values of gamma and tau(f). Parameters f(e) and K-f may change unpredictably. However, if the development of pi-structure occurs in the direction of polarization of the S-0-->S-1 transition, the values of f(e) and consequently K-f are also found to increase. If the S-1 excited state has the nature of the S-alpha((1)Lb) state then generally, gamma, f(e) and K-f are found to decrease, with tau(f) increasing. Furthermore, the K-st value in most cases of symmetrical substitution, decreases, sometimes very significantly, though the energy gap between S-1 and T-1 states becomes less. For example, the transition from 9-phenylanthracene (C-2 symmetry group) to 9,10-diphenylanthracene (D-2 symmetry group) is accompanied by a decrease in the value of K-st, by a factor of 18. This phenomenon is qualitatively treated using the first-order spin-orbit coupling theory and simulation of singlet and triplet levels.