The influence of planarity, rigidity and internal heavy atom upon fluorescence parameters and the intersystem crossing rate constant in molecules with the biphenyl basis

Fluorescence properties of 10 specially chosen aromatic compounds with the biphenyl basis, differing in the degree of planarity, rigidity and in the nature of the internal heavy atom, but family-related in pi-structure are experimentally studied at room temperature (293K). The quantum yield of fluorescence, gamma, and fluorescence decay time, pi(f), of deaerated and non-deaerated cyclohexane solutions are measured. The oscillator strength, f(e), of S0-->S1 transition, fluorescence rate constant, k(f), natural lifetime, tau0(T), and intersystem crossing rate constant, k(ST), are calculated for each compound. It was observed that the increase in planarity and rigidity causes the k(ST) value to decrease very significantly. Any disturbance of the pi-system by the torsional vibrations of the substituted part of the molecule causes the k(ST) value to increase. The introduction of a heavy atom instead of the methylene bridge of fluorene leads to the increase in the k(ST) value and to the decrease in the f(e) of the 1A-->1La transition. The absence of laser action of fluorene is explained.