Dependence of the fluorescence properties, laser properties and photochemical stability of aromatic compounds on the molecular symmetry

It is known that the spectral properties of organic compounds are, to a large extent, determined by the molecular symmetry. Numerous articles and monographs have been devoted to this problem. However, the influence of the molecular symmetry on fluorescence and, hence, laser parameters has not been fully investigated. In this paper, the fluorescence and laser properties of 20 aromatic compounds are experimentally studied at room temperature. The compounds studied are arranged in family-related π-structure pairs. In each pair, even-numbered compounds belong to a higher symmetry group than odd-numbered compounds due to symmetrical substitution. All main fluorescence parameters such as quantum yield, γ, decay time, τ_f, fluorescence rate constant, k_f (Einstein coefficient, A), and intersystem crossing rate constant, k_ST, are measured or calculated. It has been found that for most of the symmetrically substituted molecules, the value of k_ST, decreases, sometimes very significantly. For example, the transition from 9-phenylanthracene (C₂ symmetry) to 9,10-diphenylanthracene (D₂ symmetry) is accompanied by an 18-fold decrease in the value of k_ST. This phenomenon is explained by the fact that, in a molecule of higher symmetry, not all triplet states mix with the fluorescing S¹ state. It is also found that the symmetry of a molecule greatly affects laser parameters such as the threshold of laser action and the photochemical stability of a laser solution. It is observed that the threshold for even-numbered compounds is much lower and the photochemical stability, in most cases, is much higher than for odd-numbered compounds. These phenomena are discussed and explained.