The 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 lar ge extent, determined by the molecular symmetry. Numerous articles and mono graphs have been devoted to this problem. However, the influence of the mol ecular symmetry on fluorescence and, hence, laser parameters has not been f ully investigated. In this paper, the fluorescence and laser properties of 20 aromatic compounds are experimentally studied at room temperature. The c ompounds studied are arranged in family-related pi-structure pairs. In each pair, even-numbered compounds belong to a higher symmetry group than odd-n umbered compounds due to symmetrical substitution. All main fluorescence pa rameters such as quantum yield, gamma, decay time; tau(f), fluorescence rat e constant, k(f) (Einstein coefficient, A), and intersystem crossing rate c onstant, 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-phenylanth racene (C-2 symmetry) to 9,10-diphenylanthracene (D-2 symmetry) is accompan ied by an 18-fold decrease in the value of k(ST). This phenomenon is explai ned by the fact that, in a molecule of higher symmetry, not all triplet sta tes mix with the fluorescing S' state. It is also found that the symmetry o f a molecule greatly affects laser parameters such as the threshold of lase r action and the photochemical stability of a laser solution. It is observe d that the threshold for even-numbered compounds is much lower and the phot ochemical stability, in most cases, is much higher than for odd-numbered co mpounds. These phenomena are discussed and explained. (C) 2000 Elsevier Sci ence B.V. All rights reserved.