Systematic investigation of absorption, fluorescence and laser properties of some p- and m-oligophenylenes

Absorption, fluorescence and laser properties of ten selected aromatic compounds from the oligophenylene family are studied experimentally at room temperature (293 K). The first eight compounds are arranged in such way that odd numbered compounds reveal L-1(b) --> (1)A fluorescence, while even numbered compounds show L-1(a) --> (1)A fluorescence. All compounds are family related in pi-structure and are of the same degree of planarity and rigidity. The quantum yield of fluorescence, gamma, and the decay times, tau(f), of non-deaerated and deaerated cyclohexane solutions are measured. The oscillator strength, f(e), the fluorescence rate constant, K-f, natural lifetimes, tau(o)(T), and intersystem crossing rate constant, K-ST, are calculated, The lowest L-1(b) and L-1(a) singlet and L-3(a) tripler (77 K) levels are determined. Investigations showed that transition from a polyphenyl molecule which shows L-1(a) --> (1)A fluorescence to a family related in the pi-structure molecule which reveals the L-1(b) --> (1)A fluorescence is accompanied by certain changes in all the fluorescence parameters. This indicates that gamma decreases, tau(f) increases, K-f and the FWRE of the fluorescence spectrum decrease. Moreover, K-ST also decreases, sometimes very significantly. The decrease in the K-ST value is explained by the fact that matrix elements of the spin-orbit coupling of the S-alpha and T-i states are much lower in value than analogous elements of the spin-orbit coupling of S-p and T-i states. It is shown that all p-polyphenyles exhibit excellent laser action, while m-polyphenyles do not produce laser oscillation under any conditions. The values of K-ST and other fluorescence parameters measured can be used for various practical purposes and theoretical considerations. (C) 2000 Elsevier Science B.V. All rights reserved.