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

Absorption, fluorescence and laser properties of ten selected aromatic comp ounds from the oligophenylene family are studied experimentally at room tem perature (293 K). The first eight compounds are arranged in such way that o dd numbered compounds reveal L-1(b) --> (1)A fluorescence, while even numbe red compounds show L-1(a) --> (1)A fluorescence. All compounds are family r elated 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 oscill ator 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 d etermined. Investigations showed that transition from a polyphenyl molecule which shows L-1(a) --> (1)A fluorescence to a family related in the pi-str ucture molecule which reveals the L-1(b) --> (1)A fluorescence is accompani ed by certain changes in all the fluorescence parameters. This indicates th at gamma decreases, tau(f) increases, K-f and the FWRE of the fluorescence spectrum decrease. Moreover, K-ST also decreases, sometimes very significan tly. The decrease in the K-ST value is explained by the fact that <Psi(S al pha)/H-SO/Psi(Ti)> matrix elements of the spin-orbit coupling of the S-alph a and T-i states are much lower in value than analogous elements of the spi n-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 theoret ical considerations. (C) 2000 Elsevier Science B.V. All rights reserved.