The dependence of the absorption and fluorescence parameters, the intersystem crossing and internal conversion rate constants on the number of rings in polyacene molecules

The absorption and fluorescence properties of five polyacenes, including hexacene, are studied both theoretically and experimentally (293 degrees C). Because of photochemical instability, hexacene was not studied before. The quantum yields of fluorescence, gamma, and the decay times, tau(f), of non-deaerated and deaerated cyclohexane solutions are measured. The oscillator strength, f(e), of (1)A --> L-1(b) and of (1)A --> L-1(a) transitions, the fluorescence rate constant, K-f, natural lifetime, tau(0)(T), intersystem crossing, K-ST, and internal conversion, K-S, rate constants are calculated. The singlet B-1(b), L-1(b), L-1(a) and triplet L-3(a) (77 degrees K) levels, Stokes shift, Delta upsilon(St), and full width at reciprocal 'e', FWRE, of the lowest absorption bands and fluorescence spectra are experimentally determined. Systems of singlet and triplet levels are simulated (PPP-CI) and analyzed. It is found that f(e) of S-0 --> S-p transition in the row of benzene-hexacene is steadily decreasing while f(e) of S-0 --> S-alpha transition and K-ST values are steadily increasing. FWRE of the p-band and of fluorescence spectrum beginning from anthracene are decreasing steadily to hexacene. The same is true for K-f, gamma, upsilon(00) and Delta upsilon(St) values. Parameters tau(f) and K-ST values behave irregularly. From benzene --> naphthalene K-ST decreases, then it increases significantly, by a factor of 45 (naphthalene --> anthracene). In the row anthracene --> tetracene --> pentacene it decreases and then again increases (pentacene --> hexacene). In the row of compounds benzene-hexacene tau(f) changes as 31.0 --> 96.4 --> 4.4 --> 6.0 --> 7.0 --> 1.5 ns. The irregular behaviour of K-ST value is explained by considering the change in the nature of S-1 state and in the position of B-3(b)(B-3(2u)+) level. The quantum yields of fluorescence, intersystem crossing and internal conversion of pentacene and hexacene are found to be 8, 76, 16, and 1, 94.5, 4.5% respectively. (C) 1997 Elsevier Science B.V.