THE DEPENDENCE OF THE ABSORPTION AND FLUORESCENCE PARAMETERS, THE INTERSYSTEM CROSSING AND INTERNAL-CONVERSION RATE CONSTANTS ON THE NUMBEROF RINGS IN POLYACENE MOLECULES

The absorption and fluorescence properties of five polyacenes, includi ng hexacene, are studied both theoretically and experimentally (293 de grees C). Because of photochemical instability, hexacene was not studi ed before. The quantum yields of fluorescence, gamma, and the decay ti mes, 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 convers ion, 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 up silon(St), and full width at reciprocal 'e', FWRE, of the lowest absor ption bands and fluorescence spectra are experimentally determined. Sy stems of singlet and triplet levels are simulated (PPP-CI) and analyze d. It is found that f(e) of S-0 --> S-p transition in the row of benze ne-hexacene is steadily decreasing while f(e) of S-0 --> S-alpha trans ition and K-ST values are steadily increasing. FWRE of the p-band and of fluorescence spectrum beginning from anthracene are decreasing stea dily to hexacene. The same is true for K-f, gamma, upsilon(00) and Del ta upsilon(St) values. Parameters tau(f) and K-ST values behave irregu larly. 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-h exacene 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 considerin g 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 cross ing 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.