Ultrafast electron transfer in acceptor substituted bianthryl derivatives

Transient absorption spectra with subpicosecond laser excitation are reported for 9,9'-bianthryl (BA) and 9-anthryl-carbazole (C9A), their 10-cyano derivatives CBA and CC9A and for the 2-6-tert-butyl derivative of CC9A (TBCC9A). For BA and C9A, the rise time of the 690-nm absorption band assigned to the charge-transfer (CT) state, in about 20 ps in ethanol and 70 ps in butanol, is consistent with the average solvation time in these solvents. An initial fast loss of structure (less than or equal to 1 ps) in the gain spectra of BA is interpreted as due to the inertial component of the conformational relaxation within the locally excited (LE) state. A CT reaction involving both an intramolecular torsion coordinate and a solvation coordinate is proposed for BA and C9A. Quite different to the unsubstituted compounds, the CT process is found to occur during the 500-fs excitation pulse in CBA, CC9A and TBCC9A in both ethanol and butanol, The key parameter differentiating the systems is proposed to be the relative energy of the CT state with respect to the LE state, the substitution leading to a strong overlap of the two states in the Franck-Condon transition. On the other hand, diffusive solvation effects and torsion within the instantaneously populated CT state are discussed as possible causes for the picosecond-time-scale evolution of the transient spectra observed for the cyano derivatives. (C) 2000 Elsevier Science B.V. All rights reserved.