Excited state relaxation paths in 9,9 '-bianthryl and 9-carbazolyl-anthracene: a sub-ps transient absorption study

Transient absorption sprectra with sub-picosecond resolution are reported f or 9,9'-bianthryl (BA) and 9-carbazolylanthracene (C9A) in ethanol, n-butan ol and diethylether, from 320 to 800 nm. Ground-state bleaching and excited -state absorption (ESA) are seen in the UV region. In alcohols, the ESA dec ay (approximate to 20 ps in ethanol and approximate to 70 ps in butanol) ma tches the rise time of the well-known charge-transfer (CT) band in the visi ble, with a time constant comparable to the average solvation time. An addi tional change is observed for BA in the fluorescence region (400-480 nm), c haracterized by the disappearance of a structured gain contribution in less than 1 ps. This is also observed in diethylether, although the subsequent evolution is different. This ultrafast process, which is attributed to the unhindered torsional relaxation of BA out of the Franck-Condon geometry, is less clear for C9A. However, for both compounds in alcohols, the CT state formation is a second step and is more than one order of magnitude slower. In diethylether, unhindered and diffusive conformational changes are invoke d. The data support a two-coordinate (internal twisting and solvent motion) relaxation model for BA, the relative importance of these coordinates bein g strongly solvent dependent. A similar model is proposed for C9A. (C) 2000 Elsevier Science B.V. All rights reserved.