Mechanism and deactivation kinetics of S2-xanthione in acetonitrile, a quenching solvent, and of S2-exciplex measured by pico- and femtosecond laser spectroscopy

The Letter discusses the mechanism and dynamics of intra- and intermolecular electronic relaxation of the S-2 state of a xanthione molecule in acetonitrile, a quenching solvent, as well as the most important properties of the S-2-exciplex formed upon the quenching process. Despite very efficient quenching of S-2-XT and other thiones, it has not been possible so far to observe the S-2-exciplex. Therefore transient absorption system with 120 fs resolution was successfully, applied for this purpose: the S-2-exciplex was identified for the first time, its lifetime (similar to 30 ps) and transient absorption spectrum were determined as well as the mechanism of its formation and decay. Moreover, the authors observed the presence of a very fast intramolecular electronic relaxation process (S-2(v>0)-->S-0) during S-2-state deactivation which competes with intramolecular vibrational redistribution in the S-2-state. (C) 2001 Elsevier Science B.V. All rights reserved.