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 S2 state of a xanthione molecule in acetonitrile, a quenching solvent, as well as the most important properties of the S2-exciplex formed upon the quenching process. Despite very efficient quenching of S2-XT and other thiones it has not been possible so far to observe the S2-exciplex. Therefore transient absorption system with 120 fs resolution was successfully applied for this purpose: the S2-exciplex was identified for the first time, its lifetime (∼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 (S2v>0→S0) during S2-state deactivation which competes with intramolecular vibrational redistribution in the S2-state.