Ultrafast quenching of the excited S2 state of benzopyranthione by acetonitrile

Femtosecond and nanosecond transient absorption and picosecond time-correlated single photon counting techniques have been used to study the mechanism and dynamics of the efficient quenching of an aromatic thioketone, 4H-1-benzopyrane-4-thione (BPT) in the S2 state, by acetonitrile. The results suggest the involvement of two aborted processes in the quenching mechanism: exciplex formation and hydrogen abstraction. The occurrence of the latter process is supported by the observation of a clear isotope effect on going from acetonitrile to deuterated acetonitrile.