Ultrafast Quenching of the Xanthone Triplet by Energy Transfer:  New Insight into the Intersystem Crossing Kinetics

The formation and quenching of the triplet state of xanthone are studied by femtosecond techniques. As revealed by femtosecond fluorescence spectroscopy, the primarily excited 1ππ* state decays within 1.5 ps. In a transient absorption experiment, this time constant is associated with a partial rise of a triplet signature. This rise has a second and slower component with a time constant of 12 ps. In the presence of high concentrations of the quencher 1-methylnaphthalene, the slow 12 ps rise component is absent. This finding gives strong evidence that the biphasic rise of the triplet absorption of xanthone is due to a sequential mechanism, namely, a 1ππ* → 3 nπ* with fast intersystem crossing followed by a 3 nπ* → 3ππ* internal conversion. Furthermore, an analysis of the concentration dependence of the quenching kinetics allows one to pin down the intrinsic transfer time of the triplet energy from xanthone to 1-methylnaphthalene to ∼1 ps.