Picosecond studies of the intramolecular relaxation processes in isolated C60 and C70 molecules

Abstract

Time-resolved fluorescence spectra for C60 molecules embedded in Ne and Ar matrices and C70 in Ne matrices were recorded using a synchroscan streak camera. In the case of C60, the decay times of the S1 state are identical for all fluorescence bands confirming that it is a state of mixed T1g, T2g, and Gg characters. Its lifetime is determined by the intersystem crossing rate to the triplet manifold which increases in going from Ar to Ne matrices. This is attributed to a change of the dominant symmetry character of S1 in the two matrices. A transient, short-lived (∼170 ps in Ne, ∼70 ps in Ar), S3 fluorescence of dominant Gg symmetry is also observed, which feeds the S2/S1 population (of dominant T2g and T1g symmetry). Using the Energy Gap Law, a change of the S3→S2/S1 decay rate from Ne to Ar matrices is attributed to the different number of phonons needed to bridge the gap between these states and/or to an environment-dependent change of the coupling matrix element. In C70, the fluorescence from both S1 and S2 are populated at the same rate despite the 165 cm−1 energy gap between them. This is explained in terms of an intersystem crossing from the triplet state manifold. Finally, S2 decays predominantly to S1, while the latter decays to the triplet states by reversible intersystem crossing. These conclusions are confirmed by a simple kinetic model.