Electronic excitations of magnesium tetraphenylporphyrin in supersonic expansions

In this paper we report the results of our experimental study of the fluorescence excitation spectrum of Mg tetraphenylporphyrin (MgTPP) seeded in supersonic expansions of He. The spectra of the ’’isolated,’’ internally cold, bare MgTPP molecule reveal a well-resolved vibrational structure for the S0→S1 transition (the Q band), as well as for the S0→S2 transition (the Soret, B band). The vibrational structure in the S1 state reveals low-frequency, large amplitude motion, giving rise to two distinct even-parity vibrational progressions, with frequencies ω1 = 14.5 cm−1 and ω2 = 24.5 cm−1. Higher vibrational excitations in the S1 manifold of MgTPP are in good agreement with the fundamental frequencies of chlorophyll a. The spectroscopy of the Soret band of MgTPP provided information regarding the dynamics of interstate radiationless relaxation of low-lying vibrational excitations of the S2 state, which decay on the ps time scale.