Time-resolved and static optical properties of vibrationally excited porphyrins

The effects of nuclear motion on the ground state and excited state optical spectra of porphyrins are examined in a number of experiments designed to generate excess vibrational energy within the macrocycle. These include time-resolved spectroscopic measurements following ultrafast radiationless transitions, and static measurements in the gas and in the condensed phase at various temperatures. The excess vibrational energy generated by highly exothermic radiationless transitions is found to induce significant red shifts in both the ground state absorption and excited state emission features. As the excess vibrational energy is dissipated on the time scale of about 10 ps, the optical features blue shift to their steady-state spectral positions. The red shifts found in the time-resolved spectra are also observed in the ground state absorption spectra of porphyrins in the gas phase at high temperature. We consider various mechanisms for the spectral shifts, including vibrationally induced reduction of the electronic energy gap by anharmonic expansion and/or vibronic coupling.