Ultrafast Dynamics of Porphyrins in the Condensed Phase: I. Free Base Tetraphenylporphyrin

Abstract

With femtosecond resolution, using fluorescence up-conversion and transient absorption, we have carried out measurements on free base tetraphenylporphyrin (H2TPP) in benzene solution, pumping with ∼1300 cm-1 of excess vibrational energy in each of the Soret, Qy, and Qx bands, and also pumping the lowest vibrational band of Qy. From these studies, made for different excitations and at different detection wavelengths, we provide a model for describing the elementary intramolecular processes in the Soret, Qy, and Qx electronic manifolds, with the following order of time scales and couplings: electronic (femtosecond), vibrational (femtosecond−picosecond), and singlet−triplet (nanosecond). These dynamical electronic and vibrational relaxation pathways in a molecule with small dipole in nonpolar solvents can be studied without interference from solvent reorganization, as indicated by the small Stokes shift of fluorescence. Vibrationally excited Soret → {Qy,Qx} and Qy → Qx electronic relaxation occurs in less than 100 fs, within our resolution, as evidenced by the immediate rise of Qx fluorescence after Soret (397 nm) and Qy (514 and 550 nm) excitation. There are generally three distinguishable ultrafast relaxation time scales within the Qx state, which are assigned to intra- and intermolecular vibrational relaxation processes leading to thermal equilibrium in Qx, the lowest excited singlet state. The measured time scales are as follows: 100−200 fs for intramolecular vibrational energy redistribution, 1.4 ps for vibrational redistribution caused by elastic collision with solvent molecules, and 10−20 ps for thermal equilibration by energy exchange with the solvent. Decay of the equilibrated Qx population occurs on the nanosecond time scale by intersystem crossing to the triplet state.