Ultrafast Dynamics of Porphyrins in the Condensed Phase: II. Zinc Tetraphenylporphyrin
Femtosecond spectroscopic studies of zinc tetraphenylporphyrin (ZnTPP) in benzene and dichloromethane are reported, combining both fluorescence up-conversion and transient absorption measurements. The purpose is to investigate the initial electronic and vibrational relaxation of the S_1 and S_2 excited states, in a system in which interference from solvent rearrangement is insignificant as evidenced by the small Stokes shift in the fluorescence. Excitation of the low-lying singlet excited state (S_1) results in nanosecond relaxation, while excitation to S_2, the Soret band, leads to multiple electronic and vibrational relaxation time scales of S_2 and S_1 populations, from hundreds of femtoseconds to tens of picoseconds. The systematic and detailed studies reported here reveal that the Soret fluorescence band decays with a lifetime in benzene of 1.45 ps for excitation at 397 nm, while emission monitored at the same wavelength, but for two-photon 550 nm excitation, decays biexponentially with 200 fs and 1.0 ps time constants. In addition, the Soret fluorescence decay lifetime for 397 nm excitation is distinctly longer than the rise time of S_1 fluorescence for the same excitation, which varies with wavelength. These observations are consistent with the model presented here in which the Soret band structure consists of absorption from S_0 to two manifolds of states with distinct electronic and vibrational couplings to S_1 and higher electronic states. To compare with literature, we also measured the S_2 lifetime in dichloromethane and found it to be 1.9 ps, a lengthening from its value in benzene. However, the transient fluorescence intensity is greatly reduced. These observations in dichloromethane provide evidence of an ultrafast (<100 fs) channel for electron transfer from ZnTPP to dichloromethane for a subset of excited molecules in favorably oriented contact with the solvent, that is, a bifurcation of population. Finally, solvent-induced vibrational relaxation of the S_1 population following internal conversion from S_2 occurs over a range of time scales (picoseconds to tens of picoseconds) depending on the wavelength (fluorescence or transient absorption), and the observed rate indeed changes with solvent.