The dynamics and origin of the unrelaxed fluorescence of free-base tetraphenylporphyrin

Femtosecond studies of fluorescence of free-base tetraphenylporphyrin in solution were carried out using a homebuilt spectrometer which allows tracing the temporal evolution of the entire emission spectrum with the resolution of 150 fs, as well as the measurement of time-resolved fluorescence anisotropy. The S1–S0 (Qx) fluorescence was found to appear immediately after excitation, independent of the excitation wavelength. In addition, a shorter wavelength fluorescence was observed, decaying on a picosecond time scale. It was assigned to the population of the S2 (Qy) state. A model which can explain both the very fast rise of S1 emission and the picosecond decay of S2 was proposed: it assumes a reversible internal conversion between S2 and S1 states, modulated by relaxation from the higher vibronic levels of the latter.