Direct measurement of S2 -- S0 fluorescence lifetimes and anisotropy of tetraphenylporphyrins

Various tetraphenylporphyrins (zinc, magnesium, free base) were excited to the upper electronic levels of the Soret band with the second harmonic of a mode-locked Ti-sapphire laser (394 nm). An up-conversion fluorescence set-up with the time resolution of 120 fs was used to measure the decay times of the S2 fluorescence in conjunction with the risetime of the S1 fluorescence. The depopulation of the excited electronic state S2 was studied as a function of the metal ion and the solvent. The lifetimes of the electronic S2 level, measured for ZnTPP and MgTPP in different solvents were (tau) equals 1.4 - 3.4 ps. The depopulation channel from S2 to S1 was studied by measuring simultaneously the decay of S2 and the rise of S1 fluorescence. The rate constant of the process can be correlated to the energy gap between the S2 and S1 levels, which depends on the nature of the metal ions and solvents. The rotational dynamics in the Soret band was also studied by measuring the anisotropy of S2 ---> S0 fluorescence. The anisotropy decay of S2 fluorescence was found to be biexponential, with a fast component around 100 fs and a slow one (t >> 10 ps), attributed to the partial dephasing of the degenerate energy levels of the S2 state and to rotational diffusion, respectively.