Showing papers by "Gagik G. Gurzadyan published in 1999"

Direct measurement of S 2 -- S 0 fluorescence lifetimes and anisotropy of tetraphenylporphyrins

Abstract: 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 S 2 fluorescence in conjunction with the risetime of the S 1 fluorescence. The depopulation of the excited electronic state S 2 was studied as a function of the metal ion and the solvent. The lifetimes of the electronic S 2 level, measured for ZnTPP and MgTPP in different solvents were (tau) equals 1.4 - 3.4 ps. The depopulation channel from S 2 to S 1 was studied by measuring simultaneously the decay of S 2 and the rise of S 1 fluorescence. The rate constant of the process can be correlated to the energy gap between the S 2 and S 1 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 S 2 ---> S 0 fluorescence. The anisotropy decay of S 2 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 S 2 state and to rotational diffusion, respectively.