Femtosecond fluorescence spectroscopy of higher excited states

G. Gurzadyan , T. Gustavsson et T.-H. Tran-Thi

Porphyrins are among the few biological compounds showing two visible fluorescence bands assigned to the S2 ---> S0 (420 - 440 nanometers) and S1 ---> S0 (550 - 650 nanometers) transitions. Due to the very low fluorescence quantum yields (10-4) and the extremely short radiative lifetimes (a few picoseconds) of the S2 ---> S0 transition, very few experimental studies have been consecrated to the photophysical properties of the of the of the S2 state, and none treats the non-radiative relaxation from the S2 to the S1 state. Within the framework of a collaboration with Armenia (G. Gurzadyan, Armenian Academy of Sciences, Yerevan, Armenia) we have undertaken a study of the depopulation of the higher excited states (SN,S2) of several tetraphenylporphyrins with different central metal atoms (Gurzadian 3-25, 3-26).

In order to shed some new light on the details of the S2 state depopulation we have used the specially designed femtosecond lase-rinduced fluorescence upconversion spectrometer previously described. Following the decay of the S2 ---> S0 fluorescence at 430 nm at the same time as the rise of the S1 ---> S0 transition at 600 nm, we have for the first time managed to show that the S2 state disappearance is perfectly correlated with the appearance of the S1 state. This result shows that the non-radiative S2 ---> S1 relaxation mainly cascades through extremely short-lived higher vibrational levels of the S1 state.

The analysis of the short-time behavior of the S2 ---> S0 fluorescence indicates the presence of more complex processes. Firstly, one can observe an ultrafast rise in the S2 fluorescence which we assign tentatively to vibrational relaxation in the S2 state. Secondly, the fluorescence anisotropy decay displays a bi-exponential form, a fact which allows us to unveil a coherence in the upper excited state, implying that it should rather be described as a manifold of vibronic levels belonging to two or more elec-tronic states (SN,S2). The initial abnormally high value of the fluorescence anisotropy, r0 > 0,7, is interpreted as due to a electronically degenerated S2 state, and the rapid decay time (t1 = 0,2 picoseconds) corresponds to the dephasing of the coherently excited degenerate levels. The slow component in the anisotropy decay (t2 = 10 pico-seconds) is attributed to rotational diffusion.

Fluorescence decay of the S2 ---> S0 transition and fluorescence rise of the S1 ---> S0 transition of the ZnTPP porphyrin in ethanolic solution. The two curves can be described by a mono-exponential decay and rise with the same characteristic time, t = 2 .35 ps.