Photoinduced electron transfer reactions in a porphyrin-viologen complex:: Observation of S2 to S1 relaxation and electron transfer from the S2 state

Zinc-meso-tetrasulfonatophenyl-porphyrin (ZnTPPS4-) spontaneously forms a complex with methyl viologen (MV2+) in aqueous solutions. Both the free porphyrin and its complex with viologen were studied by pump-probe spectroscopy. Excitation in the porphyrin Soret band with a short laser pulse (fwhm = 200 fs) populated the upper excited-state S-2, and the subsequent processes were followed by the transient absorbance changes involved. For the free ZnTPPS,(4-) the transient absorption spectrum of the upper excited-state S-2 was different from that of the lowest excited-state S-1. The S-2 to S-1 internal conversion could therefore be followed, and a S-2 lifetime of 1.3 +/- 0.2 ps was determined. In the ZnTPPS4--MV2+ complex, excitation in the Soret band produced spectral changes attributed to the charge separated state ZnTPPS3--MV+, with a rate limited by the laser pulse (rate constant >5 x 10(12) s(-1)). This suggests that the photoinduced electron transfer from the excited porphyrin to the methyl viologen occurred directly from the S-2 state of the porphyrin, before internal conversion to S-1. The reverse electron transfer that followed regenerated the ground state reactants with a rate constant (1.3 +/- 0.2) x 10(12) s(-1).