Photoinduced Electron Transfer Reactions in a Porphyrin−Viologen Complex: Observation of S₂ to S₁ Relaxation and Electron Transfer from the S₂ State

Abstract

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 S2, 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 S2 was different from that of the lowest excited-state S1. The S2 to S1 internal conversion could therefore be followed, and a S2 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 × 1012 s-1). This suggests that the photoinduced electron transfer from the excited porphyrin to the methyl viologen occurred directly from the S2 state of the porphyrin, before internal conversion to S1. The reverse electron transfer that followed regenerated the ground state reactants with a rate constant (1.3 ± 0.2) × 1012 s-1.