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 (ZnTPPS^4-) 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 ZnTPPS^4--MV²⁺ complex, excitation in the Soret band produced spectral changes attributed to the charge separated state ZnTPPS^3--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.