Sensitized photo-redox reaction: VII. The Synthesis, FAB mass spectrum, stationary absorption, emission, transient absorption spectra, redox potential of dihydroxy-tin (IV)-mesoporphyrin dimethyl ester and its sensitized photo-reduction of methyl viologen

Abstract

In DMSO/water (4:1), photolysis of the dihydroxy-Sn (IV)-mesoporphyrin dimethyl ester (SnP)/methyl viologen (MV2+)/ethylene diamine tetraacetic acid (EDTA) ternary system produces methyl viologen cation radical with a quantum yield of 0.67, much higher than that of systems with other metal complexes of mesoporphyrin dimethyl ester. Neither EDTA nor MV2+ quenches the stationary fluorescence of SnP, implying that the reaction does not take place at the singlet state. With flash photolysis we obtain the T-T absorption spectrum of SnP (λmax 440 nm). By following the decay of this absorption, the triplet life time of SnP is estimated to be 41 μs. The life time is related to the concentration of either MV2+ or EDTA. Good linear relationships are obtained by plotting τ0τ vs. the concentration of MV2+ or EDTA (Stern-Volmer plot), from which we determine the quenching constants: kq(MV2+) =5.5 × 107 mol−1, s−1; kq (EDTA) =2.7 × 107 mol−1, s−1. The data suggests that upon photolysis of the above ternary system, both oxidative quenching and reductive quenching of the triplet state of the sensitizer are occurring. From the measured phosphorescence spectrum (λmax 704 nm) and the ground state redox, potentials (Ered1/2˜-0.84V, Eox1/2˜+1.43 V, vs. Ag/AgCl, KCl (sat.)), we obtain the redox potential of triplet SnP to be E(P+/P*T)˜-0.33 V, E(P*T+/P−)˜+0.92 V. Matching this data with the redox potential of MV2+ and EDTA, we establish the fact that during the photolysis of the SnP/MV2+/EDTA ternary system, both oxidative and reductive quenching are thermodynamically favorable processes. This is also the reason why the SnP sensitized reaction is much more efficient relative to other mesoporphyrin derivatives.