Electron transfer photosensitized by a tin lipoporphyrin in solution, micelles, and at water—organic solvent interfaces

Abstract

Electron transfer photosensitized by a tin lipoporphyrin [Sn(IV) octakis((methoxycarbonyl-methyl)- meso-tetrakis-(((eicosanyloxy)carbonyl) phenyl)-porphyrin (SnLipoP)] is investigated under various solution conditions using a donor—SnLipoP—methylviologen (MV 2+) ternary system, where the donor is triethanolamine (TEA) or ethylenediaminetetraacetic acid (EDTA). The photoreaction of SnLipoP is compared with the photoreactions sensitized by common Sn porphyrins like tin protoporphyrin IX (SnPP) and octaethylporphyrin (SnOEP). A constant photoreaction rate is observed in a water/organic solvent (hexane, benzene) two-phase system in which the porphyrin (SnLipoP, SnOEP) is in the organic solvent and MV 2+ is in the aqueous phase. The rate is monitored by the change in the UV—visible absorption spectra produced by aqueous methylviologen radical MV ·+. In contrast with the two-phase system, macroscopically homogeneous solutions (aqueous SnPP and micellar solutions of SnLipoP, SnPP and SnOEP) give pseudo-logarithmic rates. These electron-transfer processes are completely consistent with reductive primary electron transfer to the tin porphyrin and optical shielding effects. Differences in the rates for SnLipoP and the other Sn porphyrins are explained by structural differences in the porphyrins. In particular, the structure of the porphyrin influences the phase in which the porphyrin resides, its location relative to interfacial regions, and the way it interacts with itself and other system components.