Photochemical reactivity in organized assemblies. 29. Photoreactions and photoelectrochemical effects in supported multilayer assemblies. Mechanisms of interfacial reactions involving reactive supports, metalloporphyrin surface films, and solutions

Abstract

An investigation of photoprocesses occurring in molecular assemblies consisting of monolayers and multilayers of water-insoluble surfactant tin porphyrins supported on tin oxide optically transparent electrodes and other potentially reactive surfaces in contact with aqueous solutions containing potentially reducing solutes is reported. In each case the light-absorbing species is the surface-bound tin porphyrin. While irradiation of glass- or quartz-supported monolayers and multilayers of the tin porphyrins in contact with aqueous reductants such as amines, hydroquinone, of p-dimethoxybenzene leads to net photoreduction and photoaddition processes for the porphyrin to yield chlorin- and isobacteriochlorin-type products, the net reactivity of the porphyrin is totally quenched or sharply reduced when it is contained in assemblies in contact with n-type SnO/sub 2/, PtO/sub 2/, or platinum films. Several experiments were carried out which demonstrate that excited states of the porphyrin are not strongly quenched by the various supports; thus in this case the major effect of the surface is to intercept reduced species formed in the interfacial photoreaction of the porphyrins with the reducing solute. The SnO/sub 2/-supported porphyrin assemblies can serve as photoanodes for the oxidation of solution reductants; in typical experiments slides having areas ca. 3 cm/sup 2/ give photocurrents in the approx.300-nA rangemore » with a quantum efficiency of 0.3%. The low quantum efficiency observed is comparable to that measured for interfacial or homogeneous-solution reduction of the tin porphyrin by the same solutes; thus the net reactivity of the porphyrin in solution and at interfaces appears relatively comparable in these cases and is probably limited by net physical quenching of the porphyrin and triplet by the solute reductant.« less