Effects of molecular organization on photophysical behavior. 2. Photoelectrochemical and photocurrent quantum yield studies of the Langmuir-Blodgett monolayers of some surfactant porphyrins

The photoelectrochemistry of two new groups of surfactant carboxyporphyrins is described. The surface active porphyrins deposited onto a SnO2 semiconductor surface exhibit an unusual photoelectrochemical effect: a substantial and stable photocurrentlphotovoltage is generated in modified photoelectrode cells that possess porphyrins with electron-rich side chains. This is interpreted as resulting from enhanced electron injection at the illuminated interface, resulting from an enhanced through bond electron tunneling in these chains. The estimated electron-transfer rate constants (k(et)) obtained from fluorescence quenching measurements roughly agreed with the photocurrent quantum yields for these model compounds. All the model porphyrins behaved identically as a function of pH. An enhanced photocurrent quantum yield was exhibited by all the compounds on diluting their monolayers with dioleoylphosphatidylcholine, a nonfluorophore surface active compound. This is interpreted as resulting from minimization of the aggregate-mediated quenching, which enhances interfacial isoenergetic electron transfer and hence increases the photocurrent yield.