Regeneration of oxidized photosensitizers: Quantum-chemical portray of a general mechanism

The direct conversion of sun light to elec. power in mol. photoelectrochem. systems, aka Gratzel solar cells, is one of the promising "solar technologies" for utilization of abundant solar light as alternative energy source. Considering advantages of Gratzel solar cells over most com. counterparts made from silicon, this direction promise sustain ability and environmentally benign scalability to growing demands. Our DFT-calcns., which we would like to present, draw the connection between the formation of the I-I bond in I2- and the redox process of an (in-)org. donor-acceptor complex and provide electronic structure insight into all steps of a complete mechanism (see Scheme). Here we address dyes with the triphenylamine (TPA) moiety as the donor group and the cyanoacetic acid moiety as the acceptor group, as well as isolated TPA, 1-phenylindolene and Ru-dyes (N3 and analogs). A two step one-electron transfer process of the dye+ regeneration, where formation of a {dye+, I-} complex is the first, followed by coordination of a second iodide ion to the {dye+, I-} adduct affording a diiodide radical anion in {dye, I2-} is clearly portrayed by our calcns. The routes to regeneration of the organometallic dye+ via the binding of I- to the sulfur atom of the SCN group or to the bipyridyl moiety are highly analogous to the herein portrayed process of the redn. of the purely org. dye+. We thus explain the highly analogous performance of the ubiquitous redox couple based on iodine for purely org. sensitizers, as well as cyclometalate Ru-dyes with and without SCN group. Addnl., we characterized the electronic structure of a plausible charge-transfer complex, {dye+, I2-}, in which dye+ interacts with I2-. This unusual charge-transfer complex could be responsible for an alternative pathway of the one-step dye regeneration.