Redox-controlled photosensitization of nanocrystalline titanium dioxide.

Photosensitization of nanocrystalline titanium dioxide materials has been achieved by chemisorption of the pentacyanothiamineferrate(II) complex, which offers a relatively high redox potential that determines the photoelectrochemical properties of the photosensitized TiO(2). The adsorbed pentacyanoferrate complex binds to TiO(2) through the cyanide bridge and forms a new surface complex characterized by a metal-to-metal charge-transfer transition (MMCT) (Fe(II)-->Ti(IV)). The photosensitization can be observed only at low potentials at which Fe(II) moieties are present. Photocurrent switching between anodic and cathodic can be induced by varying either the photoelectrode potential or the wavelength of the incident light. Simple molecular modeling-together with spectroscopic and electrochemical measurements-allows the elucidation of the mechanism of the observed photoelectrochemical behavior.