Titanium(IV) complexes as direct TiO2 photosensitizers

Titanium dioxide photosensitization may be achieved in various ways, involving surface modification with appropriate species. The photosensitization process requires a visible light-induced electron or hole injection into conduction or valence band, respectively. Efficiency of this process depends on electronic interaction between the photosensitizer moiety (surface complex) and TiO₂ particle. At least two types of the charge injection mechanisms may be distinguished—in the first, charge is transferred from the excited state of the sensitizer molecule to the conduction or valence band while the second mechanism involves a direct molecule-to-band charge transfer (MBCT). The MBCT process can be realized by surface titanium(IV) complexes with various organic and sometimes inorganic ligands. Catechol, phthalic acid or salicylic acid derivatives, as well as cyanometallate anions, upon chemisorption at TiO₂ surface constitute an especially interesting group of ligands to yield various titanium(IV) surface complexes. Geometry of these complexes, electronic structures and possibility of their use as photosensitizers of TiO₂ are discussed on the basis of experimental data and quantum-chemical modeling. Also prospective applications of photoinduced electron transfer and photocatalytic activity of such systems are presented.