Complex and charge transfer between TiO2 and pyrroloquinoline quinone.

Pyrroloquinoline quinone (PQQ) forms a tridentate complex with coordinatively unsaturated titanium atoms on the surface of approximately 4.5 nm TiO2 particles; an association constant of K = 550 M-1 per Ti(IV)surf has been determined. Low-temperature electron paramagnetic resonance was employed in identification of localized charges and consequently produced radicals and in determination of charge-transfer processes. The photoexcitation of the PQQ-TiO2 complex results in the transfer of conduction band electrons from TiO2 to bound PQQ and the formation of the semiquinone radical. Attaching dopamine (DA) as an electron donor and PQQ as an electron acceptor on the surface of TiO2 results in spatial separation of photogenerated charges; the holes localize on dopamine and electrons on PQQ, with higher yields than for each component separately. In this triad-type assembly (PQQ-TiO2/DA) the PQQ that is bound to the particles acts as a sink for electrons allowing their almost complete scavenging even at temperature as low as 4 K.