Electron Transfer from Photoexcited TiO2 to Chelating Alizarin Molecules: Reversible Photochromic Effect in Alizarin@TiO2 under UV Irradiation

Reduction of alizarin molecules coupled to TiO2 nanoparticles (A@TiO2) occurs on UV irradiation in the presence of a sacrificial electron donor. Evidence is presented that reduction is mediated by conduction-band electrons and yields a 1,2,9,10-tetrahydroxyanthracene species which remains coupled to the TiO2 nanoparticles. The spectrum of the reduced complex displays two overlapping broad bands centred at 480 and 650 nm which can harvest visible photons besides 900 nm, in agreement with theoretical predictions by TDDFT. The potential relevance of the dual-redox behaviour of strongly TiO2 coupled anthraquinone dyes in the field of photocatalysis and in connection with their utilization in the development of dye-sensitized TiO2 solar cells is briefly discussed.