Surface modification of TiO2 nanoparticles with carotenoids. EPR study

EPR measurements demonstrate efficient charge separation on carotenoid-modi fied titanium dioxide nanoparticles (7 nn). Strong complexation of caroteno ids containing terminal carboxy groups (-CO2H) with the TiO2 surface leads to electron transfer from the adsorbed carotenoid molecule to the surface t rapping site. For these systems, EPR signals of the carotenoid radical cati ons Car(.+) and the electrons trapped on the TiO2 are observed before irrad iation (77 K). Their UV-visible spectra show an absorption band with a maxi mum near 650 nm that is characteristic of the trapped electrons. Surface mo dification of the TiO2 by other carotenoids results in the formation of a c omplex with an optical absorption band near 545 nm. These systems form char ge-separated pairs [Car(.+)... TiO2(e(tr)(-))(surf). TiO2(e(tr)(-))(latt.)] only upon 365-600 nm illumination at 77 K. Complexation of the TiO2 colloi ds with carotenoids enhances spatial charge separation, shifts the absorpti on threshold; into the visible region, and thus greatly improves the reduci ng ability of the semiconductor. Photoreduction of acceptor molecules such as 2,5-dichloro-1,4-benzoquinone, nitrobenzene, and oxygen is demonstrated.