Photoinduced electron transfer between a carotenoid and TiO2 nanoparticle.

The dynamics of photoinduced electron injection and recombination between all-trans-8'-apo-beta-caroten-8'-oic acid (ACOA) and a TiO(2) colloidal nanoparticle have been studied by means of transient absorption spectroscopy. We observed an ultrafast ( approximately 360 fs) electron injection from the initially excited S(2) state of ACOA into the TiO(2) conduction band with a quantum yield of approximately 40%. As a result, the ACOA(*)(+) radical cation was formed, as demonstrated by its intense absorption band centered at 840 nm. Because of the competing S(2)-S(1) internal conversion, approximately 60% of the S(2)-state population relaxes to the S(1) state. Although the S(1) state is thermodynamically favorable to donate electrons to the TiO(2), no evidence was found for electron injection from the ACOA S(1) state, most likely as a result of a complicated electronic nature of the S(1) state, which decays with a approximately 18 ps time constant to the ground state. The charge recombination between the injected electrons and the ACOA(*)(+) was found to be a highly nonexponential process extending from picoseconds to microseconds. Besides the usual pathway of charge recombination forming the ACOA ground state, about half of the ACOA(*)(+) recombines via the ACOA triplet state, which was monitored by its absorption band at 530 nm. This second channel of recombination proceeds on the nanosecond time scale, and the formed triplet state decays to the ground state with a lifetime of approximately 7.3 micros. By examination of the process of photoinduced electron transfer in a carotenoid-semiconductor system, the results provide an insight into the photophysical properties of carotenoids, as well as evidence that the interfacial electron injection occurs from the initially populated excited state prior to electronic and nuclear relaxation of the carotenoid molecule.