Photoinduced Electron Transfer between a Carotenoid and TiO2Nanoparticle

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Abstract
The dynamics of photoinduced electron injection and recombination between all-trans-8‘-apo-β-caroten-8‘-oic acid (ACOA) and a TiO2 colloidal nanoparticle have been studied by means of transient absorption spectroscopy. We observed an ultrafast (∼360 fs) electron injection from the initially excited S2 state of ACOA into the TiO2 conduction band with a quantum yield of ∼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 S2−S1 internal conversion, ∼60% of the S2-state population relaxes to the S1 state. Although the S1 state is thermodynamically favorable to donate electrons to the TiO2, no evidence was found for electron injection from the ACOA S1 state, most likely as a result of a complicated electronic nature of the S1 state, which decays with a ∼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 ∼7.3 μs. 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.