Photoinduced Electron Transfer between a Carotenoid and\nTiO₂ Nanoparticle

Abstract

The dynamics of photoinduced electron injection and recombination between all-<i>trans</i>-8‘-apo-β-caroten-8‘-oic acid (ACOA) and a TiO₂ colloidal nanoparticle have been studied by means of transient\nabsorption spectroscopy. We observed an ultrafast (∼360 fs) electron injection from the initially excited S₂\nstate of ACOA into the TiO₂ conduction band with a quantum yield of ∼40%. As a result, the ACOA^•+\nradical cation was formed, as demonstrated by its intense absorption band centered at 840 nm. Because\nof the competing S₂−S₁ internal conversion, ∼60% of the S₂-state population relaxes to the S₁ state. Although\nthe S₁ state is thermodynamically favorable to donate electrons to the TiO₂, no evidence was found for\nelectron injection from the ACOA S₁ state, most likely as a result of a complicated electronic nature of the\nS₁ state, which decays with a ∼18 ps time constant to the ground state. The charge recombination between\nthe injected electrons and the ACOA^•+ was found to be a highly nonexponential process extending from\npicoseconds to microseconds. Besides the usual pathway of charge recombination forming the ACOA\nground state, about half of the ACOA^•+ recombines via the ACOA triplet state, which was monitored by its\nabsorption band at 530 nm. This second channel of recombination proceeds on the nanosecond time scale,\nand the formed triplet state decays to the ground state with a lifetime of ∼7.3 μs. By examination of the\nprocess of photoinduced electron transfer in a carotenoid-semiconductor system, the results provide an\ninsight into the photophysical properties of carotenoids, as well as evidence that the interfacial electron\ninjection occurs from the initially populated excited state prior to electronic and nuclear relaxation of the\ncarotenoid molecule.