Mechanisms of Electron Injection from Retinoic Acid and Carotenoic Acids to TiO2 Nanoparticles and Charge Recombination via the T1 State As Determined by Subpicosecond to Microsecond Time-Resolved Absorption Spectroscopy: Dependence on the Conjugation Length

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Abstract
To examine the mechanisms of electron injection to TiO2 in retinoic acid (RA) and carotenoic acids (CAs), including RA5, CA6, CA7, CA8, CA9, and CA11 having the number of conjugated double bonds n = 5, 6, 7, 8, 9, and 11, respectively, their subpicosecond time-resolved absorption spectra were recorded free in solution and bound to TiO2 nanoparticles in suspension. The time-resolved spectra were analyzed by singular-value decomposition (SVD) followed by global fitting based on an energy diagram consisting of the 3Ag-, 1Bu-, 1Bu+, and 2Ag- singlet excited states, whose energies had been determined as functions of 1/(2n + 1) by the use of carotenoids with n = 9−13. It was found that electron injection took place from both the 1Bu+ and 2Ag- states in RA5, CA6, CA7, and CA8, whereas only from the 1Bu+ state in CA9 and CA11. The electron-injection efficiencies were determined, by the use of the relevant time constants determined by the SVD and global-fitting analyses, to be in the following order: RA5 ≈ CA6 < CA7 > CA8 > CA9 > CA11. To determine the mechanism of charge recombination via the T1 state, submicrosecond time-resolved absorption spectra of RA5, CA6, CA7, and CA8 bound to TiO2 nanoparticles in suspension were recorded. The SVD and global-fitting analyses lead us to a new scheme, which includes the formation of the D0•+ − T1 complex followed by transformation to both the D0•+ and T1 states. On the other hand, their one-electron oxidation potentials were determined, and their singlet and triplet levels were scaled to the conduction band edge (CBE) of TiO2. The T1 level was lower than, but closest to, the CBE in RA5, and it became lower in the order RA5, CA6, CA7, and CA8. Consistent with the energy gap between the CBE and the T1 levels, the generation of the T1 state (or in other words, charge recombination) decreased in the order RA5 > CA6 > CA7 > CA8.

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