北京理工大学 增强现实智能医疗实验室

Abstract—The excited-state dynamics of all-trans-retinoic acid (ATRA), both free in n-hexanol and bound to a TiO2 nanoparticle have been studied by use of femtosecond time-resolved visible absorption spectroscopy excited at 400 nm. Three excited singlet states, S3, S2, and S1, have been observed for the ATRA molecule, which can be assigned as 1Bu+ (π, π*), (n, π*), and 2Ag- (π, π*) singlet excited manifolds, respectively. Photoinduced electron injection from the excited singlet state S3 to the conduction band of the TiO2 nanoparticle is observed in an ATRA-sensitized TiO2 colloidal solution in hexanol; the subsequent interfacial charge recombination between the injected electron and the ATRA cation is investigated. It is found that the charge recombination is mainly via the triplet state, and the branching ratio for the population on the excited triplet state and the ground state during the charge recombination is about 6.0. The observed rate constants for the charge recombination to the triplet state and the ground state are 1/19.0 (ps-1) and 1/140 (ps-1), respectively. Treating the charge recombination as a nonadiabatic process, we can obtain a reorganization energy having a value of 0.44 eV, which indicates that the charge recombination to the ground state is in the Marcus inverted region while the charge recombination to the triplet state lies within the normal region. An apparent electronic coupling matrix element at the closest contact of ATRA and the TiO2 nanoparticle, V0T = 5118 cm-1, has been evaluated for the charge recombination to the triplet state and V0G = 2670 cm-1 for that to the ground state. It is concluded that the interfacial charge recombination is strongly coupled. The electronic coupling matrix element is expected to decay exponentially on the charge separation distance, and in the microsecond domain, the observed V(r)G decays to 3.6 cm-1, corresponding to the trapped electron migrating away from the adsorbates at a distance of 2.0 nm on the TiO2 nanoparticles. The relevance of the triplet formation to the electroluminescence device is also discussed.