Interfacial charge-transfer transitions and reorganization energies in sulfur-bridged TiO2-x-benzenedithiol complexes (x: o, m, p)

Abstract

Surface complexes formed between TiO₂ nanoparticles and enediol compounds such as 1,2-benzenediol (o-BDO) via Ti–O–C linkages show absorption of visible light due to interfacial charge-transfer (ICT) transitions. The ICT transitions take place from the π-conjugated systems to TiO₂. Recently, we reported a surface complex formed between TiO₂ and 1,2-benzenedithiol (o-BDT) via Ti–S–C linkages. This sulfur-bridged complex shows ICT transitions from the sulfur bridging atoms to TiO₂. Interestingly, it was demonstrated that the ICT transitions in the sulfur-bridged TiO₂-o-BDT complex induce photoelectric conversion more efficiently than those in the oxygen-bridged TiO₂-o-BDO complex. This result suggests that carrier recombination is suppressed with the sulfur bridging atoms. In this paper, we examine ICT transitions and reorganization energies in the sulfur-bridged TiO₂-x-BDT complexes (x: o, m, p) and compare them with those in the oxygen-bridged TiO₂-x-BDO complexes. The estimated reorganization energies for the sulfur-bridged TiO₂-x-BDT complexes (x: o, m, p) are much smaller than those for the oxygen-bridged TiO₂-x-BDO ones. Based on the Marcus theory, the small reorganization energy calculated for the TiO₂-o-BDT complex, which is less than half of that for the TiO₂-o-BDO complex, increases the activation energy of carrier recombination. The small reorganization energy is attributed to the characteristic distribution of the highest occupied molecular orbital (HOMO) on the sulfur-bridging atoms in the TiO₂-o-BDT complex, which inhibits structural changes in the benzene ring in the ICT excited state. Our work reveals the important role of the sulfur bridging atoms in the suppression of carrier recombination.