Interfacial charge-transfer transitions in a TiO2-benzenedithiol complex with Ti–S–C linkages

Interfacial charge-transfer (ICT) transitions between organic materials and inorganic semiconductors are a new mechanism for light absorption at organic–semiconductor interfaces. ICT transitions cause one-step interfacial charge separation without loss of energy. This feature is potentially useful to realize efficient organic–inorganic hybrid solar cells. ICT transitions have been examined by employing titanium dioxide (TiO2) nanoparticles chemisorbed with π-conjugated molecules via Ti–O–C linkages. Here, we report ICT transitions in a TiO2 and 1,2-benzenedithiol (BDT) complex with Ti–S–C linkages. BDT adsorbs on TiO2 by the bridging bidentate coordination of the sulfur atoms to surface titanium atoms. The TiO2–BDT complex shows ICT transitions from the BDT moiety to the conduction band of TiO2 in the visible region. The ICT transitions occur by orbital overlaps between the d orbitals of the surface titanium atoms and the π orbitals of the benzene ring. Our density-functional-theory (DFT) analysis reveals that the 3p valence orbitals of the sulfur bridging atoms contribute to more than 50% of the highest occupied molecular orbital (HOMO) and the 3d–3p(sulfur)–π interaction via the Ti–S–C linkage enhances the electronic mixing between the titanium atoms and the benzene moiety as compared to the 3d–2p(oxygen)–π via the Ti–O–C linkage. This result indicates the important role of the heavier-atom linkers for strong organic–inorganic electronic couplings.