Adsorption, Electronic, and Optical Properties of TiO₂-Pyridine Complexes: General Principles for Interfacial Charge-Transfer Transitions

Interfacial charge-transfer transitions (ICTTs) in heterogeneous complexes of organic compounds and inorganic semiconductors have recently gained increasing interest for their potential applications in photoenergy conversions and chemical sensing. ICTTs have been reported in wide band-gap inorganic semiconductors such as anatase TiO2 nanoparticles adsorbed with negatively charged electron-donating ligands. ICTTs occur from the highest occupied pi orbital of the organic component to the conduction band of the inorganic one. To generate ICTTs in the visible region strongly, the following two requirements should be satisfied: the orbital overlap between the highest occupied pi orbital and conduction band through a covalent bond and the staggered electronic structure at the organic-inorganic interface. To verify these requirements, here, we report a new type of ICTT system with pyridine (Py) derivatives. A TiO2-Py complex is quite different from the reported ICTT systems because Py is an electron-accepting ligand. We have examined the adsorption, electronic, and optical properties of the anatase TiO2-Py and anatase TiO2-Py derivative complexes. For non-substituted Py, no ICTTs take place in the visible region owing to the deep highest occupied pi orbital of the Py adsorbate. By the way, ICTTs appear in the visible region due to the introduction of a strong electrondonating group to Py via the significant elevation of an occupied pi orbital of the adsorbed Py, satisfying the above-mentioned two requirements. This result supports the general principles for ICTTs definitively.