Study of Interfacial Charge Transfer Bands and Electron Recombination in the Surface Complexes of TCNE, TCNQ, and TCNAQ with TiO2

Upon the formation of the surface complexes of dicyanomethylene compounds (TCNX: TCNE, TCNQ, and TCNAQ) with TiO2 nanoparticles, new visible light absorption bands appear due to interfacial charge-transfer transitions from the TCNX adsorbates to the conduction band of TiO2. The charge-transfer absorption band is significantly changed depending on the kind of TCNX. The spectral region is extended to the near-IR region with increase in the pi-conjugation length of TCNX. Density functional theory calculations using simple model compounds Ti(OH)(3)-O-TCNX- well reproduce the charge-transfer bands and the TCNX dependence and the experimentally observed vibrational spectrum of the TiO2-TCNQ surface complex. We then model the geminate electron recombination from TiO2 to the HOMO level of the TCNX moiety immediately following the charge-transfer transitions. The computed result indicates that the rate of the recombination process is significantly dependent on the kind of TCNX, increasing with the extent of the pi-conjugation of TCNX. This increase competes with the increase in absorbance under AM1.5 irradiation with the size of the system. In particular, the Ti(OH)(3)-O-TCNQ(-) model complex bolsters a much stronger absorbance versus TCNE and is predicted to result in a moderately higher recombination rate in the surface complex, whereas TCNAQ is expected to result in a much higher recombination rate versus TCNQ with an only moderate increase in absorbance.