Mechanism of squarylium cyanine and Ru(dcbpy)2(NCS)2 co-sensitization of colloidal TiO2

We have investigated the mechanism of squarylium cyanine (SQC) and Ru(dcbpy)(2)(NCS)(2) (N3) co-sensitization of colloidal TiO2 by means of time-resolved spectroscopies. Picosecond time-resolved fluorescence measurements combined with solvent-effect tests revealed a planar and a twisted conformer of the lowest singlet-excited state SQC ((1)SQC*). Quenching of (1)SQC* fluorescence by adsorption on TiO2 and further by co-adsorption with N3 was observed, which are ascribed to the (1)SQC*-to-TiO2 and (1)SQC*-to-N3(center dot+) electron transfer (ET) reactions based on femtosecond time-resolved absorption results. The planar and the twisted (1)SQC* conformers are able to inject electrons into the conduction band of TiO2 with the rates of 1/2.76 ns(-1) and 1/0.30 ns(-1), and to reduce N3(center dot+) with the rates of 1/2.56 ns(-1) and 1/0.28 ns(-1), respectively. The latter pair of rates are significantly larger than those of the TiO2(e(-))-to-N3(center dot+) back electron transfer (BET) reactions. In addition, ground state SQC is also found to be able to efficiently reduce N3(center dot+) with a rate constant of 1/0.32 ns(-1). These results imply that minor amount of SQC as a co-adsorbate can effectively intercept the TiO2(e(-))-to-N3(center dot+) BET, a mechanism which accounts for the improvement of light-to-electricity conversion efficiency of dye-sensitized solar cells through N3 and SQC co-sensitization (D. Zhang et al., J. Photochem. Photobiol. A: Chem. 135 (2000) 235). (c) 2006 Elsevier B.V. All rights reserved.