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 (1SQC*). Quenching of 1SQC* fluorescence by adsorption on TiO2 and further by co-adsorption with N3 was observed, which are ascribed to the 1SQC*-to-TiO2 and 1SQC*-to-N3+ electron transfer (ET) reactions based on femtosecond time-resolved absorption results. The planar and the twisted 1SQC* 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+ 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+ back electron transfer (BET) reactions. In addition, ground state SQC is also found to be able to efficiently reduce N3+ 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+ 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).