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

Abstract

We have investigated the mechanism of squarylium cyanine (SQC) and Ru(dcbpy) 2(NCS) 2 (N3) co-sensitization of colloidal TiO 2 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 TiO 2 and further by co-adsorption with N3 was observed, which are ascribed to the 1SQC*-to-TiO 2 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 TiO 2 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 TiO 2(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 TiO 2(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).