Synthesis, structure and properties of [Pt(2,2-bipyridyl-5,5-dicarboxylic acid)(3,4-toluenedithiolate)]: tuning molecular properties for application in dye-sensitised solar cells

The platinum diimine dithiolate complex, [Pt(2,2ʹ-bipyridyl-5,5ʹ-dicarboxylicacid)(3,4-toluenedithiolate)]([Pt(5,5ʹ-dcbpy)(tdt)]) and its tetrabutylammonium salt [TBA]2[Pt(5,5ʹ-dcbpy)(tdt)] have been prepared, spectroscopically and electrochemically characterised and attached on to TiO2 substrate to be used as solar cell sensitisers. A single-crystal X-ray structure was obtained for [TBA]2[Pt(5,5ʹdcbpy)(tdt)]·EtOH·EtOAc. The effect of the position of the two carboxylic acid substituents on the electrochemistry of the 5,5ʹdisubstituted complexes is discussed in comparison with the previously reported [Pt(4,4ʹ-dcbpy)(tdt)]. Electrochemical studies show no major change in the HOMO after movement of the carboxylic acid groups, consistent with assignment of the HOMO as largely dithiolate based. Movement of the carboxylic acid groups makes the diimine electronic character and hence the LUMO of the complexes different. Electrochemical studies show a change to lower energy of the LUMO represented by changes in reduction potential of the compound on moving the carboxylic acid substituents from the 4,4ʹ to the 5,5ʹ positions. Both [Pt(5,5ʹ-dcbpy)(tdt)] and [TBA]2[Pt(5,5ʹdcbpy)(tdt)] have been used as solar cell sensitisers, with the di-TBA salt giving lower dye loading but superior photovoltaic performance. The consequences of tuning the complex through the position of the carboxylic acid groups are discussed.