Regulating ancillary ligands of Ru(ii) complexes with square-planar quadridentate ligands for more efficient sensitizers in dye-sensitized solar cells: insights from theoretical investigations

In this work, we designed three dyes (Ru1, Ru2, and Ru3) by modifying the square-planar quadridentate ligand of the experimental Ru(II) complex K1, [RuL(trans-NCS)2] with L = dimethyl-6,60-bis(methyl-2-pyridylamino)-2,20-bipyridine-4,40-dicarboxylate, from a theoretical viewpoint. As is known, K1 shows obvious advantages over the famous dye N749 in light absorption ability because of its highly conjugated ancillary ligands. Density functional theory and time-dependent density functional theory methods were used to determine the geometrical structures, electronic structures and absorption spectra of the dye complexes. A quantum dynamics method in conjunction with extended Hückel theory was used to simulate the interfacial electron transfer process at the dye–TiO2 interface. The calculated results suggest that Ru1, which contains arylmethane groups, presents improved light absorption and efficient interfacial electron transfer compared with the reference dye K1. We also verified that the position of the anchoring carboxylic acid groups could largely guide the rate of interfacial electron transfer. Ru3, whose anchoring groups are attached to pyridine rings, would have significantly faster interfacial electron transfer than Ru2, whose anchoring groups are attached to the pyrrole ligands; this is because varying the position of the anchoring group results in a difference in the extent of electron donor–acceptor orbital interactions. We expect that the current study will provide some theoretical guidelines for the experimental synthesis of novel Ru(II) complex dyes.