Rational use of ligand to shift the UV-vis spectrum of Ru-complex sensitiser dyes for DSSC applications

Radiation is energy travelling through space. Sunlight radiation such as ultraviolet (UV) radiation are higher-energy radiation which are used in medicine and energy industry such as dye sensitized solar cells (DSSCs). We recently designed new Ru-complexes through novel ligands based on a recently synthesised high performance dye, i.e. [(eta 6-p-cymene)Ru(4,4',5,5'-H4tcbpy)Cl]Cl [1] (A as L1-Ru-L2) for applications in DSSCs. Two electron rich top-ligands, eta 6-N, N, N', N'-tetramethyl-p-phenylenediamine (new dye D) and eta 6- 1,4 di[(dimethylamino) ethyl] benzene (new dye E) outperform other ligands for the new high performance Ru-complexes. Our time dependent density functional theory (TD-DFT) using the CAM-B3LYP/6-311G + (d)/LANL2DZ functional and basis-set model reveals the apparent enhancement of the major UV-vis bands of metal to ligand charge transfer (MLCT) in the new dyes. The major UV-vis bands of the new dyes D and E are red shifted 35 nm and 78 nm, respectively, from the MLCT band of the reference dye A centred at 433 nm. We further discovered that the electron rich top-ligands (L1) are responsible to lift a cluster of occupied outer valence orbitals such as HOMO, HOMO-1 and HOMO-2 in the new D and E complexes, while the virtual orbitals which concentrate on the lower ligand L2 (4,4',5,5' H4 tcbpy) remain almost unchanged. The top-ligand L1 enhances the MLCT transition of the new dyes D and E through the dominant 7dz(2) electrons of the transition metal Ru in their HOMOs. The present study provides the rational of ligand control for metal complexes in broader applications such as electron transfer and electronics.