On the viability of cyclometalated Ru(II) complexes as dyes in DSSC regulated by COOH group, a DFT study

The Ru(II) complexes [Ru(bpp)(dcbpy)Cl](+) (1), [Ru(tcbpp)(bpy)Cl](+) (2), and [Ru(tc'bpp)(bpy)Cl](+) (3) (bpp = 2,6-bis(N-pyrazolyl)pyridine, dcbpy = 4,4'-dicarboxyl-bipyridine, bpy = bipyridine, tcbpp = 4-carboxyl-2,6-bis(2-carboxyl-N-pyrazolyl)pyridine, tc'bpp = 4-carboxyl-2,6-bis(4-carboxyl-N-pyrazolyl)pyridine) are studied theoretically using density functional theory (DFT) techniques to explore their properties as dye in a solar cell. The calculated geometry structure and absorption spectrum of 1 are consistent with its experimental results. The calculation results indicate which sites the COOH groups attach to can significantly influence the electronic structure of the complex. By migrating the COOH groups from the bpy ligand in 1 to bpp ligand in 2 and 3, the nature of LUMO changes from bpy-localized to bpp dominated. The calculated low-lying absorptions at λ > 370 nm of the three complexes are categorized as metal-to-ligand charge-transfer (MLCT) transitions and the transition terminates at the orbital populated by the COOH appended ligand. The atomic spin density analysis also indicates that the ligand which is modified by the COOH groups is the ideal spot for the captured electron to situate. It can be predicted that the performance of 2 and 3 in the dye-sensitized solar cell can be enhanced as compared with 1.