Density-functional analysis of the electronic structure of tris-bipyridyl Ru(II) sensitisers

Excited state transitions and energies of a series of [Ru(bpy)3]2+ type complexes incorporating the ligand, 4,4′-bis-phosphonato(methyl)-2,2′-bipyridine (dmpbpy) was investigated, and the influence of this organometallic ligand on the electronic structure of the complexes was examined using Time-Dependent Density Functional Theory (TD-DFT). Experimental data and the theoretical TD-DFT calculations were presented to support the effect of non-equivalent ligand substitution on spectral and molecular orbital (MO) energy properties on this class of tris-chelate surface sensitisers. For the series of complexes studied, it was identified that the lowest lying LUMO states were consistently found to reside on the ligand 2,2′-bipyridine (bpy) for gas phase calculations. As an implication of this, it was suggested that this could impact the effectiveness of these complexes as surface sensitisers in PEC cell applications such as the dye-sensitised solar cell (DSC) due to the lower probability of the excited state electron residing on a ligand anchored to the semiconductor substrate. However, further calculations in a solvation medium showed that the electron withdrawing nature of PO3H2 on dmpbpy saw the lowest lying LUMO states are populated on dmpbpy. This inhomogeneous distribution of electron density across non-equivalent ligands may have implications for further ‘spectral tuning’ of surface sensitisers. Despite the TD-DFT gas phase calculations not being corrected for solvent/media effects, the three longest wavelength bands associated with known charge transfer phenomena were identified. The symmetry allowed ν ¯ MLCT in the visible region was assigned as a π ψ ∗ ← dπ transition, the mid-UV spectrum ν ¯ LC was assigned π ψ ∗ ← π in origin. Whilst the near-UV shoulder on the blue side of ν ¯ MLCT showed dσ ← dπ and π∗ ← dπ transitional character and was tentatively described as ν ¯ MC/MLCT. UV–Vis absorption spectra calculated for solvated analogues containing dmpbpy indicated that the low energy transitions associated with the MLCT are subject to bathochromic shift due to solvent polarity by 0.062 eV (500 cm−1) compared with the gas phase calculations, which is more highly correlated to the observed experimental transitions.