Ground- and Excited-State Electronic Structures of the Solar Cell Sensitizer Bis(4,4‘-dicarboxylato-2,2‘-bipyridine)bis(isothiocyanato)ruthenium(II)

The ground- and excited-state electronic structures of the photosensitizer bis(4,4‘-dicarboxylato-2,2‘-bipyridine)-bis(isothiocyanato)ruthenium(II), [RuL‘₂(NCS)₂]^4- (where L‘ = 4,4‘-dicarboxylato-2,2‘-bipyridine), have been examined computationally in an effort to better understand this molecule's effectiveness in TiO₂-based photoelectrochemical cells. Density functional theory (DFT) calculations of the compound's ground state indicate that occupied molecular orbitals (MOs) localized on carboxylate groups of the bipyridyl ligands (through which the compound binds to the TiO₂ nanoparticles) energetically match the semiconductor valence band; the lowest unoccupied MOs lie above the conduction band edge and are bipyridine π* in character. These results suggest that the compound is well-positioned to bind strongly to TiO₂ and engage in electron transfer from excited states associated with the bipyridyl groups. Various excited states of the chromophore were identified using time-dependent density functional theory (TD-DFT). The TD-DFT calculations predict with significant accuracy excitation energies and corresponding oscillator strengths of transitions observed in the experimental electronic absorption spectrum in ethanol solution. Some of the calculated singlet excited states show significant electronic localization on the bipyridyl groups which, in conjunction with their energies and relatively large oscillator strengths, suggests that these states can be involved in efficient excited-state formation and subsequent electron injection into the TiO₂ conduction band. Considering both oscillator strength and spatial proximity, the most efficient electronic injection is expected at excitation energies of approximately 2.3, 3.0, and 3.2 eV. Finally, some implications of these results for the molecular engineering of solar cell sensitizers are discussed.