| Abstract
| - 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 beenexamined computationally in an effort to better understand this molecule's effectiveness in TiO2-basedphotoelectrochemical cells. Density functional theory (DFT) calculations of the compound's ground stateindicate 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 valenceband; 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 electrontransfer from excited states associated with the bipyridyl groups. Various excited states of the chromophorewere identified using time-dependent density functional theory (TD-DFT). The TD-DFT calculations predictwith significant accuracy excitation energies and corresponding oscillator strengths of transitions observed inthe experimental electronic absorption spectrum in ethanol solution. Some of the calculated singlet excitedstates show significant electronic localization on the bipyridyl groups which, in conjunction with their energiesand relatively large oscillator strengths, suggests that these states can be involved in efficient excited-stateformation and subsequent electron injection into the TiO2 conduction band. Considering both oscillator strengthand spatial proximity, the most efficient electronic injection is expected at excitation energies of approximately2.3, 3.0, and 3.2 eV. Finally, some implications of these results for the molecular engineering of solar cellsensitizers are discussed.
|
