Theoretical studies on the absorption spectra of heteroleptic ruthenium polypyridyl dyes for nanocrystalline TiO2 solar cells: Revisited with transition-component analysis

The absorption spectra of Ru complexes of the type cis-[Ru(H2dcbpy)(L)(NCS)2], where H2dcbpy = 4,4′-dicarboxy-2,2′-bipyridine and L = 1,10-phenanthroline (phen) (1) or dipyrido[3,2-a:2′,3′-c]phenazine (dppz) (2), in water were calculated by means of the time-dependent density functional theory, and the calculated spectra were subjected to transition-component analysis. Comparison of the calculated spectra of protonated, partially and fully deprotonated, and electrostatically non-compensated and fully compensated forms of 1 and 2 in water with the corresponding experimental absorption spectra in 0.01 M aqueous NaOH indicated that the predominant molecular structures are fully deprotonated anionic structures that are fully compensated with counter cations. The shapes of the calculated absorption spectra well reproduce the shapes of the corresponding experimental spectra in detail over the entire visible region. The calculated results indicated that the difference between the performances of 1- and 2-sensitized solar cells is due to differences in the contributions of the various electronic excitations that make up the absorption spectra. Transition-component analysis provided a detailed, quantitative explanation of the components of the absorption spectra of 1 and 2 and may be useful for the design and synthesis of improved sensitizers for high-performance dye-sensitized solar cells.