It is well known that photosensitizers play a vital role in the dye-sensitized solar cells (DSSCs).Understanding of their structures and photophysical properties is necessary to enhance the photoelectric conversion efficiency of DSSCs.In this work, a series of sensitizers [PcRu(RPy)(Py-COOH)] (Pc=phthalocyanine, Py=pyridine, R=COOH, CN, H, Me and OMe) as well as their one- and two-electron oxidized derivatives have been investigated using density functional theory (DFT) and time-dependent DFT.Their structural and spectroscopic properties were addressed.Full optimization demonstrates the ruthenium center exhibits octahedral geometry with six nitrogen donors.One- and two-electron oxidation slightly shortens equatorial Ru-N bond lengths relative to neutral dyes, but lengthens their axial ones.The experimentally known RuPc-1 has been calculated to evaluate performance of various functionals, basis sets, computational models of solvent effect and solvent sorts.And the TD-BLYP/SDD/PCM/Ethanol approach was used for spectral calculation in the present work.The comparison among calculated absorption spectra reveals that the neutral complexes are more suitable for photosensitizers in dye-sensitized solar cells than their oxidized derivatives.Neutral dyes were calculated to display a strong Soret absorption peak at 350 nm and a relatively weak Q band at 600 nm.They were assigned as intra-phthalocyanine π→π^* transition and Ru→Py-COOH metal-to-ligand charge transfer.As these dyes anchor to semiconductor photoanode by the carboxyl of axial pyridyl group, their π→π^* transition has no contribution to the subsequent electron injection.Further association with above ruthenium dyes missing absorption between 400～580 nm in visible region elucidates why DSSCs sensitized by them have such a low overall conversion efficiency of photo to electricity.Building on the present results, it is suggested to fabricate a sensitizer like [((COOH)_n-(Pc))Ru(L)₂], where anchoring carboxylic acids are bonded to equatorial Pc macrocycle and strong electron-donating groups are axially introduced.With this arrangement, Ru center, donating L groups and Pc macrocycle (high-lying π occupied orbitals) all serve as electron donors, while Pc ligand (low-lying π^* unfilled orbitals) and peripheral carboxylic substituents behave as electron-accepting reservoir.This in-progress work is anticipated to design dyes that improve the conversion efficiency of DSSCs.

Key words: Ru-phthalocyanine sensitizer, one- and two-electron oxidation, electronic spectrum, time-dependent density functional theory (TD-DFT)