  Recently, some new series of heteroleptic ruthenium-based dyes, the so-called
RD dyes, were designed and synthesized showing better performances compared to
the well-known homoleptic N719. In this work, using the density-functional
theory and its time-dependent extension, we have investigated the electronic
structure and absorption spectra of these newly synthesized dyes, and compared
the results to those of N3 dye to describe the variations of the properties due
to the molecular engineering of ancillary ligand. We have shown that the
calculation results of the absorption spectra for these dyes using the PBE0 for
the exchange-correlation functional are in a better agreement with the
experiment than using B3LYP or range-separated CAM-B3LYP. We have also derived
a formula based on the DFT and used it to visually describe the level shifts in
a solvent. The higher $J_{sc}$ observed in these new dyes is explained by the
fact that here, in contrast to N3, the excitation charge was effectively
transferred to the anchoring ligand. Furthermore, we have shown that the
difference dipole moment vectors of the ground and excited states can be used
to determine the charge-transfer direction in an excitation process. Finally,
the different electron lifetimes observed in these dyes is explained by
investigating the adsorption geometries and the relative orientations of iodine
molecules in different ``dye$\cdots$I$_2$'' complexes.
