(c) The hole-hopping continues toward the Ru II where the EDTA sacrificial agent can be accessed. (b) The Ru III close to FTO becomes Ru II via hole-hopping. This may indicate the following scenario depicted in Figure 1: (a) The Ru II dye close to fluorine-doped tin oxide (FTO) absorbs the light, injects an electron to the TiO 2 nanoparticle, and becomes Ru III. However, the photocurrents of all the dyes increased by a factor of 10 in the presence of EDTA (sa-DSPECs), relative to when it was absent (do-DSPECs) ( Figure S1). (ii) In the case where the electrolyte solution was loaded with ethylenediaminetetraacetic acid (EDTA) as a sacrificial agent (sa-DSPEC), the hydrogen production performance of P4 was now the lowest and P2 was the highest, in the following order P2 > P1 > P3 > P4 ( Figure S1b). (3) (i) In the dye-only system (do-DSPEC), the magnitude of photocurrent and hydrogen production was in the order P4 > P3 > P2 > P1, where that for P4 was approximately twice that of P1–P3 ( Figure S1a). Previously observed features of P1–P4 dyes were noteworthy. In the former case, the dyes were also attached to TiO 2 nanoparticles. For measuring the photocurrent, incident photon-to-current conversion efficiency (IPCE), and reorganization energy, buffered phosphate (pH = 7) in water was used, while methanol was used to solubilize the dyes for acquiring absorption and emission spectra. In our experiments, different solvents were used. P2, P3, and P4 contain electron-donating substituents (methyl, t-butyl, and n-nonyl groups, respectively) in the bipy ancillary ligand, while P1 has no substituents. (1) Our recent experimental study of DSPECs (3) investigated four ruthenium polypyridyl dyes (P1, P2, P3, and P4). The presence of electron-donating substituents in the bipyridine (bipy) ancillary ligand generally shifts the excited-state potential to more negative values, resulting in faster electron injection. (10,11) In DSSCs, the excited-state potential of the dye is designed to be appropriate for electron injection. (2,3,8,9) New structures of ruthenium dyes have been investigated in dye-sensitized solar cells (DSSCs). (1−3) Additionally, they are capable of driving water oxidation in properly designed complexes (4−7) and it is also possible without a water oxidation catalyst. Ruthenium or organic photosensitizers are promising compounds for dye-sensitized photo-electrochemical cell (DSPECs) because they absorb light in the visible spectrum and have sufficient excited-state potentials to inject electrons into the conduction band of TiO 2. These results also support experimental results for P2, which was the best compound for lateral hole-hopping in a sacrificial agent-containing system (sa-DSPEC). The largest rate was obtained for P2, which was attributed to the expansion of the highest-occupied molecular orbital toward the ancillary bipy ligands and also to the short distances between dyes on the TiO 2 surface. The calculated charge transfer rates agree well with the experimental trend. This may relate to greater electron injection in the P4 and supports experimental results of dye-only systems (do-DSPEC). The largest light-harvesting efficiency of the MLCT-anchoring state was observed in the P4-bend1 conformer, which has the lowest P4 energy. UV–vis absorption spectra by time-dependent density functional theory showed intense metal-to-ligand charge transfer to anchor bipyridine ligands (MLCT-anchoring) at 445–460 nm, which accurately reproduce experimental data. Because of the undetermined P4 conformation in the experiment, we modeled three P4 conformers with straight (P4-straight) and bent nonyl chains (P4-bend1 and bend2). These molecules, (P1), (P2), (P3), and (P4), where bipyP = 2,2′-bipyridine-4,4′-diphosphonic acid, bipy = 2,2′-bipyridine, dmb = 4,4′-dimethyl-2,2′-bipyridine, dtbb = 4,4′-di- tert-butyl-2,2′-bipyridine, and dnb = 4,4′-dinonyl-2,2′-bipyridine, are photosensitizers for applications in dye-sensitized photo-electrochemical cells (DSPECs). In this study, we present a density functional study of four ruthenium complexes by means of UV–visible spectroscopy and Marcus theory.