Density Functional Study of Metal-to-Ligand Charge Transfer and Hole-Hopping in Ruthenium(II) Complexes with Alkyl-Substituted Bipyridine Ligands

In this study, we
present a density functional study of four ruthenium
complexes by means of UV–visible spectroscopy and Marcus theory.
These molecules, [RuII(bipyP)­(bipy)2] (P1),
[RuII(bipyP)­(dmb)2] (P2), [RuII(bipyP)­(dtbb)2] (P3), and [RuII(bipyP)­(dnb)2] (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).
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). 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. The largest light-harvesting
efficiency of the MLCT-anchoring state was observed in the P4-bend1
conformer, which has the lowest P4 energy. This may relate to greater
electron injection in the P4 and supports experimental results of
dye-only systems (do-DSPEC). The calculated charge transfer rates
agree well with the experimental trend. 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 TiO2 surface. 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).