UV−visible spectra, emission spectra, and
RuIII/II reduction potentials have been measured for
cis-[Ru(bpy)2(py)(CN)]+ (bpy is 2,2‘-bipyridine;
py is pyridine), cis-Ru(bpy)2(CN)2,
[Ru(tpy)(CN)3]- (tpy is 2,2‘:6‘,2‘‘-terpyridine), [Ru(bpy)(CN)4]2-, and
[Ru(MQ+)(CN)5]2-
(MQ+ is N-methyl-4,4‘-bipyridinium
cation)
in twelve solvents. The shifts in the metal-to-ligand charge
transfer (MLCT) absorption (E
abs) or
emission
(E
em) band energies with solvent increase
linearly with the number of cyano ligands and correlate well
with
the Gutmann “acceptor number” of the solvent. Intraligand π
→ π* band energies also correlate with acceptor
number, but with only ∼30% of the shifts for the MLCT bands.
The solvent dependence arises through
mixing of the π → π* transitions with lower energy MLCT
transitions. MLCT absorption and emission
spectra are convolutions of overlapping vibronic components, and a
Franck−Condon analysis of emission
spectral profiles for
cis-Ru(bpy)2(CN)2* has been used to
evaluate the energy gap, E
0, and
χ‘0,gs, where
χ‘0,gs
is the sum of the solvent reorganizational energy for the ground state
below the excited state and the inner-sphere reorganizational energy of the low-frequency modes,
χi,L, is treated classically. Both
E
0 and
χ‘0,gs
correlate well with acceptor number with
ΔE
0/ΔAN = 44 ± 2 cm-1/AN
unit and Δχ0,gs/ΔAN = 21 ± 3
cm-1/AN unit if it is assumed that χi,L is
solvent independent. From electrochemical measurements and
the
difference in E
1/2 values for metal oxidation
and bpy reduction,
ΔΔG°es/ΔAN ≃ 70 ± 7
cm-1/AN unit with
ΔG°es the free energy of the
excited state above the ground state. These correlations show that
the energy gap
is far more sensitive to solvent than χ0,gs.
Δχ0,gs/ΔAN can also be estimated from the
relation
ΔΔG°es/ΔAN
= ΔE
0/ΔAN +
Δχ‘0,gs/ΔAN, which gives
Δχ0,gs/ΔAN = 26 ± 7 cm-1/AN unit.
The solvent reorganizational
energy of the excited state above the ground state is
χ0,es. Its variation with acceptor number can be
estimated
from the relation ΔE
abs/ΔAN −
ΔG°es/ΔAN =
Δχ0,es (=13 ± 8 cm-1/AN) or from
ΔE
abs/ΔAN −
ΔE
em/ΔAN
− Δχ0,gs/ΔAN (=14 ± 8 cm-1/AN),
if χi,L is solvent independent. These results
suggest that χ0,gs is more
sensitive to solvent than χ0,es by as much as a factor of
2. ΔE
em/ΔAN = 45 ± 3
cm-1/AN unit ≃
ΔE
0/ΔAN
= 44 ± 2 cm-1/AN unit, showing that the emission
maximum gives accurate information about the solvent
dependence of the energy gap. A model is invoked to explain the
acceptor number dependence. It is based
on electron pair donation from the lone pair on cyanide to individual
solvent molecules through donor-acceptor interactions. The model is consistent with variations in
ν(CN) with acceptor number in
cis-[Ru(bpy)2(py)(CN)]+ and in
E
1/2(RuIII/II) for the series of
complexes. In this model it is assumed that
donor−acceptor interactions are more important in the ground state
than in the excited state, consistent with
pK
a measurements, and that they are additive in
the number of cyanide ligands. These and H-bonding
interactions in related ammine c...
