The synthesis and photophysical characterization of a series of
aryl-substituted 2,2‘-bipyridyl complexes
of RuII are reported. The static and time-resolved
emission properties of
[Ru(dpb)3](PF6)2, where
dpb is 4,4‘-diphenyl-2,2‘-bipyridine, have been examined and are contrasted with those of
[Ru(dmb)3](PF6)2 (dmb
= 4,4‘-dimethyl-2,2‘-bipyridine). It is shown through analysis of electrochemical data
and detailed fitting of the emission spectrum that
the unusually large radiative quantum yield for
[Ru(dpb)3](PF6)2 in
CH3CN solution at room temperature is due
to
reduction of the degree of geometric distortion along primarily
ring-stretch acceptor mode coordinates relative to
other molecules in this class. It is proposed that the
3MLCT excited state of
[Ru(dpb)3]2+ is characterized by
a
ligand conformation in which the 4,4‘-phenyl substituents are coplanar
with the bipyridyl fragment, leading to extended
intraligand electron delocalization and a smaller average change in the
C−C bond length upon formation of the
excited state as compared to
[Ru(dmb)3]2+. These conclusions
are further supported by photophysical data on several
new molecules,
[Ru(dptb)3](PF6)2 (dptb
= 4,4‘-di-p-tolyl-2,2‘-bipyridine),
[Ru(dotb)3](PF6)2 (dotb
= 4,4‘-di-o-tolyl-2,2‘-bipyridine), and
[Ru(dmesb)3](PF6)2
(dmesb = 4,4‘-dimesityl-2,2‘-bipyridine). The systematic
increase in steric
bulk provided by this ligand series results in clear trends in
k
r, k
nr, and
S
M (the Huang−Rhys factor), consistent
with
the delocalization model. In addition, time-resolved resonance
Raman data reveal frequency shifts in ring-stretch
modes across the series supporting the notion that, as the steric bulk
of the ligand increases, the ability for the
peripheral phenyl rings to become coplanar with the bipyridyl fragment
is hindered. Ab initio calculations employing
Hartree−Fock and second-order perturbation theory on neutral and
anionic 4-phenylpyridine, put forth as a model
for the ground and excited states of
[Ru(dpb)3]2+, are also reported.
These calculations suggest a canted geometry
for the ground state, but a considerable thermodynamic driving force
for achieving planarity upon reduction of the
ligand. The canted ground-state geometry is also observed in the
single-crystal X-ray structure of the mixed-ligand
complex
[Ru(dmb)2(dpb)](PF6)2.
Finally, consideration of how this system evolves from the
Franck−Condon state
to the planar thermalized 3MLCT state is discussed
with regard to the possibility of time-resolving the onset
of
extended electron delocalization in the excited state by using
ultrafast spectroscopy.
