Influence of the “Innocent” Ligands on the MLCT Excited-State Behavior of Mono(bipyridine)ruthenium(II) Complexes: A Comparison of X-ray Structures and 77 K Luminescence Properties

The variations in the nonchromophoric ligands of [Ru(L)4bpy]2+ complexes are shown to result in large changes in
emission band shapes, even when the emission energies are similar. These changes in band shape are systematically
examined by means of the generation of empirical reorganizational energy profiles (emreps) from the observed
emission spectra (Xie, P.; et al. J. Phys. Chem. A 2005, 109, 4671), where these profiles provide convenient
probes of the differences in distortions from the ground-state structures of the 2,2-bipyridine (bpy) ligands (for
distortion modes near 1500 cm-1) in the metal-to-ligand charge-transfer (MLCT) excited states for a series of
complexes with the same ruthenium(II) bipyridine chromophore. The bpy ligand is nearly planar in the X-ray structures
of the complexes with (L)4 = (NH3)4, triethylenetetraamine (trien), and 1,4,7,10-tetraazacyclododecane ([12]aneN4).
However, for (L)4 = 5,12-rac-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, the X-ray crystal structure
shows that the bpy ligand is twisted in the ground state (a result of methyl/bpy stereochemical repulsion) and the
emrep amplitude at about 1500 cm-1 is significantly larger for this structure than for the complex with (L)4 =
1,4,8,11-tetraazacyclotetradecane, consistent with larger reorganizational energies of the bpy distortion modes in
order to form a planar (bpy-) moiety in the excited state of the former. The trien and [12]aneN4 complexes have
very nearly the same emission energies, yet the 40% smaller vibronic sideband intensity of the latter indicates that
the MLCT excited state is significantly less distorted; this smaller distortion and the related shift in the distribution
of distortion mode reorganizational energy amplitudes is apparently related to the 36-fold longer lifetime for (L)4 =
[12]aneN4 than for (L)4 = trien. For the majority (77%) of the [Ru(L)4bpy]2+ complexes examined, there is a systematic
decrease in emrep amplitudes near 1500 cm-1, consistent with decreasing excited-state distortion, with the excited-state energy as is expected for ground state−excited state configurational mixing in a simple two-state model.
However, the complexes with L = [12]aneN4, 1,4,7,10-tetraazacyclododeca-1-ene, and (py)4 all have smaller emrep
amplitudes and thus less distorted excited states than related complexes with the same emission energy. The
observations are not consistent with simple two-state models and seem to require an additional distortion induced
by excited state−excited state configurational mixing in most complexes. Because the stereochemical constraints
of the coordinated [12]aneN4 ligand restrict tetragonal distortions around the metal, configurational mixing of the
3MLCT excited state with a triplet ligand-field excited state of RuII could account for some of the variations in
excited-state distortion. The large number of vibrational distortion modes and their small vibrational reorganizational
energies in these complexes indicate that a very large number of relaxation channels contribute to the variations
in 3MLCT lifetimes and that the metal−ligand skeletal modes are likely to contribute to some of these channels.