Synthesis and spectroscopic characterization of CN-substituted bipyridyl complexes of Ru(II).

A series of ruthenium complexes having the general form Ru(bpy)(3-n)(CN-Me-bpy)(n)(2) (where bpy = 2,2'-bipyridine, CN-Me-bpy = 4,4'-dicyano-5,5'-dimethyl-2,2'-bipyridine, and n = 1-3 for complexes 1-3, respectively) have been synthesized and characterized using a variety of steady-state and nanosecond time-resolved spectroscopies. Electrochemical measurements indicate that the CN-Me-bpy ligand is significantly easier to reduce than the unsubstituted bipyridine (on the order of ∼500 mV), implying that the lowest energy (3)MLCT (metal-to-ligand charge transfer) state will be associated with the CN-Me-bpy ligand(s) in all three compounds. Comparison of the Huang-Rhys factors derived from spectral fitting analyses of the steady state emission spectra of complexes 1-3 suggests all three compounds are characterized by excited-state geometries that are less distorted relative to their ground states as compared to Ru(bpy)(3)(2); the effect of the more nested ground- and excited-state potentials is reflected in the unusually high radiative quantum yields (13% (1), 27% (2), and 40% (3)) and long (3)MLCT-state room-temperature lifetimes (1.6 μs, 2.6 μs, and 3.5 μs, respectively) for these compounds. Coupling of the π* system into the CN groups is confirmed by nanosecond step-scan IR spectra which reveal a ∼40 cm(-1) bathochromic shift of the CN stretching frequency, indicative of a weaker CN bond in the (3)MLCT excited state relative to the ground state. The fact that the shift is the same for complexes 1-3 is evidence that, in all three complexes, the long-lived excited state is localized on a single CN-Me-bpy ligand rather than being delocalized over multiple ligands.