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)](PF6)(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 similar to 500 mV), implying that the lowest energy (MLCT)-M-3 (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)](PF6)(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 (MLCT)-M-3-state room-temperature lifetimes (1.6 mu s, 2.6 mu s, and 3.5 mu s, respectively) for these compounds. Coupling of the pi* system into the CN groups is confirmed by nanosecond step-scan IR spectra which reveal a similar to 40 cm(-1) bathochromic shift of the CN stretching frequency, indicative of a weaker CN bond in the (MLCT)-M-3 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.