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 ~500 mV), implying that the lowest energy 3MLCT (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 3MLCT-state room-temperature lifetimes (1.6 μs, 2.6 μs, and 3.5 μs, respectively) for these compounds. Coupling of the p* 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 3MLCT 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. [ABSTRACT FROM AUTHOR]