Preparation, purification, and characterization of binuclear ruthenium(II) complexes: Bridging ligands based on diazafluorenes

A series of bimetallic complexes of ruthenium(II) bridged by heterocyclic ligands formed by the condensation of 4,5-diazafluoren-9-one with various diamines, hydrazine, 1,4-phenylenediamine, benzidine, and 4,4'-methylenedianiline, results in metal centers separated by various distances. The complexes give rise to metal-to-ligand charge-transfer absorptions in the 450 nm region of the visible spectrum and intraligand pi-->pi* transitions in the 300 nm region of the ultraviolet spectrum. The ruthenium(II) centers are oxidized in two closely spaced one-electron processes at potentials more positive than that for Ru(bpy)(3)(2+). The bridging ligands are reduced by two closely spaced one-electron processes at more positive potentials than that for reduction of the coordinated bipyridine ligands (similar to-1.30 V), although the diazafluorene=N-N=diazafluorene bridging ligand is reduced reversibly in two single-electron steps at E-1/2 values of -0.29 and -0.52 V. After purification by eluting the "pure" complexes over a silica gel column with various concentrations of 0.10 M NH4PF6 in methanol, emission occurs only in a glassy matrix at 77 K and at low temperatures in solution. The emission lifetimes at 77 K in a 4:1 ethanol: methanol glass are 5 +/- 1 mu s. A variable-temperature emission lifetime study reveals the presence of a low-lying state with Delta E = similar to 1500 cm(-1), and extrapolation to room temperature indicates the emission lifetimes are in the subnanosecond range.