A series of ruthenium(II) complexes of the types [Ru(bpy)(n)(dafo)(3-n)](2+) and [Ru(bpy)(n)(dafo-ketal)(3-n)](2+), where n varies between 0 and 3 and dafo is 4,5-diazafluoren-9-one, were synthesized, and their chemical, physical, and photophysical properties were examined. The coordinated dafo-ketal ligand readily forms by the direct reaction of [Ru(bpy)(n)(dafo)(3-n)](2+) complexes with ethylene glycol or by reaction of the appropriate ruthenium precursors with the correct stoichiometric amount of the dafo ligand in ethylene glycol. In each series, the visible absorption band associated with the MLCT transition shifts to the blue and the electrochemical oxidation associated with the Ru(III)/Ru(II) couple becomes more positive as II decreases. Coordinated dafo undergoes a one-electron reduction at potentials <-1 V and a second one-electron reduction at similar to-1.2 V vs SSCE. The first reduction can be associated with reduction of the carbonyl group; the second, with reduction of the bipyridine portion. Coordinated dafo-ketal only reduces at potentials <-1.2 V. The emission properties place the emitting state of the complexes on the bipyridine portion of the dafo ligand, not the carbonyl group. At 77 K in a 4:1 ethanol-methanol glass, the emission lifetimes fall in a range of 5-2 mu s as n decreases. In fluid solution, the emission lifetimes are temperature dependent with activation energies that vary from 1400 to 500 cm(-1) as n decreases. The thermally accessible state is assigned as a fourth metal-to-ligand charge transfer (MLCT ") state since the compounds are photochemically unreactive.