Structure and redox and photophysical properties of a series of ruthenium heterocycles based on the ligand 2,3-bis(2-pyridyl)quinoxaline

The synthesis, structure, and redox and photophysical properties of (Ru(bpy)/sub n/(BL)/sub 3-n/)/sup 2 +/, where n = 0-2, bpy is 2,2'-bipyridine, and BL is 2,3-bis(2-pyridyl)quinoxaline, are described. The (Ru(bpy)/sub 2/(BL))(PF/sub 6/)/sub 2/ complex crystallized in the monoclinic space group P2/sub 1//a and the cell parameters are repeated. All non-H atoms were refined anisotropically to an R value of 0.0672 for 5766 reflections with I > sigma(I). The ruthenium to nitrogen bond distances were Ru-N(pyridine) = 2.06 A (average) and Ru-N(pyrazine) = 2.096 (4) A. Absorption spectra contained bands (250-300 nm) in the ultraviolet region that were assignable to ligand ..pi.. ..-->.. ..pi..* transitions and visible bands (517-300 nm) that were assignable to d..pi.. ..-->.. ..pi..* MLCT transitions. Reduction potentials for the Ru/sup 3+/2+/ couples varied from about 1.40 to 1.70 V vs SSCE. Three reductions were observed and assigned to the one-electron reduction of each bidentate ligand. Weak luminescence was observed, and corrected emission energy maxima were located at 766 +/- 4 nm. Excited-state reduction potentials were estimated from the difference between emission energy maxima and ground-state reduction potentials. Excited-state lifetimes at room temperature in 4:1 EtOH-MeOH ranged from 167 to < 70 ns. Temperature-dependent-lifetime (90-298 K) data gave evidence for ..delta..E' values of 1100 +/- 300 cm/sup -1/. The temperature dependence was attributed to either the third MLCT state according to the localized-orbital model or the presence of a fourth CT state for (Ru(BL)/sub 2/(bpy))/sup 2 +/, since little photochemistry was found for it, and to the third and/or the fourth CT state and the dd state for both (Ru(BL)/sub 2/(bpy))/sup 2 +/ and (Ru(BL)/sub 3/)/sup 2 +/. 54 references, 7 figures, 6 tables.