Photophysical properties of covalently attached tris(bipyridine)ruthenium(2+) and Mcyclam2+ (M = nickel, H2) complexes

Absorption and emission spectra, emission quantum yields, and excited-state lifetimes of the metal-to-ligand charge-transfer (MLCT) excited state(s) of Ru(bpy)_2(6-Mebpy)^(2+), Ru(bpyh(bpy-cyclamH_2)^(4+), and Ru(bpyh)_2(bpy-cyclamNi)^(4+) are reported. The absorption and emission spectra of these complexes are similar to those of Ru(bpy)_3^(2+) in H_2O at room temperature except for small shifts in the maxima. The emission lifetime decreases by a factor of about 80 for 6-Mebpy, and the emission quantum yield by a factor of about 300 for the bpy-cyclam pendant complexes as compared to the parent Ru(bpy)_3^(2+) complex at 25 °C. Decay of the ^3MLCT excited state takes place by two independent channels: a temperature-independent pathway to the ground state and a thermally-activated pathway via a ligand-field excited state. Activation free energies, E_a, for the latter pathway were from fits of the temperature-dependent emission lifetime measurements. Smaller E_a were observed for the 6-substituted complexes (1500-2300 cm^(-1)) relative to Ru(bpy)_3^(2+)(3000-3910 cm^(-1)) in EtOH and CH_3CN, indicating that either the energy difference decreases and/or the reorganization parameter changes between the emissive ^3MLCT and the ligand-field state to which it is strongly coupled. The smaller E_a can be attributed to the increased Ru-N bond distance in the 6-substituted complexes caused steric hindrance that decreases the ligand-field strength and lowers the energy and/or reorganization parameter of the ligand-field excited state. For the bpy-cyclamNi^(2+) pendant complex, an energy-transfer pathway may also provide a deactivation channel.