PHOTOPHYSICAL PROPERTIES OF COVALENTLY ATTACHED RU(BPY)3(2+) AND MCYCLAM(2+) (M = NI, 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(bpy)2(bpy-cyclamH2)4+, and Ru(bpy)2(bpy-cyclamNi)4+ are reported. The absorption and emission spectra of these complexes are similar to those of Ru(bpy)3(2+) in H2O 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 decreases by a factor of about 300 for the bpy-cyclam pendant complexes as compared to the parent Ru(bpy)3(2+) complex at 25-degrees-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 obtained 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 CH3CN, 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 by 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-cyclamNi2+ pendant complex, an energy-transfer pathway may also provide a deactivation channel.