MONONUCLEAR AND DINUCLEAR OSMIUM(II) COMPOUNDS CONTAINING 2,2'-BIPYRIDINE AND 3,5-BIS(PYRIDIN-2-YL)-1,2,4-TRIAZOLE - SYNTHESIS, ELECTROCHEMISTRY, ABSORPTION-SPECTRA, AND LUMINESCENCE PROPERTIES

The synthesis, structure, electrochemical behavior, absorption spectra, luminescence spectra (from 90 to 298 K), luminescence lifetimes (from 90 to 298 K), and photoreactivity of the complexes Os(bpy)2(bpt)+ (1) and [Os(bpy)2]2(bpt)3+ (2), where bpy = 2,2'bipyridine and Hbpt = 3,5-bis(pyridin-2-yl)-1,2,4-triazole, are reported. The properties exhibited by 1 and 2 are compared with those of Os(bpy)3(2+) and of the analogous Ru(II) complexes. For both 1 and 2, the lowest energy absorption band and the luminescence band are attributed to Os-->bpy metal-to-ligand charge-transfer (MLCT) singlet and triplet excited states, respectively. Electrochemical oxidation is centered on the metal(s), and electrochemical reduction is centered on the ligands, with bpy being reduced at less negative potentials than bpt-. Because of the stronger sigma-donor ability of bpt- compared with bpy, the Os-->bpy CT absorption and emission bands of 1 (lambda-max(abs) = 486 nm and lambda-max(em) = 762 nm in CH3CN at room temperature) are red-shifted compared with those of the parent Os(bpy)3(2+) complex. In nitrile rigid matrix at 90 K, the emission lifetimes are 250 and 340 ns for 1 and 2, respectively. For both complexes, increasing temperature causes a decrease of the emission lifetime (55 ns for 1 and 33 ns for 2 at room temperature), but the ln (1/tau) vs 1/TAU plots between 90 and 298 K do not exhibit the highly activated decay processes characteristic of the 3MLCT-->3MC crossover. In agreement with this result, no photodecomposition was observed in CH2Cl2 solutions containing Cl- ions. The ln (1/tau) vs 1/TAU plots for 1 and 2 are noticeably different in fluid solution, indicating that an additional radiationless decay channel intervenes for 2 above 150 K.