114397-51-2Relevant academic research and scientific papers
Photophysical, photochemical, and electrochemical properties of mononuclear and dinuclear ruthenium(II) complexes containing 2,2′-bipyridine and the 3,5-bis(pyridin-2-yl)-1,2,4-triazolate ion
Barigelletti, Francesco,De Cola, Luisa,Balzani, Vincenzo,Hage, Ronald,Haasnoot, Jaap G.,Reedijk, Jan,Vos, Johannes G.
, p. 4344 - 4350 (2008/10/08)
The absorption spectra, luminescence spectra (from 77 to 298 K), luminescence lifetimes (from 77 to 298 K), luminescence quantum yields, photochemical quantum yields, and redox potentials of the complexes Ru(bpy)2(bpt)+ (1) and [Ru(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 Ru(bpy)32+. For both 1 and 2, the lowest energy absorption band and the luminescence band are attributed to Ru → 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 potentials less negative than those of bpt-. Because of the stronger σ-donor ability of bpt- compared with bpy, the Ru → bpy CT absorption and emission bands of 1 (λmaxabs = 480 nm and λmaxem = 678 nm in CH3CN at room temperature) are red-shifted compared with those of the parent Ru(bpy)32+ complex. Both absorption and emission move to higher energies in going from 1 to 2 (for 2: λmaxabs = 453 nm and λmaxem = 648 nm). In nitrile rigid matrix at 77 K the emission lifetimes are 2.8 and 3.6 μs for 1 and 2, respectively. For both complexes increasing temperature causes a strong decrease of the emission lifetime (0.16 μs for 1 and 0.10 μs for 2 at room temperature), but the ln (1/τ) vs 1/T plots between 77 and 298 K are quite different in the two cases, indicating that (i) the luminescent excited state of 1 is much more sensitive than that of 2 to the melting of the solvent matrix and (ii) the luminescent excited state of 2, but not that of 1, undergoes a thermally activated (ΔE = 3200 cm-1) radiationless transition to an upper, short-lived excited state (presumably, a triplet metal-centered level 3MC). In agreement with the expectation based on the temperature dependence of the luminescence lifetime, 2, but not 1, undergoes a photodissociation reaction in CH2Cl2 solutions containing Cl- ions. Such a photoreaction leads to 1 and Ru(bpy)2Cl2. All the observed properties show that in 2 the lowest unoccupied molecular orbital does not belong to the bpt- bridging ligand but to the terminal bpy ligands. Because of the nonequivalence of the coordinating positions of the bridging ligand, in 2 the MLCT and MC levels related to the two Ru(bpy)22+ units are not isoenergetic. It is found that the lowest excited state of 2 (responsible for the luminescence) is a 3MLCT level centered on one Ru(bpy)22+ unit, whereas photochemistry takes place from a 3MC level centered on the other Ru(bpy)22+ unit.
