A new dinucleating bridging ligand 1,2-bis(2-(1H-imidazo[4,5-f]-[1,10]phenanthrolin-2-yl)phenoxy)ethane and its dinuclear ruthenium(II) complexes: Syntheses, characterization, luminescence, and electrochemical properties

A new imidazo[4,5-f][1,10] phenanthroline-based dinucleating bridging ligand 1,2-bis(2-(1H-imidazo [4,5-f][1,10] phenanthrolin-2-yl)phenoxy)ethane (L) and its dinuclear Ru(II) complexes [(bpy)(2)Ru(L)Ru (bpy)(2)](ClO4)(4) (2) and [(phen)(2)Ru(L)Ru(phen)(2)](ClO4)(4) (3) (bpy = 2,2'-bipyridine and phen = 1,10-phenanthroline) and their characterization, photophysical, and electrochemical properties have been reported. The symmetric nature of the bridging ligand L enables the formation of dinuclear Ru(II) complexes 2 and 3 with equivalent metal centers. The compounds 2 and 3 exhibit the spin-allowed (MLCT)-M-1 (d(pi)-pi*) transition at 459 and 456 nm and Ru(II) metal centered emission at 616 and 600 nm, respectively, in fluid solution at room temperature. The emission profile and emission maxima are similar and independent of the excitation wavelength for each complex. The free bridging ligand is a blue emitter, while the complexes are red emitters in solution. The luminescence decay of the compounds 2 and 3 are mono-exponential and the luminescence lifetime is 174 and 141 ns, respectively. The photoluminescence quantum efficiency of the compound 2 is higher than that of [Ru(bpy)(3)](2+). The compounds 2 and 3 undergo metal centered oxidation and the E-1/2 value for the Ru(II)/Ru(III) redox couple is 1.32 V versus Ag/Ag+. The study demonstrates the versatility of the imidazo[4,5-f][1,10] phenanthroline-based dinucleating bridging ligand in forming dinuclear Ru(II) complexes. (C) 2011 Elsevier B. V. All rights reserved.