Absorption spectra, luminescence properties and electrochemical behaviour of ruthenium(II) complexes containing bis(pyridyl)triazole ligands

Abstract

Four novel complexes of general formula Ru(L)2(T)2+ (1, Lbpy, Ttrz; 2, Lbpy, Ttrz-Q; 3, Lbiq, Ttrz; 4, Lbiq, Ttrz-Q; bpy=2,2′-bipyridine, biq=2,2′-biquinoline; trz=4-amino-3,5-di-2-pyridyl-4H-1,2,4-triazole; trz-Q=4(4′-N,N-dimethylamino-phenyl)imino-3,5-di-2-pyridyl-4H-1,2,4-triazole) have been synthesized, and their absorption spectra, luminescence properties (both in fluid solution at room temperature and in rigid matrix at 77 K), and electrochemical behaviour have been investigated. The absorption spectra of the complexes show intense absorption bands in the UV region (ϵ in the range 104–105 M−1 cm−1) that are assigned to ligand- centred transitions and moderately intense absorption bands in the visible (ϵ in the range 103–104 M−1 cm−1) that are attributed to metal-to-ligand charge transfer (MLCT) transitions. The absorption bands in the visible of the biq-containing complexes are at lower energies than those of the bpy-containing ones. The four complexes emit from a MLCT excited state both at 77 K and at room temperature, with lifetimes in the range 10−5– 10−6 s and 10−7–10−8 s, respectively. The luminescence lifetimes and quantum yields are practically the same for 1 and 3 and for 2 and 4, respectively, indicating that the presence of the N,N-dimethylamino unit on the triazole ligand does not affect the radiative and radiationless rate constants of the chromophores and does not cause an electron-transfer quenching process. On electrochemical oxidation, 1 and 3 exhibit a reversible one-electron wave at +1.22 and +1.37 V versus SCE, respectively, that are assigned to metal-centred oxidations, while 2 and 4 undergo two successive one-electron oxidations at +1.30 and +1.56 V (2) and +1.30 and +1.71 V (4). By comparison with the redox behaviour of the free ligands, in both 2 and 4 the first process is attributed to oxidation of the N,N-dimethylamino moiety, and the second one to metal-centred oxidation. Two reversible reduction processes occur in all the complexes at about −1.15 and −1.40 V (1 and 2) and at about −0.60 and −0.85 V (3 and 4). Such processes are assigned as bpy- and biq-centred reductions, respectively. The positive shift of the metal-centred oxidation on passing from 1 and 3 to 2 and 4 is attributed to electronic ‘communication’ between the chromophoric metal unit and the electron-donor N,N-dimethylamino group across the triazole ligand and the conjugate NCHC6H4bridge, and to an electrostatic term.