Dinuclear iridium(III) complexes consisting of back-to-back tpy-(ph)n-tpy bridging ligands (n=0, 1 or 2) and terminal cyclometallating trientate N-C-N ligands

Three dinuclear iridium(III) complexes consisting of a conjugated bis-tpy type bridging ligand and cyclometallating capping tridentate ligands of the 1,3-di-2-pyridylbenzene family have been prepared (tpy, 2,2',6',2 ''-terpyridine). The two tpy units of the bridge are connected via their back-positions (4') either directly or with a p-phenylene or p-biphenylene spacer. The synthesis relies on the reaction between the dinuclear [Ir(dpb)Cl-2](2) complex (dpb-H) = 1,3-dipyridyl-4,6-dimethylbenzene) and the corresponding bis-tpy ligand. Electrochemical measurements afford metal-centered oxidation and ligand-centered reduction potentials; from the oxidation steps, no evidence is obtained for a strong coupling between the two iridium(III) subunits of the dinuclear species. For all complexes, ground-state absorption data in the 380 nm to visible region show a trend which is consistent with the presence of charge-transfer (CT) transitions involving different degrees of electronic delocalization at the bridging ligands. (dpb)Ir(tpy-tpy)Ir(dpb)(4+) exhibits an appreciable luminescence at room temperature (phi = 3.0 x 10(-3); tau = 3.3 ns), whereas no emission from the other binuclear complexes is detected. All binuclear complexes luminesce at 77 K, and a metal-to-ligand CT nature for (dpb)Ir(tpy-tpy)Ir(dpb)(4+) is suggested, whereas a ligand-centered (LC) emission is proposed for (dpb)Ir(tpy-(ph)(2)-tpy)Ir(dpb)(4+) on the basis of the comparison with the phosphorescence properties of the free bridging ligand, tpy-(ph)2-tpy. Transient absorbance experiments at room temperature afford the absorption spectra and lifetimes of the nonemissive excited states. For (dpb)Ir(tpy-ph-tpy)Ir(dpb)(4+) and (dpb)Ir(tpy-(ph)(2)-tpy)Ir(dpb)(4+), the spectra exhibit a broad profile peaking around 780 nm, quite intense in the case of (dpb)Ir(tpy-(ph)(2)-tpy)Ir(dpb)(4+), and lifetimes of 160 and 440 ps, respectively.