Dinuclear Iridium(III) Complexes Consisting of Back-to-Back tpy−(ph)n−tpy Bridging Ligands (n = 0, 1, or 2) and Terminal Cyclometallating Tridentate N−C−N Ligands
journal contributionposted on 25.12.2006 by Audrey Auffrant, Andrea Barbieri, Francesco Barigelletti, Jean-Paul Collin, Lucia Flamigni, Cristiana Sabatini, Jean-Pierre Sauvage
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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)Cl2]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 (φ = 3.0 × 10-3; τ = 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.