Novel dinuclear luminescent compounds based on iridium(III) cyclometalated chromophores and containing bridging ligands with ester-linked chelating sites

Article abstract of DOI:10.1021/ic000212x

The syntheses and study of the spectroscopic, redox, and photophysical properties of a new set of species based on Ir(III) cyclometalated building blocks are reported. This set includes three dinuclear complexes, that is, the symmetric (with respect to the bridging ligand) diiridium species [(ppy)₂Ir(μ-L-OC(O)-C(O)O-L)Ir(ppy)₂] [PF₆]₂ (5; ppy = 2-phenyl-pyridine anion; L-OC(O)-C(O)O-L = bis[4-(6′-phenyl-2,2′-bipyridine-4′-yl)phenyl] benzene-1,4-dicarboxylate), the asymmetric diiridium species [(ppy)₂Ir(μ-L-OC(O)-L)Ir(ppy)₂] [PF₆]₂ (3; L-OC(O)-L = 4-{[(6′-phenyl-2,2′-bipyridine-4′-yl) benzoyloxy]phenyl}-6′-phenyl-2,2′-bipyridine), and the mixed-metal Ir-Re species [(ppy)₂Ir(μ-L-OC(O)-L)Re(CO)₃Br][PF₆] (4). Syntheses, characterization, and spectroscopic, photophysical, and redox properties of the model mononuclear compounds [Ir(ppy)₂(L-OC(O)-L)][PF₆] (2) and [Re(CO)₃(L-COOH)Br] (6; L-COOH = 4′-(4-carboxyphenyl)-6′-phenyl-2,2′-bipyridine) are also reported, together with the syntheses of the new bridging ligands L-OC(O)-L and L-OC(O)-C(O)O-L. The absorption spectra of all the complexes are dominated by intense spin-allowed ligand-centered (LC) bands and by moderately intense spin-allowed metal-to-ligand charge-transfer (MLCT) bands. Spin-forbidden MLCT absorption bands are also visible as low-energy tails at around 470 nm for all the complexes. All the new species exhibit metal-based irreversible oxidation and bipyridine-based reversible reduction processes in the potential window investigated (between +1.80 and -1.70 V vs SCE). The redox behavior indicates that the metal-based orbitals are only weakly interacting in dinuclear systems, whereas the two chelating halves of the bridging ligands exhibit noticeable electronic interactions. All the complexes are luminescent both at 77 K and at room temperature, with emission originating from triplet MLCT states. The luminescence properties are temperature- and solvent-dependent, in accord with general theories: emission lifetimes and quantum yields increase on passing from acetonitrile to dichloromethane fluid solution and from room-temperature fluid solution to 77 K rigid matrix. In the dinuclear mixed-chromophore species 3 and 4, photoinduced energy transfer across the ester-linked bridging ligands seems to occur with low efficiency.