Dinuclear Iridium(III) Complexes Linked by a Bis(β-diketonato) Bridging Ligand: Energy Convergence versus Aggregation-Induced Emission

Novel iridium(III)/iridium(III) and indium(III)/platinum(II) di-nuclear complexes, [{Ir(ppyFF)(2)}(2)(mu(2)-L)] (4) and [{Ir(ppyFF)(2)]-{mu(2)-L){Pt(ppy)}] (5) [ppyFF = 2-(2,4-difluorophenyl)pyridine, ppy = 2-phenylpyridine, L = 1,3-bis(3-phenyl-3-oxopropanoyl)benzene], linked by an L bridging ligand were prepared, and their photophysical properties were investigated in solution and in the solid state The photophysical properties of mononuclear iridium(III) and platinum(II) complexes, [Ir(ppyFF)(2)(dbm)] (1) and [Pt(ppy)(dbm)] (2) bearing a dibenzoylmethane (dbm) ligand were also compared Whereas the UV/Vis absorption spectra of 4 and 5 show independent light absorption at each metal-centered moiety, the photoluminescence spectra of 4 and 5 display almost identical features, but very weak emissions in solution at both room temperature and 77 K The weak emission in solution is found to mainly originate from a (3)LX state of the L bridging ligand, which reflects the occurrence of efficient energy convergence from the triplet states of the Pt(ppy) and Ir(ppyFF) moieties to the (3)LX state of L. By contrast, intense orange-red emission, that is, aggregation-induced emission, is produced in the solid state of 4 and 5 Inspection of the crystal-packing structures of 5 reveals that strong intermolecular pi-pi interactions between the adjacent pyridine rings of ppyFF ligands in the Ir-centered moieties are responsible for the emissive metal-to-ligand ligand charge-transfer [(3)M(LL)CT] state of the solid-state dinuclear systems The electrochemical properties of 4 and 5 further indicate that the first two reductions occur at the dbm moieties of the L bridging ligand linked to each metal center, which is consistent with the fact that the lowest-energy excited state of the L bridging ligand dominates the excited-state properties of 4 and 5 in solution