A New Class of Luminescent Tricarbonyl Rhenium(I) Complexes Containing Bridging 1,2-Diazine Ligands: Electrochemical, Photophysical, and Computational Characterization

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Abstract
A novel class of luminescent tricarbonyl rhenium(I) complexes of general formula [Re2(μ-X)2(CO)6(μ-diaz)] (X = halogen and diaz = 1,2-diazine) was prepared by reacting [ReX(CO)5] with 0.5 equiv of diazine (seven different ligands were used). The bridging coordination of the diazine in these dinuclear complexes was confirmed by single-crystal X-ray analysis. Cyclic voltammetry in acetonitrile showed for all the complexes (but the phthalazine derivative) a chemically and electrochemically reversible ligand-centered reduction, as well as a reversible metal-centered bielectronic oxidation. With respect to the prototypical luminescent [ReCl(CO)3(bpy)] complex, the oxidation is more difficult and the reduction easier (about +0.3 V), so that a similar highest occupied molecular orbital–lowest unoccupied molecular orbital gap is observed. All of the complexes exhibit photoluminescence at room temperature in solution, with broad unstructured emission from metal-to-ligand charge-transfer states, at λ in the range 579–620 nm. Lifetimes (τ = 20–2200 ns) and quantum yields (Φ up to 0.12) dramatically change upon varying the bridging ligand X and the diazine substituents: in particular, quantum yields decrease in the series Cl, Br, and I and in the presence of substituents at the α positions of the pyridazine ring. A combined density functional and time-dependent density functional study of the geometry, relative stability, electronic structure, and photophysical properties of all the pyridazine derivatives was performed. The nature of the excited states involved in the electronic absorption spectra was ascertained, and trends in the energy of the highest occupied and lowest unoccupied molecular orbitals upon changing the pyridazine substituents and the bridging halogen ligands were discussed. The observed emission properties of these complexes were shown to be related to a combination of steric and electronic factors affecting their ground-state geometry and their stability.

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