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

Abstract

A novel class of luminescent tricarbonyl rhenium(I) complexes of general formula [Re2(mu-X)2(CO)6(mu-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 lambda in the range 579-620 nm. Lifetimes (tau=20-2200 ns) and quantum yields (Phi 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 alpha 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.

Highly Emitting Neutral Dinuclear Rhenium Complexes as Phosphorescent Dopants for Electroluminescent Devices

“…Thick lines: CV including first oxidation and reduction peaks; thin lines: first oxidation peak at 2 V s À1 . findings of the previous study [6] on the related [Re 2 (m-Cl) 2 (CO) 6 (mdiazine)] complexes. For comparison, the data concerning the complex bearing unsubstituted pyridazine (compound 0) are also reported in Table 1 and Figure 1.…”

“…All the complexes display four intense bands in the n(CO) IR region, as typical of this class of compounds. [6,7] The 1 H resonances of the ortho-hydrogens of coordinated diazines exhibit a significant downfield shift (0.3-0.6 ppm) with respect to the corresponding free diazines. [10] …”

“…The first reduction peak, which is centered on the diazine ligand, [6] is monoelectronic and reversible, both from the chemical and the electrochemical point of view. [11] The peak potentials regularly shift in the negative direction with an increase in the number of alkyl substituents, consistently with an increasingly electron-rich aromatic site.…”

“…[2] Iridium(III) complexes have been mainly exploited, [3] but also other transition metals have been investigated, including rhenium(I). [4,5] The synthesis of a new family of tricarbonyl rhenium(I) complexes has recently been reported, [6] which exhibit intense photoluminescence in the range 580-620 nm, with the highest quantum yields ever reported for neutral tricarbonyl rhenium complexes (up to ca. 10%).…”

“…Lifetimes and emission intensities strongly vary on changing the ancillary ligands (halide [6] or hydride [7] ) or the substituents on the diazines, as a result of a wide variability of the rate of non-radiative decays. Computational studies [6] indicate that the emission intensity can be correlated to an easily computable parameter, such as the instantaneous interaction energy, [8] thus showing the connection between the accessibility of radiationless relaxation pathways and the stiffness of the Re 2 -diazine scaffold, as expected for transitions involving MLCT states. On this ground it was possible to predict that highly emitting species might be obtained on using electron rich 1,2-diazines suitably substituted in the b positions.…”