Synthesis, structure, photophysical and electrochemiluminescence properties of Re(i) tricarbonyl complexes incorporating pyrazolyl–pyridyl-based ligands

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Abstract
Three rhenium carbonyl complexes 1–3 were synthesized by reaction of the appropriate bidentate pyrazolyl–pyridyl-based ligand L1, L2 (L1 = 2-[1-{4-(bromomethyl)benzyl}-1H-pyrazol-3-yl]pyridine; L2 = 1,4-bis(3-(2-pyridyl)pyrazol-1-ylmethyl)benzene) with [Re(CO)5Cl] in toluene. They were characterized by elemental analyses, ESI–MS, 1H spectroscopy, and X-ray crystallography for 1 and 2. Compounds 1–3 exhibit bright yellow–green luminescence in the solid state and in solution at 298 K with the lifetimes in the microsecond range. It is noteworthy that the luminescent quantum efficiencies of compounds 1–3 are between 0.040 and 0.051, which are much higher than that of the [Re(bpy)(CO)3Cl] complex (= 0.019) (M. M. Richter et al., Anal. Chem., 1996, 68, 4370; J. Van Houten et al., J. Am. Chem. Soc., 1976, 98, 4853). Electrogenerated chemiluminescence (ECL) was observed in solutions of these complexes in the absence or presence of coreactant tri-n-propylamine (TPrA) or 2-(dibutylamino)ethanol (DBAE) by stepping the potential of a Pt disk working electrode. The ECL spectra are identical to the photoluminescence spectra, indicating that the chemical reactions following electrochemical oxidation or reduction form the same 3MLCT excited states as that generated in the photoluminescence experiments. In most cases, the ECL quantum efficiencies of complexes 1–3 are comparable to that of the [Re(L)(CO)3Cl] (L = bpy or phen) system. Oxygen tends to substantially decrease ECL intensities of the three rhenium complexes–TPrA system, which could allow them to be used as oxygen sensors.

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