Photochemical synthesis of intensely luminescent isocyano rhenium(I) complexes with readily tunable structural features

A new class of readily tunable isocyano rhenium(I) diimine luminophores, cis,cis-[Re(CO)₂(CNR)₂- (N-N)]⁺ (R=2,4,6-Cl₃C₆H₂, 4-ClC₆H ₄, 4-Br-2,6-(CH₃)₂C₆H₂, 2,6-(CH₃)₂C₆H₃, 4-[(CH ₃)₂N]C₆H₄, 4-(C₆H ₅)C₆H₄, 4-nBuC₆H₄, tBu), has been synthesized in high yield by a highly selective photochemical substitution reaction. These complexes were characterized by ¹H NMR and IR spectroscopy, mass spectrometry, and elemental analysis. The X-ray crystal structures of one of the complexes and one of the precursor complexes for the photosubstitution reaction were also determined. As the isocyanide ligands are readily tunable, complexes with excellent solubility in benzene or other nonpolar solvents could be designed through slight modification of the isocyanide ligands with a short nBu substituent. With the characteristic strong infrared absorptions of the carbonyl (C≡O) and isocyanide (C≡N) stretches as well as the high solubility of the reactant and product in benzene, which is the solvent for the photoreaction, the photosubstitution reaction of [Re(CO)₃(nBuC₆H₄NC)₂Br] with 4,4'-di-tert-butyl-2,2'-bipyridine was also studied by in situ IR spectroscopy. The photophysical and electrochemical properties of these complexes were also investigated. Except for the complex with [(CH₃)₂N]C ₆H₄NC ligands, all complexes displayed intense luminescence with quantum yields of up to 0.37 in degassed CH₂Cl ₂ solution at room temperature. These emissions were assigned as the phosphorescence derived from the metal-to-ligand charge transfer [dπ(Re)→π*(N-N)] excited state. The emissive excited states of these complexes have also been characterized by transient absorption spectroscopic studies. The capability of tuning the emissive excited-state energy through the modification of the isocyanide ligands could be reflected by the significant shifting of the phosphorescence from 530 to 620 Nm with the same phenanthroline ligand. Tunable luminophore: A new class of rhenium(I) diimine luminophores, cis,cis-[Re(CO)₂(CNR)₂(N-N)]⁺ (R=2,4,6-Cl₃C₆H₂, 4-ClC₆H ₄, 4-Br-2,6-(CH₃)₂C₆H₂, 2,6-(CH₃)₂C₆H₃, 4-[(CH ₃)₂N]C₆H₄, 4-(C₆H ₅)C₆H₄, 4-nBuC₆H₄, tBu), has been synthesized by a selective photochemical substitution reaction. This photosubstitution was studied by in situ IR spectroscopy (see picture; phen=phenanthroline). Most complexes display intense metal-to-ligand charge transfer phosphorescence with quantum yields of up to 0.37. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.