Red-emitting neutral rhenium(i) complexes bearing a pyridyl pyridoannelated N-heterocyclic carbene

Two novel rhenium(i) tricarbonyl complexes of general formula fac-[Re(N^C:)(CO)3X] are herein presented, where N^C: is the pyridoannelated N-heterocyclic carbene (NHC) arising from 2-(2-pyridinyl)imidazo[1,5-a]pyridinium hexafluorophosphate proligand, namely [pyipy]PF6, and X being Cl and Br. The synthetic pathway is a one-pot reaction that starts from the azolium salt as the NHC source and [Re(CO)5X] to yield the desired charge-neutral fac-[Re(pyipy)(CO)3X] complexes (1-2). Both complexes were thoroughly characterized by spectroscopic, electrochemical, theoretical investigation as well as X-ray diffraction analysis. They display a rather similar electronic absorption spectrum in dilute CH2Cl2 solution, which is characterized by a broad profile extending into the blue region. This lowest-lying absorption band is attributed to a transition with admixed metal-to-ligand charge transfer and intraligand charge transfer (1MLCT/1ILCT) character. Degassed samples of the complexes display moderate (Φ ≈ 1.5%) and long-lived (τ = 12.8-13.4 μs) red photoluminescence with highly structured profile independent of the nature of the ancillary halogen ligand and little sensitivity to the solvent polarity, highlighting the markedly different nature of the emitting excited state in comparison with the lowest-lying absorption. Indeed, photoluminescence is ascribed to a long-lived excited state with metal-perturbed triplet ligand-centred (3LC) character as supported by both experimental and density functional theory (DFT) investigations.