Structural and Photophysical Studies of Phosphorescent Three-Coordinate Copper(I) Complexes Supported by an N‑Heterocyclic Carbene Ligand

A series of four neutral luminescent three-coordinate
Cu­(I) complexes
(IPr)­Cu­(N∧N), where IPr is a monodentate N-heterocyclic
carbene (NHC) ligand (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene)
and N∧N denotes monoanionic pyridyl-azolate ligands,
have been synthesized and characterized. A monomeric, three-coordinate
geometry, best described as distorted trigonal planar, has been established
by single-crystal X-ray analyses for three of the derivatives. In
contrast to the previously reported (IPr)­Cu­(N∧N)
complexes, the compounds described here display a perpendicular orientation
between the chelating N∧N ligands and the imidazolylidene
ring of the carbene ligand. The geometrical preferences revealed by
X-ray crystallography correlate well with the NMR data. The conformational
behavior of the complexes, investigated by variable-temperature 1H NMR spectroscopy, indicate free rotation about the CNHC–Cu bond in solution. The complexes display broad,
featureless luminescence at both room temperature and 77 K, with emission
maxima that vary between 555 and 632 nm depending on sample conditions.
Luminescence quantum efficiencies of the complexes in solution (Φ
≤ 17%) increase markedly in the solid state (Φ ≤
62%). On the basis of time-dependent density functional theory (TD-DFT)
calculations and the experimental data, luminescence is assigned to
phosphorescence from a metal-to-ligand charge-transfer (MLCT) triplet
state admixed with ligand-centered (LC) character.