Linear Carbene Pyridine Copper Complexes with Sterically Demanding N,N′-Bis(trityl)imidazolylidene: Syntheses, Molecular Structures, and Photophysical Properties

The sterically demanding carbene ITr (N,N'-bis(triphenylmethyl)imidazolylidene) was used as a ligand for the preparation of luminescent copper(I) complexes of the type [(ITr)Cu(R-pyridine/R'-quinoline)]BF4 (R = H, 4-CN, 4-CHO, 2,6-NH2, and R' = 8-CI, 6-Me). The selective formation of linear, bis(coordinated) complexes was observed for a series of pyridine and quinoline derivatives. Only in the case of 4-cyanopyridine a one-dimensional coordination polymer was formed, in which the cyano group of the cyanopyridine ligand additionally binds to another Cu atom in a bridging manner, thus leading to a trigonal planar coordination environment. In contrast, employing sterically less demanding monotrityl-substituted carbene 3, no (NHC)Cu-pyridine complexes could be prepared. Instead, a bis-carbene complex [(3)(2)Cu]PF6 was obtained which showed no luminescence. All linear pyridine/quinoline coordinated complexes show weak emission in solution but intense blue to orange luminescence doped with 10% in PMMA films and in the solid state either from triplet excited states with unusually long lifetimes of up to 4.8 ms or via TADF with high radiative rate constants of up to 1.7 x 10(5) s(-1) at room temperature. Combined density functional theory and multireference configuration interaction calculations have been performed to rationalize the involved photophysics of these complexes. They reveal a high density of low-lying electronic states with mixed MLCT, LLCT, and LC character where the electronic structures of the absorbing and emitting state are not necessarily identical.