A Cyclic Alkyl(amino)carbene as Two-Atom π-Chromophore Leading to the First Phosphorescent Linear CuI Complexes

The members of a series of linear and trigonal copper(I) complexes bearing a cyclic alkyl(amino) carbene (CAAC) ligand show surprising photophysical properties compared to those of the corresponding N-heterocyclic carbene (NHC) complexes. Whereas the linear NHC complexes [CuX(NHC)] are almost non-emissive, [CuX(CAAC)] (X= Cl, Br, I) and [Cu(CAAC)(2)] PF6 show very bright emissions from their triplet excited states in the blue to green region, displaying quantum yields of up to 65% in the solid state, even though the pi-acceptor comprises only the carbene C and N atoms with no other pi conjugation. [Cu(CAAC)(2)] PF6 is the fastest Cu-I-based triplet state emitter characterized to date, not dis-playing thermally activated delayed fluorescence (TADF), with an intrinsic lifetime of only 10.6 mu s, that is, k(r)= 9.4 V 10(4) s(-1), competitive with many Pt-II-and Ir-III-based emitters. In order to test the stability of such linear copper CAAC complexes in devices, some of our compounds have been applied in proof-of-principle organic light-emitting diodes (OLEDs). This case study thus demonstrates for the first time the use of CAACs as suitable pi-chromophores for Cu-I-based phosphorescent emitters, and their implementation in OLEDs underlines the general applicability of this class of ligands in materials science.