Abstract
Conformationally flexible "Carbene-Metal-Amide" (CMA) complexes of copper and gold have been developed based on a combination of sterically hindered cyclic (alkyl)(amino)carbene (CAAC) and 6- and 7-ring heterocyclic amide ligands. These complexes show photoemissions across the visible spectrum with PL quantum yields of up to 89% in solution and 83% in host-guest films. Single crystal X-ray diffraction and photoluminescence (PL) studies combined with DFT calculations indicate the important role of ring structure and conformational flexibility of the amide ligands. Time-resolved PL shows efficient delayed emission with sub-microsecond to microsecond excited state lifetimes at room temperature, with radiative rates exceeding 106 s-1. Yellow organic light-emitting diodes (OLEDs) based on a 7-ring gold amide were fabricated by thermal vapor deposition, while the sky-blue to warm-white mechanochromic behavior of the gold phenothiazine-5,5-dioxide complex enabled fabrication of the first CMA-based white light-emitting OLED.
Highlights
Two-coordinate coinage metal complexes with linear geometry (L)MX (L 1⁄4 carbene; M 1⁄4 Cu, Ag or Au; X 1⁄4 anionic ligand) have recently emerged as a new class of strongly photoemissive materials.[1,2,3,4]. Their effectiveness is based on a combination of ligands with complementary donor and acceptor properties: on the one hand, a carbene ligand capable of acting as both a strong electron donor and effective p-acceptor, and, on the other hand, an anionic ligand X that on photochemical or electrical excitation enables charge transfer to the acceptor orbital of the carbene
We report the use of these emitters in organic light-emitting diodes (OLEDs), including the rst case of a device based on a mechanochromic CMA material, an application enabled by the conformational exibility of the amide ligand
Au1 shows a mechanochromic response due to its ability to crystallize in two different modi cations: monoclinic crystals show a bright featureless sky-blue emission at 475 nm, whereas the orthorhombic form emits warm-white light with a maximum at 540 nm accompanied by a minor peak at 475 nm (30% photoluminescence quantum yields (PLQY)) (Fig. 6)
Summary
The emission mechanism of CMAs has been the subject of several theoretical and spectroscopic investigations.[16,17,18,19] Theoretical calculations revealed that the highest occupied molecular orbital (HOMO) is located mostly on the carbazole, while the lowest unoccupied molecular orbital (LUMO) comprises mainly the Ccarbene p-orbital.[13,15] Excitation is a ligand-to-ligand charge transfer process (LLCT) from the carbazole to the carbene ligand involving mainly a HOMO / LUMO transition, with only a minor (#5% to HOMO, 7–15% to LUMO) contribution of the metal orbitals. Edge Article between ground (mgs) and excited states (mes) (Scheme 1) This process differs from that operative in most metalbased phosphors where metal-to-ligand charge transfer (MLCT) is dominant.[20,21,22,23,24,25,26,27,28]. The electron-donor capacity of these amido ligands varies signi cantly, so that the luminescence wavelengths can be tuned to cover the visible spectrum from blue to deep red.[33] We report the use of these emitters in OLEDs, including the rst case of a device based on a mechanochromic CMA material, an application enabled by the conformational exibility of the amide ligand
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