Carbene–Metal Complexes As Molecular Scaffolds for Construction of through-Space Thermally Activated Delayed Fluorescence Emitters

Abstract

The through-space charge transfer (CT) process is observed in Cu(I) carbene−metal−amide complexes, where conventional imidazole or imidazoline N-heterocyclic (NHC) carbene fragments act as inert linkers and CT proceeds between a metal-bound carbazole donor and a distantly situated carbenebound phenylsulfonyl acceptor. The resulting electron transfer gives a rise to efficient thermally activated delayed fluorescence (TADF), characterized with high photoluminescence quantum yields (Φ PL up to 90%) and radiative rates (k r ) up to 3.32 × 10 5 s −1 . The TADF process is aided by fast reverse intersystem crossing (rISC) rates of up to 2.56 × 10 7 s −1 . Such emitters can be considered as hybrids of two existing TADF emitter design strategies, combining low singlet−triplet energy gaps (ΔE ST ) met in all-organic exciplex-like emitters (0.0062−0.0075 eV) and small, but non-negligible spin−orbital coupling (SOC) provided by a Cu atom, like in TADF-active organometallic complexes.

A Green Synthesis of Carbene‐Metal‐Amides (CMAs) and Carboline‐Derived CMAs with Potentin vitroandex vivoAnticancer Activity

“…Finally, the Au‐based complex 6 a bearing another widely used NHC, IMes (IMes= N,N ′‐bis‐(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene), was also obtained in high yield. To the best of our knowledge, a heterocyclic amine‐based CMA with the metal/NHC combination of 5 a was only very recently reported for the first time, [24] while a heterocyclic amine‐based CMA with the metal/NHC combination of 6 a is reported here for the first time.…”

“…The latter cases have received comparatively much less attention because of limited potential for OLED materials applications, even though recent advances show that modifications of the acceptor imidazol(in)ylidene ligand enable throughspace TADF. [24] Importantly, applications beyond TADF are possible when using imidazol(in)ylidene-based CMAs, with recent examples being long-lived room temperature phosphorescence (RTP), [18] and photocatalysis by triplet energy transfer. [22] For the latter application, the choice of the carbene ligand and the metal, Au in particular, is paramount as weaker π-acceptors enable longer-lived triplet states and the strong spin-orbit coupling (SOC) of the heavy metal atom enables efficient intersystem crossing (ISC), thus these compounds emit solely via phosphorescence, in addition to being more photolytically stable than their Ag congeners.…”

“…The ligand combination in the former case renders these materials ideal for Thermally Activated Delayed Fluorescence (TADF) applications. The latter cases have received comparatively much less attention because of limited potential for OLED materials applications, even though recent advances show that modifications of the acceptor imidazol(in)ylidene ligand enable through‐space TADF [24] . Importantly, applications beyond TADF are possible when using imidazol(in)ylidene‐based CMAs, with recent examples being long‐lived room temperature phosphorescence (RTP), [18] and photocatalysis by triplet energy transfer [22] .…”

An Air‐ and Moisture‐stable Zinc(II) Carbene Dithiolate Dimer Showing Fast Thermally Activated Delayed Fluorescence and Dexter Energy Transfer Catalysis**

“…This approach provides the additional advantage of various coordination spheres of the same metal centre being accessible, which greatly influences the nature, energy and reactivity of the excited states. Extensive work on trigonally and linearly coordinated Cu I compounds, in particular, has shown that very efficient triplet state decay can be obtained either by phosphorescence, [20,24–26] or thermally activated delayed fluorescence (TADF) [10,21,27–29] . For this, the ideal molecular structure should allow for the formation of low‐energy ligand‐to‐ligand charge transfer (LLCT) states with some metal‐to‐ligand (ML)CT admixture to mediate spin‐orbit coupling (SOC).…”

Observation of long-lived phosphorescence in Au(i) complexes bearing chromophoric (N-heterocyclic carbene) ligands

“…18,19 In these complexes, a cyclic (alkyl)(amino)carbene (CAAC) ligand serves as electron acceptor from the (amido) metal(I) fragment, leading to conformationally dependent delayed fluorescence. One example is the work of Traskovskis and co-workers 20 who demonstrate that sulfonyl-substituents in the N-aryl moieties of (NHC)Cu(I) and -Ag(I) complexes reinforce charge transfer, resulting in thermally activated delayed fluorescence, in another instance of carbene-modulated luminescence. Thompson and co-workers described a digold(I)-amide complex where the supporting carbene ligand is a binuclear gold(I) construct.…”