A Luminescent Two‐Coordinate Au^IBimetallic Complex with a Tandem‐Carbene Structure: A Molecular Design for the Enhancement of TADF Radiative Decay Rate

Abstract

A luminescent bimetallic AuI complex comprised of N‐heterocyclic carbene (NHC) and carbazole (Cz) ligands, that is, (NHC’)Au(NHC)AuCz has been synthesized and studied. Both carbene ligands in the bimetallic complex act as electron acceptors in tandem to increase the energy separation between the ground and excited state, which is higher than those found in either monometallic analogue, (NHC)AuCz and (NHC’)AuCz. A coplanar geometry designed into the tandem complex ensures sufficient electronic coupling between the π‐orbitals of the ligands to impart a strong oscillator strength to the singlet intra‐ligand charge‐transfer (1ICT) transition. Theoretical modelling indicates that the emissive ICT excited state involves both NHC ligands. The tandem complex gives blue luminescence (λmax=480 nm) with a high photoluminescent quantum yield (ΦPL=0.80) with a short decay lifetime (τ=0.52 μs). Temperature‐dependent photophysical studies indicate that emission is via thermally assisted delayed fluorescence (TADF) and give a small singlet‐triplet energy difference (ΔEST=50 meV, 400 cm−1) consistent with the short TADF lifetime.

Toward rational design of TADF two-coordinate coinage metal complexes: understanding the relationship between natural transition orbital overlap and photophysical properties

“…As revealed by the diffraction results, the molecules present linear two-coordination geometry with near coplanar orientation of NHC and carbazole ligands in agreement with data from analogous cMa derivatives. [29][30][31]48 The dihedral angles between the two ligands planes range from 0.31 to 12.71. The C NHC -M bond is longer than the M-N Cz bond in Me-Cu, but they become near equal in Me-Ag.…”

“…25,[28][29][30]37 Investigations have been carried out focusing on two-coordinate TADF complexes experimentally and theoretically, to develop structure-property relationships and strategies to achieve high radiative (k r ) and low non-radiative (k nr ) decay rates. [29][30][31]38,[43][44][45][46][47][48] TADF molecules fall into two basic categories, depending on whether they have slow or fast rates for intersystem crossing (ISC). 12 Organic TADF materials generally have slow ISC rates (k ISC = 10 5 -10 8 s À1 ) owing to weak spin-orbital coupling (SOC), which makes fluorescent radiative decay k S 1 r À Á competitive with intersystem crossing.…”

“…25,28–30,37 Investigations have been carried out focusing on two-coordinate TADF complexes experimentally and theoretically, to develop structure–property relationships and strategies to achieve high radiative ( k r ) and low non-radiative ( k nr ) decay rates. 29–31,38,43–48…”

Synthesis of MAuAg (M = Ni, Pd, or Pt) and NiAuCu Heterotrimetallic Complexes Ligated by a Tritopic Carbanionic N-Heterocyclic Carbene

“…Very recently, Thompson and co-workers have demonstrated that digold complexes comprised of d-NHCs D and carbazole ligands exhibited an enhanced luminescence efficiency. The d-NHCs could act as electron acceptors to increase the energy separation between the ground and excited states, which is higher than that observed in the monometallic analogue …”

Carbene-metal complexes as molecular scaffolds for construction of through-space TADF emitters

“…16,17 Due to a favorable electronic level configuration and metal involvement TADF lifetimes below 1.0 μs can be achieved. 18,19 Here we present a novel approach towards TADF emitters that can be considered as a hybrid case between the previously discussed all-organic through-space and metal-assisted designs (Fig. 1).…”