Tetradentate Gold(III) Complexes as Thermally Activated Delayed Fluorescence (TADF) Emitters: Microwave‐Assisted Synthesis and High‐Performance OLEDs with Long Operational Lifetime

Abstract

Structurally robust tetradentate gold(III)‐emitters have potent material applications but are rare and unprecedented for those displaying thermally activated delayed fluorescence (TADF). Herein, a novel synthetic route leading to the preparation of highly emissive, charge‐neutral tetradentate [C^C^N^C] gold(III) complexes with 5‐5‐6‐membered chelate rings has been developed through microwave‐assisted C−H bond activation. These complexes show high thermal stability and with emission origin (3IL, 3ILCT, and TADF) tuned by varying the substituents of the C^C^N^C ligand. With phenoxazine/diphenylamine substituent, we prepared the first tetradentate gold(III) complexes that are TADF emitters with emission quantum yields of up to 94 % and emission lifetimes of down to 0.62 μs in deoxygenated toluene. These tetradentate AuIII TADF emitters showed good performance in vacuum‐deposited OLEDs with maximum EQEs of up to 25 % and LT95 of up to 5280 h at 100 cd m−2.

Highly efficient carbazolylgold(iii) dendrimers based on thermally activated delayed fluorescence and their application in solution-processed organic light-emitting devices

“…[34][35][36] Following our OLED studies on gold(III) systems, [34][35][36] Che and co-workers very recently reported tridentate and tetradentate ligand-containing gold(III) complexes that demonstrated EL performance and operational stability in vacuum-deposited OLEDs. [41][42][43] These small-molecular based gold(III) complexes were found to exhibit TADF properties, as supported by their fast radiative decay rate constant (k r ) in the order 10 5 s À1 , completely separated frontier orbitals, variable temperature emission and excited-state decay studies. It is worth noting that a large k r value of $1.36 Â 10 5 s À1 could also be found in our recently reported alkynylgold(III) dendrimers, 39 indicating that the emission is not purely originating from the triplet excited state.…”

“…It is worth noting that the excited state lifetimes of 1 are almost unchanged in the range of 200 K (1.3 ms) to 320 K (1.2 ms), suggesting that the thermal upconversion process is almost complete under these temperature conditions. 56 As the ISC rate constants (k ISC ) of structurally related cyclometalated gold(III) complexes are found to be in the picosecond regime, [41][42][43] much faster than that of the rate constant for uorescence decay (k  ), the DE ST of these complexes can be estimated by tting the variable-temperature emission data to the full kinetic model employed by Dias, Penfold, and Monkman. 57 1)) and k B is the Boltzmann constant.…”

“…It is worth noting that a large k r value of $1.36 Â 10 5 s À1 could also be found in our recently reported alkynylgold(III) dendrimers, 39 indicating that the emission is not purely originating from the triplet excited state. [41][42][43] Recent reports have demonstrated that the photophysical properties of cyclometalated gold(III) complexes can be signicantly enriched upon replacing the more common C^N^C type of ligand framework with the C^C^N moiety, [44][45][46] owing to the suppressed non-radiative decay rate constant via the distorted triplet state. Taking advantage of the rich photoluminescence behavior found in the solid state, herein we report the synthesis of rstand second-generation carbazolylgold(III) dendrimers (2 and 3), which are higher generation dendrimers of a previously reported structurally related small-molecule carbazolylgold(III) complex, [Au{4-t BuC^C(4-t BuC 6 H 4 )^N (py) }(Cbz)] (1).…”

Recent Advances in Metal-TADF Emitters and Their Application in Organic Light-Emitting Diodes

“…Since Cu(I) complexes generally lack thermal and electrochemical stability, efforts have been directed to develop 2nd and 3rd row transition-metal-TADF emitters such as those of Pd(II), Ag(I), Au(I), and Au(III) to meet the stringent requirements of emitters for practical OLEDs (Li G. et al, 2019 ). Gratifyingly, high external quantum efficiencies (EQEs) of up to 27.5 and 25.0% have been achieved in OLEDs with Au(I) and Au(III) TADF emitters, respectively (Di et al, 2017 ; Zhou et al, 2020 ), reflecting Au-TADF emitters as an emerging new class of competitive, emissive dopant in OLED industry.…”

“…Solution-processed OLEDs fabricated with these emitters showed EQE of up to 11.1%. Che and co-workers developed new strategies for synthesizing tetradentate Au(III) complexes with O-bridged/spiro-arranged C ∧ C ∧ N ∧ C ligand by microwave induced C-H activation (Zhou et al, 2020 ). By rationally varying the substituent(s) on the ligand, the emissive excited states of the Au(III) emitters are changed from triplet intraligand ( 3 IL) excited states with k r of ~10 3 s −1 , to TADF from ILCT excited states.…”