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The efficient harvesting of triplet excitons is crucial to the realization of high-performance organic light-emitting diodes (OLEDs). Herein, we show that coordination of donor-acceptor (D-A) type molecules to a metal atom in a monodentate fashion can lead to thermally activated delayed fluorescence (TADF) emissions with wide color tunability only through varying the non-coordinating acceptor moiety. A panel of TADF gold(I) complexes with emission maxima (λmax) of 545–645 nm from metal perturbed intraligand charge-transfer (MPICT) excited states have been developed. Synergetic effects of heavy atom-induced spin-orbit coupling (SOC), steric-induced donor-acceptor twisting and suppressed intramolecular motions lead to high emission efficiencies of 65%–85% in doped films with delayed fluorescence lifetime of as short as 2.0 µs. Transient absorption spectroscopic studies on selected complexes determined the kISCto be 6.5 × 109s−1. Theoretical calculations confirmed the participation of minor d orbital into the lowest excited state, which led to an SOC value of 5.19 cm−1between the lowest-lying singlet and triplet excited states. The yellow to deep red solution-processed OLEDs based on the new gold(I) complexes incorporated with various D-A ligands demonstrated promising performances. This study validates a modular design for TADF metal complexes, which will broaden the choices of metal centers and allow for facile color tuning viasimple ligand synthesis.

Herein we investigated the luminescence mechanism of one “carbene–metal–amide” copper compound with thermally activated delayed fluorescence (TADF) using density functional theory (DFT)/multireference configuration interaction, DFT, and time-dependent DFT methods with the polarizable continuum model. The experimentally observed low-energy absorption and emission peaks are assigned to the S1state, which exhibits clear interligand and partial ligand-to-metal charge-transfer character. Moreover, it was found that a three-state (S0, S1, and T1) model is sufficient to describe the TADF mechanism, and the T2state should play a negligible role. The calculated S1–T1energy gap of 0.10 eV and proper spin–orbit couplings facilitate the reverse intersystem crossing (rISC) from T1to S1. At 298 K, the rISC rate of T1→ S1(∼106s–1) is more than 3 orders of magnitude larger than the T1phosphorescence rate (∼103s–1), thereby enabling TADF. However, it disappears at 77 K because of a very slow rISC rate (∼101s–1). The calculated TADF rate, lifetime, and quantum yield agree very well with the experimental data. Methodologically, the present work shows that only considering excited-state information at the Franck–Condon point is insufficient for certain emitting systems and including excited-state structure relaxation is important.

Carbene–Au–amide (CMA) type complexes, in which the amide and carbene ligands act as an electron donor (D) and acceptor (A), respectively, can exhibit strong delayed fluorescence (DF) from a ligand to ligand charge transfer (LLCT) excited state. Although the coplanar donor–acceptor (D-A) conformation has been suggested to be a crucial factor favoring radiative decay of the charge-transfer excited state, the geometric structural factor underpinning the excited-state mechanism of CMA complexes remains an open question. We herein develop a new class of carbene–Au–carbazolate complexes by introducing large aromatic substituents onto the carbazolate ligand, the presence of which are conceived to restrict the rotation of the Au–N bond and thus confine a twisted D-A conformation in both ground and excited states. A highly twisted D-A orientation is found for the complexes in their crystal structures. Photophysical studies reveal that the twisted conformation induces a decrease in the gap (ΔEST) between the lowest singlet excited state (S1) and the triplet manifold (T1) and thus a faster reverse intersystem crossing (RISC) from T1to S1at the expense of oscillator strength for an S1radiative transition. In comparison with the coplanar analogue, the twisted complexes exhibit comparable or improved DF with quantum yields of up to 94% and short emission lifetimes down to sub-microseconds. The tuning of excited-state dynamics has been well interpreted by density functional theory (DFT) and time-dependent DFT (TDDFT) calculations, which unveil much faster RISC rates for twisted complexes. Solution-processed organic light-emitting diodes (OLEDs) based on the new CMA complexes show promising performances with almost negligible efficiency rolloff at a brightness of 1000 cd m–2. This work implies that neither a coplanar ground-state D-A conformation nor a dynamic rotation of the M–N bond is the key to the realization of efficient DF for CMA complexes.

Monovalent coinage metal complexes with a carbene–metal–amide (CMA) structure have emerged as an appealing class of thermally activated delayed fluorescence (TADF) emitters. The relative donor–acceptor orientation has been identified as a crucial factor that determines the TADF properties. In this work, two Au(I)-based CMA complexes with sandwich structures, namely, Au-DPhand Au-DDPXZ, were designed and synthesized. The benzene (Ph) and O-bridged triphenylamine (DPXZ) having different electron-donating capabilities were introduced to the carbazole (Cz) fragment. Theoretical and photophysical studies were performed to investigate the electronic structures and photophysical properties of the CMA complexes. Both complexes show tiny ΔESTvalues (0.01 and 0.02 eV) and thus faster reverse intersystem crossing rates owing to the good separation of frontier molecular orbitals. However, the photoluminescence quantum yields are lower for the present sandwich-type complexes (58% for Au-DDPXZand 71% for Au-DPh) than for the CMA analogues with a coplanar or less twisted structure. Together with their lifetimes of 1.14 and 1.69 μs, the radiative decay rates were determined to be 3.43–6.23 × 105s–1. The theoretical calculations manifest a decreased oscillator strength for the S1radiative decay arising from the separation of frontier orbitals. This work reveals that the TADF properties of CMA complexes are affected by various parameters and that a suitable donor–acceptor torsion angle is necessary for optimizing the TADF efficiency.

N- and B-Embedded polycyclic aromatic hydrocarbons with a multiresonance (MR) character have fascinating photophysical properties including high photoluminescence quantum yields and ultrapure emission color purity. The combination of MR character and heavy metal atom for developing photoluminescent transition metal complexes is of great interest but has rarely been investigated. Herein a platinum(II) complex, namely, Pt(BICPy)(acac), containing a MR moiety in the cyclometalated ligand has been synthesized and characterized. The complex displays strong narrowband phosphorescence with a quantum yield of 53% and a lifetime of 26 μs. Theoretical calculations unveiled the enhanced spin–orbit coupling effect induced by metal coordination. This work provides a way for harvesting the triplet excitons of MR type molecules.