Metal‐based thermally activated delayed fluorescence (TADF) is conceived to inherit the advantages of both phosphorescent metal complexes and purely organic TADF compounds for high‐performance electroluminescence. Herein a panel of new TADF Au(I) emitters has been designed and synthesized by using carbazole and pyrazine‐fused nitrogen‐heterocyclic carbene (NHC) as the donor and acceptor ligands, respectively. Single‐crystal X‐ray structures show linear molecular shape and coplanar arrangement of the donor and acceptor with small dihedral angles of <6.5°. The coplanar orientation and appropriate separation of the HOMO and LUMO in this type of molecules favour the formation of charge‐transfer excited state with appreciable oscillator strength. Together with a minor but essential heavy atom effect of Au ion, the complexes in doped films exhibit highly efficient (Φ∼0.9) and short‐lived (<1 μs) green emissions via TADF. Computational studies on this class of emitters have been performed to decipher the key reverse intersystem crossing (RISC) pathway. In addition to a small energy splitting between the lowest singlet and triplet excited states (ΔEST), the spin‐orbit coupling (SOC) effect is found to be larger at a specific torsion angle between the donor and acceptor planes which favours the RISC process the most. This work provides an alternative molecular design to TADF Au(I) carbene emitters for OLED application.

The Pd complex PdN3N exhibits an unusual dual emission of roomtemperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF), but the mechanism is elusive. Herein, we employed both density functional theory (DFT) and time-dependent DFT (TD-DFT) methods to explore excited-state properties of this Pd complex, which shows that the S 0 , S 1 , T 1 , and T 2 states are involved in the luminescence. Both the S 1 → T 1 and S 1 → T 2 intersystem crossing (ISC) processes are more efficient than the S 1 fluorescence and insensitive to temperature. However, the direct T 1 → S 1 and T 2 -mediated T 1 → T 2 → S 1 reverse ISC (rISC) processes change remarkably with temperature. At 300 K, these two processes are more efficient than the T 1 phosphorescence and therefore enable TADF. Importantly, the T 1 → S 1 rISC and T 1 phosphorescence rates are comparable at 300 K, which leads to dual emissions of TADF and RTP, whereas these two channels become blocked at 100 K so that only the T 1 phosphorescence is recorded experimentally.

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 (ΔE ST ) between the lowest singlet excited state (S 1 ) and the triplet manifold (T 1 ) and thus a faster reverse intersystem crossing (RISC) from T 1 to S 1 at the expense of oscillator strength for an S 1 radiative 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.

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 S1 state, 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 T2 state should play a negligible role. The calculated S1–T1 energy gap of 0.10 eV and proper spin–orbit couplings facilitate the reverse intersystem crossing (rISC) from T1 to S1. At 298 K, the rISC rate of T1 → S1 (∼106 s–1) is more than 3 orders of magnitude larger than the T1 phosphorescence rate (∼103 s–1), thereby enabling TADF. However, it disappears at 77 K because of a very slow rISC rate (∼101 s–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.

We have employed the QM(CASPT2//CASSCF)/MM method to explore the excited-state isomerization and decay mechanism of a single-bond-rotation locked photoactive yellow protein (PYP) chromophore in wild-type and mutant proteins. The S 1 state is a spectroscopically bright state in the Franck−Condon region. In this state, there exist two excited-state isomerization pathways separately related to the clockwise and anticlockwise rotations of the C=C bond. The clockwise path is favorable because of a small barrier of 2 kcal/mol and uses a novel bicycle-pedal unidirectional photoisomerization mechanism in which the involved two dihedral angles rotate asynchronously because of the reinforced hydrogen-bonding interaction between the chromophore and Cys69. Near the twisted S 1 minimum, the chromophore hops to the S 0 state via the S 1 /S 0 conical intersection. Finally, the R52A mutation has small effects on the excited-state properties and photoisomerization of the locked PYP chromophore. The present work provides new insights for understanding the photochemistry of PYP chromophores in protein surroundings.