The two-coordinate carbene−metal−amide complexes have attracted a great deal of attention due to their remarkable thermally activated delayed fluorescence (TADF) properties, giving them promise in organic light-emitting diode application. To reveal the inherent mechanism, we take CAAC−Cu(I)−Cz and CAAC−Au(I)−Cz as examples to investigate the photophysical properties in solution and solid phases by combining quantum mechanics/molecular mechanics approaches for the electronic structure and the thermal vibration correlation function formalism for the excited-state decay rates. We found that both intersystem crossing (ISC) and its reverse (rISC) are enhanced by 2−4 orders of magnitude upon aggregation, leading to highly efficient TADF, because (i) the metal proportion in the frontier molecular orbitals increases, leading to an enhanced spin−orbit coupling strength between S 1 and T 1 , and (ii) the reaction barriers for ISC and rISC are much lower in solution than in aggregate phases through a decrease in energy gap ΔE ST and an increase in the relative reorganization energy through bending the angle ∠C2−Cu−N1 for T 1 . We propose a pump−probe time-resolved infrared spectroscopy study to verify the mechanism. These findings can clarify the ongoing dispute over the understanding of the high TADF quantum efficiency for two-coordinate metal complexes.

It is generally believed that the electron-withdrawing cyano group in the olefin linkage would inhibit the stability and πconjugation of covalent organic frameworks (COFs), which raises concerns about their optoelectronic properties. However, the structure−activity relationship between the structure of olefin linkages and properties of COFs is still inconclusive. In this work, imine-, vinylene-, and acrylonitrile-linked COFs with identical triphenyltriazine building blocks were designed and synthesized. Our work demonstrated that construction of acrylonitrile linkages not only enhanced the chemical stability and photostability but also led to remarkable optoelectronic properties with a record fluorescence quantum yield of 35.37% in the solid state. Further, the acrylonitrile linkage endows TTAN-COF/Pt NPs with superior and durable photocatalytic activity in both the hydrogen evolution reaction (11.94 mmol g −1 h −1 ; BET surface area, 739.28 m 2 g −1 ) and aerobic oxidation reaction. This work demonstrates that the acrylonitrile linkage can significantly enhance the optoelectronic properties and photocatalytic activities of COFs compared with the highly π-conjugated vinylene linkage, providing a valuable reference for the design of optoelectronic functional materials.

Thermally activated delayed fluorescence (TADF) materials are competitive candidates toward electrically pumped organic lasing, because of its ability to suppress triplet accumulations by reverse intersystem crossing (RISC), especially, the multiresonance TADF (MR-TADF) compounds featuring narrow-band emission and high photoluminescence quantum yields. The goal of this work is to theoretically screen out promising electrically pumped organic laser compounds over both MR-TADF and conventional TADF molecules. We calculate the photophysical parameters over 21 organic TADF molecules to determine if the electrically pumped lasing criteria can be met, i.e., no substantial absorption/annihilation processes caused by excitons and polarons near the S1 emission wavelength. The selection criteria include large oscillator strength of S1, large net emission cross-section, long S1 lifetime, and large reverse intersystem crossing rate. We are able to conclude that DABNA-2, m-Cz-BNCz, ADBNA-Me-Mes, and ADBNA-Me-Tips MR-TADF molecules are prospective candidates for electrically pumped lasing based on our theoretical protocol, and we believe this work would immediately benefit this field with better and more efficient molecular design of TADF gain materials.

It is generally perceived that fast reverse intersystem crossing rate of T1→S1 (krisc) is crucial for efficient organic thermally activated delayed fluorescence (TADF) emitters. We demonstrate here that for transition...

Compared with isoalloxazine, the core chromophore of biologically important flavins, alloxazine exhibits much lower fluorescence quantum yield and larger intersystem-crossing quantum yield. However, its efficient radiationless relaxation pathways are still elusive. In this work, we have used the QM­(MS-CASPT2//CASSCF)/MM method to explore the mechanistic photophysics of alloxazine chromophore in aqueous solution. On the basis of the optimized minima, conical intersections, and crossing points in the lowest 1ππ*, 1 nπ*, 3ππ*, and 3 nπ* states, we have proposed three energetically possible nonadiabatic relaxation pathways populating the lowest 3ππ* triplet state from the initially populated excited 1ππ* singlet state. The first is the direct 1ππ*→ 3ππ* intersystem crossing via the 1ππ*/3ππ* crossing point. The second is an indirect 1ππ* → 3ππ* intersystem crossing relayed by the dark 1 nπ* singlet state. In this route, the 1ππ* system first decays to the 1 nπ* state via the 1ππ*/1 nπ* conical intersection, followed by an 1 nπ*→ 3ππ* intersystem crossing at the 1 nπ*/3ππ* crossing point to arrive at the final 3ππ* state. The third is similar to the second one; but its intersystem crossing is relayed by the 3 nπ* triplet state. The 1ππ* system first decays to the 3 nπ* state via the 1ππ*/3 nπ* crossing point; the generated 3 nπ* state is then de-excited to the 3ππ* state through the 3 nπ*→ 3ππ* internal conversion at the 3 nπ*/3ππ* conical intersection. According to the classical El-Sayed rule, we suggest the second and third paths play a much more important role than the first one in the formation of the lowest 3ππ* state.