structural relaxation and vibronic coupling of the excited states. For pure organic materials, donor-acceptor typed thermally activated delayed fluorescence (TADF) emitters are able to realize 100% internal quantum efficiency via efficient reverse intersystem crossing (RISC) process, but the geometrical deformation between D and A units typically causes broad intramolecular charge-transfer emission with substantial Stokes shift in devices. [2] A promising molecular design strategy termed multiresonance TADF (MR-TADF) that fully addressed the aforementioned obstacle has been lately proposed by Hatakeyama et al. [3] This is made possible by atomic separation of frontier molecular orbitals (FMOs) via opposite resonance effect of electron-rich nitrogen and electron-deficient boron in a rigid molecular framework, which promises i) limited excited-state reorganization and minimized bonding/antibonding characteristics of FMOs, leading to compressed full width at half maximum (FWHM) of emission band; ii) adequate separation and overlap of the electron/hole wavefunctions to induce small singlet-triplet energy difference (ΔE ST ) and intense oscillating strength (f), leading to high luminescence efficiency. [4][5][6][7] An additional merit for most MR-TADF emitters is their tendency to adopt horizontal anisotropic orientation in vacuum-evaporated organic thin films, which would benefit light extraction from device and enhance efficiency. [5,8] For instance, by employing a state-of-the-art MR-TADF emitter ν-DABNA, maximum external quantum efficiency (EQE max ) up to 34.4% with high color purity (FWHM = 18 nm and Commission International de l'Éclairage (CIE) coordinates of (0.12, 0.11)) was realized in deep-blue OLED. [8,9] An important issue to be addressed for MR-TADF emitters is the severe aggregation-caused quenching (ACQ) and spectral broadening in solid state due to intense π-π interactions. [6,10,11] In a recent photophysical study, Monkman's group unveiled the strong prevalence for ν-DABNA to form less emissive aggregates/excimers even at extremely low doping concentrations, resulting in broadened linewidth and lower-than-expected device performances. [12] These findings offer a clear explanation to the fact that, though many MR-TADF emitters present narrowband photoluminescence (PL) emission with quantum Multiresonance thermally activated delayed fluorescence (MR-TADF) emitters manifest great potential for organic light-emitting diodes (OLEDs) due to their high exciton-utilization efficiency and narrowband emission. Nonetheless, their tendency toward self-quenching caused by strong interchromophore interactions would induce doping sensitivity and deteriorate the device performances, and effective strategy to construct quenching-resistant emitters without sacrifycing color purity is still to be developed. By segregating the planar MR-TADF skeleton using two bulky carbazolyl units, herein a highly emissive molecule with enhanced quenching resistance is reported. The steric effect largely removes the formation of de...

Multi-resonance TADF (MR-TADF) emitters are promising for high-resolution OLEDs, but the concurrent optimization of excited-state dynamics and color purity remains a tough challenge. Herein, three deep-blue MR-TADF compounds (BN1-BN3) featuring gradually enlarged ring-fused structures and increased rigidity are accessed by lithiumfree borylation in high yields from the same precursor, with all the emitters possessing CIE y coordinates below 0.08. Structure-property investigations demonstrate a strategic improvement of the oscillator strength (f osc ) and acceleration of the reverse intersystem crossing (RISC) process by extending the π-skeleton, where BN3 realizes a maximum external quantum efficiency (EQE) of 37.6 % and reduced roll-off, thus showing the best efficiency reported for deep-blue TADF OLEDs. The internal regulation of the efficiency and color purity of these compounds validate the general effectiveness to achieve advanced deep-blue narrowband emitters with higher-order boron/nitrogen-based MR motifs.

Multiresonance thermal activated delayed fluorescence (MR-TADF) materials with an efficient spin–flip transition between singlet and triplet excited states remain demanding. Herein, we report an MR-TADF compound (BN–Se) simultaneously possessing efficient (reverse) intersystem crossing (ISC/RISC), fast radiative decay, close-to-unity quantum yield, and narrowband emission by embedding a single selenium atom into a common 4,4′-diazaborin framework. Benefitting from the high RISC efficiency accelerated by the heavy-atom effect, organic light-emitting diodes (OLEDs) based on BN–Se manifest excellent performance with an external quantum efficiency of up to 32.6% and an ultralow efficiency roll-off of 1.3% at 1000 cd m–2. Furthermore, the high ISC efficiency and small inherent energy loss also render BN–Se a superior photosensitizer to realize the first example of visible (λex > 450 nm)-to-UV (λem < 350 nm) triplet–triplet annihilation upconversion, with a high efficiency (21.4%) and an extremely low threshold intensity (1.3 mW cm–2). This work not only aids in designing advanced pure organic molecules with fast exciton dynamics but also highlights the value of MR-TADF compounds beyond OLED applications.

light-emitting diodes (OLEDs) due to their low production-cost as well as the capability to harvest "dark" triplet excitons via a reverse intersystem crossing (RISC) mechanism. [1][2][3][4] Facilitating of RISC is made by reduction of the energy gap (ΔE ST ) between the lowest excited singlet and triplet (S 1 and T 1 ) states, typically achieved in twisted donor-acceptor (D-A) typed molecules with separated frontier molecular orbitals (FMOs) at the expense of low oscillator strength (f) and photoluminescence quantum yield (Φ PL ). [5][6][7][8] This contradiction undoubtedly sets an obstacle to obtain highly efficient emitter, and the large structural reorganization occurred in the excited state would cause broad emission spectra and increased non-radiative decay channels, eventually weakening the performances of corresponding devices. [9][10][11][12][13][14][15] Instead of the conventional D-A configured emitters, a unique category of multi-resonance TADF (MR-TADF) molecules based on fused polycyclic aromatics was lately proposed by Hatakeyama et al. to mitigate the aforementioned issues. [16][17][18][19][20][21][22] The complementary resonance effects of electrondeficient B and electron-rich N/O atoms within the framework separates the FMOs to induce short-range charge-transfer (SR-CT), concurrently offering a small ΔE ST and a high radiative decay rate (10 7 -10 8 s −1 ) from S 1 to ground (S 0 ) state. [17,23] The planar nature with high rigidity guaranteed narrow full width at half maximum (FWHM) emissions, and could also induce favorable horizontal dipole orientation of the emitters to boost optical out-coupling efficiency. [24] Thus far, the majority of the blue MR-TADF emitters were constructed from the basic motif DABNA (Scheme 1) due to synthetic feasibility and their decent quantum efficiencies. [25] Replacement of the diphenylamino subunits into carbazolyl-derived BN-CZ as a new core skeleton with bathochromic emission, which was adopted to realize full-color electroluminescence (EL) with high color purity and maximum external quantum efficiencies (EQE max s) above 20% by peripheral electronic modulation. [24,[26][27][28][29] Nonetheless, most MR-TADF devices encountered severe triplet-related efficiency loss at high luminance/current density stemmed from the intrinsic structural planarity and long-delayed lifetime, and relied on the involvement of sensitizing host to avoid triplet Multi-resonance thermally activated delayed fluorescence (MR-TADF) offers an exceptional solution for narrowband organic light-emitting diode devices in terms of color purity and luminescence efficiency, while the development of new MR skeleton remains an exigent task. It is hereby demonstrated that a simple modification of the B (boron)−N (nitrogen) framework by sp 3 -carbon insertion will significantly bathochromic shift the short-range charge-transfer emission, boost the reverse intersystem crossing process, and improve the device performances. The bis(acridan)phenylene-based skeleton developed in this contribution presen...

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