Simple Acridan‐Based Multi‐Resonance Structures Enable Highly Efficient Narrowband Green TADF Electroluminescence

Abstract

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...

Narrowing the Electroluminescence Spectra of Multiresonance Emitters for High-Performance Blue OLEDs by a Peripheral Decoration Strategy

“…This design enables the emitter to realize a small Δ E ST and suppressed structural relaxation in the excited states concurrently, resulting in TADF with a narrowband emission as well as high photoluminescence quantum yields (PLQYs). A series of MR-TADF materials based on the B–N or carbonyl–nitrogen framework have been developed for high-performance TADF OLEDs with a small fwhm of as low as 18 nm and an external quantum efficiency (EQE) of up to 34%. − Nevertheless, compared to the conventional D–A-type TADF materials, the development of MR-TADF compounds is still in its infancy. The relationship between the chemical structures and their photophysical properties, including the efficiency and fwhm, still lags behind.…”

“…Decorating the B–N-based framework with substituent moieties is one of the most effective strategies to realize MR-TADF emitters with high efficiency, tunable emission, as well as a narrow bandwidth. For example, by rigidifying the peripheral phenyl groups of DABNA-1 with a sp 3 -carbon or heteroatom linkage, a series of MR-TADF materials have been developed with green emission. ,, The peripheral decoration of a carbazole-based MR-TADF skeleton BNCz with electron-withdrawing or -donating moieties at suitable positions has also been proved effective to realize the narrowband emission at longer wavelengths. ,− ,, However, the study of blue MR-TADF emitters with a peripheral decoration strategy is still rare. In addition, though many reported MR-TADF emitters exhibited extremely small fwhm’s in a dilute solution state, a few of them achieved electroluminescence (EL) spectra with the fwhm being smaller than 25 nm (0.14 eV) to meet the requirements of visual verisimilitude levels.…”

The Rise of 1,4‐BN‐Heteroarenes: Synthesis, Properties, and Applications

“…Notably, MR‐TADF materials 151 and 153 with sp 3 ‐carbon bridge were also reported by Yang and co‐workers independently in the same year. [ 59 ] The nonplanar framework in these two compounds significantly boosted the RISC process, resulting in superior device performances (EQEs up to 28.2%) without utilizing any sensitizer. Furthermore, the introduction of bulkier bis(acridan)phenylene groups in 153 were proved effective to boost the efficiency and alleviate roll‐off behavior (1.8% at 100 cd m −2 ).…”

Steric Modulation of Spiro Structure for Highly Efficient Multiple Resonance Emitters

“…Yang et al. compared methyl‐ and phenyl‐substituted acridan‐based donors in affecting MR‐TADF features, in which the installation of the bulkier periphery by phenyl dangling groups can boost the external quantum efficiency (EQE) to approximately 28 % and alleviate roll‐off behavior [4g] . Very recently, Duan et al.…”