Efficient thermally activated delayed fluorescence materials based on the multiple resonance effect were synthesized by nucleophilic substitution and electrophilic C–H borylation, featuring a small energy gap between the singlet and triplet state, high photoluminescence quantum yield, and narrow emission band. Organic light-emitting diodes employing these materials as emitters exhibited sky-blue emission with a high external quantum efficiency of 21.4% and a narrow full width at half-maximum of 33 nm.

An expanded heterohelicene consisting of three BN 2 -embedded [4]helicene subunits (V-DABNA-Mes) has been synthesized by one-shot triple borylation. The key to success is the excessive use of boron tribromide in an autoclave. Based on the multiple resonance effect of three boron and six nitrogen atoms, V-DABNA-Mes exhibited a narrowband sky-blue thermally activated delayed fluorescence with a full width at half-maximum of 16 nm. The resonating π-extension minimized the singlet−triplet energy gap and enabled rapid reverse intersystem crossing with a rate constant of 4.4 × 10 5 s −1 . The solution-processed organic lightemitting diode device, employed as an emitter, exhibited a narrowband emission at 480 nm with a high external quantum efficiency of 22.9%.

Ultra-narrowband blue multi-resonance-induced thermally activated delayed fluorescence (MR-TADF) materials (V-DABNA and V-DABNA-F), consisting of three DABNA subunits possessing phenyl or 2,6-difluorophenyl substituents on the peripheral nitrogen atoms are synthesized by one-shot triple borylation. Benefiting from the inductive effect of fluorine atoms, the emission maximum of V-DABNA-F (464 nm) is blueshifted from that of the parent V-DABNA (481 nm), while maintaining a small full width at half maximum (FWHM, 16 nm) and a high rate constant for reverse intersystem crossing (6.5 × 10 5 s −1 ). The organic light-emitting diodes (OLEDs) using V-DABNA and V-DABNA-F as emitters are fabricated by vapor deposition and exhibit blue emission at 483 and 468 nm with small FWHMs of 17 and 15 nm, corresponding to Commission Internationale d'Éclairage coordinates of (0.09, 0.27) and (0.12, 0.10), respectively. Both devices achieve high external quantum efficiencies of 26.2% and 26.6% at the maximum with minimum efficiency roll-offs of 0.9% and 3.2%, respectively, even at 1000 cd m −2 , which are record-setting values for blue MR-TADF OLEDs.

Electrophilic C–H borylation of arenes using boron triiodide has been developed. This reaction proceeded smoothly in the absence of additives, and the diiodoboryl group was installed at the most sterically accessible carbon, where the HOMO is localized to a certain extent. Moreover, regioselective multiple borylation of polycyclic aromatic compounds was achieved by using excess boron triiodide. The borylated intermediates were transformed into a variety of arylboron compounds such as arylboronates, boronic acids, and trifluoroborates.

Metrics & MoreArticle Recommendations* sı Supporting InformationThe full width at half-maximum of the electroluminescence spectrum in an OLED device employing V-DABNA-Mes as an emitter was miscalculated to be 27 nm. The corrected value is 17 nm (93 meV) as shown in Figure 4 below. ■ ASSOCIATED CONTENT* sı Supporting InformationThe Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.2c11135. Synthesis, analytical and crystal data, photoelectron yield, UV/vis absorption, and NMR spectra, DFT studies, fabrication of OLED devices, Cartesian coordinates (PDF)