Azaborine as a Versatile Weak Donor for Thermally Activated Delayed Fluorescence

Abstract

Extensive research has been devoted to the development of thermally activated delayed fluorescence emitters, especially those showing pure-blue emission for use in lighting and full-color display applications. Toward that goal, herein we report a novel weak donor, 1,4azaborine (AZB), with complementary electronic and structural properties compared to the widely used dimethylacridan (DMAC) or carbazole (Cz) donors. Coupled with a triazine acceptor, AZB-Ph-TRZ is the direct structural analogue of the high-performance and well-studied green TADF emitter DMAC-TRZ and has ΔE ST = 0.39 eV, a photoluminescence quantum yield (Φ PL ) of 27%, and λ PL = 415 nm in 10 wt % doped mCP films. The shortened analogue AZB-TRZ possesses redshifted emission with a reduced singlet−triplet gap (ΔE ST = 0.01 eV) and fast reverse intersystem crossing (k RISC of 5 × 10 6 s −1 ) in mCP. Despite a moderate Φ PL of 34%, OLEDs with AZB-TRZ in mCP showed sky-blue emission with CIE 1931 (x,y) of (0.22,0.39) and a maximum external quantum efficiency (EQE max ) of 10.5%. Expanding the chemist's toolkit for the design of blue donor− acceptor TADF materials will enable yet further advances in the future, as AZB is paired with a wider range of acceptor groups.

Understanding the Key Requirements for Ultra-Efficient Sensitisation in Hyperfluorescence OLEDs.

“…Starting from our recent work using the high-performance TADF emitter DMAC-TRZ as a benchmark, 17,31 we find that despite the lower non-HF device performance of ACRSA, the latter enjoys a considerable performance boost in HF-OLEDs, with an almost tripling of the EQEmax. 10,32 Similar results are also obtained using the recently reported deeper blue TADF emitter AZB-TRZ, 33 which exhibits very fast rISC but is intrinsically limited by low PLQY. Surprisingly, despite the photoluminescence spectra of ACRSA and DMAC-TRZ being energetically similar, ACRSA exhibits significantly higher overall FRET efficiency in HF systems, even when compared to the deeper blue AZB-TRZ sensitizer which has better FRET spectral overlap.…”

“…AZB-TRZ (Figure 4e) has a considerably different balance of decay rates compared to ACRSA and DMAC-TRZ, with its intrinsic performance limited instead by a fast non-radiative decay rate and low PLQY (Table S2). 33 Similar to DMAC-TRZ, a clear distribution of dihedral angles is observed through the time dependent emission spectra (Figure S5a) leading to limited spectral overlap of specific conformers (redshifted species) with the terminal emitter. The spectral overlap of the steadystate emission spectra with ν-DABNA is nonetheless higher compared to ACRSA and DMAC-TRZ sensitisers (Figure S4), leading to the highest J factor (Table S3) and FRET radius (Table 2) in this series of sensitisers.…”

Structure Engineering of Acridine Donor to Optimize Color Purity of Blue Thermally Activated Delayed Fluorescence Emitters

“…In this work, fluorophenyl (F‐Ph) and trifluoromethylphenyl (CF 3 ‐Ph) were used as substituents and grafted into the 2,7‐sites of DMAC to design new donors 2,7‐F‐Ph‐DMAC and 2,7‐CF 3 ‐Ph‐DMAC, and finally to develop blue TADF emitters. It is anticipated that the introduction of the electron withdrawing group F‐Ph or CF 3 ‐Ph on DMAC ring not only weakens the electron‐donating ability of DMAC donors, facilitating a hypsochromic shift in emission and enhancing blue color purity, but also preserves outstanding electroluminescent properties [30–32] . The widely used triphenyltriazine (TRZ), i. e. cyaphenine [33–36] , was selected as acceptor to prepare two D‐A type TADF emitters, namely 10‐(4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)phenyl)‐2,7‐bis(2‐fluorophenyl)‐9,9‐dimethyl‐9,10‐dihydroacridine (2,7‐F‐Ph‐DMAC‐TRZ) and 10‐(4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)phenyl)‐9,9‐dimethyl‐2,7‐bis(2‐(trifluoromethyl)phenyl)‐9,10‐dihydroacridine (2,7‐CF 3 ‐Ph‐DMAC‐TRZ, structures shown in Scheme 1).…”

“…It is anticipated that the introduction of the electron withdrawing group F-Ph or CF 3 -Ph on DMAC ring not only weakens the electron-donating ability of DMAC donors, facilitating a hypsochromic shift in emission and enhancing blue color purity, but also preserves outstanding electroluminescent properties. [30][31][32] The widely used triphenyltriazine (TRZ), i. e. cyaphenine [33][34][35][36] , was selected as acceptor to prepare two D-A type TADF emitters, namely 10-(4-(4,6-diphenyl-1,3,5-triazin-2yl)phenyl)-2,7-bis(2-fluorophenyl)-9,9-dimethyl-9,10-dihydroacridine (2,7-F-Ph-DMAC-TRZ) and 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9,9-dimethyl-2,7-bis(2-(trifluoromethyl)phenyl)-9,10-dihydroacridine (2,7-CF 3 -Ph-DMAC-TRZ, structures shown in Scheme 1). Meanwhile, the previously reported TADF emitter, 2,7-Ph-DMAC-TRZ, was synthesized as a reference for comparison.…”

Borylation–Reduction–Borylation for the Formation of 1,4-Azaborines

“…This is consistent with the lower isolated yield observed for 4d relative to 4a – 4c . Nevertheless, accessing 4d with a N -benzyl group is important as it can be deprotected to form the N -H-1,4-azaborine for use in subsequent reactions as a number of us previously have reported …”