Molecular Design and Synthesis of Narrowband Near-Ultraviolet and Pure Deep-Blue Thermally Activated Delayed Fluorescence Materials by an Ether Group Strategy

Abstract

Boron-containing polycyclic aromatic hydrocarbons are promising materials for the development of displays due to their multiple-resonance thermally activated delayed fluorescence (MR-TADF) with narrowband emission. However, except for electrophilic aromatic borylation reactions, synthetic strategies for the generation of boron-containing MR-TADF molecules remain virtually unexplored. In particular, the synthesis of MR-TADF emitters that exhibit narrow near-ultraviolet and pure deep-blue emission constitutes a challenging task. Here, we present a directed tri-ortho-lithiation-borylation approach that provides a new family of N,N-bridge-type triphenylboranes that bear phenylimino groups instead of the methylene groups at the 8- and 14-positions and ether groups instead of the hydrogen atoms at the 3- and 19-positions of 1-borapentacyclohenicosanonaene. The effects of the electron-donating resonance of the oxygen atoms of the ether groups and the incorporation of oxygen atoms in the six-membered cycle allow the precise tuning of the HOMO-LUMO energy gaps, resulting in narrowband near-ultraviolet and pure deep-blue TADF with Commission-International-de-l'Éclairage coordinates (CIE_{x , y}) of (0.142-0.160, 0.029-0.063) for the photoluminescence (PL) and (0.146-0.160, 0.026-0.053) for the electroluminescence (EL). The CIE_{x , y} for the EL meet the BT.2020 requirement for the blue primary of ultrahigh-definition displays.

Keywords: Boron; Deep blue; Polycyclic aromatic hydrocarbons; Thermally activated delayed fluorescence; Ultraviolet.

Figure 1

a) Chemical structures of 1‐borapentacyclohenicosanonaene and DABNA‐1. Protocols for the synthesis of N,N‐bridge‐type triphenylboranes via b),c) previously reported electrophilic borylation reactions and d) directed tri‐ortho‐lithiation–borylation reactions followed by further derivatization developed in this study.

Figure 2

Chemical structures and theoretical‐calculation results of a)–d) DABNA‐1, e)–h) 2, and i)–l) 6. b),f),j) Side views of the S₀‐optimized geometries, c),g),k) frontier molecular orbitals (isovalue = 0.04), and d),h),l) energy‐level diagrams with spin–orbit‐coupling matrix elements (SOCMEs) at the T₁‐optimized geometries. Transition energies for S₁, T₁, and T₂ were calculated at the SCS‐CC2/def2‐TZVP//TPSSh/6–311G(d,p) level; SOCMEs were calculated at the TPSSh/TZP//TPSSh/6–311G(d,p) level.

Scheme 1

Synthesis of N,N‐bridge‐type triphenylboranes 2–6. a) For 3: 1‐BuBr (3.0 mol. eq.), NaH (4.0 mol. eq.), DMF, 120 °C, 24 h. b) For 4: Et₃SiCl (3.0 mol. eq.), NaH (4.0 mol. eq.), 1,4‐dioxane, 90 °C, 5 h. c) For 5: CH₂BrCH₂Br (1.1 mol. eq.), K₂CO₃ (3.0 mol. eq.), DMF, 120 °C, 24 h. d) For 8: Tf₂O (2.2 mol. eq.), ⁱ Pr₂NEt (3.0 mol. eq.), CH₂Cl₂, 0 °C, 18 h.

Figure 3

Molecular structures a) 2 and b) 6 in the crystalline state with thermal ellipsoids at 50% probability; hexane molecules in the single crystals of 6 are omitted for clarity.

Figure 4

Photophysical properties of 2–6. a) UV–vis absorption and b) normalized PL spectra in toluene solution (1 × 10⁻⁵ M, 293 K, λ _ex = 320 nm). c) Normalized PL spectra, d),f) transient PL decay curves at time scales of 100 ns and 680 µs, respectively, and e) the corresponding CIE _x,y diagram in PMMA (3 wt.%‐doped film, 293 K, λ _ex = 320 nm).

Figure 5

EL characteristics of OLED devices fabricated using 2–6. a) Device structure with energy‐level diagram, b) normalized EL spectra with EQE dependence on the luminescence (inset), and c) the corresponding CIE _x,y diagram.