Borylation and rearrangement reactions of azasilaanthracenes to afford B,N-doped nanographenes

Abstract

An air-stable B₃,N₃-containing dibenzobisanthene (8) was prepared in 29% yield by heating a 1,3,5-tri(azasilaanthryl)benzene (5) with BBr₃ (180 °C). Under these conditions, the reaction does not stop after threefold SiMe₂/BBr exchange but proceeds further via two rearrangement and two intramolecular C–H borylation steps. Some mechanistic details were unveiled by using smaller model systems and applying lower reaction temperatures. According to X-ray crystallography, compound 8 has a helically distorted scaffold. Due to its multiple resonance structure, it shows a narrow-band blue-green emission (λ_em = 493 nm; Φ_PL = 84%; FWHM = 0.20 eV; THF); samples measured in PMMA gave prompt and delayed fluorescence lifetimes of 10.7 ns and 136 μs, respectively. The optical properties of 8 and of structurally related species were also investigated by quantum-chemical means: most of these compounds exhibit a small energy gap ΔE_ST between the lowest excited singlet (S₁) and triplet (T₁) states and a non-negligible spin–orbit coupling (SOC) between S₁ and T₁/T₂, demonstrating their potential as thermally activated delayed fluorescence (TADF) emitters.