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

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.