Highly emissive planarized B,N-diarylated benzonaphthoazaborine compounds for narrowband blue fluorescence

Highly fluorescent blue emitters with high color purity are of great significance for optical applications. Herein, a series of planarized B,N-diarylated benzonaphthoazaborine compounds, namely, BzNp (1), BuBzNp (2), Bu(2)BzNp (3), Bu(2)BzMeNp (4), and Bu(2)BzBuNp (5), where electron-donating (t)Bu and Me groups are differently introduced into the B-Ph, N-Ph, or benzoazaborine rings, are prepared and characterized. All compounds exhibit low-energy absorptions (λ(abs) = 462–467 nm) and emissions (λ(PL) = 472–478 nm) remarkably red-shifted compared with those found for the pristine dibenzoazaborine compound (404 and 415 nm, respectively). Although the expansion of π-conjugation in the azaborine ring by replacing one phenyl ring with a naphthyl ring is mainly responsible for the redshifts, the emission is also fine-tuned by attached alkyl groups, which have a greater impact on the B-centered LUMO level at the azaborine ring than at the B-Ph ring. The bandgap control and emission tuning are further supported by electrochemical and theoretical studies. Notably, blue to sky-blue fluorescence of all compounds exhibits unitary photoluminescence quantum yields, narrow full width at half maximum values (∼20 nm), and small Stokes shifts (∼11 nm), indicating strong emissions with high color purity.