Synthesis, Structure, and Properties of Two Planar BN-Benzofluorenes

Two planar BN doped benzofluorenes (<b>BN-BkF</b> and <b>BN-BjF</b>) were synthesized separately through palladium-catalyzed intramolecular C-C or C-N coupling reactions. The structures of both <b>BN-BkF</b> and <b>BN-BjF</b> are unambiguously confirmed by X-ray crystallographic analysis. Moreover, the BN unit doping leads to a much lower HOMO level and higher LUMO level as compared to their carbon analogues <b>BkF</b> and <b>BjF</b>. In comparison to <b>BkF</b> and <b>BjF</b>, both <b>BN-BkF</b> and <b>BN-BjF</b> exhibit blue shifts in their ultraviolet-visible (UV-vis) absorption and fluorescence emission spectra. Furthermore, halogenation of <b>BN-BkF</b> and <b>BN-BjF</b> afforded monohalogenated BN-benzofluorenes in good yields. These monohalogenated BN-benzofluorenes can serve as convenient intermediates for palladium-catalyzed coupling reactions to yield a series of functionalized BN-benzofluorene derivatives. The UV-vis absorption and emission spectroscopies in dichloromethane of these BN-benzofluorene derivatives were studied, and the photophysics of these compounds exhibited a high degree of substituent dependency.