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

Two planar BN doped benzofluorenes (BN-BkF and BN-BjF) were synthesized separately through palladium-catalyzed intramolecular C-C or C-N coupling reactions. The structures of both BN-BkF and BN-BjF 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 BkF and BjF. In comparison to BkF and BjF, both BN-BkF and BN-BjF exhibit blue shifts in their ultraviolet-visible (UV-vis) absorption and fluorescence emission spectra. Furthermore, halogenation of BN-BkF and BN-BjF 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.