Ammonium Arylspiroborate Compounds: Synthesis, Crystal Structure, Fluorescence Properties, and Antibacterial Activity

A series of R-substituted ammonium arylspiroborate compounds (R = CH3, H, F, Cl, Br) have been synthesized via a facile one-pot method. The structures of these new kinds of boron compounds were characterized by IR, NMR spectra, elemental analysis and X-ray diffraction techniques, and their possible formation mechanism was proposed. Surprisingly, the as-synthesized boron compounds showed multifunctional character with not only excellent fluorescence properties (high quantum yields and large Stokes shift) in both the liquid and solid state but also high antibacterial activity against both Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria. The chlorine-substituted boron compound even possessed antibacterial activity comparable, with that of commercial antibiotics. More importantly, the fluorescence properties and antibacterial activity of these boron compounds could be easily mediated by the variation of the substituents, which was proved to regulate the conjugated system, modify the hydrophobic character, and adjust the electric charge on the surface of boron compounds. In addition, the influence of different factors including solvent (i.e., polarity, concentration), temperature, and pH on the fluorescence properties of boron compounds was discussed in detail. This work provides a new strategy for the design of multifunctional organoboron compounds and the mediation of their properties.