A New Boron-Rhodamine-Containing Carboxylic Acid as a Sugar Chemosensor

We propose a boron-rhodamine-containing carboxylic acid (BRhoC) substance as a new sugar chemosensor. BRhoC was obtained by the Friedel-Crafts reaction of 4-formylbenzoic acid and N,N-dimethylphenylboronic acid, followed by chloranil oxidation. In an aqueous buffer solution at pH 7.4, BRhoC exhibited an absorption maximum (Abs_max) at 621 nm. Its molar absorption coefficient at Abs_max was calculated to be 1.4 × 10⁵ M^-1 cm^-1, and it exhibited an emission maximum (Em_max) at 644 nm for the excitation at 621 nm. The quantum yield of BRhoC in CH₃OH was calculated to be 0.16. The borinate group of BRhoC reacted with a diol moiety of sugar to form a cyclic ester, which induced a change in the absorbance and fluorescence spectra. An increase in the D-fructose (Fru) concentration resulted in the red shift of the Abs_max (621 nm without sugar and 637 nm with 100 mM Fru) and Em_max (644 nm without sugar and 658 nm with 100 mM Fru) peaks. From the curve fitting of the plots of the fluorescence intensity ratio at 644 nm and 658 nm, the binding constants (K) were determined to be 2.3 × 10² M^-1 and 3.1 M^-1 for Fru and D-glucose, respectively. The sugar-binding ability and presence of a carboxyl group render BRhoC a suitable building block for the fabrication of highly advanced chemosensors.