(rac)-1,1′-Binaphthyl-Based Simple Receptors Designed for Fluorometric Discrimination of Maleic and Fumaric Acids

(rac)-1,1'-Binaphthyl-based simple receptors 1 and 2 have been designed, synthesized and studied theoretically. The receptors utilize naphthyridine as the binding motifs for complexation of dicarboxylic acids in CHCl(3). The emission of the BINOL moiety was monitored experimentally to ascertain the selectivity and sensitivity of the receptors. Receptor 1 distinguishes maleic acid from isomeric fumaric acid by exhibiting different fluorescence behavior and demonstrates stronger binding in the excited state. Modulation of the binding sites of 1 leads to a new receptor structure 2, which was found to be less efficient in distinguishing maleic from fumaric acid, fluorometrically. Both 1 and 2 also recognize other hydroxy di- and tricarboxylic acids. The binding interactions were monitored by (1)H NMR, fluorescence and UV-vis spectroscopic methods. Structures of apo-hosts, guests and host-guest complexes were determined using force-field based conformational searching. Low energy ensembles were grouped into geometrically similar families, and low energy structures from each family were verified using B3LYP/6-31G*/PB-SCRF(CHCl(3)) calculations. The atomistic calculations provide insight into the differential dicarboxylic acid binding behavior of receptors 1 and 2.