The selective turn-on recognition of fluoride ions using 5-aryl-rhodanines: Colorimetric & fluorescent detection

The main goal of this study is to obtain 5-Ar-Rhs by Knoevenagel condensation without using any catalyst as an ion-sensing organic ligand and investigate their sensing abilities. The 5-Ar-Rhs were obtained as the main product from the no-catalyst reactions carried out at high temperatures. Following synthesis, the anion-sensing properties of 5-Ar-Rhs were studied using colorimetric detection, and spectroscopic methods. The results showed that the 5-Ar-Rhs is only selective for F¯¯ion in the presence of different ions. The binding ratios of 5-Ar-Rhs to F¯ were found to be 1:1 according to Job's plot experiments. The binding constant of 3A and 3B were calculated and found to be 7.05 × 105 M−1 and 1.45 × 104 M−1 for F¯, respectively. The LOD values of 3A and 3B were calculated and found to be 22.4 µM and 27.3 µM, respectively. In addition, theoretical studies by DFT methods were also carried out to optimize the structure and explained the fluoride anion binding mechanism of 5-Ar-Rhs 3A and 3B. As a result, temperature and solvent-controlled Knoevenagel condensation reactions of the 3-NH2-Rh (1), which has a reactive amino group, were examined. Additionally, we can understand that 3A and 3B are good selective candidate sensors that can be used for fluoride detection from sensing studies.