Synthesis And Charecterization Of Schiff Bases As Anion Sensors

Abstract

This study demonstrates how Sensors 1 - 4 were synthesized in one step from the condensation of appropriate salicylaldehyde and derivative mixed with company of furfurylamine without solvent at room temperatures. Sensor 1 was synthesized under irradiation from a microwave. A schiff base is a compound with a functional group that contains carbon-nitrogen double bonded with a nitrogen atom connected to a group of aryl or alkyl. These sensors are in two parts; one is the anion binding part which is based on furfurylamine part and the other is a conjugated aromatic chromophore which converts binding-induced-changes into optical signals. The colorimetric and anion sensing properties of 1 - 4 towards anions such as F¯, CN¯, SCN¯, AcO¯, OH¯, HSO₄¯, H₂PO₄¯, Br¯, Cl¯, ClO₄¯, N₃¯ and Cystine in CH₃CN have been studied. Optical examination of solutions of the sensors before and after addition of F¯, CN¯, OH¯, H₂PO₄¯ and AcO¯ ions resulted in intense colorimetric changes that were clearly observable to the naked eye. Upon binding to F¯, CN¯, OH¯, H₂PO₄¯ and AcO¯ the sensors shown large bathochromic shifts in their UV-Vis and fluorescence emission spectra. Colorimetric and UV-Vis experiments showed that 1 and 2 had a strong selectivity for F¯ while sensor 2 had strong selectivity for CN¯ too. Sensor 3 and 4 did not have strong selectivity and detection for any of anions; sensor 3 displays fluorescence intensity toward N₃¯ and H₂PO₄¯. Adding other anions to the acetonitrile solutions of the sensors resulted in very weak colorimetric and spectral changes. Job's plots showed a 1:2 binding between Sensor 1 and F¯ and Sensor 2 with F¯, CN¯ and AcO¯ ions, while 3 and 4 showed non-binding and detecting toward anions. In CH3CN-H2O mixture (9:1, v/v), Sensor 1 and 2 dismissed F¯ detection. The binding constants of 1 and 2 were determined by UV/Vis titration in CH3CN and analyzed by Benesi-Hildebrand expression. Indication the calculated binding constants shows that the presence of selective anions decrease in the order of 2> 1. Fluorescence experiments indicated that Sensors 1 and 2 showed selective binding for F¯ and AcO¯ while 2 showed selective binding to the CN¯ too; Sensor 3 showed selective binding to the N3¯ and H₂PO₄¯. The extent of conjugation, nature and position of the electron withdrawing -NO2 substituent in the structure of the Sensor 2 was observed to improve anion selectivity. X-ray structures of 2 indicated that the compounds existed as imino-nitrophenol zwitterions in solid state. The 1H-NMR and 13C-NMR results showed that 1 and 2 existed as zwitterions in solution form. Proton transfer mechanisms have been recognized due to the changes in the absorption and fluorescence spectra. In ground states, two steps process has been observed: the development of the sensor-anion hydrogen-bond complex [LH…X] in addition to the anion induced deprotonation of the complex form L and HX2. In the excited states, the excited-state intermolecular proton transfer assisted in the deprotonation of the sensors. Figure 1: Shows Sensor 1, 2, 3, 4