Synthesis And Colorimetric Characterization Of Salicylidene Derivatives As Anion Sensors

Abstract

The synthesis and characterization of (E)-2-(2-hydroxybenzylideneamino)phenol (sensor 1), (E)-2-(2-hydroxybenzylideneamino)-5-nitrophenol (sensor 2), (E)-2-(2-hydroxybenzylideneamino)-4-nitrophenol (sensor 3), (E)-2-(2-hydroxy-5-nitrobenzylideneamino)-4-nitrophenol (sensor 4), (E)-2-(2- hydroxy-5-nitrobenzylideneamino)phenol (sensor 5) and (E)-2-((furan-2-ylmethylimino)methyl)phenol (sensor 6) are described. Sensors 1 - 6 were synthesized in one step from the condensation of salicylaldehyde or salicylaldehyde derivative and an appropriate aminophenol without solvent under irradiation from a microwave. These sensors comprises two parts; one is the anion binding part which is based on imine-phenol moieties and the other is a conjugated aromatic chromophore which converts binding induced changes into optical signals. The colorimetric and anion sensing properties of 1 - 6 towards anions such as F-, AcO-, H2PO4-, Br-, Cl-, ClO4- and HSO4- in CH3CN have been investigated. Visual inspection of solutions of the sensors before and after addition of F-, AcO- and H2PO4- ions resulted in dramatic colorimetric changes that were clearly visible to the naked eye. Upon binding to F-, AcO- and H2PO4-, the sensors displayed large bathochromic shifts in their UV/vis and fluorescence emission spectra. Colorimetric and UV/vis experiments showed that 1 and 6 had a strong selectivity for F- while 2, 3, 4 and 5 had a strong affinity for F-, AcO- and H2PO4-. Addition of Br-, Cl-, ClO4- and HSO4- to the acetonitrile solutions of the sensors resulted in very weak colorimetric and spectral changes. Job's plots showed a 1:2 binding between 1, 2, 4, 5 and 6 with F- while 3 showed a 1:1 binding to F-, AcO- and H2PO4- ions. In CH3CN-H2O mixture (9:1, v/v), sensor 3 discriminated AcO- from F- and H2PO4-. The binding constants of 1 - 6 were determined by UV/vis titration in CH3CN and analyzed by Benesi-Hildebrand expression. The calculated binding constants in the presence of selective anions were indicated to decrease in the order of 4 > 3 >5 > 2> 1 > 6. The selectivity of the studied anion sensors was also shown to decrease in line with the following order of the studied anions; AcO- > F- > H2PO4-. Fluorescence experiments indicated that sensors 1, 2 and 6 showed selective binding for F- whereas 3, 4 and 5 showed selective binding to the more basic ions (F-, AcO- and H2PO4-). The extent of conjugation, nature and position of the electron withdrawing -NO2 substituent in the structure of the sensors was observed to enhance anion selectivity. X-ray structures of 3 and 4 indicated that the compounds existed as iminio-phenolate zwitterions in solid state. The 1H-NMR and 13C-NMR results indicated that 3 and 4 existed as zwitterions in solution form. From the changes in the absorption and fluorescence spectra, proton transfer mechanisms have been deduced. In ground states, a two step process has been observed: first, the formation of the sensor-anion hydrogen-bond complex [LH...X] and secondly, the anion-induced deprotonation of the complex to form L- and HX2. In the excited states, the excited-state intermolecular proton transfer aided in the deprotonation of the sensors.