Synthesis of new luminescent bis-azo-linkage Schiff bases containing amino-phenol and its derivative. Part I: Studying of their tautomeric, acidochromic, thermochromic, ionochromic, and photolüminesence properties

In this work, two novel bis-azo-linkage Schiff bases (H4L1 and H4L2) containing conjugated group (phenol ring in H4L1 and nitro-phenol ring in H4L2) were synthesized by using new azo-aldehyde compound AZ-1 (3,3′-(4-nitro-1,2-phenylene)bis(diazene-2,1-diyl)bis(2-hydroxy-5-nitrobenzaldeyde). They were characterized by elemental analysis and IR, 1H/13C NMR, UV–vis, and masss pectral techniques. Prototropic tautomeric equilibrium (azo-imine ↔ hydrazone-imine or azo-imine ↔ azo-amine), and acido-, thermo-, ionochromism in these Schiff bases were investigated by UV–vis spectroscopy. Photochromism study indicated that DMSO solution of H4L1 and H4L2 exhibits strong green luminescence at room temperature upon photoexcitation, attributed to exist as hydrazone-imine form in the excited state. Emission wavelength was 505 and 523 nm in photoluminescence spectra. On the other hand, azo-imine tautomer was stabilized by increasing of temperature in the ground state in solution for both H4L1 and H4L2. As well, H4L2 showed a selective and sensitive colorimetric sensing toward acid-base and anions (F−, CH3COO−, C2O42- and SO32-).