UV-Vis spectroscopic and colorimetric anion detection and fluorescence properties of new 3-amino-4-hydroxybenzenesulfonic acid-based Schiff bases depending on the strength and position of the electron donor substitution

Abstract

In this study, imine compounds derived from 3-amino-4-hydroxybenzenesulfonic acid were synthesized and their structures were determined using spectroscopic techniques. The compounds were analyzed using various spectroscopy methods, and the experimental UV-Vis data matched the theoretical predictions. 3-(2,5-dihydroxybenzylideneamino)-4-hydroxybenzenesulfonic acid (Compound 1) displayed lower stability, higher reactivity, and easier photoexcitation due to a smaller HOMO-LUMO energy gap. The investigated compounds showed promise as chemosensors for anions, providing visible detection in daylight conditions. 4-hydroxy-3-(2-hydroxy-3-methoxybenzylideneamino)- benzenesulfonic acid (Compound 3) exhibited selective fluorescence at specific wavelengths. The imine compounds interacted with DNA through electrostatic interactions. Moreover, the investigated compounds demonstrated higher antioxidant activity compared to BHT, with 4-hydroxy-3-(2-hydroxy-3-methoxybenzylideneamino)- benzenesulfonic acid (Compound 3) displaying the highest activity. However, fluorescence measurements indicated that the emission signals were strongly influenced by the position and strength of the electron-donating group. Adding a hydroxy or methoxy moiety near the -OH group on the phenyl ring decreased the fluorescence signal due to intersystem crossing and intramolecular charge transfer mechanisms, respectively. These findings were supported by femtosecond transient absorption spectroscopy measurements. The results emphasize the significance of substituents in imines derived from 3-amino-4-hydroxybenzenesulfonic acid in determining their biological activities, as well as their optical and sensor properties.