Tuning of reversible thermochromic properties of salicylaldehyde Schiff bases through the substitution of methoxy and nitro groups

Abstract In this study, in order to explore the new kinds of reversible thermochromic materials, four salicylaldehyde Schiff bases (BSP1-4) substituted with methoxy and nitro groups were synthesized and their reversible thermochromic properties were investigated. All compounds were characterized by HNMR, IR, UV–vis, TG-DSC, and SEM techniques. Through HNMR, IR and UV–vis spectra, four target products display different colors. The UV–Vis spectra show that the type and position of the substituents of BSPs can control the absorption wavelength by changing the electronic transition mode, which leads to the difference of the color of the samples. With the increase of light colored samples, the content of the ΔE1 gradually increases until the maximum value of 48.6. TG-DSC result indicates good thermal stability of the BSP below 350 °C. SEM images show that the surface morphologies of four BSP samples are different, including flake, rod and strip. The thermochromic properties of the BSP samples were evaluated using variable temperature FT-IR spectroscopic, solid UV–Vis spectra and response time. DFT calculations were conducted to rationalize the optical behavior of the BSP samples in Enol and Ketone form. The results of variable temperature FT-IR support the hypothesis that the reversible thermochromic mechanism of salicylaldehyde Schiff base is proton transfer. Among all of BSPs, the BSP1 has the best reversible thermochromic cycle stability (ΔE2 = 2.6) and the shortest response time (10s) which can be used as the new promising thermochromism materials.