Multi-stimuli responsive behavior of two Schiff base complexes with high contrast multicolor switching and wearable applications for rapid detection of HCl and NH3 vapor

In this work, we successfully synthesized two Schiff base (HL) complexes, ZnLCl2 (1) and [Cd2L2Cl4]·CH3CN (2), via the one-pot reaction of metal salts with 4-[bis(4-bromophenyl)amino]benzaldehyde and 3-hydroxy-2-naphthoic acid hydrazide. Complex 2 exhibits not only acidochromic properties but also NH3-induced discoloration behavior, which is rarely reported among triphenylamine derivatives or complexes. However, complex 1 shows no color change upon fuming with HCl or NH3 vapor. The experimental results show that acidochromism may be due to the decomposition of the structure by HCl vapor, while NH3-induced discoloration is mainly attributed to the phase change of 2 from crystalline to amorphous state. In addition, complexes 1 and 2 showed high-contrast mechanochromic luminescence (MCL) change from yellow to orange and green to yellow, respectively. Complexes 1 and 2 also exhibit electrochromic behavior with color change from yellow to brown. Moreover, complex 2 was successfully made into test paper for mechanical stimuli and the rapid detection of HCl and NH3 vapor. The complex 2 can be fixed on a mask coat to fabricate a 2-based wearable hazardous gas sensor, suggesting that 2 is expected to develop into the next generation wearable sensors for detection of HCl and NH3 vapor.