A Schiff base with excited-state intramolecular proton transfer and its Zinc (II) complex: Mechanochromic luminescence and acid-base stimuli response

A new Schiff base compound (HL & BULL;CH3COOH) with D-A structure and its zinc(II) complex (1) were successfully obtained. Due to the introduction of diethylamino and rotatable aromatic rings, both HL & BULL;CH3COOH and the complex adopt loosely packing patterns. HL & BULL;CH3COOH exhibits keto-form emission due to the excited-state intramolecular proton transfer (ESIPT) process, while complex 1 shows enol-form emission. They all show reversible mechanochromic luminescence (MCL) behavior, which is caused by the phase transformation from crystalline state to amorphous state revealed by Powder X-ray diffraction (PXRD) studies, and is accompanied by the departure of CH3COOH molecules for HL & BULL;CH3COOH. HL & BULL;CH3COOH and complex 1 can be protonated under the fumigation of HCl, resulting in the acidochromic properties, accompanied with the phase transformation. HCl also destroyed the interactions between CH3COOH and Schiff base molecules, leading to the departure of acetic acid molecules. More interestingly, HL & BULL;CH3COOH also exhibited NH3-stimuli response properties due to the prohibition of the ESIPT process.