Multistimuli Responsive Behaviors of Salicylaldehyde Hydrazone Zn(II) Complexes in Solution, Solid State, and PMMA Film

Zn(II) complexes based on salicylaldehyde hydrazone ligands have emerged as a promising class of stimuli-responsive luminescent materials. Herein, three Zn(II) complexes, namely, [Zn₂(HL¹)₂(py)₂]·H₂O (1a), Zn₂(HL¹)₂(py)₂ (1b), and Zn₂(HL²)₂(py)₂ (2), were synthesized by self-assembly with Zn(II) and salicylaldehyde hydrazone under solvothermal conditions. Complexes 1a and 2 exhibit reversible mechanochromic luminescence (MCL) during grinding and fuming processes due to the transformation between the crystalline phase and amorphous phase. In addition, slight grinding induces a crystal phase transformation from 1a to 1b. Comprehensive structural analysis and experimental results indicate that grinding may destroy the coordination bond, leading to the dissociation of the pyridine molecule from the metal center. Besides, the existence of weak interaction between molecules is easy to destroy upon external mechanical force, resulting in MCL with a high contrast. Moreover, DMSO solutions of complexes 1a and 2 both turn from yellow to colorless accompanied by luminescence quenching after irradiating with white light. 1a shows reversible photochromism between the switching of white light and 365 nm UV illumination with a short response time, while 2 shows a prolonged and irreversible response, potentially attributed to the steric hindrance effects. However, complex 2 exhibits a more obvious photochromic property in the solid state than complex 1a. Its color changes from yellow to brown upon irradiation with 365 nm UV light, attributed to the formation of radicals generated by photoinduced electron transfer from the hydroxyl oxygen atom to the coordinating pyridine molecule. The PMMA film of complex 2 also exhibits a stimuli response under the irradiation of UV or white light. These salicylaldehyde hydrazone Zn(II) complexes, exhibiting MCL and photochromic properties, demonstrate significant potential as advanced multistimuli responsive materials for various applications.