Multi-stimuli-responsive Zn(ii)-Schiff base complexes adjusted by rotatable aromatic rings

Abstract

Multifunctional luminescent materials have attracted intensive interest. However, the mechanisms behind them are still to be explored. In this work, three Zn(II) complexes based on Schiff bases (HL¹ and HL²) that contain rotatable aromatic rings were designed and prepared. They exhibited different mechanochromic luminescence (MCL) and acidochromism. The polymorphous ZnL¹₂ and ZnL^1a₂ crystallize in different crystal systems with different conformations. The ligands in ZnL¹₂ adopt a more twisted conformation than those in ZnL^1a₂. ZnL¹₂ exhibits MCL with high contrast, while ZnL^1a₂ exhibits a negligible MCL property. This may be due to the looser packing of the complex induced by the more twisted conformation of the ligand HL¹. ZnL¹₂ could undergo crystal phase transformation into ZnL^1a₂ by grinding/fuming cycles. To increase the flexibility of the ligand, a methylene group was introduced to result in HL², which can improve the mechanochromic luminescence effect of the Zn(II) complex with high color contrast. The ligands involved in coordination generally adopt a more twisted conformation than those free ligands due to the steric hindrance, resulting in more obvious MCL for complexes. By comparing the luminescence of ligands and their complexes under acid–base stimulation, it is found that the acidochromic properties could be attributed to the generation of ligands at the surface of complexes via the gaseous HCl-solid Zn(II) complex reaction. The high contrast mechanochromic and acidochromic luminescence properties would lead to promising potential applications of these complexes in smart fluorescent materials, and would also provide some ideas for the design of multi-stimuli responsive molecules.