Zn(II) Complexes Based on a Schiff Base: Mechanochromism- and Solvent Molecule-Dependent Acidochromism

Abstract

A new Schiff base molecule (HL) with a triphenylamine group and its three zinc(II) complexes (1, 1-THF, and 1-DMF) were synthesized and characterized. As expected, three kinds of loosely packed structures that are susceptible to mechanical stimulation were adjusted through twisted conformations and weak intermolecular interactions (C–H···π, π···π, and hydrogen bonds). They all exhibit remarkable mechanochromic luminescence (MCL) behavior with high contrast upon mechanical grinding due to the phase transition between a crystalline and an amorphous state, as confirmed by powder X-ray diffraction (PXRD) studies. Especially, grinding of 1-THF was accompanied by the departure of lattice tetrahydrofuran (THF) molecules. Further investigation revealed that 1-THF and 1-DMF showed outstanding acidochromic behavior. The PXRD and thermogravimetric analysis (TGA) results proved that fumigation with HCl could destroy the interaction between the solvent and ligand molecules, resulting in the phase transition from the crystalline state to the amorphous state, which is essential for their acidochromic properties. The acidochromic mechanism is different from the reported protonation and deprotonation process. Both HL and complex 1 exhibit no acidochromism. This work will shed light on the mechanism of acidochromic properties of Schiff base zinc(II) complexes and expand the potential solid-state stimuli-responsive luminescence materials.