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The development of stimulus-responsive luminescent materials, especially those based on a single compound exhibiting multicolor and high-contrast (Δλem ≥ 100 nm) chromic properties, is a critical challenge. In this work, we synthesized and characterized a zinc(II) complex (1). As expected, 1 displays aggregation-induced emission enhancement (AIEE) in THF/H2O mixtures, and remarkable multicolor switching under external stimuli in the solid state. Complex 1 shows reversible mechanochromic luminescence behavior with a large wavelength shift (Δλem = 100 nm) during the grinding–fuming cycles, due to the phase transformation between the crystalline and amorphous states. More impressively, 1 exhibits obvious acidochromic properties (Δλem = 130 nm) which originate from the adsorption of vapor and a gas–solid reaction on the crystal surface. Furthermore, 1 exhibits electrochemical oxidation behavior accompanied by quenching of yellow-green emission due to the overlap of an emission band and an absorption band. The above-mentioned color changes under ambient light can also be observed by the naked eye during the mechanical, acid–base vapor and electrical stimulation. Based on the high-contrast and multicolor switching, complex 1 was successfully developed into test papers and films in the field of rapid detection of mechanical stimuli and HCl/NH3 vapors. [ABSTRACT FROM AUTHOR]

An acetonitrile solvate of a Schiff base molecule (HL) with acetonitrile (HL·2CH3CN) and its Cd(II) complex (Cd(HL)2Cl2, 1) was designed and synthesized. The different conformation of HL in HL·2CH3CN is adjusted by the hydrogen bonding O–H⋯O between HL molecules, together with N–H⋯N between HL molecules and acetonitrile molecules. Meanwhile, the conformation of HL in complex 1 is adjusted by coordination interactions between Zn ions and atoms O and N, together with hydrogen bonding N–H⋯Cl. The presence of the triphenylamine group makes HL·2CH3CN and complex 1 loosely packed. Upon grinding, HL·2CH3CN and complex 1 both showed high-contrast mechanochromic luminescence (MCL) change from blue to green and cyan to yellow, respectively. These changes can be eliminated by fumigation with organic vapor. The results of powder X-ray diffraction (PXRD) show that their MCL is due to the phase transformation from a crystalline state to an amorphous state. In addition, HL·2CH3CN and complex 1 also exhibited high-contrast acidochromism upon exposure to HCl and NH3 vapor. X-ray photoelectron spectroscopy (XPS) and PXRD studies show that the protonation of the –NH– group together with phase transformation from a crystalline state to an amorphous state is attributed to the fluorescence switching. For HL·2CH3CN, the protonation process was accompanied by the departure of acetonitrile molecules. The emission of HL was restored to the amorphous state rather than the original crystalline state emission after further exposure to NH3 vapor, while 1-HCl could revert to the original crystalline state emission. In addition, HL·2CH3CN and complex 1 have been successfully used to produce writable and acid-responsive test papers. [ABSTRACT FROM AUTHOR]

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 ZnL1a2 crystallize in different crystal systems with different conformations. The ligands in ZnL12 adopt a more twisted conformation than those in ZnL1a2. ZnL12 exhibits MCL with high contrast, while ZnL1a2 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 ZnL1a2 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. [ABSTRACT FROM AUTHOR]