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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]

• A new viologen complex was successfully synthesized and characterized. • Under UV irradiation, the samples heated at 120 and 180℃showed different colors. • We construct a "AND" logic-gate strategy, different inputs lead to disparate results. Electron transfer is crucial for the rapid response of viologen-based complexes and investigating the factors that affect electron transfer has always been a major challenge. Herein, a new viologen-based complex, formulated as {[Zn 5 (cbbbc) 3 (BTC) 3 (μ 1 , 1 -OH)(H 2 O) 5 ]·22H 2 O} n , with a three-dimensional framework was successfully designed and synthesized by viologen ligand 1-(3-carboxybenzyl)-4,4′-bipyridinium chloride (Hcbbpy), trimesic acid (H 3 BTC) and Zn(NO 3) 2 ·6H 2 O. In particular, complex 1 does not exhibit photochromic properties. However, samples of complex 1 heated at 120 and 180 ℃ display tunable colors under UV light irradiation. The experimental results indicate that the compound's light response is less sensitive with higher water content in the lattice. The polarity and hydrogen bonding ability of lattice water in viologen-based complexes can interact with the donor of viologen, leading to a reduction in electron transfer. This study demonstrates the impact of lattice water on electron transfer in viologen-based complexes and offers insights for designing faster response complexes in the future. The light response of the complexes becomes less sensitive as the water content in the lattice increases. This is because the lattice water hinders the electron transfer of the complexes. [Display omitted] [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]

Three AIE (aggregation-induced emission)-ESIPT (excited-state intramolecular proton transfer) active 2-(2-hydroxyphenyl)benzothiazole derivatives, HL1, HL2 and HL3 with one, two and three rotatable phenyl groups, were obtained and characterized. Their AIE properties in THF/HEPES solution were investigated in detail. HL2 shows the best AIE performance with 71-fold fluorescence enhancement, while HL3 only shows a 9-fold enhancement. With the AIE property, HL1 and HL2 could act as fluorescence chemosensors to detect Cu2+ ions via the "turn off" mode in THF/HEPES media. With the ESIPT property, HL1 and HL2 could also detect Zn2+ ions via the "turn on" mode in EtOH/HEPES media. During the detection process, both demonstrate rapid response and high contrast before and after the addition of metal ions. The species formed in the detection system were investigated. The results of X-ray single-crystal diffraction confirm that Zn2+ is coordinated with the oxygen atom and Schiff base nitrogen atom instead of the benzothiazole nitrogen atom in the tetrahedron geometry. Moreover, the chemosensors were successfully constructed into handy fluorescence test papers for Cu2+ and Zn2+ detection. [ABSTRACT FROM AUTHOR]

A novel Schiff base ligand HL ·CH 3 COOH with excited-state intramolecular proton transfer (ESIPT) and its zinc (Ⅱ) complex 1 were successfully synthesized, and both of them show high-contrast mechano- and acidochromic luminescence. [Display omitted] A new Schiff base compound (HL ·CH 3 COOH) 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 ·CH 3 COOH and the complex adopt loosely packing patterns. HL ·CH 3 COOH 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 CH 3 COOH molecules for HL ·CH 3 COOH. HL ·CH 3 COOH 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 CH 3 COOH and Schiff base molecules, leading to the departure of acetic acid molecules. More interestingly, HL ·CH 3 COOH also exhibited NH 3 -stimuli response properties due to the prohibition of the ESIPT process. [ABSTRACT FROM AUTHOR]

Multistimuli responsive materials based on the viologen coordination polymers have attracted much interest in the past few years due to the increasing demand for smart materials. In this paper, based on a viologen ligand 1-(3-carboxybenzyl)- 4,4′-bipyridinium chloride (Hcbbpy) and zinc(II) ion, a new crystalline complex, namely, {[Zn(cbbpy)Cl 2 ]·H 2 O} n (1) has been designed and synthesized. In complex 1 , every cbbpy ligand as a bridge links two Zn2+ ions via carboxylate O atom and pyridine N atom to extend to a 1D zigzag chain. The 1D chains are extended to 3D supramolecular structure through π···π stacking and hydrogen bonding. More importantly, 1 exhibits fast photochromic behaviors in response to UV and X-ray due to the formation of viologen radicals. Complex 1 also shows photo-modulated luminescence property in the solid state. In addition, complex 1 can be deposited on filter paper and displays the potential application as inkless print material. [Display omitted] • A new viologen complex was successfully synthesized and characterized. • Complex 1 shows fast response to UV light and X-ray irradiation. • Complex 1 can be deposited on filter paper and displays as inkless print material. [ABSTRACT FROM AUTHOR]

Organic molecules and some coordination polymers that are responsive to external stimuli (such as light, electrical potential, heat, guest inclusion or pressure) often generate structural isomerization, such as ring-opening/closing, tautomerism and cis-trans isomerization. While for viologen derivatives, generally, electron transfer occurs upon the photoirradiation without a significant structural change. Herein we present a new viologen-based Zn-complex (1) which undergoes single-crystal-to-single-crystal (SCSC) transformation to 1G upon crushing. Both 1 and 1G could change to 1Y when stored under air in the dark at ambient for one week. The colorless crystal 1 undergoes a two-step color change to pink (1P) and brown (1Z) upon irradiation with UV for 40 s and 15 min, respectively. More importantly, the complex 1G also has a two-step coloring phenomenon under UV light. The colorless crystals of 1G change into blue (1Q) after irradiation for 15 s and finally become brown (1Z′) with the increase of irradiation time. While 1Y only undergoes one coloration from colorless to cyan 1Y′ after UV irradiation. The possible pathways for electron transfer were confirmed to be through intermolecular hydrogen bonding according to the results of crystal structure and the photochromic behavior of the ground samples. 1Z and 1Z′ could change color to yellow 1H and 1H′ in the dark, respectively. Only the changes between 1 and 1P , 1Z and 1H , 1G and 1Q , 1Z′ and 1H′ , 1Y and 1Y′ are reversible, which implies that the crystal structures varied upon photoirradiation because 1Z and 1Z' can revert to neither the original crystals 1 and 1G , nor the intermediate 1P and 1Q. These findings help to understand multiple photochromism and variations of crystal structures under different external stimuli via SCSC transformations. In addition, complex 1 shows high selectivity for primary aromatic amine aniline among various amines. [Display omitted] • Complex 1 undergoes a single-crystal to single-crystal transformation to 1G upon crush with structure change. • Both 1 and 1G could undergo a two-step coloring phenomenon under UV light. • 1 and 1G change to 1Y when stored in the dark for one week. • Complex 1 shows high sensitivity to aniline. • The pathways for electron transfer of the two-step were discussed in detail. [ABSTRACT FROM AUTHOR]

A multi-binding site chemosensor, N-(3-methoxy-2-hydroxybenzylidene)-3-hydroxy-2-naphthahydrazone (H3L), with excited-state intramolecular proton transfer (ESIPT) behaviour was prepared and characterized. It possesses no aggregation-induced emission (AIE) characteristics but can detect Cd2+ and Zn2+ ions selectively in the "off–on" mode based on the AIE of their complexes in the media of THF/HEPES and THF/H2O, respectively, which will provide a new strategy for target detection based on AIE. The detection limits of Zn2+ and Cd2+ were 9.85 × 10−9 M and 1.27 × 10−7 M, respectively. The aggregates of the complexes formed in the detection system were confirmed by DLS data and SEM images. The corresponding Zn2+ (1) and Cd2+ (2) complexes were prepared to investigate the response mechanism. Powder X-ray diffraction and single crystal X-ray diffraction proved that complex 1 is the species formed in the detection system. The chemosensor coordinates with the Cd2+ and Zn2+ ions in different formation and coordination modes, leading to the emission position of the aggregates at 560 and 645 nm, respectively, based on which Cd2+ ions were successfully differentiated from Zn2+ ions. Moreover, the detection of Cd2+ and Zn2+ ions was realized qualitatively via test paper and quantitatively in water. [ABSTRACT FROM AUTHOR]

In this work, we successfully synthesized two Schiff base (HL) complexes, ZnLCl 2 (1) and [Cd 2 L 2 Cl 4 ]·CH 3 CN (2), via the one-pot reaction of metal salts with 4-[bis(4-bromophenyl)amino]benzaldehyde and 3-hydroxy-2-naphthoic acid hydrazide. Complex 2 exhibits not only acidochromic properties but also NH 3 -induced discoloration behavior, which is rarely reported among triphenylamine derivatives or complexes. However, complex 1 shows no color change upon fuming with HCl or NH 3 vapor. The experimental results show that acidochromism may be due to the decomposition of the structure by HCl vapor, while NH 3 -induced discoloration is mainly attributed to the phase change of 2 from crystalline to amorphous state. In addition, complexes 1 and 2 showed high-contrast mechanochromic luminescence (MCL) change from yellow to orange and green to yellow, respectively. Complexes 1 and 2 also exhibit electrochromic behavior with color change from yellow to brown. Moreover, complex 2 was successfully made into test paper for mechanical stimuli and the rapid detection of HCl and NH 3 vapor. The complex 2 can be fixed on a mask coat to fabricate a 2 -based wearable hazardous gas sensor, suggesting that 2 is expected to develop into the next generation wearable sensors for detection of HCl and NH 3 vapor. [Display omitted] • Complexes 1 and 2 showed reversible mechanochromic luminescence behavior. • 2 exhibits acidochromic properties and NH 3 -induced discoloration behavior. • Complexes 1 and 2 also exhibit electrochromic behavior. • 2 can be made into test paper for repeated writing and HCl/NH 3 vapor detecting. • 2 can be made wearable sensor for detection of HCl and NH 3 vapor. [ABSTRACT FROM AUTHOR]

Nowadays, photoactive viologen units have been widely encapsulated in metal-organic frameworks and assembled into multifunctional materials with photochromism and other light regulating properties. A novel host-guest MOF material, {[Zn(bcbpy) 0.5 (IPA)]·CH 3 CN} n (1), has been constructed by a viologen ligand 1,1′-bis((3-carboxylatobenzyl)-4,4′-bipyridinium)-dichloride (H 2 bcbpy·2Cl), isophthalic acid (IPA) and zinc(II) chloride. Complex 1 exhibits UV and X-ray dual photochromism and photo-controlled luminescence properties. Complex 1 also exhibits a temperature-dependent phosphorescence from cyan to yellow when the temperature increases from 77 to 297 K. 1T will be obtained when 1 is heated in air at 230 °C for 30 min, 1T shows less sensitive photochromic property than 1 due to its longer C–H⋯O hydrogen bond distance for electron transfer pathway. In addition, complex 1 has the potential application as an inkless and erasable printing material due to the excellent photochromic property. [Display omitted] • A UV and X-ray dual photochromic complex 1 was synthesized and characterized. • Complex 1 exhibits photo-controlled luminescence properties. • Complex 1 exhibits temperature-dependent phosphorescence properties. • Complex 1 has the potential to be used as an inkless and erasable printing material. [ABSTRACT FROM AUTHOR]