Solvent-Modulated Self-Assembly of Naphthalenediimide-Based Cd(II) Complexes and the Controllable Photochromism via Conformational Isomerization.

Rational regulation of the properties of photochromic materials is a challenging and meaningful work. In the present work, NDI-based complexes, namely, [Cd0.5(NDI)(HBDC)]·H2O (1) and a series of conformational isomers of {[Cd(NDI)0.5(BDC)]·MeCN}n (2), were synthesized by varying the solvent conditions (H2BDC = terephthalic acid, NDI = N,N'-bis(3-pyridylcarbonylhydrazine)-1,4,5,8-naphthalene diimide). Complex 1 exhibits a 0D mononuclear structure without photochromic behavior due to the bad conjugation of the naphthalene diimide moiety. The conformational isomers of complex 2 manifest a 3D network, showing ultra-fast photo-induced intermolecular electron transfer photochromic behavior under X-ray, UV, and visible light. However, they show different photochromic rates and coloring contrast upon photoirradiation, which originates from their difference in the distances of lone pair(COO)···π(NDI). This was realized via controlling the solvent ratio in the reaction system. In addition, compared to UV/X-ray light, 2 exhibits greater sensitivity to visible light and is an organic-inorganic hybrid material with photomodulated luminescence. Based on the excellent performance, complex 2 can be applied to filter paper, showing potential applications as an inkless printing medium and selective perception of ammonia and amine vapors in the solid state via different visual color changes.

Multistimuli-Responsive Materials Based on Zn(II)-Viologen Coordination Polymers and Their Applications in Inkless Print and Anticounterfeiting.

Recently, stimuli-responsive materials have attracted great attention, while most of them respond to single or two stimuli. Thus, it is essential to design multifunctional stimuli-responsive materials and develop their applications. The strategy that constructing high-dimensional coordination polymers facilitates the application scope of a viologen-based photochromic system is put forward and confirmed for the first time. Herein, a novel multistimuli-responsive viologen-based Zn-MOF with a two-dimensional framework has been successfully designed and synthesized. Complex 1 exhibits chromic behavior under a variety of external stimuli such as 365 nm UV, X-rays, heat, electricity, and ethylamine. More interestingly, the crystal state of complex 1 displays dual fluorescence and room-temperature phosphorescence (RTP) emission and emits a yellow afterglow when turning off the UV lamp. In addition, Eu(III)-functionalized hybrids, Eu3+@Zn-MOF, were prepared by coordinated postsynthetic modification based on viologen complexes for the first time. The sample of Eu3+@Zn-MOF inherits the photochromic characteristics of the viologen complexes and gives the distinctive fluorescence of the europium ions. Based on the multicolor switching of 1 and Eu3+@Zn-MOF, their possible practical utilization was successfully developed in the fields of inkless, erasable print media, electrochromic information tag printing, information encryption, and anticounterfeiting.

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

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.

Multistimulus Response of Two Tautomeric Zn(II) Complexes and Their White-Light Emission Based on Different Mechanisms.

A triphenylamine (TPA)-based 2H-quinazoline Zn(II) complex (Q-TPA-Zn) exhibiting dual fluorescence and phosphorescence emission in the solid state was designed and prepared. It possesses mechanochromic luminescence and thermochromic luminescence properties. In the solid state, the white afterglow luminescence could be observed at 77 K (CIExy: 0.27, 0.33) while cyan luminescence could be observed at 297 K. After thermolysis at 300 °C, Q-TPA-Zn could be transformed into Schiff base complex S-TPA-Zn with white fluorescence in the powder state (CIExy: 0.32, 0.38), in methanol (CIExy: 0.32, 0.39), and in dimethylformamide (CIExy: 0.26, 0.32) at room temperature. This arises from dual emission of normal* emission and tautomeric* emission induced by excited-state intramolecular proton transfer (ESIPT) from the benzimidazole NH group to the Schiff base N atom. Q-TPA-Zn could also be transformed into its isomeric form, S-TPA-Zn, through photochemical ring-opening reaction upon irradiation under 365 nm in the solution, exhibiting high-contrast photochromic luminescence. Interestingly, S-TPA-Zn could further be transformed into its zwitterionic isomer after continuous irradiation. The same ring-opening reaction could also take place for the orgainc compound Q-TPA via heating or 365 nm irradiation. The ring-opening reaction mechanism and ESIPT emission were interpreted via theoretical calculation.

A Multistimuli Responsive Crystalline Cd(II)-Viologen Coordination Polymer with Single-Crystal-Single-Crystal Transformation.

It is necessary to develop stable and fast multistimuli responsive materials due to the growing demand in our daily life. In this work, a new viologen-based Cd-complex (1) exhibits multiple thermochromic and photochromic behaviors through 10 states with 7 colors. For example, it responds to both Cu Kα/Mo Kα X-ray sources and UV dual light quickly with a color change from colorless to dark blue (1X) (Cu Kα/Mo Kα X-ray sources) and cyan (1-UV) (UV light), respectively. Interestingly, it exhibits a three-step coloration phenomenon when heated, which is unprecedented in viologen compounds. Crystal 1 undergoes a color change to pink, blue, and brown under 130, 180, and 240 °C, respectively. In addition, upon fumigation, both 1P and 1Q undergo a decoloration process to colorless (1K) and yellow (1T), respectively. Four more states (1P, 1K, 1T, and 1O) obtained via dehydration-hydration treatment are all photochromic. More importantly, via single-crystal-single-crystal transformation (SC-SC), the photochromic and thermochromic behaviors of 1 were investigated from the molecular level, which is also rather rare for thermochromic species. The detailed electron donor and the pathways for electron transfer were clearly given according to the results of crystal structure. The colorful states upon external stimuli may be attributed to the multiple pathways for electron transfer.

Multistimuli Responsive Solid-State Emission of a Zinc(II) Complex with Multicolour Switching.

The development of smart luminescent materials, especially those stimulus-responsive fluorescent materials that can switch between different colors repeatedly under external stimulation based on a single molecule, is of great significance but a challenge. In this work, a novel zinc(II)-Schiff base complex (ZnL2) was obtained and characterized. Upon exposure to the HCl and NH3 vapors, it displayed remarkable tricolor acidochromic behavior with high contrast and rapid response under the ambient light as well as UV light (365 nm). The XPS analyses of ZnL2 crystals before and after HCl/NH3 fuming show that the acidochromism originates principally from the adsorption of vapor and the gas-solid reaction equilibrium on the crystal surface. The reddish-brown color of the HCl-fumigated ZnL2 crystals could be attributed to the generation of HL at the surface of ZnL2, and red-shifted emission could be ascribed to the self-absorption effect. The single crystal X-ray diffraction data indicate that these processes cause slight changes in the molecular conformation and crystal packing. ZnL2 shows reversible mechanochromic luminescence behavior between yellow and orange emission during the grinding-fuming/heating cycles due to the modulation between amorphous and crystalline states. Moreover, ZnL2 was successfully made into test paper for the rapid detection of HCl/NH3 vapors and mechanical stimuli.

ESIPT-AIE active Schiff base based on 2-(2'-hydroxyphenyl)benzo-thiazole applied as multi-functional fluorescent chemosensors.

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.

A multi-binding site hydrazone-based chemosensor for Zn(ii) and Cd(ii): a new strategy for the detection of metal ions in aqueous media based on aggregation-induced emission.

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.