A photochromic and scintillation Eu-MOF with visual X-ray detection in bright and dark environments

The detection of X-rays has always been a frontier of scientific research. An Eu-MOF with both X-ray-induced photochromic and scintillation properties has been synthesized through the combination of a photochromism-active viologen ligand and rare earth Eu element with high-efficiency absorption of X-rays. In a bright environment, Eu-MOF exhibits different color changes under high-energy X-rays and low-energy X-rays, which can effectively distinguish X-rays. Eu-MOF can also be used for X-ray detection by scintillation properties in dark environments. This work provides a new perspective on the design of multifunctional materials that can perform simple X-ray detection in different environments.

Light-Controlled Aggregation and Gelation of Viologen-Based Coordination Polymers

Ditopic bis-(triazole/pyridine)viologens are bidentate ligands that self-assemble into coordination polymers. In such photo-responsive materials, light irradiation initiates photo-induced electron transfer to generate π-radicals that can self-associate to form π-dimers. This leads to a cascade of events: processes at the supramolecular scale associated with mechanical and structural transition at the macroscopic scale. By tuning the irradiation power and duration, we evidence the formation of aggregates and gels. Using microscopy, we show that the aggregates are dense, polydisperse, micron-sized, spindle-shaped particles which grow in time. Using microscopy and time-resolved micro-rheology, we follow the gelation kinetics which leads to a gel characterized by a correlation length of a few microns and a weak elastic modulus. The analysis of the aggregates and the gel states vouch for an arrested phase separation process, a new scenario to supramolecular systems.

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.

A novel multichromic Zn(II) cationic coordination polymer based on a new flexible viologen ligand exhibiting aniline detection in the solid state

In this work, a novel multichromic cationic coordination polymer, named [Zn4(BTC)3(bcbpy)2]·5H2O (1), based on a new flexible viologen ligand 1,1'-bis(3-cyanobenzyl)-[4,4'-bipyridine]-1,1'-diium (H2bcbpy·2Cl), Zn(NO3)2·6H2O and pyromellitic acid (H4BTC) was synthesized. Compound 1 has good photosensitive activity and can respond to sunlight at room temperature. The colour of compound 1 changes rapidly in response to UV light and blue ray irradiation within 5 s. We rarely obtained the crystal structures after irradiation under UV light and blue ray. At the same time, compound 1 shows the hydrochromism phenomenon when heated at 120 °C, and it also shows the ability of detecting aniline and NO2- under low-concentration conditions (10-4 M).

Near-infrared photothermal conversion of stable radicals photoinduced from a viologen-based coordination polymer

A highly stable radical photoinduced from a viologen-based coordination polymer exhibited a remarkable near-infrared photothermal effect with good recyclability.

Metal-dependent photosensitivity of three isostructural 1D CPs based on the 1,1′-bis(3-carboxylatobenzyl)-4,4′-bipyridinium moiety

The comparison of the photochromic behaviors of three CPs indicates that metal ions play an important role in regulating their photochromic properties.

Sun, UV and X-ray triple photochromic properties of three coordination polymers based on 1,1′-bis(3-carboxylatobenzyl)-4,4′-bipyridinium ligand

Reversible color changes for three compounds can be observed in the air upon sunlight, UV-light and X-ray irradiation.

Photochromic properties of three 2D MOFs based on 1-carboxyethyl-4,4′-bipyridinine

Viologen units have been widely used to impart metal–organic frameworks (MOFs) with photochromic properties. However, construction of a stable photochromic system in viologen MOFs has not been fully explored. Herein, we report three examples of MOFs, namely, {[Cd(CEbpy)(m-BDC)(DMF)]·2H₂O}_n (1), {[Cd(CEbpy)(p-BDC)(H₂O)]·H₂O}_n (2), and {[Zn(CEbpy)(p-HBDC)(p-BDC)_0.5]·H₂O}_n (3) based on benzenedicarboxylic acids (m-H₂BDC = 1,3-benzenedicarboxylic acid, p-H₂BDC = 1,4-benzenedicarboxylic acid) and a viologen-derived ligand 1-carboxyethyl-4,4′-bipyridine (L = CEbpy). As expected, the incorporation of the viologen unit into the frameworks results in the predefined photochromism upon both sunlight and UV-light. Compounds 1–3 feature a two-dimensional (2D) layered structure and are all photochromic due to the formation of CEbpy radicals by photoinduced electron transfer (PET). The aggregates build an interesting stable crystalline framework that exhibits long-lived color constancy in the solid state.

Viologen units have been widely used to impart metal–organic frameworks (MOFs) with photochromic properties.

A novel photochromic coordination polymer based on a robust viologen ligand exhibiting multiple detection properties in the solid state

A new photochromic and porous Cd(ii) coordination polymer (CP) exhibiting multiple detection properties for amines and benzenes in the solid state.

Photochromism of supramolecular assemblies based on benzenecarboxylate donors and viologen acceptors

Similar viologen acceptors, a regular change in donor moieties and photochromism provide an ideal model for elucidating D–A matching rules.

A series of host–guest coordination polymers containing viologens: syntheses, crystal structures, thermo/photochromism and factors influencing their thermo/photochromic behaviors

Some viologen-containing compounds displaying thermo/photochromic behaviors which can be used as inkless print media and anti-counterfeiting materials have been reported.

Differentiable Detection of Volatile Amines with a Viologen-Derived Metal-Organic Material

The development of selective sensing materials for amine detection has received considerable attentions because amines have high toxicity and exist widely. In this article, we demonstrate for the first time that a degree of discriminative detection of alkylamines can be achieved by a metal-organic coordination material. The material is derived from Cd^II and 4,4'-bipyridinium-1,1'-bis(phenylene-3-carboxylate), shows 1D channels lined with electron-deficient viologen chromophores, and exhibits different colors upon contact with amine vapors of different molecular sizes and types (primary, secondary, and tertiary). The vapochromism is attributable to electron transfer from the amine group to viologen. The discrimination between amines is because the analyte-receptor interactions, which either directly mediate or indirectly affect electron transfer, are influenced by the number of the N-H bonds in the amine molecule, the size of the amine molecule relative to the receptor channel and the steric hindrance for the electron donor-acceptor contacts. The material also shows reversible photo- and hydrochromism owing to stimuli-induced reversible electron transfer. The compound can be deposited in paper simply by spraying the mixture solution of the starting metal salt and the ligand. The paper can be used as portable test strips for visual and differentiable detection of amines and as erasable inkless printing medium.

A photochromic zinc-based coordination polymer for a Li-ion battery anode with high capacity and stable cycling stability

A zinc-based coordination polymer with photochromic 4,4′-bipyridinium derivatives as redox active building units is demonstrated as an anode material for Li-ion batteries.

An inorganic–organic hybrid photochromic material with fast response to hard and soft X-rays at room temperature

The first X-ray-induced photochromic material, with the coloration time being within 5 min when exposed to Mo-K_α and Al-K_α X-rays, was designed and synthesized.

A two-dimensional CdII coordination polymer based on naphthalenediimide: synthesis, crystal structure and photochromic properties

Naphthalenediimides, a class of organic dyes with an expanded π-electron-deficient plane, have attracted considerable interest because of their photoinduced electron transfer from neutral organic moieties to stable anionic radicals. This makes them excellent candidates for organic linkers in the construction of photochromic coordination polymers. Such a photochromic two-dimensional coordination polymer has been prepared using N,N'-bis(pyridin-4-ylmethyl)naphthalene-1,8:4,5-bis(dicarboximide) (DPMNI). In crystallization tubes, upon slow diffusion of an MeOH solution of cadmium perchlorate into a CHCl₃ solution of DPMNI, the complex poly[[bis[μ₂-2,7-bis(pyridin-4-ylmethyl)benzo[imn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetrone-κ²N:N']bis(perchlorato-κO)cadmium(II)] chloroform tetrasolvate], {[Cd(C₂₆H₁₆N₄O₄)₂(ClO₄)₂]·4CHCl₃}_n, (I), was obtained. The asymmetric unit contains one Cd²⁺ cation, two DPMNI ligands, two coordinated ClO₄^- anions and four CHCl₃ solvent molecules. Each Cd²⁺ cation is interconnected by four DPMNI linkers to generate a neutral two-dimensional naphthalenediimide coordination network with all the ClO₄^- anions above or below this plane. Strong interlaminar anion-π interactions between the coordinated ClO₄^- anions and the imide rings of an adjacent layer lead to a three-dimensional supramolecular structure. Compound (I) exhibits reversible photochromic behaviour and photocontrolled tunable luminescence properties, which may originate from the photoinduced electron-transfer generation of radicals in the DPMNI ligand.

Photochromic and photomodulated luminescence properties of two metal–viologen complexes constructed by a tetracarboxylate-anchored bipyridinium-based ligand

Photochromic and photomodulated luminescence properties of two metal–viologen complexes constructed using a tetracarboxylate-anchored bipyridinium-based ligand are reported.

Improving coloration time and moisture stability of photochromic viologen–carboxylate zwitterions

The coordination compound [Zn₂LCl₄]_n (ZnLCl) showed greater moisture stability and faster color change than the viologen ligand L (N,N′-dipropionate-4,4′-bipyridinium).

Impact of Lattice Water on Solid-State Electron Transfer in Viologen Pseudopolymorphs: Modulation of Photo- and Piezochromism

Stimuli-induced solid-state electron transfer (ET) underlies the use of viologen compounds as responsive materials, but unequivocal structure-property correlations for solid-state ET are still lacking. With different pseudopolymorphic solids derived from N,N'-bis(4-carboxylphenyl)viologen ([H₂bcpV]²⁺), here we report a systematic study on photo- and piezochromic properties associated with ET. We show that the higher the water content in the lattice, the less sensitively the compounds respond to light and pressure. It is proposed that the lattice water does not act as an electron donor but serves to change the ET energetics through its unique polarity and hydrogen bonding capability. The impedimental impact of water on solid-state ET of viologen compounds has not yet been recognized and elucidated prior to this work. The study also suggests that pressure is more powerful than light in inducing ET.

Synthesis and properties of electron accepting star-shaped phosphaviologen oligomers

A straightforward synthetic route toward redox-active tri- and tetrastar phosphaviologen oligomers that show the intriguing ability of storing up to 8 electrons is reported.