A non-porous Fe(II) complex for the colorimetric detection of hazardous gases and the monitoring of meat freshness

Food quality monitoring and freshness assessment are critical for ensuring food safety at a large scale. Ammonia is used as an important indicator of protein rich food spoilage state. However, current ammonia gas sensors suffer from insufficient sensitivity and selectivity, or sophisticated instrumentation, hindering their practical application in in-situ and real-time food quality monitoring. To overcome such limitations, an innovative nonporous colorimetric complex 1 has been synthesized and investigated for the detection of NH_3(g) and its volatile organic derivatives including aliphatic amines, 1,2-diaminopropane_(g), isobutylamine_(g) and ethylenediamine_(g), etc. The sensor operates colorimetrically at room temperature without energy input, with a detection limit to ammonia_(g) of 105 ppb, and show excellent reusability. The colorimetric detection mechanism involves a partial spin state change of Fe(II) ions upon exposure to amines in the gas phase. In addition, the complex was utilized as real-time monitoring of meat freshness using a smartphone. Thus, chemosensor 1 is considered as a ground breaking new-generation portable electronic nose for vapors of volatile organic compounds discrimination at room temperature.

Applications of Viologens in Organic and Inorganic Discoloration Materials

Viologen derived from 4,4'-bipyridine has attracted much attention because of its color changing properties with electron transfer, unique redox stability and structural diversity. These characteristics have led to its successful use in various applications, in particular in color-changing materials. In the past few years, researchers have been working on the syntheses of viologen-based color-changing functional materials, and such materials have been widely used in many fields. In photochromic materials, it is used as anti-counterfeiting material; in thermochromic, it is used as memory storage material, and in electrochromic, it is used as a battery material. This Review discusses the progress of viologen in organic and inorganic discoloration materials in recent years. The syntheses of viologen and its derivatives are summarized, and its application in the field of discoloration materials is introduced.

Photo/Electrochromic Dual Responsive Behavior of a Cage-like Zr(IV)-Viologen Metal-Organic Polyhedron (MOP)

Stable stimulus-responsive materials are highly desirable due to their widespread potential applications and growing demand in recent decades. Despite the fact that viologen derivatives have long been known as excellent photochromic and electrochromic materials, the development of stable viologen-based multifunctional smart materials with short coloration times remains an exciting topic. To obtain photochromic and electrochromic dual responsive materials, embedding the viologen ligand into a robust metal oxide cluster to increase its stability and sensitivity is an effective strategy. Herein, a viologen-based metal-organic polyhedron (MOP) {[Zr₆L₃(μ₃-O)₂(μ₂-OH)₆Cp₆]·8Cl·CH₃OH·DMF} [Zr-MOP-1; H₂L·2Cl = 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium dichloride, and Cp = η⁵-C₅H₅] was successfully prepared and characterized. It consists of trinuclear Zr-oxygen secondary building units and exhibits reversible photochromic and electrochromic dual responsive behaviors. As expected, the designed robust viologen-based nanocage with a V₂E₃ (V = vertex, and E = edge) topology can maintain its stability and rapid photo/electrochromic behaviors with an obvious reversible change in color from purple (brown) to green, mainly due to the enclosed cluster structure and the abundant free viologen radicals that originate from the effective Cl → N and O → N electron transfers. Spectroelectrochemistry and theoretical calculations of this Zr-MOP were also performed to verify the chromic mechanism.

Synthesis of an advanced metal-guided photochromic system for molecular keypad lock: detailed experimental findings and theoretical understanding

The dye-containing Schiff base metal complex is a new member of the photochromic family with advantages, such as long-wavelength absorption, high molar absorption coefficient, quick-photoresponse, and excellent fatigue resistance.

Two new photochromic supramolecular compositions based on viologen: photocontrolled fluorescence, aniline detection and inkless erasable printing performance

Two multifunctional supramolecular components synthesized by two different viologen ligands and pyromellitic acid.

Multifunctional Viologen-Derived Supramolecular Network with Photo/Vapochromic and Proton Conduction Properties

A supramolecular network [H₄bdcbpy(NO₃)₂·H₂O] (H₄bdcbpy = 1,1′-Bis(3,5-dicarboxybenzyl)-4,4′-bipyridinium) (1) was prepared by a zwitterionic viologen carboxylate ligand in hydrothermal synthesis conditions. The as-synthesized (1) has been well characterized by means of single-crystal/powder X-ray diffraction, elemental analysis, thermogravimetric analysis and infrared and UV-vis spectroscopy. This compound possesses a three-dimensional supramolecular structure, formed by the hydrogen bond and π–π interaction between the organic ligands. This compound shows photochromic properties under UV light, as well as vapochromic behavior upon exposure to volatile amines and ammonia, in which the electron transfer from electron-rich parts to the electron-deficient viologen unit gives rise to colored radicals. Moreover, the intensive intermolecular H-bonding networks in 1 endows it with a proton conductivity of 1.06 × 10⁻³ S cm⁻¹ in water at 90 °C.

Viologen-derived material showing photochromic, visually oxygen responsive, and photomodulated luminescence behaviors

A new 2D viologen-derived Zn(ii) photochromic material exhibits photochromic, visually oxygen responsive, and photomodulated luminescent behaviors.

Sonochemical synthesis of a trinuclear Cu(ii) complex with open coordination sites for the differentiable optical detection of volatile amines

A discrete trinuclear Cu(ii) complex, namely, [Cu₃(pzdc)₂(dpyam)₂(H₂O)₄] (1) (H₃pzdc = pyrazole-3,5-dicarboxylic acid, dpyam = 2,2′-dipyridylamine) was simply synthesized by the sonochemical process and structurally characterized. The single-crystal X-ray diffraction analysis revealed that three adjacent Cu(ii) centers are linked via two bridging pzdc ligands to form a trinuclear Cu(ii) unit. Each trinuclear Cu(ii) unit contains open coordination sites with two trigonal bipyramidal Cu(ii) centers and one elongated octahedral geometry. Moreover, the open coordination site of 1 was occupied by a small molecule, leading to the guest-induced structural transformation with chromism that was verified by FT-IR, UV-vis diffuse reflectance spectra, elemental analysis, PXRD, and SEM techniques. Compound 1 exhibits color change along with structural transformation in methanol media and after the dehydration process. Also, 1 shows different color responses after exposure to different amine vapors. In addition, compound 1 was conveniently deposited onto a filter paper by a sonochemical method used as a portable test strip for the discriminative qualitative detection of amines.

A trinuclear Cu(ii) complex with open coordination sites for the differentiable optical detection of volatile amines.

Printable UV-Light Sensor for Human Eye Protection

Light-emitting diode based electronic screens emit near-ultraviolet radiation, which causes harm to the human eye after prolonged exposure. Thus, it is of paramount importance to prepare a sensitive and adjustable visible near-ultraviolet sensor for retinal warning. Herein, a series of bipyridine derivatives were synthesized to investigate effects of substituent groups and anions on photochromic properties via both experimental and theoretical studies. The introduction of dual hydrogen bonding urea onto substituted groups significantly accelerated the photochromic rate due to strong intermolecular interactions, which reduces molecular spacing and promotes the electron-transfer effect. Moreover, the photochromic rate was tuned by changing the size of the anion. Larger anions widen the molecular spacing and weaken the electron transfer and eventually lead to a decrease in the photochromic rate. Finally, bipyridine derivatives were printed on a polyethylene terephthalate film or paper as a sensitive, adjustable, and visible sensor to monitor near-ultraviolet radiation emitted by an light-emitting diode screen.