Bipyridinium derivative-based coordination polymers: From synthesis to materials applications

Photocatalyzed Oxidative Tandem Reaction Mediated by Bipyridinium for Multifunctional Derivatization of Alcohols

The multifunctional derivatization of alcohols has been achieved by the bipyridinium-based conjugated small molecule photocatalysts with redox center and Lewis acid site. Besides exhibiting high activity in the selective generation of aldehydes/ketones, acids from alcohols through solvent modulation, this system renders the first selective synthesis of esters via an attractive cross-coupling pattern, whose reaction route is significantly different from the traditional condensation of alcohols and acids or esterification from hemiacetals. Following the oxidization of alcohol to aldehyde via bipyridinium-mediated electron and energy transfer, the Lewis acid site of bipyridinium then activates the aldehyde and methanol to obtain the acetal, which further reacts with methanol to generate ester. This method not only demonstrates a clear advantage of bipyridinium in diverse catalytic activities, but also paves the way for designing efficient multifunctional small molecule photocatalysts. This metal- and additive-free photocatalytic esterification reaction marks a significant advancement towards a more environmentally friendly, cost-effective and green sustainable approach, attributed to the utilization of renewable substrate alcohol and the abundant, low-cost air as the oxidant. The mildness of this esterification reaction condition provides a more suitable alternative for large-scale industrial production of esters.

Photochromic radical states in 3D covalent organic frameworks with zyg topology for enhanced photocatalysis

Covalent-organic frameworks (COFs) with photoinduced donor-acceptor (D-A) radical pairs show enhanced photocatalytic activity in principle. However, achieving long-lived charge separation in COFs proves challenging due to the rapid charge recombination. Here, we develop a novel strategy by combining [6 + 4] nodes to construct zyg-type 3D COFs, first reported in COF chemistry. This structure type exhibits a fused Olympic-rings-like shape, which provides a platform for stabilizing the photoinduced D-A radical pairs. The zyg-type COFs containing catalytically active moieties such as triphenylamine and phenothiazine (PTZ) show superior photocatalytic production rates of hydrogen peroxide (H2O2). Significantly, the photochromic radical states of these COFs show up to 400% enhancement in photocatalytic activity compared to the parent states, achieving a remarkable H2O2 synthesis rate of 3324 μmol g-1 h-1, which makes the PTZ-COF one of the best crystalline porous photocatalysts in H2O2 production. This work will shed light on the synthesis of efficient 3D COF photocatalysts built on topologies that can facilitate photogenerating D-A radical pairs for enhanced photocatalysis.

Zero-dimensional halide hybrid bulk glass exhibiting reversible photochromic ultralong phosphorescence

Dynamically responsive materials, capable of reversible changes in color appearance and/or photoemission upon external stimuli, have attracted substantial attention across various fields. This study presents an effective approach wherein switchable modulation of photochromism and ultralong phosphorescence can be achieved simultaneously in a zero-dimensional organic-inorganic halide hybrid glass doped with 4,4´-bipyridine. The facile fabrication of large-scale glasses is accomplished through a combined grinding-melting-quenching process. The persistent luminescence can be regulated through the photochromic switch induced by photo-generated radicals. Furthermore, the incorporation of the aggregation-induced chirality effect generates intriguing circularly polarized luminescence, with an optical dissymmetry factor (glum) reaching the order of 10-2. Exploiting the dynamic ultralong phosphorescence, this work further achieves promising applications, such as three-dimensional optical storage, rewritable photo-patterning, and multi-mode anti-counterfeiting with ease. Therefore, this study introduces a smart hybrid glass platform as a new photo-responsive switchable system, offering versatility for a wide array of photonic applications.

Cationic Europium-Organic Framework for Chromatographic Column Separation of Ionic Dyes and Stimuli-Responsive Chromic Properties

A novel 3D europium-based cationic framework (Eu-CMOF) has been constructed solvothermally by employing a viologen derivative as an organic functional building unit. Notably, Eu-CMOF demonstrates its capability as a proficient aqueous-phase ion-exchange host, facilitating the remarkable rapid chromatographic column separation of new coccine and malachite green (NC3-/MG+), as well as new coccine and methylene blue (NC3-/MLB+), in mere 2 to 4 min. Adsorption thermodynamics and kinetics of anionic dyes demonstrate that Eu-CMOF exhibits a higher adsorption capacity for NC3-, as evaluated by the Langmuir model, reaching a value of 173 mg·g-1. The pseudo-second-order rate constant is determined to be 3.84 × 10-3 mg-1·g·min-1. Additionally, Eu-CMOF displays reversible photochromic and amine- and ammonia-induced vapochromic behaviors. Further mechanistic studies reveal that these chromic behaviors are primarily attributed to the generation of free viologen radical stimulated by Xe-light or electron-rich amine/ammonia. This research contributes to the development of advanced materials with applications in rapid chromatographic separation and stimuli-responsive chromic properties, showcasing the potential of Eu-CMOF as a versatile platform for practical applications.

Hydro-Photo-Thermo-Responsive Multicolor Luminescence Switching of a Ternary MOF Hybrid for Advanced Information Anticounterfeiting

Developing smart materials capable of solid-state multicolor photoluminescence (PL) switching in response to multistimuli is highly desirable for advanced anticounterfeiting. Here, a ternary MOF hybrid showing hydro-photo-thermo-responsive multicolor PL switching in the solid state is presented. This hybrid is constructed by co-immobilizing Eu3+ and methyl viologen (MV) cations within an anionic MOF via the cation-exchange approach. The confined guest cations are well arranged in the framework channels, facilitating the synergistic realization of stimuli-responsive multiple PL color-switching through intermolecular coupling. The hybrid undergoes a rapid and reversible PL color-switching from red to blue upon water simulation, which is achieved by activating the blue emission of the framework linker while simultaneously quenching the Eu3+ emission. Furthermore, the hybrid displays photo-thermo-responsive PL switching from red to dark. UV-light irradiation or heating triggers the chromic conversion of MV to its colored radical form, which exhibits perfect spectral overlap with Eu3+, thus activating Förster resonance energy transfer (FRET) from Eu3+ to MV radicals and quenching the Eu3+ emission. Inspired by these results, PL morse patterns are designed and fabricated using a novel triple-level encryption strategy, showcasing the exciting potential of this hybrid in advanced anticounterfeiting applications.

Series of Viologen-Based Metal-Organic Polyhedra with Photo/Electrochromic Behavior for Inkless Printing and UV Detection

The fabrication of molecular crystalline materials with fast, multistimuli-responsive behavior and the construction of the corresponding structure-activity relationship are of extraordinary significance for the development of smart materials. In this context, three multistimuli-responsive functional metal-organic polyhedra (MOP), {[Dy2(bcbp)3(NO3)1.5(H2O)7]·Cl4.2·(NO3)0.3·H2O}n (1), {[Dy2(bcbp)4(H2O)8]Cl6}n (2), and {[Eu2(bcbp)4(H2O)10]Cl6·H2O}n (3; bcbp = 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium), were successfully prepared and characterized. All of the compounds exhibit rapid and reversible photochromic and electrochromic dual-responsive behaviors. Furthermore, benefiting from the well-defined crystal structure and different responsive behaviors, the photoinduced electron transfer (PIET) process and structure-activity relationship were explored. In addition, considering the excellent photochromic performance, function filter paper and smart organic glass were successfully prepared and used for ink-free printing and UV light detection.

Superior Electrochemical Water Splitting and Energy-Storage Performances of In Situ Fabricated Charge-Separated Metal Organophosphonate Single Crystals

The design and exploration of advanced materials as a durable multifunctional electrocatalyst toward sustainable energy generation and storage development is the most perdurable challenge in the domain of renewable energy research. Herein, a facile in situ solvothermal approach has been adopted to prepare a methylviologen-regulated crystalline metal phosphonate compound, [C12H14N2][Ni(C11H11N2)(H2hedp)2]2•6H2O (NIT1), (H4hedp = 1-hydroxyethane 1,1-diphosphonic acid) and well characterized by several techniques. The as-prepared NIT1 displays excellent bifunctional electrocatalytic activity with dynamic stability toward oxygen evolution reaction (η10 = 288 mV) and hydrogen evolution reaction (η10 = 228 mV) in alkaline (1.0 M KOH) and acidic mediums (0.5 M H2SO4), respectively. Such a low overpotential and Tafel slope (68 mV/dec for OER; 56 mV/dec for HER) along with long-term durability up to 20 h of NIT1 make it superior to benchmark the electrocatalyst and various nonprecious metal-based catalysts under similar experimental condition. Further, the electrochemical supercapacitor measurements (in three-electrode system) reveal that the NIT1 electrode possesses much higher specific capacity of 187.6 C g-1 at a current density of 2 A g-1 (272 C g-1 at 5 mV s-1) with capacitance retention of 75.2% over 10,000 cycles at 14 A g-1 (Coulombic efficiency > 99%) in 6 M KOH electrolyte medium. Finally for a practical application, an asymmetric supercapacitor device (coin cell) is assembled by NIT1 material. The as-fabricated device delivers the maximum energy density of 39.4 Wh kg-1 at a power density of 450 W kg-1 and achieves a wide voltage window of 1.80 V. Notably, the device endures a remarkable cycle performance with cyclic retention of 92% (Coulombic efficiency > 99%) even after 14,000 charge/discharge cycles at 10 A g-1. Nevertheless, the extraordinary electrochemical activities toward OER and HER as well as the high-performance device fabrication for LED illumination of such a noble metal-free lower-dimensional charge-transfer compound are truly path breaking and would be promising for the development of advanced multifunctional materials.

One-pot Tandem Synthesis and Spontaneous Product Separation of N-heterocycles based on Bifunctional Small-molecule Photocatalyst

Homogeneous and heterogeneous reactions wherein the resulting products remain dissolved in solvents generally require complicated separation and purification process, despite the advantage of heterogeneous systems allowing retrieval of catalysts. Herein, we have developed an efficient approach for the one-pot tandem synthesis of quinazolines, quinazolinones and benzothiadiazine 1,1-dioxides from alcohols and amines utilizing a bifunctional bipyridinium photocatalyst with redox and Lewis acid sites using air as an oxidant. Through solvent-modulation strategy, the photocatalytic system exhibits high performance and enables most products to separate spontaneously. Consequently, the homogeneous catalyst can be reused by direct centrifugation isolation of the products. Notably, the method is also applicable to the less active substrates, such as heterocyclic alcohols and aliphatic alcohols, and thus provides an efficient and environmentally friendly photocatalytic route with spontaneous separation of N-heterocycles to reduce production costs and meet the needs of atomic economy and green chemistry.

A viologen-derived luminescent material exhibiting photochromism, photocontrolled luminescence and selective detection of Cr2O72- in aqueous solution

Stable viologen-derived multifunctional smart materials exhibit widespread practical applications in many areas. In this study, a viologen-derived material with 4-fold interpenetrating diamondoid network, {[Cd(1,4-ndc)(cpbpy)]·2H2O}n, was successfully constructed based on asymmetrical N-(3-carboxyphenyl)-4,4'-bipyridinium (cpbpy) and 1,4-naphthalenedicarboxylic acid (1,4-H2ndc). The compound shows reversible photochromic behavior under a xenon lamp, which are proved by UV-vis spectra and EPR characterizations. Moreover, the compound with good photoluminescence properties displays photocontrolled luminescence quenching behaviors. Owing to its good water stability, the compound is then applied in luminescence sensing for the detection of Cr2O72- in aqueous solution. The corresponding luminescence quenching constant for Cr2O72- is KSV = 4.33 × 104 M-1, and the detection limit is 3.66 μM. Systematic investigations on the luminescence quenching mechanism suggest that the inner filter effect resulted in the selective detection of Cr2O72-. This study provides inspiration for the design and synthesis of target luminescent crystalline materials with rigid and asymmetric viologen-derived ligands.

A Versatile Strategy for the Generation of Air-stable Radical-functionalized Materials

The exploration of a facile approach to create structurally versatile substances carrying air-stable radicals is highly desired, but still a huge challenge in chemistry and materials science. Herein, a non-contact method to generate air-stable radicals by exposing pyridine/imidazole ring-bearing substances to volatile cyanuric chloride vapor, harnessed as a chemical fuel is reported. This remarkable feat is accomplished through a nucleophilic substitution reaction, wherein an intrinsic electron transfer event transpires spontaneously, originating from the chloride anion (Cl- ) to the cationic nitrogen (N+ ) atom, ultimately giving rise to pyridinium/imidazolium radicals. Impressively, the generated radicals exhibit noteworthy stability in the air over one month owing to the delocalization of the unpaired electron through the extended and highly fused π-conjugated pyridinium/imidazolium-triazine unit. Such an approach is universal to diverse substances, including organic molecules, metal-organic complexes, hydrogels, polymers, and organic cage materials. Capitalizing on this versatile technique, surface radical functionalization can be readily achieved across diverse substrates. Moreover, the generated radical species showcase a myriad of high-performance applications, including mimicking natural peroxidase to accelerate oxidation reactions and achieving high-efficiency near-infrared photothermal conversion and photothermal bacterial inhibition.

Photochromism and Photomagnetism in Two Ni(II) Complexes Based on a Photoactive 2,4,6-Tris-2-Pyridyl-1,3,5-Triazine Ligand

It is still challenging to construct novel photochromic and photomagnetic materials in the field of molecular materials. Herein, the 2,4,6-tris-2-pyridyl-1,3,5-triazine (TPTz) molecule was found to display photochromic properties under room temperature light irradiation. Two mononuclear structures, [Ni(H2O)(TPTz)(C2O4)]·2H2O (1; C2O42- = oxalate) [Ni(H2O)(TPTz)(C2O4)]·0.5H2O (2), and one chain compound [Ni(TPTz)(H2-HEDP)]·2H2O (3; HEDP = hydroxyethylidene diphosphonate) were obtained by assembling TPTz with polydentate O-ligands (oxalate and phosphonate) and the paramagnetic Ni2+ ions. The electron-transfer (ET)-dominated photochromism was observable in 1 and 2 after light irradiation with the photogeneration of relatively stable radicals, and the resultant photochromism was demonstrated via UV-vis, photoluminescence, X-ray photoelectron spectra, electron paramagnetic resonance spectra, and molecular orbital calculations. Due to the denser stacking interactions between the adjacent organic molecules, 2 exhibited a faster photochromic rate than 1. Compared with 1 and 2, compound 3 did not show photochromic behavior, which was deciphered by the theoretical calculations for all of the compounds. Importantly, the magnetic couplings appeared between photogenerated radicals and paramagnetic Ni2+ ions, resulting in a scarcely photomagnetic phenomenon of 1 and 2 in the Ni-based electron transfer photochromic materials. This work enriches the available kind of ligands for the design of ET photochromic materials, putting forward a method to tune the electron transfer photochromic efficiency in the molecular materials.

Tunable Chromic Properties of Viologen-Metal Polymers Modulated by Coordination Modes for Inkless Erasable Printing

Inkless and erasable printing (IEP) based on chromic materials holds great promise to alleviate environmental and sustainable problems. Metal-organic polymers (MOPs) are bright platforms for constructing IEP materials. However, it is still challenging to design target MOPs with excellent specific functions rationally due to the intricate component-structure-property relationships. Herein, an effective strategy was proposed for the rational design IEP-MOP materials. The stimuli-responsive viologen moiety was introduced into the construction of MOPs to give it potential chromic behaviors and two different coordination models (i. e. bilateral coordination model, M1 ; unilateral coordinated model, M2 ) based on the same viologen ligand were designed. Aided by theoretical calculations, model M1 was recommended secondarily as a more suitable system for IEP materials. Along this line, two representative viologen-ZnII MOPs 1 and 2 with models M1 and M2 were synthesized successfully. Experiments exhibit that 1 does have quicker stimuli response, stronger color contrast and longer radical lifetime compared to 2. Significantly, the obtained 1-IEP media brightly inherits the excellent chromic characteristics of 1 and the flexibility of the paper at the same time, which achieves most daily printing requirements, as well as enough resolution and durability to be used in identification by smart device.

Boosting Solid-State Luminescence of Thiazolothiazole Viologen by Incorporating Metal Halide Clusters to Hinder π-Stacking

A new strategy is developed herein to improve the solid fluorescence of thiazolothiazole viologen by using the ZnCl42- cluster as a scaffold to hinder π-stacking. Importantly, the Cl···H bonds are formed in the solid state to sustain the framework and can be automatically dissociated when dissolved in H2O, thus having no impact on the strong emission in aqueous solution. As such, the first case of organic-inorganic viologen-zinc halide named 4PV·ZnCl4 was designed and synthesized, and a significant increase in photoluminescence quantum yield (ΦF) is realized from 4PV·2Br (ΦF = 0%) to 4PV·ZnCl4 (ΦF = 27.0%) in solid and from 97% to 98% in H2O. 4PV·ZnCl4 also displays pH stimuli-responsive naked-eye chromic behavior and photoluminescence with different coloring states and intensities. The multifunctional performance of 4PV·ZnCl4 provides a prerequisite for carrying different information, expanding their promising application in multilevel information encryption.

Dual Photo-/Electrochromic Pyromellitic Diimide-Based Coordination Polymer

By the combination of N,N'-bis(carboxymethyl)-pyromellitic diimide (H2CMPMD, 1) and zinc ions, a novel PMD-based coordination polymer (CP), [Zn(CMPMD)(DMF)1.5]·0.5DMF (2) (DMF = N,N'-dimethylformamide), has been prepared and characterized. 1 and 2 exhibit completely different photochromic properties, which are mainly reflected in the photoresponsive rate (5 s for 1 vs 1 s for 2) and coloration contrast (from colorless to light green for 1 vs green for 2). This phenomenon should be attributed to the introduction of zinc ions and the consequent formation of the distinct interfacial contacts of electron donors (EDs) and electron acceptors (EAs) (dn-π = 3.404 and 3.448 Å for 1 vs dn-π = 3.343, 3.359, 3.398, and 3.495 Å for 2), suggesting a subtle modulating effect of metal ions on interfacial contacts, photoinduced intermolecular electron transfer (PIET) and photochromic behaviors. Interestingly, the photochromic performance of 2 can be enhanced after the removal of coordinated DMF, which might be ascribed to the decrease of the distance of EDs/EAs caused by lattice shrinkage, which further improves the efficiency of PIET. Meanwhile, 2 displays rapid electrochromic behavior with an obvious reversible color change from colorless to green, which can be used in an electrochromic device. This work develops a new type of EA for the construction of stimuli-responsive functional materials with excellent dual photo-/electrochromic properties.

Multi-Stimuli-Responsive Chromic Behaviors of an All-in-One Viologen-Based Cd(II) Complex

A one-dimensional Cd(II) chain coordination polymer constructed by an electron-deficient viologen-anchored carboxylate ligand was successfully synthesized. Owing to the favorable stimuli-chromic properties of viologen, the title compound shows reversible photochromism, thermochromism, electrochromism, and naked-eye-detectable differentiable vapochromic response to different volatile amines. The chromic behaviors of it are ascribed to the formation of viologen radicals triggered by external stimuli. And the differentiated response to volatile amines is attributed to the size effect of the amines as well as the steric hindrance effect of forming α/β Cv-H···Namines interactions of the viologen unit to further affect the occurrence of electron transfer. Such an all-in-one crystalline material might have more practical applications in photoelectric, erasable inkless printing, light printing, and volatile amine detection fields.

A viologen-based Cd(II) coordination polymer as a multifunctional platform for photochromism, chemochromism and a broad range of fluorescence pH sensing

A multi-responsive Cd(II) coordination polymer (1) has been constructed by introducing a viologen derivative as both the framework backbone and ligand side pendant. Notably, compound 1 exhibits intriguing properties, including photochromism, methanol-assisted photochromism and chemochromism to ammonia. Furthermore, compound 1 also displays fluorescence pH sensing ability in a wide pH range.

Piezochromism and Conductivity Modulations under High Pressure by Manipulating the Viologen Radical Concentration

Manipulating the radical concentration to modulate the properties in solid multifunctional materials is an attractive topic in various frontier fields. Viologens have the unique redox capability to generate radical states through reversible electron transfer (ET) under external stimuli. Herein, taking the viologens as the model, two kinds of crystalline compounds with different molecule-conjugated systems were designed and synthesized. By subjecting the specific model viologens to pressure, the cross-conjugated 2-X all exhibit much higher radical concentrations, along with more sensitive piezochromic behaviors, compared to the linear-conjugated 1-X. Unexpectedly, we find that the electrical resistance (R) of 1-NO3 decreased by three orders of magnitude with the increasing pressure, while that in high-radical-concentration 2-NO3 remained almost unchanged. To date, such unusual invariant conductivity has not been documented in molecular-based materials under high pressure, breaking the conventional wisdom that the generations of radicals are beneficial to improve conductivity. We highlight that adjusting the molecular conjugation modes can be used as an effective way to regulate the radical concentrations and thus modulate properties rationally.

Conductive Viologen Hydrogel Based on Hyperbranched Polyamidoamine for Multiple Stimulus-Responsive Drug Delivery

The emergence of precision medicine and personalized pharmacotherapy has led to the development of advanced drug delivery systems that can respond to multiple stimuli. Conductive hydrogels have excellent electrical signal responsiveness and drug storage capabilities; however, current conductive hydrogels suffer from poor mechanical properties, low ionic conductivity, and high voltage. Herein, a covalently crosslinked viologen hydrogel was prepared using electroactive hyperbranched polyamidoamine (EHP) as the crosslinking center in a polymeric network. Attributed to its unique molecular architecture, this hydrogel exhibits improved mechanical properties (high tensile strength and desirable stretchability up to 1280%). Approvable ionic conductivity, biocompatibility, antibacterial properties, and wearable strain-sensing performance were also disclosed, ascribed to the participation of versatile viologen groups in the hydrogel structure. This hydrogel exhibited high efficiency in drug release (81.6%) at a lower voltage of -1.2 V. Moreover, fascinating pH-stimulus drug release behavior was also demonstrated in both acidic and alkalescent environments owing to the dramatic conformational transition of EHP. This work provides a new design strategy for conductive hydrogels for multiple stimulus-responsive drug delivery systems.

Sensitive Current Sensor Based on a Lanthanide Framework with Lewis Basic Bipyridyl Sites for Cu2+ Detection

A new Yb-based three-dimensional metal-organic framework with free Lewis basic sites, [Yb₂(ddbpdc)₃(CH₃OH)₂] (referred to as ACBP-6), from YbCl₃ and (6R,8R)-6,8-dimethyl-7,8-dihydro-6H-[1,5]dioxonino[7,6-b:8,9-b']dipyridine-3,11-dicarboxylic acid (H₂ddbpdc) was synthesized by a conventional solvothermal method. Two Yb³⁺ are connected by three carboxyl groups to form the [Yb₂(CO₂)₅] binuclear unit, which is further bridged by two carboxyl moieties to produce a tetranuclear secondary building unit. With further ligation of the ligand ddbpdc^2-, a 3-D MOF with helical channels is constructed. In the MOF, Yb³⁺ only coordinates with O atoms, leaving the bipyridyl N atoms of ddbpdc^2- unoccupied. The unsaturated Lewis basic sites make this framework possible to coordinate with other metal ions. After growing the ACBP-6 in situ into a glass micropipette, a novel current sensor is formed. This sensor shows high selectivity and a high signal-to-noise ratio toward Cu²⁺ detection with a detection limit of 1 μM, due to the stronger coordination ability between the Cu²⁺ and the bipyridyl N atoms.

Viologen-functionalized magnetic material for the removal of Iodine and benzanthracene in an aqueous solution

The development of magnetically active adsorbents for effective iodine removal would be highly desirable to address environmental pollution and remediation. Herein, we demonstrated the synthesis of Vio@SiO₂@Fe₃O₄ as an adsorbent via surface functionalisation of electron-deficient bipyridium (viologen) units on the surface of magnetically active silica-coated magnetite (Fe₃O₄) core. This adsorbent was characterised thoroughly using various analytical techniques, such as field emission scanning electron microscopy (FESEM), thermal gravimetric analysis, Fourier transform infrared spectroscopy (FTIR), field emission transmission electron microscopy (FETEM), Brunauer-Emmett-Teller (BET) analysis and X-ray photon analysis (XPS). The removal of triiodide from the aqueous solution was monitored via the batch method. It revealed that the complete removal was achieved upon stirring for 70 min. The thermally stable and crystalline Vio@SiO₂@Fe₃O₄ displayed efficient removal capacity even in the presence of other competing ions and at different pHs. The adsorption kinetics data were analysed following the pseudo-first-order and pseudo-second-order models. Further, the isotherm experiment showed that the maximum uptake capacity of iodine is 1.38 g/g. It can be regenerated and reused over multiple cycles to capture iodine. Further, Vio@SiO₂@Fe₃O₄ displayed a good removal capacity toward toxic polyaromatic, Benzanthracene (BzA) pollutant with an uptake capacity of 2445 μg/g. This effective removal of toxic pollutants iodine/benzanthracene was attributed to the strong non-covalent electrostatic and π-π interaction with electron-deficient bipyridium units.

Pyridinium-derived polycationic covalent organic polymers for aromatic C-H bond photocatalytic oxidation

Using oxygen in the air as the sole oxidant to oxidize hydrocarbons into high value-added compounds is a highly promising synthesis strategy with economic advantages. However, the oxidation of hydrocarbons with molecular oxygen under mild conditions is challenging due to the large C-H bond energy in hydrocarbons. Herein, a metal-free two-dimensional covalent organic polymers (COP) functionalized by photoactive pyridinium units has been developed for heterogeneous photocatalytic oxidation of hydrocarbons. This is the first kind of COPs material that can achieve photocatalytic oxidation of hydrocarbons without any additives or stoichiometric oxidants except for the oxygen in the air.

Crystal structure of bis(6-carboxyhexyl)-4,4′-bipyridinium dibromide – 2,6-dihydroxynaphthalene (1/2), C42H46Br2N2O8

C42H46Br2N2O8, monoclinic, P21/n (no. 14), a = 7.5393(9) Å, b = 10.7586(12) Å, c = 24.073(3) Å, β = 91.275(3)°, V = 1952.1(4) Å3, Z = 2, R gt(F) = 0.0446, wR ref(F 2) = 0.1037, T = 293(2) K.

Gradual Size Enlargement of Aluminum-Oxo Clusters and the Photochromic Properties

Metallic clusters, assembled by functional motifs, possess the attribute of regulating the properties by changing inorganic and organic components. In this work, a series of aluminum-oxo clusters, [Al₆O(dmp)₄(Hdmp)₂]·2ⁱPrOH [Al₆-1, H₃dmp = 2,2-bis(hydroxymethyl)propionic acid], [Al₆(H₂thmmg)₆]·2DMF·2H₂O [Al₆-2, H₅thmmg = N-tris(hydroxymethyl)methylglycine], [Al₈(OH)₄(NAP-OH)₁₂(MeO)₇(MeOH)]Cl·7MeCN·3MeOH (Al₈, HNAP-OH = 3-hydroxy-2-naphthoic acid), and [Al₁₀(NA)₁₀(MeO)₂₀] (Al₁₀, HNA = nicotinic acid), were obtained based on different carboxylic acids, realizing metallic ring size enlargement from 5.91 to 9.32 Å. They all exhibit good chemical stability. Importantly, the Al₈ cluster displays obvious photochromic behavior from pale yellow to orange yellow, originating from the generation of photoinduced radicals in the metal-assisted ligand-ligand electron transfer process of 3-hydroxy-2-naphthoic acid (HNAP-OH). This work enriches the metal ring cluster chemistry and reports the example of the aluminum-oxo cluster-based photochromic material, developing a novel system of photochromic materials.

The Methylene Spacer Matters: The Structural and Luminescent Effects of Positional Isomerism of n-Methylpyridyltriazole Carboxylate Semi-Rigid Ligands in the Structure of Zn(II) Based Coordination Polymers

Two Zn(II) coordination polymers (CPs) based on n-methylpyridyltriazole carboxylate semi-rigid organic ligands (n-MPTC), with n = 3 (L1) and 4 (L2), have been prepared at the water n-butanol interphase by reacting Zn(NO₃)₂·4H₂O with NaL1 and NaL2. This allows us to systematically investigate the influence of the isomeric positional effect on their structures. The organic ligands were obtained by saponification from their respective ester precursors ethyl-5-methyl-1-(pyridin-3-ylmethyl)-1H-1,2,3-triazole-4-carboxylate (P1) and ethyl-5-methyl-1-(pyridin-4-ylmethyl)-1H-1,2,3-triazole-4-carboxylate (P2), resulting in their corresponding sodium salt forms, 3-MPTC, and 4-MPTC. The structure of the Zn(II) CPs determined by single-crystal X-ray diffraction reveals that both CPs have 2D supramolecular hydrogen bond networks. The 2D supramolecular network of [Zn(L1)]n (1) is built up by hydrogen bond interactions between oxygen and hydrogen atoms between neighboring n-methylpyridyltriazole molecules, whereas in [Zn(L2)·4H₂O]n (2) the water molecules link 1D polymeric chains forming a 2D supramolecular aggregate. The structures of 1 and 2 clearly show that the isomeric effect in the semi-rigid ligands plays a vital role in constructing the Zn(II) coordination polymers, helped by the presence of the methylene spacer group, in the final structural conformation. The structures of 1 and 2 significantly affect their luminescent properties. Thus, while 2 shows strong emission at room temperature centered at 367 nm, the emission of 1 is quenched substantially.

A Novel Molecular Assembly of a Cobalt-Sulfate Coordination Polymer and Melamine: A Manifestation of Magnetic Anisotropy

A novel molecular assembly of a cobalt-sulfate coordination polymer and melamine is synthesized under acidic conditions. Bar-shaped pink monocrystals as long as 1 mm are found to align along magnetic field lines in the proximity of a strong magnet. Magnetometry shows no hysteresis at temperatures down to 2 K but instead magnetic anisotropy and antiferromagnetic coupling. X-ray diffraction on a single crystal reveals that the cobalt-sulfate chains are along the shortest lattice vector or the crystal's long axis. The crystal alignment along the magnetic flux can be attributed to single-ion anisotropy that results in longitudinal antiferromagnetic coupling along the chain. Both structurally and magnetically isotropic crystals of metal-organic hybrid materials can be highly useful as elemental components in magneto-optics.

Enhancement of Long-Lived Persistent Room-Temperature Phosphorescence and Anion Exchange with I- and SCN- via Metal-Organic Hybrid Formation

An in-depth understanding of structure-property relationships and the construction of multifunctional stimuli-responsive materials are still difficult challenges. Herein, we discovered a 4,4'-bipyridinium derivative with both photochromism and dynamic afterglow at 77 K for the first time. A one-dimensional (1D) Cd(II) coordination polymer (1) assembled by only a 4,4'-bipyridinium derivative and cadmium chloride showed photochromism, room-temperature phosphorescence (RTP), and electrochromism. Interestingly, we found that 1 underwent single-crystal-to-single-crystal transformation during the anion exchange process, and the color of the crystal changed from colorless to yellow (1-SCN^-) within 10 min. Complex 1 exhibited photochromism, whereas 1-SCN^- did not. The difference in the photochromic behavior between the two complexes was ascribed to the electron transfer pathway between the carboxylate groups and viologen. The DFT calculation based on the crystal structure of 1-SCN^- indicated that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were mainly located on bipyridine and cadmium atoms, eliminating the possibility of electron transfer, whereas for complex 1, electron transfer was probable from O and Cl atoms to pyridinium N atoms in viologen as demonstrated by density of states (DOS) calculations. In addition, complex 1 was successfully made into test paper for the rapid detection of I^- and SCN^- and displayed potential applications in inkless printing, multiple encryption, and anticounterfeiting.

The modulation effect of auxiliary ligands on photochromic properties of 3D naphthalene diimide coordination polymers

Two novel naphthalene diimide (NDI) coordination polymers (CPs), [Cd(NicNDI)(4,4'-SBC)] (1) and [Cd(NicNDI)(2,2'-BPC)] (2) (NicNDI = (3-pyridylacylamino)-1,4,5,8-naphthalene diimide, 4,4'-SBC = 4,4'-stilbene dicarboxylic acid, 2,2'-BPC = 2,2'-biphenyl dicarboxylic acid), were designed and prepared by the combination of electron-deficient NicNDI and electron-rich aromatic carboxylic acid ligands in the presence of cadmium ions. The usage of aromatic carboxylic acid ligands with different conjugation degrees, sizes, shapes and charge densities leads to the generation of distinct interpenetrated three-dimensional (3D) frameworks. Interestingly, photochromism of 1 and weak photoactivity of 2 should be attributed to the introduction of different auxiliary ligands and consequently the formation of distinct interfacial contacts of electron donors (EDs)/electron acceptors (EAs) (d_π-π = 3.427 Å, infinite -ED-EA-ED-EA- for 1vs. d_π-π = 3.634 Å, discrete ED-EA-ED for 2), suggesting a subtle modulating effect of auxiliary ligands on interfacial contacts, photoinduced intermolecular electron transfer (PIET) and photoresponsive behaviors.

The Construct and Interpretation of Chelated Coordination Polymers and Their Use in Nanomaterials Research

Presently, an important step from basic research to practical applications is synthesizing nanostructured materials. Metal-organic structures, as well as coordination polymers, are a diverse group of materials with a wide range of potential and properties applications. It has been difficult to get these materials into commercial use because of many drawbacks. Polymers containing chelated units are described and assessed for their advancements and problems in preparation, properties, and structure. Here, a proposed approach based on designing coordination polymeric materials with chelated units using the metal-ligand approach (CPM-CU-MA) has been introduced for a columnar-layered plan, supramolecular components, and building levels. Nanocomposite materials can be formed through the thermal transformation of coordination polymers based on donor atoms. The polymeric metal chelates (PMCs) are categorized according to luminescent coordination polymer (LCoP) development. It is classified as macrocyclic intracomplex, polynuclear, and molecular according to its macrostructure. Supramolecular networks (SMNs) can be transformed into a coordination polymer by introducing cyclo-dehydrogenation of natural building blocks on a surface. The structure-property connections of LCPs can influence a framework of liquid crystal display (LCP) that has been given based on LC phase modulators with a large modulation depth and has useful applications in LC lens. In the spatial organization of PMCs, the main focus is on the commonalities and contrasts between higher- and lower-molecular-weight chelates based on molecularly imprinted sensors (MISs) and nanomaterial sensors for a wide range of uses. New functional nanoparticles based on the molecular components have exciting potential, as demonstrated by these findings based on parameters risk factors for human health, hazards reduction in the environment, lack of cost-effectiveness, environmental sustainability, and bioavailability of polymers with an overall performance of 95.3%.

Photoactive Anthracene-9,10-dicarboxylic Acid for Tuning of Photochromism in the Cd/Zn Coordination Polymers

Electron transfer photochromic materials with photo-triggered radicals have received huge interest from chemists due to their potentialities in anticounterfeiting, displays, energy conversion, and information storage. However, utilizing the sole carboxylic acid to synthesize novel electron transfer photochromic species is still confronted with huge challenges. Herein, an acentric three-dimensional network Cd₂(ADC)₂(DMF)₂(H₂O) (1; ADC = anthracene-9,10-dicarboxylate; DMF = N,N-dimethylformamide) and a two-dimensional layer Zn(ADC)(H₂O)·DMA·H₂O (2; DMA = N,N-dimethylacetamide) were synthesized and characterized via a photoactive H₂ADC ligand. Both compounds exhibited electron transfer photochromism with the formation of radical photoproducts at the solid state, which was revealed by IR, UV-Vis absorption, photoluminescence and electron spin resonance spectra, and magnetic susceptibility measurements. Density functional theory calculations for 1 showed that the coloration process is a metal-assisted ligand-to-ligand electron transfer process between adjacent ADC molecules, and photogenerated stable radicals are delocalized over the ADC components. Compared with 1, the shorter distances between ADC components via coordination bonds promoted 2 to exhibit a higher coloration efficiency and larger quantity of photogenerated radicals. Furthermore, both compounds showed unexpected radical-actuated photochromism in aqueous solution. This work showed that the carboxylic acid ligands, without viologen acceptors, could construct the electron transfer photochromic complexes, showing a novel kind of ligand for the design of hybrid photochromic materials.

Photochromic and electrochromic properties of a viologen-based multifunctional Cd-MOF

A pair of novel multifunctional MOF materials [Cd₂(L)₂(D/L-Lm)(H₂O)₂]·5H₂O (denoted as D/L-Cd-MOF) has been synthesized by combining the viologen-functionalized ligand H₂L⁺Cl^- and chiral D/L-tartaric acid (H₂Lm) with a simple solvothermal method. Due to the unique photoelectric response properties of the viologen units, reversible photochromic and electrochromic properties have been combined in D/L-Cd-MOF, which points to a new way of designing and constructing multifunctional photoelectric materials.

Hybrid Photochromic Lanthanide Phosphonate with Multiple Photoresponsive Functionalities

Hybrid photochromic materials (HPMs) with specific photoresponsive functionality have applications in many fields. The photoinduced electron-transfer (ET) strategy has been proved to be effective in the synthesis of HPMs with diverse photomodulated properties. The exploitation of new electron acceptors (EAs) is meaningful for promoting the development of HPMs. In this work, we introduced a rigid tetraimidazole derivative, 3,3,5,5-tetra(imidazol-1-yl)-1,1-biphenyl (TIBP) as a potential EA, into a metal-diphosphonate (1-hydroxyethylidene-1,1-diphosphonic acid, H₄-HEDP) system to explore HPMs and finally obtained a hybrid metal phosphonate (H₄-TIBP)_0.5·[Dy(H-HEDP) (H₂-HEDP)]·H₂O (1). 1 features anionic chains composed of diphosphonate and Dy³⁺ ions. The extra charge is balanced by protonated TIBP cations, which exist in the void of adjacent chains and form H-bonds with O_phosphonate (N-H···O). Upon photostimulation with a Xe lamp (300 W), the crystalline sample 1 exhibited coloration by changing from colorless to pale yellow because of the presence of photoinduced radicals that originated from the ET from O_phosphonate to N_TIBP. Along with the coloration, photomodulated fluorescence, magnetism, and proton conductivity were also detected in the photoactivated samples. Different from the reported HPMs based on polypyridine derivatives and photoactive species such as pyridinium and naphthalimide derivatives as EAs, our study provides a new category of EA units to yield HPMs with fascinating photoresponsive functionality via the assembly of polyimidazole derivatives and phosphonate-based supramolecular building blocks.

Improved Effect of Metal Coordination on Molecular Oxygen Activation for Selective Aerobic Photooxidation

A pyridinium-based complex with environment-friendly and earth-abundant Zn^II ion was synthesized and explored as a green catalyst applied in activating molecular oxygen for the simple and efficient photooxidation of alcohols into aldehydes under additive-free and mild conditions. The metal coordination was conducive to promoting the electron transfer efficiency and introducing the heavy-atom effect for the increased generation of ⋅O₂ ^- and ¹ O₂ . Accordingly, improved photocatalytic performance of this complex compared to its precursor, no matter activity or selectivity, was obtained, facilitating the transformation of alcohols into aldehydes in a sustainable way.

Lattice solvent controlled photochromism of tripyridyl-triazine-based zinc bromide complexes

Two tripyridyl-triazine-based zinc complexes exhibit lattice solvent controlled and solvent-induced photochromic properties.

Inserting protonated phenanthroline derivatives to interchain voids of anionic halometallate units to generate hybrid materials with tunable photochromic performance

Hybrid photochromic materials (HPMs) have potential application in many fields like display, protection, information storage. The generation of HPMs with tunable photochromic performance is meaningful for the availability of smart...

Ultrafast visible-light photochromic properties of naphthalenediimide-based coordination polymers for visual detecting/filtering blue light

Two novel naphthalenediimide-based (NDI-based) coordination polymers (CPs) [Cd∙(3-PMNDI)∙(HNDC)2∙(DMF)] (1) and [Cd∙(4-PMNDI)∙(NDC)]∙DMF (2) (H2NDC = 1,4-naphthalenedicarboxylic acid, 3-/4-PMNDI = N,N′-bis(3-/4-pyridylmethyl)-1,4,5,8-naphthalenediimide, DMF = N,N′-dimethylformamide), have been designed and synthesized, which are constructed...