Redox-Responsive Photochromism and Fluorescence Modulation of Two 3D Metal-Organic Hybrids Derived from a Triamine-Based Polycarboxylate Ligand

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.

Leveraging Crystalline and Amorphous States of a Metal-Organic Complex for Transformation of the Photosalient Effect and Positive-Negative Photochromism

Uncovering differences between crystalline and amorphous states in molecular solids would both promote the understanding of their structure-property relationships, as well as inform development of multi-functional materials based on the same compound. Herein, for the first time, we report an approach to leverage crystalline and amorphous states of a zero-dimensional metal-organic complex, which exhibited negative and positive photochromism, due to the competitive chemical routes between photocycloaddition and photogenerated radicals. Furthermore, different polymorphs lead to the on/off toggling of photo-burst movement (photosalient effect), indicating the controllable light-mechanical conversion. Three demos were further constructed to support their application in information encryption and anti-counterfeiting. This work provides the proof-of-concept of a state- and polymorph-dependent photochemical route, paving an effective way for the design of new dynamically responsive systems.

A radical photochromic metal-organic framework for boosting NIR-II photothermal conversion and therapy

This work demonstrates for the first time that a photochromic metal organic framework (pMOF) can be employed as a promising class of NIR-II photothermal material based on the photoinduced donor-acceptor intermolecular charge transfer process. After further surface-modification, such UV-activated pMOF-a nanoparticles allow the strong inhibition of 4T1 cancer cells under 1064 nm laser irradiation.

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.

Controlled Photoinduced Generation of "Visual" Partially and Fully Charge Separated States in Viologen Analogues

Charge-separated states with a lifetime scale of seconds or longer not only favor studies using various steady-state analysis techniques but are important for light-energy conversion and other applications. Through a steric-hindrance-induced method, unprecedented photoinduced generation of a partially charge separated (PCS) state with a lifetime of days has been detected in the "visual" mode during the decay of excited states to a commonly observed fully charge separated (FCS) state for viologen analogues. One pale yellow 4,4'-bipyridine-based metalloviologen compound, with an interannular dihedral angle of 1.84° in 4,4'-bipyridine, directly decays to the purple FCS state after photoexcitation. The other pale yellow compound, with a similar coordination framework but a larger interannular dihedral angle (33.74°), changes first to a yellow PCS state and then relaxes slowly (in the dark in Ar, ca. 2 days; 70 °C in Ar, ca. 1 h) to the purple FCS state. The two-step coloration phenomenon is unprecedented for viologen compounds and their analogues and also rather rare for other photochromic species. EPR and Raman data reveal that photoinduced charge separation first generates univalent zinc and radicals and then the received electron in Zn(I) slowly distributes further to 4,4'-bipyridine. Reduction of π-conjugation and a direct to indirect change in band gap account for the prolongation of the relaxation process and the capture of the PCS state. These findings help to understand and control decay processes of excited states and provide a potential design strategy for multicolor photochromism, light-energy conversion with high efficiency, or other applications.

Open-Framework Zn Compound with Cationic-π Interaction: Photochromism and Benzene Series Detection

Photochromic open-framework compounds are of potential application in detection/sensors. The issue of improving the detection limits has received much attention. We synthesized a new open-framework Zn compound, namely, compound 1 ([Zn(MQ)(IPA)Cl]·5H₂O) (MQ = N-methyl-4,4'-bipyridinium, IPA = m-phthalic acid), which showed a 1D channel structure by a cationic-π interaction. It is noteworthy that this compound is an effective detector for aniline though luminescence emission, which exhibited unprecedented detection limits in photochromic open-framework compounds.

A Porous CoII-MOF for CO2 Cycloaddition and the Protective Effect against Staphylococcus aureus Systemic Infection in the Department of Ultrasound

A new metal-organic framework (MOF) based on CoII ions as nodes, [Co2(H2O)3(cada)](DMF)4, which has coordinated water molecules at the occupied CoII sites along with a suitable pore environment, was constructed by reaction of 5,5′-(9H-carbazole-2,7-diyl)diisophthalic acid (H4cada) and Co(NO3)2·6H2O in a water and DMF mixed solvent. The resulting activated MOF 1ais able to uptake considerable amounts of CO2 at room temperature, and be further used for the efficient conversion of epoxides along with CO2 into cyclic carbonates under mild conditions without a co-catalyst. To control intra-hospital cross-infection in the Department of Ultrasound, the anti-bacterial activity of the compound was assessed in a systemic Staphylococcus aureus infection mouse model. The survival rate of systemic Staphylococcus aureus infected mice after compound treatment was determined to evaluate protective effect of the compound invivo. The number of colony-forming units (CFUs) in the organs of infected mice was also counted for further verification.

Metal-dependent photochromic performance in two isostructural supramolecular chains

The introduction of nonphotochromic coplanar 1,10-phenanthroline into a metal carboxylate system produces two isostructural supramolecular chains with metal-dependent photochromic performance.

Encapsulating organic guest cations in anionic MOFs that exhibit multi-responsive photochromism and photocontrolled luminescence

Two MOF-based host–guest compounds were prepared by incorporating viologen into an anionic framework, and exhibited photochromism upon irradiation with various light sources. Moreover, their luminescence properties can be switched by irradiation.

Two viologen-based photoluminescent compounds: excitation-wavelength-dependent and photoirradiation-time-dependent photoluminescent switches

We synthesized two isostructural multi-coloured photoluminescent coordination polymers. They exhibit excitation-wavelength-dependent photoluminescence emission and photoirradiation-time-dependent photoluminescence emission in solid-state.

The first 2,6-di(1,6-naphthyridin-2-yl)pyridine-based redox photochromic coordination polymer platform with selective vapochromism for trolamine

The first redox photochromic coordination polymer system with photosensitive 2,6-di(1,6-naphthyridin-2-yl)pyridine ligand has been developed, exhibiting stepwise photo- and vapochromism towards UV irradiation and amino-attached vapor fumigation.

Photochromism of metal-organic frameworks based on carbazole-dicarboxylic acid and bipyridine: sensing adjustment by controlling strut-to-strut energy transfer

In this paper, two energy-transfer photochromic metal-organic frameworks (MOFs) {[Zn(L)_0.5(bpy)]·H₂O·DMF}_n (1) and {[Zn(L)_0.5(bpe)]·2H₂O·DMF}_n (2) (H₄L = 9,9'-(1,4-phenylenebis(methylene))bis(9H-carbazole-3,6-dicarboxylic acid), bpy = 4,4'-bipyridine, bpe = 4,4'-vinylenedipyridine) were designed and synthesized. Both 1 and 2 showed similar pillared-paddle wheel type frameworks with bpy and bpe as the chromophore, respectively, and L^4- as the antenna-type light harvester, yielding strut-to-strut energy transfer (antenna behavior) within the well-ordered structures. Among them, 1 displayed excellent energy-transfer photochromic behavior under UV light accompanied by color transformation from colorless to purple. In addition, the photochromic behavior of 1 has obvious, fast, controllable and reversible characteristics. On the other hand, 2 showed a different energy-transfer photochromic behavior in the aspects of color changing, gamut, and sensitivity. The variation has been ascribed to the substitution of chromophore bpy in 1 with bpe in 2, which influences the efficiency of energy transfer within the MOFs. Therefore, with the structural diversity and tunability of MOFs, the sensitivity, color, and gamut of energy-transfer of the photochromic MOFs can be tuned by the appropriate choice of the constitutions of MOFs. This work will provide useful guidance for developing novel energy-transfer photochromic MOF materials.

A viologen-based multifunctional Eu-MOF: photo/electro-modulated chromism and luminescence

A novel viologen-based multifunctional Eu-MOF integrating photochromism, photomodulated fluorescence, and electrochromic and electroluminochromic properties was investigated.

A zwitterionic ligand-based water-stable metal–organic framework showing photochromic and Cr(vi) removal properties

A water-stable In-MOF based on the zwitterionic viologen ligand was synthesized. It exhibits photochromic property and Cr(vi) removal performance.

Photochromic Lanthanide(III) Materials with Ion Sensing Based on Pyridinium Tetrazolate Zwitterion

Lanthanide(III) ion (Ln(III)) sensing has become a major research area owing to its intriguing prospect in clinic, biology, and environmental studies. However, the existing methods have limitations like requirement of expensive instrumentation, long analytical times, and sample pretreatments, revealing the necessity of other methods. In this work, by using N-methyl-4-pyridinium tetrazolate (mptz) zwitterion as an electron acceptor, we obtained several new Ln(III) compounds with electron-transfer (ET) photochromic properties: [Ln(NO₃)₃(H₂O)₄]·mptz [Ln = Sm (1), Eu (2), Gd (3), La (4), Ce (5), Pr (6), Nd (7), Tb (8), Dy (9), Ho (10), Er (11), Tm (12), Yb (13), Lu (14)]. Notably, different Ln(III) ions in these compounds can be visually identified by their different photoinduced color changes related to the ET process. This work may not only contribute to the more understanding of the structure-photosensitivity relationships of pyridinium-based compounds, but also provide a new approach for Ln(III) ion sensing.

Photochromism and photomagnetism in crystalline hybrid materials actuated by nonphotochromic units

Using the pillar-layer strategy, two isomorphic crystalline coordination compounds with obvious photochromic and photomagnetic behaviors actuated by nonphotochromic ligands were synthesized.

Tunable photochromic properties of hybrid solids controlled by the conjugated length of non-photochromic units

The photochromic performance of two hybrid zincophosphite frames could be tuned via enlarging the conjugated length of the non-photochromic bipyridine unit.

Two novel multichromic coordination polymers based on a new flexible viologen ligand exhibiting photocontrolled luminescence properties and sensitive detection for ammonia

Two novel Cd-CPs based on a new viologen ligand exhibiting photochromism, thermochromism, photocontrolled luminescence properties, thermocontrolled luminescence properties and ammonia detection.

Hierarchically controllable photoreaction of a coordination polymer based on quaternized 1,2-bis(4'-pyridyl)ethylene

A multiply photoactive compound has been developed based on quaternized 1,2-bis(4'-pyridyl)ethylene, which exhibits a hierarchical photoreaction by controlling light intensity. Weak ultraviolet light can effectively promote the cycloaddition reaction with fluorescence enhancement, while stronger ultraviolet light can stimulate a complex response of cycloaddition and electron-transfer which shows turn-off luminescence and coloration. Photofacilitated bleaching of the colored products has been observed for the first time in the pyridinium family.

The influence of the secondary building linker geometry on the photochromism of naphthalenediimide-based metal–organic frameworks

The photochromism of three MOFs based on NDI chromophores is mainly induced by two kinds of non-covalent intermolecular interactions.

A ZnII-MOF Assembled by Triazine-Based Polycarboxylate and 4,4′-Bipy: Structure, Fluorescent Properties, and Water Vapour Adsorption

A new flexible triazine-based polycarboxylate metal–organic framework, {[Zn4(TTHA)(4, 4′-bipy)2(OH)2(H2O)]·H2O}n (1), H6TTHA = 1,3,5-triazine-2,4,6-triamine hexaacetic acid, has been synthesised under hydrothermal conditions and structurally characterised by X-ray diffraction, infrared spectroscopy, elemental analysis, and X-ray single-crystal diffraction. Structural studies indicate that 1 exhibits a 3D network with a 4,6-connected topology, formed by the interconnection of tetranuclear units and the ligands. The fluorescence emission properties of 1 and the free ligands were investigated in the solid state at room temperature. Moreover, the fluorescence lifetime (τ) and quantum yield (Φf) were obtained to further characterise the fluorescence properties. In addition, the water vapour adsorption of 1 was first studied for H6TTHA based complexes, which shows a general water adsorption capacity.

Smart nanoporous metal–organic frameworks by embedding photochromic molecules – state of the art and future perspectives

The opportunities, properties and perspectives of smart photochromic materials, which are obtained by embedment of stimuli-responsive molecules in nanoporous MOFs (metal–organic frameworks), both in bulk and in thin films, are reviewed.

Structure of a Cd(ii) mixed-ligand coordination polymer: single crystalline conductance switch involving photoinduced electron transfer and photocoloration

A novel Cd(ii)-MOF with tunable photophysical behavior has been developed. The control of color, fluorescence emission and conductivity is achieved in a single crystalline conductance switch by incorporating donor and acceptor building blocks into a molecular wire.

Energy-dependent photochromism at room temperature for visually detecting and distinguishing X-rays

A radiochromic material with energy-dependent X-ray-induced photochromism was obtained by incorporating a radiosensitive ligand into an electron-transfer photochromic metalloviologen system.

Synthesis and characterization of a photochromic magnesium(II) coordination polymer based on a naphthalene diimide ligand

Naphthalene diimides, which are planar, chemically robust and redox-active, are an attractive class of electron-deficient dyes, which can undergo a single reversible one-electron reduction to form stable radical anions in the presence of electron donors upon irradiation. This makes them excellent candidates for organic linkers in the construction of photochromic coordination polymers. Such a photochromic one-dimensional linear coordination polymer has been prepared using N,N′-bis(3-carboxyphenyl)naphthalene-1,8:4,5-tetracarboximide (H2BBNDI). Crystallization of H2BBNDI with magnesium nitrate in an N,N′-dimethylformamide (DMF)/ethanol/H2O mixed-solvent system under solvothermal conditions afforded the one-dimensional coordination polymer catena-poly[[bis(dimethylformamide-κO)magnesium(II)]-bis[μ-N-(3-carboxylatophenyl)-N′-(3-carboxylphenyl)naphthalene-1,8:4,5-tetracarboximide-κ2 O:O′]], [Mg(C28H13N2O8)2(C3H7NO)2] n . The asymmetric unit contains half of a magnesium cation, one HBBNDI− ligand and one DMF molecule. Two partially deprotonated HBBNDI− ligands bridge two magnesium cations to form a one-dimensional chain. Strong inter-chain π–π interactions between the naphthalene rings of the HBBNDI− ligand and the imide rings of adjacent chains provide a two-dimensional structure. The supramolecular three-dimensional framework is stabilized by π–π interactions between naphthalene rings of neighbouring two-dimensional supramolecular networks. The complex exhibits a reversible photochromic behaviour, which may originate from the photoinduced electron-transfer generation of radicals in the HBBNDI− ligand.