Viologen Guest-Mediated Luminescence Emission Tuning and Photochromic Behavior by a Series of Viologen@Zn-MOF Materials

The encapsulation of various guest molecules into the pores of metal-organic frameworks (MOFs) to form hybrid materials has attracted significant attention due to their unique spatial distribution and certain preferential geometry of the guests inside the MOFs. This arrangement often results in the guests exhibiting unique physical and chemical properties due to their intramolecular interactions with the host. In this article, five viologen derivatives were introduced as guests in a Zn-MOF with different benzene ring lengths, resulting in the formation of host-guest three-dimensional (3D) MOFs. The five compounds exhibited guest-dependent emission wavelength, color, and excellent photochromic behavior upon ultraviolet (UV) light radiation due to the distinct electronic transfer and π···π stacking interactions between the viologen guests and the host framework. This study provides a host-guest strategy for designing color-tunable luminescent and highly sensitive photochromic materials.

One-Dimensional Chain Viologen-Based Lanthanide Multistimulus-Responsive Materials with Photochromism, Photoluminescence, Photomagnetism, and Ammonia/Amine Vapor Sensing

In this work, a series of novel multistimulus-responsive lanthanide coordination polymers {[LnL(H₂O)₄]Cl₃·3H₂O}_n (Ln = Dy, Tb, Eu) constructed using a dicarboxylic acid viologen derivative L (L = N,N'-4,4'-bipyridiniodipropionate) and LnCl₃·6H₂O were prepared. All materials showed positive responses to UV light, and the photochromic phenomena accompanied by significant photoquenching of photoluminescence could be observed through a photoelectron transfer mechanism. Strikingly, the Dy analogue displayed photomagnetic behavior, as well as responded positively to small molecules of inorganic ammonia/organic amines. Furthermore, the good photoresponsive and ammonia/amine vapor-responsive properties of the Dy-based material were further fulfilled in dual-function papers involving erasable inkless printing and visual amine detection applications. This work aims to advance the development of multistimulus-responsive multifunctional materials incorporating viologen derivates and versatile lanthanide ions and further enriches the research in this field.

A novel three-position molecular switch based on the transformations of a cyano-substituted pyrrol-2-one derivative

A new nitrile-rich heterocycle showing a reversible naked-eye response to basic and nucleophilic agents as well as to temperature was characterized.

An electron-deficient naphthalene diimide-based metal-organic framework for detecting electron-rich molecules through photo-/chemo-induced chromism

A novel naphthalene diimide-based metal-organic framework (MOF) {[Zn(3-DPMNI)_0.5(NDC)]·3DMF} (1@DMF), (H₂NDC = 2,6-naphthalenedicarboxylic acid, DPMNI = N,N'-bis(3-pyridylmethyl)-1,4,5,8-naphthalene diimide, DMF = N,N'-dimethylformamide), has been synthesized, which shows a 3D pillar-layer architecture built of carboxylate layers and naphthalene diimide pillars. The compound exhibits outstanding photochromic performance due to photoinduced electron transfer (ET) between the electron-rich guest molecules and electron-deficient host framework (host-guest ET). Of note, the host framework of 1 cannot show a macroscopic color change owing to the absence of the ET pathway. Nevertheless, it exhibits discriminative photochromic behavior in the presence of electron-rich solvents, which is mainly attributed to different electron-donating abilities of guest solvents and distinct interfacial contacts of electron donors/acceptors. Furthermore, the MOF can also show discriminative ET chemochromic response to different sizes and shapes of organic amines, which can be potentially used for the visual detection of electron-rich organic amines, especially n-butylamine (n-BUA).

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.

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.

Photochromic and photocontrolled luminescent rare-earth D–A hybrid crystals based on rigid viologen acceptors

Due to the introduction of a strong electron donor, CoIII(CN)63−, into the structure, the rare-earth donor–acceptor (D–A) hybrid crystal shows enhanced photochromism. The coordinative Eu3+ cation is also beneficial toward improving the luminescence.

Construction of a viologen-derived 2D material with photochromism, photoswitchable luminescence and inkless and erasable prints via sunlight irradiation

A Zn(ii) material based on thiophenedicarboxylate and viologen derivative mixed ligands is synthesized. Thanks to pyridinium unit embedded in its structure, the material exhibits a rapid photochromic response via sunlight irradiation.

A multifunctional colorimetric sensor originating from a cadmium naphthalene diimide-based metal–organic framework: photochromism, hydrochromism, and vapochromism

A new cadmium metal–organic framework with photocontrolled luminescence properties and multiple chromic properties was synthesized for used as a multifunctional colorimetric sensor and provide a new route to design functional MOFs.

Three-component D-A hybrid heterostructures with enhanced photochromic, photomodulated luminescence and selective anion-sensing properties

As an emerging class of hybrid complexes, donor-acceptor (D-A) hybrid heterostructures with advantages of both photoactive organic and inorganic components have provided an excellent platform for the fabrication of multifunctional photoactive materials. In this context, we have demonstrated three novel host-guest D-A hybrid heterostructures, {[Ln(BCEbpy)(H₂O)₄][Co^III(CN)₆]·4H₂O}_n (1 (Eu), 2 (Dy), 3 (Sm)), based on the anionic Co(CN)₆^3- and cationic coordination layers assembled from a viologen functionalized tecton and Ln(NO)₃. Due to the introduction of an electron donor, Co^III(CN)₆^3-, the unique hybrid exhibits a highly sensitive and reversible photochromic transformation from light-yellow to brown upon UV-Vis irradiation. More interestingly, accompanied with this photochromic process, hybrid 1 simultaneously possesses a photomodulated fluorescence behaviour. In addition, hybrid 1 shows high sensitivity and selectivity towards Cr₂O₇^2- anions with a fairly small LOD of ca. 9.6 × 10^-6 M.

Constructing a 3D-layered energetic metal–organic framework with the strong stacking interactions of hydrogen-bridged rings: the way to an insensitive high energy complex

Constructing 3D-layered EMOFs with strong stacking interactions of hydrogen-bridged rings to enhance their packing efficiency and energetic performances.