Distributable reactivity of a dual photochemically active molecule functionalized by a carboxyl coordination unit has been achieved based on anion-assisted coordination assembly. The anions induce the photoactive ligand to align at different orientations in three-dimensional metal–organic frameworks, thus allowing the supramolecular systems to show photocycloaddition with simultaneous luminescence transformation or undergo photoinduced electron transfer accompanied by coloration–decoloration under alternating light and thermal stimuli.

Two three-dimensional (3D) zeolitic open-framework aluminoborates (ABOs) [Zn(ma)(en)2][AlB5O10] (1) and [Zn(ma)(en)2][AlB6O11(OH)] (2) (ma = methylamine, en = ethylenediamine) were successfully made under solvothermal conditions, which represent the first examples using similar metal complex (MC) with mixed amines as the structure-directing agents in ABOs. Notice that the central atom in the MC is coordinated by mixed amines is uncommon. However, they exhibit distinctly different structures: 1 crystallizes in a centrosymmetric [Al(B5O10)]n2n– zeolitic framework built by pentaborate (B5O10) clusters and AlO4 tetrahedra, exhibiting a 4-connected cag topology, while 2 contains hexaborate (B6O11(OH)) clusters and AlO4 groups, further alternately joined to form a noncentrosymmetric [AlB6O11(OH)]n2n– zeolitic framework with 7-/9-, 8-/10-ring helical channels and large 13-ring channels, showing a dia topology. Nonlinear optical (NLO) determination showed that 2 exhibits good second-harmonic generation response of ∼1.9 × KH2PO4 and is phase-matchable, combining the short UV cutoff edge, making 2 a potential UV NLO material.

A flexible bipyridinium ligand has been introduced into a series of isomorphous metal–organic frameworks to yield porous materials with subtriangular 1D channel and cagelike pore space. The introduction of bipyridinium molecules into the porous frameworks leads to the formation of 1D channel with positively charged surface, which shows high affinities to polar molecules such as methanol, ethanol, and water vapor. Additionally, the bipyridinium molecules possess conjugated pyridyl rings connected by carbon–carbon single bonds, which can rotate freely to keep twisted or large-conjugated planar configuration by controlling coordination condition during the self-assembly process. The cagelike pore space formed by these flexible bipyridinium molecules contains arene–arene stacked pyridyl rings and thus renders abundant π–π interaction sites for efficient adsorption of benzene and toluene molecules, and is also large enough to accommodate cyclohexane molecules. These metal–organic frameworks show efficient adsorption for benzene, toluene, and cyclohexane molecules through high utilization efficiency of unique pore space, and display rarely broad-spectrum adsorptivity to various organic pollutants with hydrophilic/hydrophobic, conjugated/nonconjugated characteristics.

•Two novel bipyridinium-based coordination polymers have been synthesized and structurally characterized.•Compound 1 display photochromic behavior while compound 2 is photoinert.•The influence of different substituent groups in same site of auxiliary ligands on the photochromic behavior has been studied.Two bipyridinium-based coordination polymers, {[Cd(BCbpy)2(o-BDC)]·13H2O} (1) and {[Cd(BCbpy)2(SA)]·6H2O} (2), have been synthesized and structurally characterized. Single-crystal X-ray diffraction analysis shows that the two compounds have similar one-dimensional coordination polymeric chain structures but different packing patterns. Compound 1 displays fast eye-detectable photochromic behavior while compound 2 is photoinert under 365 nm irradiation. The influence of different substituent groups in same site of auxiliary ligands on the photochromic behaviors of the two compounds has been studied in detail.The effect of different substituent groups in the same position of auxiliary ligands on the photochromic behaviors of bipyridinium-based coordination polymers has been explored.

AbstractA viologen-based Borromean entangled porous framework was found to be sensitive to both CuKα and MoKα X-ray sources, showing rapid photochromic response and recovery within one minute. The X-ray-induced photochromic process is accompanied by a reversible single-crystal-to-single-crystal (SC-SC) structural transformation, an unprecedented phenomenon for X-ray sensitive materials. The complex can be further processed into portable thin films for detecting the dose of the X-ray exposure. Moreover, the photochromism can occur over a broad temperature range of 100–333 K, both in the form of single crystals and thin films, making it a potential candidate for practical indoor and outdoor applications.

An olefin-containing pyridinium derivative, 1-(4-carboxybenzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium chloride (HBCbpeCl) has been introduced into a novel 2D metal–organic framework with rare right- and left-handed spiral nanotubular channels arranged alternately in the structure. The 1D channel is modified by positively-charged pyridinium segments, which shows high affinities to Lewis basic molecules such as methanol, ethanol, and water vapor. Additionally, the BCbpe molecules in the structure have been directed through cation–π interactions to be aligned antiparallel to each other with the CC double bond arranged properly for [2 + 2] photocycloaddition, which can be triggered to undergo structural transformation from 2D to 3D. This post-cycloaddition modification strategy has been used to separate alcohols and water, which includes a successful adjustment of large tailor-made channels by the [2 + 2] cycloaddition reaction. The gas-chromatographic separation experiments show that ethanol can be better separated from water through a column filled with a post-cycloaddition modified sample than the as-synthesized one.

The self-assembly of a photoactive 1,1′-bis(4-carboxybenzyl)-4,4′-bipyridinium ligand with cadmium chloride in the presence of a strong electron-donating oxalate anion has successfully afforded a photochromic coordination polymer, which shows high sensitivity not only to ultraviolet-visible radiation but also to X-rays.

The past ten years have evidenced the rapid development of bipyridinium-based coordination polymers, however, the design and synthesis of photo-switchable luminescent materials with a non-destructive readout capability based on bipyridinium ligands are still at the primary stage. In this work, two new coordination polymers with a bipyridinium-functionalized m-benzenedicarboxyl ligand were synthesized for this purpose. By using anions with different geometries, the molecular linkage and packing structure have been adjusted, and thus allow the two title compounds to exhibit eye-detectable color development with different photoresponsive rates, and distinct luminescence behaviors. Furthermore, compound 2 is the first reported Zn-based luminescent coordination polymer with a non-destructive readout capability. These results can not only help to further understand the structure–photosensitivity relationships of bipyridinium-based systems but also guide the design and synthesis of more luminescent materials with a non-destructive readout capability.

Two non-centrosymmetric alkaline-earth metal borates, Ca2[B5O8(OH)]2[B(OH)3]·H2O (1) and Ba2[B6O9(OH)2][B3O3(OH)4]·(OH)·4H2O (2), have been synthesized under solvothermal conditions. Both of them exhibit layered structures constructed from two fundamental building blocks (FBBs), respectively. 1 features porous double-layers made by the acentric single A-layer and B-layer in a face-to-face arrangement. Ca–O layers gather the B–O double-layers together resulting in the chiral structure of 1. Compound 2 contains two different FBBs, which is rare in borate. Hexaborate B6 clusters are responsible for the expansion of inorganic B6-layers while the isolated triborate B3 clusters are arranged in supermolecular B3-layers. Ba1-B6-layers and Ba2-B3-layers are alternately arranged in the same direction generating the NCS structure of 2. Both 1 and 2 are phase-matchable materials with second-harmonic generation (SHG) responses of ∼1.3 and ∼0.8 times that of KDP (KH2PO4), respectively. UV-vis diffuse reflectance spectra reveal that they have short cut-off edges lower than 200 nm. These characteristics make them potential deep-UV NLO materials. The calculations reveal that the SHG efficiency mainly originates from the synergistic effect of the distorted MOn (M = Ca, Ba; n = 7, 10, 11) polyhedra and the asymmetric π-conjugated of BO3 units.

A multi-stimuli responsive small molecule organic material has been developed, which for the first time exhibits dual-wavelength response to render photocoloration under visible light and turn-on luminescence under UV light. The colored product can be bleached and shows turn-on luminescence again when re-exposed to UV light.

Two isomorphous halogen-bridged metal–organic frameworks have been solvothermally synthesized based on the bipyridinium ligand and structurally characterized. The two compounds show an eye-detectable color development upon light irradiation, but different coloration degrees. The relationship between the structure and photosensitivity has been studied in detail. The photochromic properties of the bipyridinium unit have been used to modulate the luminescence under light illumination. Compound 1 is the first example showing photo-modulated luminescence switching featuring a non-destructive readout capability based on transition metals.

Metal–organic frameworks (MOFs) as newly emerged materials have experienced rapid development in the last few years. The modular synthesis procedure allows integrating functional groups in their frameworks with varied applications. Due to the easy modification of the backbone and highly charged characteristics with interesting electron-active properties, the use of bipyridinium derivatives as synthons for the fabrication of functional metal–bipyridinium frameworks (MBPFs) has attracted increased interest over the past few years. Various bipyridinium-bearing ligands have been designed for the construction of functional MBPFs, and some of them present intriguing properties for potential applications including photochromism and photoswitching, sensing, molecule adsorption and separation. This perspective aims to highlight the recent progress in this area, and seeks to uncover promising ideas that will underscore future advancements at both the fundamental and applied levels.

An olefin-containing pyridinium derivative, 1-(4-carboxybenzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium chloride (HBCbpeCl) has been introduced into a novel 2D metal–organic framework with rare right- and left-handed spiral nanotubular channels arranged alternately in the structure. The 1D channel is modified by positively-charged pyridinium segments, which shows high affinities to Lewis basic molecules such as methanol, ethanol, and water vapor. Additionally, the BCbpe molecules in the structure have been directed through cation–π interactions to be aligned antiparallel to each other with the CC double bond arranged properly for [2 + 2] photocycloaddition, which can be triggered to undergo structural transformation from 2D to 3D. This post-cycloaddition modification strategy has been used to separate alcohols and water, which includes a successful adjustment of large tailor-made channels by the [2 + 2] cycloaddition reaction. The gas-chromatographic separation experiments show that ethanol can be better separated from water through a column filled with a post-cycloaddition modified sample than the as-synthesized one.

A multi-stimuli responsive small molecule organic material has been developed, which for the first time exhibits dual-wavelength response to render photocoloration under visible light and turn-on luminescence under UV light. The colored product can be bleached and shows turn-on luminescence again when re-exposed to UV light.

Two non-centrosymmetric alkaline-earth metal borates, Ca2[B5O8(OH)]2[B(OH)3]·H2O (1) and Ba2[B6O9(OH)2][B3O3(OH)4]·(OH)·4H2O (2), have been synthesized under solvothermal conditions. Both of them exhibit layered structures constructed from two fundamental building blocks (FBBs), respectively. 1 features porous double-layers made by the acentric single A-layer and B-layer in a face-to-face arrangement. Ca–O layers gather the B–O double-layers together resulting in the chiral structure of 1. Compound 2 contains two different FBBs, which is rare in borate. Hexaborate B6 clusters are responsible for the expansion of inorganic B6-layers while the isolated triborate B3 clusters are arranged in supermolecular B3-layers. Ba1-B6-layers and Ba2-B3-layers are alternately arranged in the same direction generating the NCS structure of 2. Both 1 and 2 are phase-matchable materials with second-harmonic generation (SHG) responses of ∼1.3 and ∼0.8 times that of KDP (KH2PO4), respectively. UV-vis diffuse reflectance spectra reveal that they have short cut-off edges lower than 200 nm. These characteristics make them potential deep-UV NLO materials. The calculations reveal that the SHG efficiency mainly originates from the synergistic effect of the distorted MOn (M = Ca, Ba; n = 7, 10, 11) polyhedra and the asymmetric π-conjugated of BO3 units.

The past ten years have evidenced the rapid development of bipyridinium-based coordination polymers, however, the design and synthesis of photo-switchable luminescent materials with a non-destructive readout capability based on bipyridinium ligands are still at the primary stage. In this work, two new coordination polymers with a bipyridinium-functionalized m-benzenedicarboxyl ligand were synthesized for this purpose. By using anions with different geometries, the molecular linkage and packing structure have been adjusted, and thus allow the two title compounds to exhibit eye-detectable color development with different photoresponsive rates, and distinct luminescence behaviors. Furthermore, compound 2 is the first reported Zn-based luminescent coordination polymer with a non-destructive readout capability. These results can not only help to further understand the structure–photosensitivity relationships of bipyridinium-based systems but also guide the design and synthesis of more luminescent materials with a non-destructive readout capability.

Metal–organic frameworks (MOFs) as newly emerged materials have experienced rapid development in the last few years. The modular synthesis procedure allows integrating functional groups in their frameworks with varied applications. Due to the easy modification of the backbone and highly charged characteristics with interesting electron-active properties, the use of bipyridinium derivatives as synthons for the fabrication of functional metal–bipyridinium frameworks (MBPFs) has attracted increased interest over the past few years. Various bipyridinium-bearing ligands have been designed for the construction of functional MBPFs, and some of them present intriguing properties for potential applications including photochromism and photoswitching, sensing, molecule adsorption and separation. This perspective aims to highlight the recent progress in this area, and seeks to uncover promising ideas that will underscore future advancements at both the fundamental and applied levels.

Two isomorphous halogen-bridged metal–organic frameworks have been solvothermally synthesized based on the bipyridinium ligand and structurally characterized. The two compounds show an eye-detectable color development upon light irradiation, but different coloration degrees. The relationship between the structure and photosensitivity has been studied in detail. The photochromic properties of the bipyridinium unit have been used to modulate the luminescence under light illumination. Compound 1 is the first example showing photo-modulated luminescence switching featuring a non-destructive readout capability based on transition metals.