Li-Xuan Cai

Reversible luminescence switch in a photochromic metal-organic framework.

Eu(III) ions have been introduced into a photoactive viologen system to yield a polyrotaxane-like metal-organic framework, which exhibits reversible photochromism and luminescence modulation with a non-destructive readout capability in the solid state.

Introduction of lewis acidic and redox-active sites into a porous framework for ammonia capture with visual color response.

Based on the Lewis acidic site and redox ability of bipyridinium ligand, a porous framework with an adsorption advantage for ammonia over water and color response ability has been constructed. The compound is highly stable and flexible to external stimuli, exhibiting reversible single-crystal-to-single-crystal transformations, in response to temperature change and NH3 capture. More attractively, the title compound shows obvious color change from yellow to dark blue when exposed to ammonia vapor within just a few seconds, indicating a strong ability to function as a visual colorimetric absorbent for ammonia.

Supramolecular isomer-dependent photochromism and emission color tuning of bipyridinium salts

A study of the supramolecular isomer-dependent photoresponse and emission behaviors of a carboxybenzyl-substituted bipyridinium salt in the solid state is presented. Two supramolecular isomers (A1 and A2) are obtained by self-assembly with the same starting materials at different concentrations. The flexible conformation of the bipyridinium molecule results in different crystal packing geometries, which further influence and alter their photochromic behaviors owing to the crucial differences in the distance and orientation between the donor and acceptor moieties. Remarkably, the introduction of carboxyl groups into the bipyridinium molecule leads to intramolecular charge transfer (ICT) emissions in the visible region. A strong fluorescence emission can be detected in A1, and reversible on/off and multicolor (yellow, olivine and green) fluorescence switching, via photo-irradiation in the solid state, has been observed. Considering the easy chemical modification and the intriguing multicolor photoswitching properties, the present system shows great potential for use in solid photonic devices.

Structural diversity of the mixed-ligand system Mn-cpdba-2,2′-bpy controlled by temperature

A series of Mn-cpdba-2,2′-bpy complexes, {[Mn(cpdba)(2,2′-bpy)]}n1, {[Mn2(cpdba)2(2,2′-bpy)(H2O)2]·3H2O}n2, {[Mn2(cpdba)2(2,2′-bpy)2(H2O)2]·H2O}n3, {[Mn(cpdba)(2,2′-bpy)]·1.5H2O}n4, {[Mn4(cpdba)4(2,2′-bpy)3(H2O)2]}n5 (H2cpdba = 4-(4-carboxyphenylamino)-3,5-dinitrobenzoic acid; 2,2′-bpy = 2,2′-bipyridine) have been synthesized at different temperatures under hydrothermal conditions. Complex 1 has a 2D (4,4) network with alternate left and right-handed helical chains. With increasing temperature to 150 °C, complex 2, which has a complicated 3D framework constructed by a unique building block Mn3(cpdba)6(2,2′-bpy)2, can be obtained. Complex 3 that has been reported previously was obtained at 160 °C. This complex exhibits a 1D chain structure which is extended into a 3D supermolecular framework by π–π stacking interactions. Complex 4 was detected as a by-product in the synthesis of 3. X-Ray single crystal diffraction analysis reveals the formation of 2D layer network containing [Mn(CO2) + Mn(CO2)]n double-helices. With continuing increase to 170 °C, complex 5 featuring 3D framework similar to 2, with the decrease in the coordinated water and lattice water molecules as well as increase in the regularity of the framework, was formed. Influence of the 2,2′-bpy co-ligand on the coordination configuration of the cpdba2− ligand has been discussed. The magnetic susceptibility measurements of 1 and 5 reveal that two complexes exhibit antiferromagnetic coupling interactions.

Reversible luminescence switching between single and dual emissions of bipyridinium-type organic crystals

An excitation-wavelength-dependent luminescence behavior of a bipyridinium derivative, together with its luminescence switching between single- and dual-emissions upon reversible solid state structural transformation, has been presented.

Polycatenation‐Driven Self‐Assembly of Nanoporous Frameworks Based on a 1D Ribbon of Rings: Regular Structural Evolution, Interpenetration Transformation, and Photochemical Modification

A series of nanoporous frameworks constructed by a polycatenated isoreticular 1D ribbon of rings have been developed. The orientation of catenated ribbons can be fine tuned by varying counter anions, which allows both pore size and shape to be systematically adjusted in a pre-synthetic process. Distinct from conventional pore construction modes in which the organic linkers are alternately connected by metal nodes into a 3D periodic arrangement, the present polycatenation approach represents an alternative for constructing soft porous materials with tunable pore metrics and functions. Furthermore, these porous structures can interconvert into each other based on an anion-exchange process, accompanied by the transformation of the interpenetrating structures in different dimensional networks, which is unusual in porous frameworks. In addition, such a porous framework can be post-synthetically modified by a photoinduced [2+2] cycloaddition reaction, which not only achieves the surface modification (from conjugated to non-conjugated inner surface), but also triggers the structural transformation from low dimension to high dimension. Such a post-modification process reinforces the pore architecture through a covalent locking effect and has a great impact on the adsorption properties.

Auxiliary ligand-controlled photochromism and decolourization of two bipyridinium-based metal–organic hybrid materials with various water clusters

Two bipyridinium-based metal–organic hybrid materials, {[Cd(Bpyen)0.5(p-BDC)(H2O)Br]·5H2O} (1) and {[Cd(Bpyen)(o-BDC)Br2]·8H2O} (2), have been synthesized. 1 is a 2D coordination polymer while 2 shows 1D chain-like structure. Both compounds exhibit rapid color change under light irradiation. However, the decolourization of the photochromic products is very different. The influence of the auxiliary ligands on the photochromism and decolourization process of the bipyridinium-based hybrid materials has been explored. Interestingly, water molecules are encapsulated in these two compounds and form novel water aggregates consisting of novel water building units, such as the (H2O)10 water cluster in 1 and the (H2O)12 water cages in 2 through hydrogen-bond interactions. The effect of these water aggregates on the stability of the two compounds has been studied.

Anion-induced structural transformation involving interpenetration control and luminescence switching

Three metal–organic coordination compounds with different interpenetrating modes, [Cd(BCbpy)(BDC)]·3H2O (1), [Cd2(BCbpy)2(BDC)Cl2][Cd(BCbpy)2(BDC)]·18H2O (2), and [Cd(BCbpy)Cl2]·3H2O (3) (HBCbpyCl = 1-(4-carboxybenzyl)-4,4-bipyridinium chloride, H2BDC = 1,4-benzenedicarboxylic acid), have been synthesized and structurally characterized. Compound 1 exhibits a 2D + 2D → 2D interpenetrating array with the coexistence of polyrotaxane and polycatenane structures. Compound 2 is an unprecedented example of a 2D + 2D + 1D → 2D interpenetrating structure which involves 2D + 2D → 2D parallel interpenetration and a 2D + 1D → 2D polyrotaxane. In the case of compound 3, the absence of the rod-like BDC ligand prevents threading through the ring-like units, no polyrotaxane structure is formed, and the compound features infinite 1D ring-containing chains arranged in parallel without any interpenetration. Interestingly, the structural transformation can be achieved through a solvent-mediated anion exchange process when compound 1 or 2 is exposed to the aqueous solution containing chloride ions. Upon progressive replacement of the BDC2− ligands by chloride ions, the complicated network topology is simplified and undergoes an evolution from interpenetration to non-interpenetration, accompanied by a visually observed on–off–on fluorescence switching.

π-Conjugation-directed highly selective adsorption of benzene over cyclohexane

Crystalline materials with large tailor-made channels have been made by an isoreticular synthesis strategy, which includes a successful adjustment of the π-conjugation over a framework skeleton allowing target recognition for aromatic molecules. The selective absorption of benzene over cyclohexane has been achieved, with a significant increase in benzene uptake with the improvement of the degree of π-conjugation.

Targeted Functionalization of Porous Materials for Separation of Alcohol/Water Mixtures by Modular Assembly

Three isoreticular hydrogen-bonded frameworks with functionalized pore structures were constructed by a modular self-assembly process in which a series of amino acids with various substituents serve as facile exchange subassemblies to decorate the pore wall. The ordered amino acid side-chain groups in the pore channels play an important role in determining the adsorption behavior of the framework materials, and ensure exclusive adsorption of methanol/water over ethanol. Gas-chromatographic separation experiments demonstrated that alcohols can be efficiently separated from ternary water/methanol/ethanol mixtures and revealed a key influence of the adsorbate-host framework interaction on the practical separation performance of mixtures.