Reversible Luminescent Switching in an Organic Cocrystal: Multi-Stimuli-Induced Crystal-to-Crystal Phase Transformation

A luminescent cocrystal system is reported to undergo crystal-to-crystal phase transformation from yellow-emitting polymorph I to green-emitting polymorph II, triggered by THF fuming or heating, and the green emission can recover to the initial yellow emission by grinding. The established spectroscopic and crystallographic analyses demonstrate that the phase transition occurred and benefits from the combined effect of similar molecular arrange sequence and unique alteration of intermolecular interactions from halogen/hydrogen bonds in I to π–π stacking in II. Furthermore, I and II exhibit red-shift emission under hydrostatic pressure. The emission of I and II shows a red-shift and recovers towards the initial emission upon acid–base fuming. This is a rare example of reversible luminescent switching of cocrystal based upon crystal-to-crystal phase transition, and provides an alternative strategy to develop multi-stimuli responsive materials.     

Synthesis, Crystal Structure and Luminescent Property of a One-dimensional Europium Citrate Complex

A new compound, [Eu(Hcit)(H2O)2]·H2O]n (1, Hcit3-= C(OH)(COO-)(CH2COO-)2), has been synthesized under hydrothermal reactions of europium oxide, MnCl2·4H2O and citric acid at 120 ℃ for three days. The compound was characterized by single-crystal X-ray diffraction analyses, IR and TGA. Complex 1 crystallizes in monoclinic, space group P21/n with a = 6.179(1), b = 9.688(2), c = 16.990(3) , β = 91.98(3)°, Z = 4, V = 1016.4(3) 3, C6H11EuO10, Mr = 395.11, Dc = 2.582 g/cm3, μ = 6.218 mm-1, F(000) = 760, R = 0.0183 and wR = 0.0411. Single-crystal X-ray analysis reveals that complex 1 displays 1D ladder chains along the a axis, with dinuclear Eu2O2 units serving as "steps" and carboxylate groups as "uprights", which are connected by hydrogen bonds. The solid-state luminescent property of complex 1 was investigated at room temperature. Upon excitation at 394 nm, compound 1 exhibits interesting luminescent properties with several intense bands in the visible region and the most intense and sharp emission being in the red region at 615 nm. The TGA and XRD results prove that complex 1 undergoes facile thermal decomposition to form Eu2O3 at about 870 ℃.     

Reversible Phase Transition of Porous Coordination Polymers

Porous coordination polymers or metal–organic frameworks with reversible phase-transition behavior possess some attractive properties, and can respond to external stimuli, including physical and chemical stimuli, in a dynamic fashion. Their phase transitions can be triggered by adsorption/desorption of guest molecules, temperature changes, high pressure, light irradiation, and electric fields; these mainly include two types of transitions: crystal–amorphous and crystal–crystal transitions. These types of porous coordination polymers have received much attention because of their interesting properties and potential applications. Herein, reversible phase transition porous coordination polymers are summarized and classified based on different stimuli sources. Corresponding typical examples are then introduced. Finally, examples of their applications in gas separation, chemical sensors, guest molecule encapsulation, and energy storage are also presented.     

Ln3+‐Mediated Self‐Assembly of a Collagen Peptide into Luminescent Banded Helical Nanoropes

Design of biomimetic peptides to achieve the desired properties of natural collagen has much potential to build functional biomaterials. A collagen‐peptide/Ln³⁺ system has been constructed and self‐assembled to form helical nanoropes with a distinct periodic banding pattern characteristic of natural collagen. The fully reversible self‐assembly is specifically mediated by lanthanide ions, but not by other commonly used divalent metal ions. Lanthanide ions not only provide an external biocompatible stimulus of the assembly, but also play as a functional unit to endow the assembled materials with easily tunable photoluminescence. To our knowledge, this is the first report of collagen‐peptide‐based materials with exquisite nanorope structure and excellent photoluminescent features. These novel luminescent nanomaterials may have great potential in cell imaging, medical diagnostics, and luminescent scaffolds for cell cultivation.     

Enzymatic Hydrolysis-Responsive Supramolecular Hydrogels Composed of Maltose-Coupled Amphiphilic Ureas

Maltose is a ubiquitous disaccharide produced by the hydrolysis of starch. Amphiphilic ureas bearing hydrophilic maltose moiety were synthesized via the following three steps: I) construction of urea derivatives by the condensation of 4-nitrophenyl isocyanate and alkylamines, II) reduction of the nitro group by hydrogenation, and III) an aminoglycosylation reaction of the amino group and the unprotected maltose. These amphiphilic ureas functioned as low molecular weight hydrogelators, and the mixtures of the amphipathic ureas and water formed supramolecular hydrogels. The gelation ability largely depended on the chain length of the alkyl group of the amphiphilic urea; amphipathic urea having a decyl group had the highest gelation ability (minimum gelation concentration=0.4 mM). The physical properties of the supramolecular hydrogels were evaluated by measuring their thermal stability and dynamic viscoelasticity. These supramolecular hydrogels underwent gel-to-sol phase transition upon the addition of α-glucosidase as a result of the α-glucosidase-catalyzed hydrolysis of the maltose moiety of the amphipathic urea.     

Solvent-Driven Reversible Phase Transition of a Pillared Metal–Organic Framework

Over the last decade, the controllable reversible phase transition of functional materials has received growing interest as it shows unique suitability for various technological applications. Although many metal–organic frameworks (MOFs) possess a lamellar structure, the reversible structural transformation of MOFs between their three-dimensional (3D) phase and two-dimensional (2D) phase remains a largely unexplored area. Herein, we report for the first time a europium MOF with unprecedented reversible morphology in different solvents at room temperature. This europium MOF displayed a 3D nanorod morphology in organic solvent and a 2D nanobelt architecture in water. As a proof of concept for potential applications of this reversible-phase-transition MOF, we were able to use a delamination recovery method to load dye molecules that previously could not be loaded into europium MOFs.     

Halogen-Modulated Reversible Phase Transition in Organic-Inorganic Hybrid Perovskites

The tunable structure, abundant raw materials, and ease of preparation have made molecular dielectric crystals popular for use in device design. In spite of this, some known molecular switching materials have a low phase transition temperature and a low dielectric constant, which limit their applications. Therefore, designing and synthesizing molecular-based phase transition compounds with high phase transition temperature and superior properties is especially important. In this work, we use 3-chloropropan-1-ammonium hydrochloride and SbCl₃/SbBr₃ inorganic salts as building blocks to synthesize compounds (CPA)₂(BPA)₂Sb₂Br₁₀ ( 1 ) (CPA=3-chloropropan-1-ammonium, BPA=3-bromopropan-1-ammonium) and (CPA)₂Sb₂Cl₈ ( 2 ). Compound 1 has a high phase transition temperature (407.45 K). Dielectric measurements and differential scanning calorimetry (DSC) confirm the structural phase transition in compound 1 , and no fatigue decay is observed after several dielectric cycles. In addition, compounds 1 and 2 possess semiconductor properties. The findings of this study provide new directions for the design and application of multifunctional molecular dielectric materials.     

Phase Transitions in Pure and Hybrid Hydrogels: A Fluorescence Study

Summary: Recent observations on phase transitions around the critical point showed that the critical exponents differed drastically from percolation results and classical results for pure hydrogel systems, depending on monomer concentration. In addition to pure hydrogels, the sol-gel phase transition during radical crosslinking copolymerization of acrylamide (AAm) and N-isopropylacrylamide (NIPA) hybrid was studied by using the steady state fluorescence (SSF) technique. N,N′-methylenebis(acrylamide) (BIS) and ammonium persulfate (APS) were used as crosslinker and initiator, respectively. Pyranine (trisodium 8-hydroxypyrene-1,3,6-trisulfonate acid, HPTS) was added as a fluoroprobe for monitoring the polymerization. It was observed that pyranine binds to AAm and NIPA chains on the initiation of the polymerization, thus the fluorescence spectra of the bonded pyranines shift to shorter wavelengths. Fluorescence spectra of the bonded pyranines allowed to monitor the sol-gel phase transition without disturbing the system mechanically and to test the universality of the sol-gel transition as a function of polymer concentration ratios. The observations around the gel point of PAAm-PNIPA hybrid show that the gel fraction exponent β obeyed the percolation result.     

Reversible Luminescent Switching in an Organic Cocrystal: Multi‐Stimuli‐Induced Crystal‐to‐Crystal Phase Transformation

A luminescent cocrystal system is reported to undergo crystal‐to‐crystal phase transformation from yellow‐emitting polymorph I to green‐emitting polymorph II, triggered by THF fuming or heating, and the green emission can recover to the initial yellow emission by grinding. The established spectroscopic and crystallographic analyses demonstrate that the phase transition occurred and benefits from the combined effect of similar molecular arrange sequence and unique alteration of intermolecular interactions from halogen/hydrogen bonds in I to π–π stacking in II. Furthermore, I and II exhibit red‐shift emission under hydrostatic pressure. The emission of I and II shows a red‐shift and recovers towards the initial emission upon acid–base fuming. This is a rare example of reversible luminescent switching of cocrystal based upon crystal‐to‐crystal phase transition, and provides an alternative strategy to develop multi‐stimuli responsive materials.     

Charge‐Transfer Phase Transition of a Cyanide‐Bridged FeII/FeIII Coordination Polymer

Heterometallic Prussian blue analogues are known to exhibit thermally induced charge transfer, resulting in switching of optical and magnetic properties. However, charge‐transfer phase transitions have not been reported for the simplest FeFe cyanide‐bridged systems. A mixed‐valence Fe^II/Fe^III cyanide‐bridged coordination polymer, {[Fe(Tp)(CN)₃]₂Fe(bpe)?5 H₂O}_n, which demonstrates a thermally induced charge‐transfer phase transition, is described. As a result of the charge transfer during this phase transition, the high‐spin state of the whole system does not change to a low‐spin state. This result is in contrast to FeCo cyanide‐bridged systems that exhibit charge‐transfer‐induced spin transitions.     

Synthesis,Structure and Luminescent Property of an Europium(Ⅲ) Coordination Polymer

A novel europium(Ⅲ) coordination polymer, [Eu3(pydc)3(SO4)(H2O)5(ox)0.5]·3H2O(1, H2 ox = oxalic acid, H2 pydc = pyridine-2,6-dicarboxylic acid), has been synthesized hydrothermally from the self-assembly of lanthanide ions(Eu3+) with the flexible oxalic acid and the rigid pyridine-2,6-dicarboxylic acid ligands, respectively. It crystallizes in the triclinic system, space group P1 with a = 11.225(4), b = 12.390(5), c = 13.752(5)A, α = 89.228(7), β = 71.142(6), γ = 75.552(6)°, Dc = 2.317 g/cm3, μ = 5.480 mm-1, F(000) = 1150, Z = 2, the final R = 0.0351 and w R = 0.0949 for 8782 observed reflections with I 2σ(I). Structural analysis reveals that complex 1 exhibits a three-dimensional metal-organic framework via π-π stacking interactions. In addition, complex 1 displays a red fluorescence emission in the solid state at room temperature, which corresponds to the 5D0 → 7F2 transition of Eu3+ ions.     

Self‐Healing Behavior in a Thermo‐Mechanically Responsive Cocrystal during a Reversible Phase Transition

The molecular‐level motions of a coronene‐based supramolecular rotator are amplified into macroscopic changes of crystals by co‐assembly of coronene and TCNB (1,2,4,5‐tetracyanobenzene) into a charge‐transfer complex. The as‐prepared cocrystals show remarkable self‐healing behavior and thermo‐mechanical responses during thermally‐induced reversible single‐crystal‐to‐single‐crystal (SCSC) phase transitions. Comprehensive analysis of the microscopic observations as well as differential scanning calorimetry (DSC) measurements and crystal habits reveal that a thermally‐reduced‐rate‐dependent dynamic character exists in the phase transition. The crystallographic studies show that the global similarity of the packing patterns of both phases with local differences, such as molecular stacking sequence and orientations, should be the origin of the self‐healing behavior of these crystals.     

Arylazo-3,5-dimethylisoxazoles: Azoheteroarene Photoswitches Exhibiting High Z-Isomer Stability,Solid-State Photochromism,and Reversible Light-Induced Phase Transition

Reversibly photoswitchable phenylazo-3,5-dimethylisoxazole and 37 aryl-substituted derivatives were synthesized. Excellent photoswitching ability of these compounds in solution and the solid state was demonstrated. Through kinetics studies by means of NMR spectroscopy, high Z-isomer stability was demonstrated. Interestingly, the majority of the derivatives showed light-induced contrasting color changes in solution and the solid state. Besides, many of the derivatives exhibit partial phase transition upon UV irradiation. The highlight of this class of photoswitches is the reversible light-induced phase transition between solid and liquid phases in the parent compound, which can be used in patterned crystallization. These results show that this new class of azoheteroarene based photoswitches has opportunities to be useful in various domains.     

Re(I) Complexes as Backbone Substituents and Cross-Linking Agents for Hybrid Luminescent Polysiloxanes and Silicone Rubbers

This study focuses on the synthesis of hybrid luminescent polysiloxanes and silicone rubbers grafted by organometallic rhenium(I) complexes using Cu(I)-catalyzed azido-alkyne cycloaddition (CuAAC). The design of the rhenium(I) complexes includes using a diimine ligand to create an MLCT luminescent center and the introduction of a triple C≡C bond on the periphery of the ligand environment to provide click-reaction capability. Poly(3-azidopropylmethylsiloxane-co-dimethylsiloxane) (N₃-PDMS) was synthesized for incorporation of azide function in polysiloxane chain. [Re(CO)₃(MeCN)(5-(4-ethynylphenyl)-2,2′-bipyridine)]OTf (Re1) luminescent complex was used to prepare a luminescent copolymer with N₃-PDMS (Re1-PDMS), while [Re(CO)₃Cl(5,5′-diethynyl-2,2′-bipyridine)] (Re2) was used as a luminescent cross-linking agent of N₃-PDMS to obtain luminescent silicone rubber (Re2-PDMS). The examination of photophysical properties of the hybrid polymer materials obtained show that emission profile of Re(I) moiety remains unchanged and metallocenter allows to control the creation of polysiloxane-based materials with specified properties.     

钼基吡啶类有机-无机杂化晶体材料的合成、可逆相变及介电性质

以钼酸铵、4-二甲氨基吡啶为原料,在乙醇、甲醇、HBr和水的混合溶液中通过溶剂挥发法成功合成有机-无机杂化晶体材料(C7H11N2)4[MoOBr4(H20)]2Br2(1).并通过变温红外光谱、单晶X射线衍射、粉末X射线衍射、热重、介电和差热分析等测试方法对晶体进行结构、热能及电性能表征分析.结构显示阴-阳离子在空间...     

Surface Molding of Microscale Hydrogels with Microactuation Functionality

This work describes the fabrication of numerous hydrogel microstructures (μ‐gels) via a process called “surface molding.” Chemically patterned elastomeric‐assembly substrates were used to organize and manipulate the geometry of liquid prepolymer microdroplets, which, following photo‐initiated crosslinking, maintained the desired morphology. By adjusting the state of strain during the crosslinking process, a continua of structures could be created using one pattern. These arrays of μ‐gels have stimuli‐responsive properties that are directly applicable to actuation where the basis shape and array geometry of the μ‐gels can be used to rationally generate microactuators with programmed motions. As a method, “surface molding,” represents a powerful addition to the soft‐lithographic toolset that can be readily applied to the simultaneous synthesis of large numbers of geometrically and functionally distinct polymeric microstructures.     

Reversible Phase Transitions in a Buckybowl Monolayer

Like penguins on ice , buckybowl molecules move closer together when cooled on a copper surface (see model of a corannulene molecule adsorbed on Cu(111)). Upon heating, the molecules spread out into the original crystal phase again. The lower density at room temperature can be explained by the increase in entropy owing to the excitation of bowl vibrations at the surface.

     

Pathway-Dependent Phase Transitions of Supramolecular Self-assemblies Containing Cationic Amphiphiles with Azobenzene and Disulfide Groups

There have been several attempts to construct supramolecular chemical systems that mimic the phase transitions in living systems. However, most of these phase transitions are one-to-one and induced by one stimulus or chemical; there have been few reports on the pathway-dependent phase transition of supramolecular self-assemblies in multi-step. To induce multistep phase transitions, molecular crystals were prepared that contained a cationic amphiphile bearing azobenzene and disulfide groups. A reducing agent caused the crystals to become vesicles, and adjacent, non-touching vesicles fused under UV and subsequent visible light. Adding a reducing agent to the worm-like aggregates that were generated after UV irradiation of the original crystals resulted in the growth of sheet-like aggregates. ¹H NMR and fluorescence anisotropy measurements showed that a series of phase transitions was induced by changes in the phase structures from molecular conversions of the reactive amphiphiles. The multiple pathway-dependent phase transitions of supramolecular self-assemblies can provide a methodology for developing new stimuli-responsive materials that exhibit the desirable properties under specific circumstances from a systems chemistry viewpoint.