Reversible Assembly of Microgels by Metallo-Supramolecular Chemistry

The combination of supramolecular chemistry and soft colloids as microgels represents an ambitious way to develop multi-versatile colloidal assemblies. Hereafter, terpyridine-functionalized poly(N-isopropylacrylamide) (PNiPAM) microgel building blocks are shown to undergo an assemble–freeze–disassemble process. The microgel assemblies, which are controlled by monitoring the attractive and repulsive potentials between the soft colloidal particles, are then frozen by forming inter-particle metal–terpyridine bis-complexes upon addition of the metallic cation (such as Fe^II, Co^II). By oxidation of the metal–terpyridine bis-complex links, the aggregates open up, which is due to the complex dissociation releasing the connected particles in the form of single microgels. We extended our work to the development of 1D filaments and 2D membranes materials made of soft particles connected via supramolecular chemistry.     

Synthesis,characterization, and micellization studies of coil‐rod‐coil and ABA ruthenium(II) terpyridine assemblies with π‐conjugated electron acceptor systems

A series of Ru^II heterodinuclear complexes of ABA ‐type with electron‐deficient bis‐terpyridines as building blocks was synthesized by (R‐tpy)Ru^IIICl₃ complexation. These compounds were characterized by NMR spectroscopy, MALDI‐TOF, ESI‐TOF mass spectrometry, and elemental analysis. The results were compared with a coil‐rod‐coil Ru^II metallo‐supramolecular copolymer, which was synthesized by bis‐complex formation between a hydrophilic ω‐terpyridine poly(ethylene glycol) Ru^II mono‐complex and a hydrophobic bis‐terpyridine‐functionalized rigid core. This amphiphilic Ru^II triblock copolymer showed self‐assembly to clusters and micelles in aqueous solution, which was studied by transmission electron microscopy and dynamic light scattering. Applying velocity sedimentation experiments the number of amphiphilic Ru^II ABA triblock copolymer molecules within the micelles could be estimated. Finally, the photophysical properties of the Ru^II supramolecular assemblies were investigated by UV–vis spectroscopy. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Supramolecular Polymers Self‐Assembled from trans‐Bis(pyridine) Dichloropalladium(II) and Platinum(II) Complexes

Two structurally similar trans‐bis(pyridine) dichloropalladium(II)‐ and platinum(II)‐type complexes were synthesized and characterized. They both self‐assemble in n‐hexane to form viscous fluids at lower concentrations, but form metallogels at sufficient concentrations. The viscous solutions were studied by capillary viscosity measurements and UV/Vis absorption spectra monitored during the disassembly process indicated that a metallophilic interaction was involved in the supramolecular polymerization process. For the two supramolecular assemblies, uncommon continuous porous networks were observed by using SEM and TEM revealed that they were built from nanofibers that fused and crosslinked with the increase of concentration. The xerogels of the palladium and platinum complexes were carefully studied by using synchrotron radiation WAXD and EXAFS. The WAXD data show close stacking distances driven by π–π and metal–metal interactions and an evident dimer structure for the platinum complex was found. The coordination bond lengths were extracted from fitting of the EXAFS data. Moreover, close Pt^II–Pt^II (Pd^II–Pd^II) and Pt?Cl (Pd?Cl) interactions proposed from DFT calculations in the reported oligo(phenylene ethynylene) (OPE)‐based palladium(II) pyridyl supramolecular polymers were also confirmed by using EXAFS. The Pt^II–Pt^II interaction is more feasible for supramolecular interaction than the Pd^II–Pd^II interaction in our simple case.     

Thermosensitive and Switchable Terpyridine‐Functionalized Metallo‐Supramolecular Poly(N‐isopropylacrylamide)

Metallo‐supramolecular polymers offer attractive possibilities to combine the properties of polymers with the characteristics offered by the metal–ligand coordination. Here we present for the first time the combination of metal‐bis(terpyridine) complexes and lower critical solution temperature (LCST) polymers that can be switched by addressing either the thermosensitive polymer or the metal complex. We describe a new strategy for the synthesis of poly(N‐isopropylacrylamide) (PNIPAM) end functionalized with a terpyridine moiety, which is further used for the preparation of Fe^II and Zn^II‐bis(terpyridine PNIPAM). The comparison of the LCST behavior of the uncomplexed ligands and their metal complexes that bear different counter ions is included. Furthermore, the switchability of the synthesized Fe^II system is demonstrated by a decomplexation reaction followed by the characterization of the uncomplexed ligand.

     

Hydrothermal Synthesis of a 3D Polymeric Cobalt(II) Carboxylate Derivative from 1,2,4,5‐Benzenetetracarbonitrile

The metal‐organic complexes Co₂(terpy)₂(btec)·H₂O 1 (terpy = 2,2′:6′,2″‐terpyridine, btec = 1,2,4,5‐benzenetetracarboxylate) was synthesized by hydrothermal synthesis method, using 1,2,4,5‐benzenetetracarbonitrile, terpy and CoAc₂·4H₂O. Single crystal X‐ray diffraction showed that each btec^4– ligand links four Co^II atoms and each Co^II atom links to two btec^4– ligands forming a 1D double‐chain structure. Furthermore, the chains pack together through short face–face π–π interactions forming a 3D supramolecular structure. Additionally, the magnetic measurements show antiferromagnetic interactions among metal ions for compound 1 .     

Dynamic control of volume phase transitions of poly(N‐isopropylacrylamide) based microgels in water using hydrazide‐aldehyde chemistry

We demonstrate that the volume phase transition temperature (VPTT) of copolymer microgel particles made from N‐isopropylacrylamide (NIPAm) and methacryloyl hydrazide (MH) can be tailored in a reversible manner upon the reaction of the hydrazide functional groups with aldehydes. The microgels were synthesized by precipitation polymerization in water. Due to the water‐soluble nature of the MH monomer, the VPTT at which the microgel particles contract shifts to higher values by increasing the incorporated amounts of methacryloyl hydrazide from 0 to 5.0 mol %. The VPTT of the copolymer microgel dispersions in water can be fine‐tuned upon addition of hydrophobic/hydrophilic aldehydes, which react with the hydrazide moiety to produce the hydrazone analogue. This hydrazone formation is reversible, which allows for flexible, dynamic control of the thermo‐responsive behavior of the microgels. The ability to “switch” the VPTT was demonstrated by exposing hydrophilic streptomycin sulfate salt incubated microgel particles to an excess of a hydrophobic aldehyde, that is benzaldehyde. The temperature at which these microgels contracted in size upon heating was markedly lowered in these aldehyde exchange experiments. Transformation into benzaldehyde hydrazone derivatives led to assembly of the microgel particles into small colloidal clusters at elevated temperatures. This control of supracolloidal cluster formation was also demonstrated with polystyrene particles which had a hydrazide functionalised microgel shell. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1745–1754     

Syntheses,Crystal Structures,and Properties of Metal(II) Coordination Polymers based on Flexible Ligand 1,3‐Bis(4‐phenoxy)benzenedicarboxylic Acid and Bis(benzimidazole)

Three new coordination polymers, [Zn(PBDC)(bbbm)_0.5]_n ( 1 ), [Co(PBDC)(bbbm)]_n ( 2 ), and [Cd(PBDC)(bbbm)]_n ( 3 ) were prepared via hydrothermal reactions of different metal(II) nitrates with flexible 1,3‐bis(4‐phenoxy)benzenedicarboxylic acid (H₂L) and 1,1‐(1,4‐butanediyl)bis(benzimidazole) ligand. All these complexes were fully characterized by elemental analysis, FT‐IR, thermogravimetric analysis (TGA), powder X‐ray diffraction, and single‐crystal X‐ray diffraction. Structure analyses revealed that complex 1 has a 2D→2D twofold interpenetrating framework simplified by a 4‐connected sql net with point symbol (4⁴.6²), whereas complexes 2 and 3 are isostructural and exhibit a 2D→2D twofold interpenetrating framework rationalized as a three‐connected hcb net with point symbol (6³). Complexes 1 – 3 further expand to 3D supramolecular structures through non‐covalent C–H ··· O interactions. Additionally, the luminescent and magnetic properties of some of these complexes were studied. Complex 3 presents ideal photoluminescent behavior, whereas complex 2 shows antiferromagnetic coupling between the central Co^II ions, suggesting its latent application in magnetic material.     

Synthesis,Structures, and Luminescent Properties of Three Coordination Polymers Constructed from Tricarboxylate and Bisbenzoimidazole Ligand

The hydrothermal reaction of the tricarboxylate ligand 5‐(carboxymethoxy)isophthalic acid (H₃L) with Zn^II, Cd^II, and Sn^II salts in the presence of the bisbenzoimidazole coligand 1, 3‐bis(benzimidazol‐2‐yl)‐2‐oxapropane (bbop) afforded the coordination polymers, [Zn(HL)(bbop)]_n ( 1 ), [Cd(HL)(bbop)]_n ( 2 ), and {[H₂(bbop)][Sn₂L₂]}_n ( 3 ). The complexes were characterized by elemental analyses, IR spectroscopy, single‐crystal X‐ray diffraction analyses, thermogravimetric analyses, and fluorescence properties. The structures of complexes 1 and 2 are constructed by 1D chains and show strong blue luminescence emission. The structure of complex 3 is a 2D anionic dilayer, and shows a vase‐like porous structure occupied by the bulky [H₂(bbop)]²⁺ cation, which is an uncommon structural feature in transition metal coordination polymers. The three complexes are further connected by hydrogen bonds to form 3D supramolecular architectures.     

Crystallization behavior of soft, attractive microgels

The equilibrium phase behavior and the dynamics of colloidal assemblies composed of soft, spherical, colloidal particles with attractive pair potentials have been studied by digital video microscopy. The particles were synthesized by precipitation copolymerization of N-isopropylacrylamide (NIPAm), acrylic acid (AAc), and N,N'-methylene bis(acrylamide) (BIS), yielding highly water swollen hydrogel microparticles (microgels) with temperature- and pH-tunable swelling properties. It is observed that in a pH = 3.0 buffer with an ionic strength of 10 mM, assemblies of pNIPAm-AAc microgels crystallize due to a delicate balance between weak attractive and soft repulsive forces. The attractive interactions are further confirmed by measurements of the crystal melting temperatures. As the temperature of colloidal crystals is increased, the crystalline phase does not melt until the temperature is far above the lower critical solution temperature (LCST) of the microgels, in stark contrast to what is typically observed for phases formed due to purely repulsive interactions. The unusual thermal stability of pNIPAm-AAc colloidal crystals demonstrates an enthalpic origin of crystallization for these microgels.     

Facile Self‐Assembly of Metallo‐Supramolecular Ring‐in‐Ring and Spiderweb Structures Using Multivalent Terpyridine Ligands

A series of metallo‐supramolecular ring‐in‐ring structures was generated by assembling Cd^II ions and the multivalent terpyridine ligands ( L^1‐3 ) composed of one 60°‐bent and two 120°‐bent bis(terpyridine)s with varying alkyl linker lengths. The mechanistic study for the self‐assembly process excluded an entropically templated pathway and showed that the intramolecularly complexed species is the key intermediate leading to ring‐in‐ring formation. The next‐generation superstructure, a spiderweb, was produced in quantitative yield using the elongated decakis(terpyridine) ligand ( L⁵ ).     

Lead‐sensitive PNIPAM microgels modified with crown ether groups

A series of lead‐sensitive poly(N‐isopropylacrylamide) microgels with pendant crown ether groups were prepared. Their cation‐sensitive behaviors were studied by dynamic light scattering. When ionic strength is not controlled, adding salts causes the microgel particles to deswell. However, when the salt effect is ruled out by keeping a constant ionic strength, adding Pb²⁺ results in much larger swelling. The Pb²⁺‐induced swelling was explained by the formation of host–guest complex between Pb²⁺ and the pendant crown ether groups, which increases the hydrophilicity of the polymer and accordingly the degree of swelling. The lead sensitivity of the microgels increases with increasing crown ether content. For the modified microgel with the highest crown ether content, it swells to ～430% of its original volume at [Pb²⁺] = 10 mM. Other cations also increase the swelling degree of the modified microgels. The extent of the cation‐induced swelling mainly depends on their affinity to the pendant crown ether groups. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4120–4127, 2010     

Superior Electron‐Transport Ability of π‐Conjugated Redox Molecular Wires Prepared by the Stepwise Coordination Method on a Surface

Electronic conductivity of molecular wires is a critical fundamental issue in molecular electronics. π‐Conjugated redox molecular wires with the superior long‐range electron‐transport ability could be constructed on a gold surface through the stepwise ligand–metal coordination method. The β^d value, indicating the degree of decrease in the electron‐transfer rate constant with distance along the molecular wire between the electrode and the redox active species at the terminal of the wire, were 0.008–0.07 Å^?1 and 0.002–0.004 Å^?1 for molecular wires of bis(terpyridine)iron and bis(terpyridine)cobalt complex oligomers, respectively. The influences on β^d by the chemical structure of molecular wires and the terminal redox units, temperature, electric field, and electrolyte concentration were clarified. The results indicate that facile sequential electron hopping between neighboring metal–complex units within the wire is responsible for the high electron‐transport ability.     

Synergistic Effects of Metals in a Promising RuII−PtII Assembly for a Combined Anticancer Approach: Theoretical Exploration of the Photophysical Properties

Ru^II?Pt^II complexes are a class of bioactive molecules of interest as anticancer agents that combine a light‐absorbing chromophore with a cisplatin‐like unit. The results of a DFT and TDDFT investigation of a Ru^II complex and its conjugate with a cis‐PtCl₂ moiety reveal that a synergistic effect of the metals makes the assembly a promising multitarget anticancer drug. Inspection of type I and type II photoreactions and spin–orbit coupling computations reveals that the cis‐PtCl₂ moiety improves the photophysical properties of the Ru^II chromophore, ensuring efficient singlet oxygen generation and making the assembly suitable for photodynamic therapy. At the same time, the Ru^II chromophore promotes a new alternative activation mechanism of the Pt^II ligand via a triplet metal‐to‐ligand charge transfer (³M LCT) state, before reaching the biological target. The importance of the supramolecular architecture is accurately derived, opening interesting new perspectives on the use of bimetallic Ru^II?Pt^II assemblies in a combined anticancer approach.     

微凝胶胶体晶体研究进展

Poly(N-isopropylacrylamide)(PNIPAM)微凝胶粒子是一种软的胶体粒子.和单分散的SiO_2、PS、PMMA等硬的胶体粒子一样,单分散的PNIPAM微凝胶粒子也可以自组装成为高度有序的胶体晶体.微凝胶粒子软物质的特性及其对外部刺激的响应性赋予其不同于硬球的组装行为.微凝胶胶体晶体的高度有序结构及其刺激响应性使其在诸多领域有重要用途.本文分别介绍了三维及二维微凝胶胶体晶体组装的研究进展,并对已开发的基于微凝胶胶体晶体的应用进行了总结.     

Design of Lead(II) Metal–Organic Frameworks Based on Covalent and Tetrel Bonding

Three solid materials, [Pb( HL )(SCN)₂] ? CH₃OH ( 1 ), [Pb( HL )(SCN)₂] ( 2 ), and [Pb( L )(SCN)]_n ( 3 ), were obtained from Pb(SCN)₂ and an unsymmetrical bis‐pyridyl hydrazone ligand that can act both as a bridging and as a chelating ligand. In all three the lead center is hemidirectionally coordinated and is thus sterically optimal for participation in tetrel bonding. In the crystal structures of all three compounds, the lead atoms participate in short contacts with thiocyanate sulfur or nitrogen atoms. These contacts are shorter than the sums of the van der Waals radii (3.04–3.47 Å for Pb ??? S and 3.54 Å for Pb ??? N) and interconnect the covalently bonded units (monomers, dimers, and 2D polymers) into supramolecular assemblies (chains and 3D structures). DFT calculations showed these contacts to be tetrel bonds of considerable energy (6.5–10.5 kcal mol^?1 for Pb ??? S and 16.5 kcal mol^?1 for Pb ??? N). A survey of structures in the CSD showed that similar contacts often appear in crystals of Pb^II complexes with regular geometries, which leads to the conclusion that tetrel bonding plays a significant role in the supramolecular chemistry of Pb^II.     

Coordination‐Driven Macrocyclization for Locking of Photo‐ and Thermal cis→trans Isomerization of Azobenzene

Both trans and cis isomers of azobenzene‐linked bis‐terpyridine ligand L1 were incorporated in rigid macrocycles linked by Fe^II(tpy)₂ (tpy: terpyridine) units. The complex of the longer trans‐ L1 is dinuclear [(trans‐ L1 )₂ ? Fe^II₂], whereas the complex of the shorter cis‐ L1 is mononuclear [cis‐ L1? Fe^II]. The complex cis‐ L1? Fe^II was not only thermally stable but also photochemically inactive. These results indicate a perfectly locked state of cis‐azobenzene. The stable macrocyclic structure of cis‐ L1? Fe^II causes locking of the isomerization. To the best of our knowledge, this is first example of dual locking of photo‐ and thermal isomerization of cis‐azobenzene.     

Swelling Kinetics of Microgels Embedded in a Polyacrylamide Hydrogel Matrix

Composite hydrogels—macroscopic hydrogels with embedded microgel particles—are expected to respond to external stimuli quickly because microgels swell much faster than bulky gels. In this work, the kinetics of the pH‐induced swelling of a composite hydrogel are studied using turbidity measurements. The embedded microgel is a pH‐ and thermosensitive poly(N‐isopropylacrylamide‐co‐acrylic acid) microgel and the hydrogel matrix is polyacrylamide. A rapid pH‐induced swelling of the embedded microgel particles is observed, confirming that composite hydrogels respond faster than ordinary hydrogels. However, compared with the free microgels, the swelling of the embedded microgel is much slower. Diffusion of OH^? into the composite hydrogel film is identified as the main reason for the slow swelling of the embedded microgel particles, as the time of the pH‐induced swelling of this film is comparable to that of OH^? diffusion into the film. The composition of the hydrogel matrix does not significantly change the characteristic swelling time of the composite hydrogel film. However, the swelling pattern of the film changes with composition of the hydrogel matrix.     

Energy Transfer in a Hybrid IrIII Carbene–PtII Acetylide Assembly for Efficient Hydrogen Production

A new heterometallic supramolecular complex, consisting of an iridium carbene‐based unit appended to a platinum terpyridine acetylide unit, representing a new Ir^III–Pt^II structural motif, was designed and developed to act as an active species for photocatalytic hydrogen production. The results also suggested that a light‐harvesting process is essential to realize the solar‐to‐fuel conversion in an artificial system as illustrated in the natural photosynthetic system.     

A Cytostatic Ruthenium(II)–Platinum(II) Bis(terpyridyl) Anticancer Complex That Blocks Entry into S Phase by Up‐regulating p27KIP1

Cytostatic agents that interfere with specific cellular components to prevent cancer cell growth offer an attractive alternative, or complement, to traditional cytotoxic chemotherapy. Here, we describe the synthesis and characterization of a new binuclear Ru^II–Pt^II complex [Ru(tpy)(tpypma)Pt(Cl)(DMSO)]³⁺ (tpy=2,2′:6′,2′′‐terpyridine and tpypma=4‐([2,2′:6′,2′′‐terpyridine]‐4′‐yl)‐N‐(pyridin‐2‐ylmethyl)aniline), VR54, which employs the extended terpyridine tpypma ligand to link the two metal centres. In cell‐free conditions, VR54 binds DNA by non‐intercalative reversible mechanisms (K_b=1.3×10⁵ M ^?1) and does not irreversibly bind guanosine. Cellular studies reveal that VR54 suppresses proliferation of A2780 ovarian cancer cells with no cross‐resistance in the A2780CIS cisplatin‐resistant cell line. Through the preparation of mononuclear Ru^II and Pt^II structural derivatives it was determined that both metal centres are required for this anti‐proliferative activity. In stark contrast to cisplatin, VR54 neither activates the DNA‐damage response network nor induces significant levels of cell death. Instead, VR54 is cytostatic and inhibits cell proliferation by up‐regulating the cyclin‐dependent kinase inhibitor p27^KIP1 and inhibiting retinoblastoma protein phosphorylation, which blocks entry into S phase and results in G1 cell cycle arrest. Thus, VR54 inhibits cancer cell growth by a gain of function at the G1 restriction point. This is the first metal‐coordination compound to demonstrate such activity.     

Monodentate and bridging behaviour of the sulfur‐containing ligand 4′‐[4‐(methylsulfanyl)phenyl]‐4,2′:6′,4′′‐terpyridine in two discrete zinc(II) complexes with acetylacetonate

The Zn complexes bis(acetylacetonato‐κ²O,O′)bis{4′‐[4‐(methylsulfanyl)phenyl]‐4,2′:6′,4′′‐terpyridine‐κN¹}zinc(II), [Zn(C₅H₇O₂)₂(C₂₂H₁₇N₃S)₂], (I), and {μ‐4′‐[4‐(methylsulfanyl)phenyl]‐4,2′:6′,4′′‐terpyridine‐κ²N¹:N^1′′}bis[bis(acetylacetonato‐κ²O,O′)zinc(II)], [Zn₂(C₅H₇O₂)₄(C₂₂H₁₇N₃S)], (II), are discrete entities with different nuclearities. Compound (I) consists of two centrosymmetrically related monodentate 4′‐[4‐(methylsulfanyl)phenyl]‐4,2′:6′,4′′‐terpyridine (L1) ligands binding to one Zn^II atom sitting on an inversion centre and two centrosymmetrically related chelating acetylacetonate (acac) groups which bind via carbonyl O‐atom donors, giving an N₂O₄ octahedral environment for Zn^II. Compound (II), however, consists of a bis‐monodentate L1 ligand bridging two Zn^II atoms from two different Zn(acac)₂ fragments. Intra‐ and intermolecular interactions are weak, mainly of the C—H...π and π–π types, mediating similar layered structures. In contrast to related structures in the literature, sulfur‐mediated nonbonding interactions in (II) do not seem to have any significant influence on the supramolecular structure.