Three-Component Self-Assembly of a Series of Triply Interlocked Pd12 Coordination Prisms and Their Non-Interlocked Pd6 Analogues

Template-assisted formation of multicomponent Pd6 coordination prisms and formation of their self-templated triply interlocked Pd12 analogues in the absence of an external template have been established in a single step through Pd?N/Pd?O coordination. Treatment of cis-[Pd(en)(NO3)2] with K3tma and linear pillar 4,4'-bpy (en=ethylenediamine, H3tma=benzene-1,3,5-tricarboxylic acid, 4,4'-bpy=4,4'-bipyridine) gave intercalated coordination cage [{Pd(en)}6(bpy)3(tma)2]2[NO3]12 (1) exclusively, whereas the same reaction in the presence of H3tma as an aromatic guest gave a H3tma-encapsulating non-interlocked discrete Pd6 molecular prism [{Pd(en)}6(bpy)3(tma)2(H3tma)2][NO3]6 (2). Though the same reaction using cis-[Pd(NO3)2(pn)] (pn=propane-1,2-diamine) instead of cis-[Pd(en)(NO3)2] gave triply interlocked coordination cage [{Pd(pn)}6(bpy)3(tma)2]2[NO3]12 (3) along with non-interlocked Pd6 analogue [{Pd(pn)}6(bpy)3(tma)2](NO3)6 (3'), and the presence of H3tma as a guest gave H3tma-encapsulating molecular prism [{Pd(pn)}6(bpy)3(tma)2(H3tma)2][NO3]6 (4) exclusively. In solution, the amount of 3' decreases as the temperature is decreased, and in the solid state 3 is the sole product. Notably, an analogous reaction using the relatively short pillar pz (pz=pyrazine) instead of 4,4'-bpy gave triply interlocked coordination cage [{Pd(pn)}6(pz)3(tma)2]2[NO3]12 (5) as the single product. Interestingly, the same reaction using slightly more bulky cis-[Pd(NO3)2(tmen)] (tmen=N,N,N',N'-tetramethylethylene diamine) instead of cis-[Pd(NO3)2(pn)] gave non-interlocked [{Pd(tmen)}6(pz)3(tma)2][NO3]6 (6) exclusively. Complexes 1, 3, and 5 represent the first examples of template-free triply interlocked molecular prisms obtained through multicomponent self-assembly. Formation of the complexes was supported by IR and multinuclear NMR (1H and 13C) spectroscopy. Formation of guest-encapsulating complexes (2 and 4) was confirmed by 2D DOSY and ROESY NMR spectroscopic analyses, whereas for complexes 1, 3, 5, and 6 single-crystal X-ray diffraction techniques unambiguously confirmed their formation. The gross geometries of H3tma-encapsulating complexes 2 and 4 were obtained by universal force field (UFF) simulations.

Tuning DNA-amphiphile condensate architecture with strongly binding counterions

Electrostatic self-assembly of colloidal and nanoparticles has attracted a lot of attention in recent years, since it offers the possibility of producing novel crystalline structures that have the potential to be used as advanced materials for photonic and other applications. The stoichiometry of these crystals is not constrained by charge neutrality of the two types of particles due to the presence of counterions, and hence a variety of three-dimensional structures have been observed depending on the relative sizes of the particles and their charge. Here we report structural polymorphism of two-dimensional crystals of oppositely charged linear macroions, namely DNA and self-assembled cylindrical micelles of cationic amphiphiles. Our system differs from those studied earlier in terms of the presence of a strongly binding counterion that competes with DNA to bind to the micelle. The presence of these counterions leads to novel structures of these crystals, such as a square lattice and a root 3 x root 3 superlattice of an underlying hexagonal lattice, determined from a detailed analysis of the small-angle diffraction data. These lower-dimensional equilibrium systems can play an important role in developing a deeper theoretical understanding of the stability of crystals of oppositely charged particles. Further, it should be possible to use the same design principles to fabricate structures on a longer length-scale by an appropriate choice of the two macroions.

Coordination-Driven Self-Assembly of 2D-Metallamacrocycles Using a New Carbazole-Based Dipyridyl Donor: Synthesis, Characterization, and C-60 Binding Study

A new carbazole-based 90 degrees dipyridyl donor 3,6-di(4-pyridylethynyl)carbazole (L) containing carbazole-ethynyl functionality is synthesized in reasonable yield using the Sonagashira coupling reaction. Multinuclear NMR, electrospray ionization-mass spectrometry (ESI-MS), including single crystal X-ray diffraction analysis characterized this 90 degrees building unit. The stoichiometry combination of L with several Pd(II)/Pt(II)-based 90 degrees acceptors (1a-1d) yielded 2 + 2] self-assembled metallacycles (2a-2d) under mild conditions in quantitative yields 1a = cis-(dppf)Pd(OTf)(2); 1b = cis-(dppf)Pt(OTf)(2); 1c = cis-(tmen)Pd(NO3)(2); 1d = 3,6-bis{trans-Pt(C C) (PEt3)(2)(NO3))carbazole]. All these macrocycles were characterized by various spectroscopic techniques, and the molecular structure of 2a was unambiguously determined by single crystal X-ray diffraction analysis. Incorporation of ethynyl functionality to the carbazole backbone causes the resulted macrocycles (2a-2d) to be pi-electron rich and thereby exhibit strong emission characteristics. The macrocycle 2a has a large internal concave aromatic surface. The fluorescence quenching study suggests that 2a forms a similar to 1:1 complex with C-60 with a high association constant of K-sv = 1.0 X 10(5) M-1.

Self-Assembly of an Octanuclear Platinum(II) Tetragonal Prism from a New Pt-4(II) Organometallic Star-Shaped Acceptor and Its Nitroaromatic Sensing Study

2 + 4] self-assembly of a pyrene-functionalized Pt-8(II) tetragonal prism (2) is achieved using a newly designed star-shaped organometallic acceptor (1) in combination with an amide-based ``clip'' donor (L). The propensity of this prism (2) as a selective sensor for nitroaromatics (2,4-dinitrotoluene, 1,3,5-trinitrotoluene, and picric acid), which are the chemical constituents of many commercial explosives, has been examined.

From Molecular to Supramolecular: An Exploration into the Modes of Self-assembly in Conformationally Locked Polycyclitols

This brief account highlights the notable findings of our investigation into the supramolecular chemistry of conformationally locked polycyclitols in the solid state. The study was aimed at analyzing the crystal packing and unraveling the modalities of non-covalent interactions (particularly, intramolecular vis-a-vis intermolecular OH center dot center dot center dot O hydrogen bonds) in polyols. The know-how obtained thereof, was successfully utilized to engineer self-assemblies of designer polycyclitols, having hydrogen bond donors and acceptors fettered onto a trans-decalin scaffold. The results seek to draw particular attention to the intrinsic attribute of this rigid carbocyclic framework to lock functional groups into spatially invariant positions and bring potential intramolecular hydrogen bonding partners into favorable interaction geometry to engender predictability in the self-assembly patterns.

Coordination self-assembly of tetranuclear Pt(II) macrocycles with an organometallic backbone for sensing of acyclic dicarboxylic acids

Coordination self-assembly of a series of tetranuclear Pt(II) macrocycles containing an organometallic backbone incorporating ethynyl functionality is presented. The 1 : 1 combination of a linear acceptor 1,4-bistrans-Pt(PEt3)(2)(NO3)(ethynyl)]benzene (1) with three different dipyridyl donor `clips' (L-a-L-c) afforded three 2 + 2] self-assembled Pt-4(II) macrocycles (2a-2c) in quantitative yields, respectively L-a = 1,3-bis-(3-pyridyl)isothalamide; L-b = 1,3-bis(3-pyridyl)ethynylbenzene; L-c = 1,8-bis(4-pyridyl)ethynylanthracene]. These macrocycles were characterized by multinuclear NMR (H-1 and P-31); ESI-MS spectroscopy and the molecular structures of 2a and 2b were established by single crystal X-ray diffraction analysis. These macrocycles (2a-2c) are fluorescent in nature. The amide functionalized macrocycle 2a is used as a receptor to check the binding affinity of aliphatic acyclic dicarboxylic acids. Such binding affinity is examined using fluorescence and UV-Vis spectroscopic methods. A solution state fluorescence study showed that macrocycle 2a selectively binds (K-SV = 1.4 x 10(4) M-1) maleic acid by subsequent enhancement in emission intensity. Other aliphatic dicarboxylic acids such as fumaric, succinic, adipic, mesaconic and itaconic acids caused no change in the emission spectra; thereby demonstrating its potential use as a macrocyclic receptor in distinction of maleic acid from other aliphatic dicarboxylic acids.

In-situ impedance spectroscopy of layer-by-layer self-assembly of polyelectrolytes

In-situ impedance spectroscopy of layer-by-layer self-assembly of weak polyelectrolytes is presented. Interdigitated capacitors with active area of 1×1 mm2 and electrode spacing of 5 μm are fabricated and used for this purpose. Measurement results indicate that the impedance decreases with increase in number of polyelectrolyte layers. About 2.5% of relative change in magnitude of impedance at 104.7 KHz is seen for four bi-layers of Poly(Allylamine Hydrochloride) (PAH)/Poly(Acrylic acid) (PAA). An electrical equivalent for polyelectrolyte binding is obtained.

Efficacious Anticancer Drug Delivery Mediated by a pH-Sensitive Self-Assembly of a Conserved Tripeptide Derived from Tyrosine Kinase NGF Receptor

We present herein a short tripeptide sequence (Lys-Phe-Gly or KFG) that is situated in the juxtamembrane region of the tyrosine kinase nerve growth factor (Trk NGF) receptors. KFG self-assembles in water and shows a reversible and concentration-dependent switching of nanostructures from nanospheres (vesicles) to nanotubes, as evidenced by dynamic light scattering, transmission electron microscopy, and atomic force microscopy. The morphology change was associated with a transition in the secondary structure. The tripeptide vesicles have inner aqueous compartments and are stable at pH7.4 but rupture rapidly at pH approximate to 6. The pH-sensitive response of the vesicles was exploited for the delivery of a chemotherapeutic anticancer drug, doxorubicin, which resulted in enhanced cytotoxicity for both drug-sensitive and drug-resistant cells. Efficient intracellular release of the drug was confirmed by fluorescence-activated cell sorting analysis, fluorescence microscopy, and confocal microscopy.

Self-assembly of discrete metallamacrocycles employing half-sandwich octahedral diruthenium(II) building units and imidazole-based ligands

Equimolar combination of a series of binuclear half-sandwich p-cymene ruthenium(II) building units Ru-2(mu-eta(4)-C2O4)(MeOH)(2)(eta(6)-p-cymene)(2)](OTf)(2) 1a](OTf)(2), Ru-2(mu-eta(4)-N,N'-diphenyloxamidato)( MeOH)(2)(eta(6)-p-cymene)(2)](OTf)(2) 1b](OTf)(2) and Ru-2(mu-eta(4)-C6H2O4)(MeOH)(2)(eta(6)-p-cymene)(2)](OTf)(2) 1c](OTf)(2) separately with imidazole-based ditopic ligands (L-1-L-2) in methanol yielded a series of tetranuclear metallamacrocycles 2-7](OTf)(4), respectively L-1 = 1,4-bis(imidazole-1-yl)benzene; L-2 = 4,4'-bis(imidazole-1-yl)biphenyl; OTf- = O3SCF3-]. Similarly, the reaction of Ru-2(mu-eta(4)-C2O4)(MeOH)(2)(eta(6)-p-cymene)2](OTf)(2) 1a](OTf)(2) with a triazine-based tritopic ligand 1,3,5-tris(imidazole-1-yl) triazine (L3) in 3: 2 M ratio afforded an unexpected tetranuclear macrocycle 8](OTf)(4) instead of an expected trigonal prismatic cage 8a](OTf)(6). All the self-assembled macrocycles 2-8](OTf)(4) were isolated in moderate to high yields and were fully characterized by multinuclear H-1, F-19] NMR, IR and electrospray ionization mass spectrometry (ESI-MS). In addition, X-ray diffraction study on the single crystals of 3](OTf)(4) and 8](OTf)(4) also indicated the formation 2 + 2] self-assembled macrocycles. Despite the possibility of formation of different conformational isomeric macrocycles (syn-and anti) and polymeric product due to free rotation of ligand sites of imidazole linkers, the selective formation of single conformational isomer (anti) as the only product is quite interesting. Furthermore, the photo-and electrochemical properties of these assemblies have been studied using UV/Vis absorption and cyclic voltammetry analysis. (c) 2013 Elsevier B.V. All rights reserved.

Template-free multicomponent coordination-driven self-assembly of Pd(II)/Pt(II) molecular cages

Recent years have seen a tremendous increase in the interest for constructing hollowed-out molecular frameworks, for their potential uses. Metal-ligand coordination-driven self-assembly has provided multitudes of opportunities in the formation of molecular architectures of desired shapes and sizes, with the help of the information already coded in the components. This article summarizes the recent developments in the construction of multicomponent molecular cages through this process, with a focus on the decreasing relevance of templates, and use of these systems in catalysis/host-guest chemistry.

Beyond the ``Coffee Ring'': Re-entrant Ordering in an Evaporation-Driven Self-Assembly in a Colloidal Suspension on a Substrate

We study the phenomenon of evaporation-driven self-assembly of a colloid suspension of silica microspheres in the interior region and away from the rim of the droplet on a glass plate. In view of the importance of achieving a large-area, monolayer assembly, we first realize a suitable choice of experimental conditions, minimizing the influence of many other competing phenomena that usually complicate the understanding of fundamental concepts of such self-assembly processes in the interior region of a drying droplet. Under these simplifying conditions to bring out essential aspects, our experiments unveil an interesting competition between ordering and compaction in such drying systems in analogy to an impending glass transition. We establish a re-entrant behavior in the order disorder phase diagram as a function of the particle density, such that there is an optimal range of the particle density to realize the long-range ordering. The results are explained with the help of simulations and phenomenological theory.

Self-assembly of Chloro-Bridged Arene-Ruthenium Based Rectangle: Synthesis, Structural Characterization and Sensing Study

Self-assembly of a chloro-bridged half-sandwich p-cymene ruthenium(II) complex Ru-2(mu-Cl-2)(eta(6)-p-cymene)(2)Cl-2] 1 with linear ditopic donor L; trans-1,2-bis(4-pyridyl) ethylene] in presence of 2 eq. AgNO3 in CH3CN yielded a chloro-bridged molecular rectangle 2. The rectangle 2 was isolated as nitrate salt in high yield (90 %) and characterized by infra-red, H-1 NMR spectroscopy including ESI-MS analyses. Molecular structure of 2 was determined by single crystal X-ray diffraction study The diffraction analysis shows that 2 adopts a tetranuclear rectangular geometry with the dimensions of 5.51 angstrom x 13.29 angstrom and forming an infinite supramolecular chain with large internal porosity arising through multiple pi-pi and CH-pi interactions between the adjacent rectangles. Furthermore, rectangle 2 is used as selective receptor for phenolic-nitroaromatic compounds such as picric acid, dinitrophenol and nitrophenol.

Component Selection in the Self-Assembly of Palladium(II) Nanocages and Cage-to-Cage Transformations

Dynamic supramolecular systems involving a tetratopic palladium(II) acceptor and three different pyridine-and imidazole-based donors have been used for self-selection by a synergistic effect of morphological information and coordination ability of ligands through specific coordination interactions. Three different cages were first synthesized by two-component self-assembly of individual donor and acceptor. When all four components were allowed to interact in a reaction mixture, only one out of three cages was isolated. The preferential binding affinity towards a particular partner was also established by transforming a non-preferred cage into a preferred cage by interaction with the appropriate ligand. Computational studies further supported the fact that coordination interaction of imidazole moiety to Pd-II is enthalpically more preferred compared to pyridine, which drives the selection process. Analysis of crystal packing of both complexes indicated the presence of strong hydrogen bonds between nitrate and water molecules and also H-bonded 3D networks of water. Both complexes exhibit promising proton conductivity (10(-5) to ca. 10(-3) Scm(-1)) at ambient temperature under a relative humidity of circa 98% with low activation energy.

Hydrophobic hydration driven self-assembly of curcumin in water: Similarities to nucleation and growth under large metastability, and an analysis of water dynamics at heterogeneous surfaces

As the beneficial effects of curcumin have often been reported to be limited to its small concentrations, we have undertaken a study to find the aggregation properties of curcumin in water by varying the number of monomers. Our molecular dynamics simulation results show that the equilibrated structure is always an aggregated state with remarkable structural rearrangements as we vary the number of curcumin monomers from 4 to 16 monomers. We find that the curcumin monomers form clusters in a very definite pattern where they tend to aggregate both in parallel and anti-parallel orientation of the phenyl rings, often seen in the formation of beta-sheet in proteins. A considerable enhancement in the population of parallel alignments is observed with increasing the system size from 12 to 16 curcumin monomers. Due to the prevalence of such parallel alignment for large system size, a more closely packed cluster is formed with maximum number of hydrophobic contacts. We also follow the pathway of cluster growth, in particular the transition from the initial segregated to the final aggregated state. We find the existence of a metastable structural intermediate involving a number of intermediate-sized clusters dispersed in the solution. We have constructed a free energy landscape of aggregation where the metatsable state has been identified. The course of aggregation bears similarity to nucleation and growth in highly metastable state. The final aggregated form remains stable with the total exclusion of water from its sequestered hydrophobic core. We also investigate water structure near the cluster surface along with their orientation. We find that water molecules form a distorted tetrahedral geometry in the 1st solvation layer of the cluster, interacting rather strongly with the hydrophilic groups at the surface of the curcumin. The dynamics of such quasi-bound water molecules near the surface of curcumin cluster is considerably slower than the bulk signifying a restricted motion as often found in protein hydration layer. (C) 2014 AIP Publishing LLC.