Reaction Kinetics and the Development of Catalytic Processes. Colloids and the Depletion Interaction. Henk N. Quantum Dot Solar Cells. Jiang Wu. Subsecond Annealing of Advanced Materials. Wolfgang Skorupa. Denise Gabuzda. Lasers in Materials Science. Marta Castillejo. Encyclopedia of Nuclear Physics and its Applications. Reinhard Stock.
Amorphous Nanophotonics. Carsten Rockstuhl. In-situ Materials Characterization. Heinz Graafsma. Nanostructured Materials for Magnetoelectronics. Bekir Aktas. Springer Handbook of Crystal Growth. Govindhan Dhanaraj. Excitonic and Photonic Processes in Materials. Jai Singh. Studying Kinetics with Neutrons. Imaging and Manipulating Molecular Orbitals. Leonhard Grill. Emilio Scalise. Edoardo Baldini. Ellipsometry of Functional Organic Surfaces and Films. Karsten Hinrichs. Non-equilibrium Phenomena in Confined Soft Matter. Simone Napolitano.
Joseph Woicik. Alex Frano. Low-Dimensional Functional Materials. Reinhold Egger. Physical Properties of Nanorods. Roman Krahne. Progress in Nanophotonics 2. Motoichi Ohtsu. Clusters in Nuclei, Volume 3. Christian Beck. Complex Plasmas. Michael Bonitz. Inorganic Scintillators for Detector Systems. Paul Lecoq. Jeroen A. Zhiming M. Soft Matter at Aqueous Interfaces. Peter Lang.
Novel Functional Magnetic Materials. Arcady Zhukov. Youichi Murakami. Adsorption and Diffusion in Nanoporous Materials. Rolando M. Shinichiro Seki. New Trends in Atomic and Molecular Physics. Man Mohan. Local Structural Characterisation. Duncan W. Nanoparticulate Materials. Kathy Lu. Topological Structures in Ferroic Materials. Jan Seidel. Photoelectron Spectroscopy. Shigemasa Suga. Studies have demonstrated that helicates 12 - 15 are able to bind to double-helical DNA Single-strand cleavage was provoked by visible-light irradiation in solutions of pBR plasmid containing a helicate This is a very interesting and promising application because specially designed helicates may perform thermal or light-induced strand cleavage multiple binding to DNA.
Not only 2,2'-bipyridine based ligands are capable of attaining helical structures in the presence of metal ions, but many similar polytopic heterocyclic ligands can form double helical complexes. Complexation studies were also performed with ligands 19 - 21 Chart 2 , which contain a sequence of bipyridine and terpyridine units The unsymmetrical ligand 20 is able to form two homoduplex heterotrinuclear complexes as a result of the parallel or antiparallel alignment of the ligands. These results are of interest because the pairing determined by the metal ions between the bipyridine and terpyridine units offers the possibility of coding by means of the design of specific coding sequences, metal-ion mediated translation and finally replication by generation of the complementary strand from the constituent subunits, analogous to nucleic acid replication.
Other different ligands have been employed in order to explore different levels of molecular programming leading to the formation of double helical structures Chart 3 32, In addition, alkylthio quinquepyridines were used by Potts et al. In the same manner, double-helical ruthenium complexes were synthesized This versatility to different metal readings occurs because this ligand can be seen as having two linked groups, one bidentate bipyridine and one tridentate terpyridine The same idea was explored with other oligopyridines, such as the sexipyridine 24 59 and the septipyridine 25 Recently, the syntheses of double-stranded helical complexes derived from hexa n -propylthio novipyridine were reported In the complexation of these ligands, the selection of a fraction of all donor groups in a determined ligand by a metal ion is performed according to its preferred geometry 30e.
Another important aspect in the study of the helicates is the nature of the bridge between the bipyridine units. The junction elements may show, not only enough flexibility to allow the ligand the description of a helical shape around the metals, but also the rigidity to communicate the helicity from the metal to its neighbor s and to avoid the binding of the metal bonded to the first site with the other binding site in the same ligand. Many other studies have demonstrated the crucial function of the bridge between bipyridine units 65 and that between catechol ligands able to form triple helicates Some researchers have revealed that 1,3-phenylene can be used with success as a rigid bridge for constructing dinuclear double-stranded helicates 69, This result is due to the fact that the rigid spacer favored formation of the double helical precursor in relation to the heterochiral complex.
Formats and Editions of Inorganic nanoarchitectures by organic self-assembly [pewahomaci.tk]
The work presented until now shows that three essential aspects are crucial in the formation of a metallohelicate: the nature of the binding site in the ligand, the junction between the binding sites and the configuration of the coordination metal. The combination of these three features may be employed in the construction of more complex inorganic architectures. In the same manner, the design of ligands possessing combined binding components, i. One example was recently given by ligand 27 , which has oligo-bipyridine units connected by means of a bispyridyl-pyridazine unit.
Its central cavity contains four PF 6 - anions, as well as solvent molecules. Since the complex is highly positively charged, it should act as a receptor possessing strong association constants with anionic species. Structural modifications in ligand 27 , as well as the combination of different units 75 and the design of other programming systems 76 , may throw light on the multisubroutine self-assembled processes and yield a variety of architectures with increasing complexity. Although inorganic triple helical structures have been known since the end of the fifties 77 , systematic studies involving triple-stranded metallohelicates started only in the last fifteen years, with the intensive work concerning the design of ligands possessing catecholate or 1-hydroxypyridonate as binding units for ferric ion sequestering Raymond and co-workers have demonstrated that compound 29 Chart 5 forms a complex [Fe 2 29 3 ] in basic medium In another pioneering work on the design of triple-stranded helicates, Shanzer and co-workers synthesized compounds 30 and Many segmental ligands have been designed in order to yield triple-stranded helical structures upon complexation with lanthanide ions Many ligands have been synthesized with the purpose of generating triple helical architectures through complexation with appropriate metal ions The steric information stored in the oligobipyridines 4 - 11 Chart 1 was found to yield double-helicates by complexation with metal ions having tetrahedral coordination.
Steric effects due to the disubstitution at 6,6'- positions prevent the fixation of metal ions with octahedral coordination, which would lead to the formation of triple-helicates. However, formation of these helicates is made possible through changes in the steric instruction. By addressing this question, Elliott and co-workers have synthesized the bis bipyridines 33 - 38 Chart 5 , where the bipyridine units are connected by the 4,4'-positions The connection of bipyridine units via the 5,5'-positions was shown to be another approach to achieve triple helical structures.
The same studies were carried out with ligand 40 These results also illustrate very well the importance of the information stored in the ligand to give the correct structure. Its structure was determined by X-ray crystallography and showed that the three strands of ligand 42 are wrapped around each other and held together by three metal ions, forming the triple helicate. Another interesting feature, the self-recognition, originates from the steric information needed for the assembly of double- and triple-helicates.
Thus, the mixture of two programmed molecular systems provokes a very clean assembly of two well-defined helical complexes from a mixture containing four species, in a process involving 11 different elements. The reaction of an oligo-tridentate or a tris bipyridine ligand with metal ions able to perform an octahedral reading does not necessarily yield the corresponding double- or triple-stranded helicates.
Circular helicates have received increasing interest, due to their specific features. In these circular architectures, each ligand extends over adjacent metal centers and the strands wrap around each other, in order to yield the helical structure. In addition, these structures can be considered metallohelicate analogs of the circular DNA established in some viruses. The reaction of the ligand with FeCl 2 produces compound 45 , a circular double-stranded helicate Figure 13 This pentanuclear complex, a torus having an outer diameter between two opposed carbon atoms of ca.
All metal ions have a distorted octahedral coordination sphere occupied by three bipyridine groups, two terminal and one central, each from a different ligand strand.
Thus, the use of anions such as SO 4 2- , SiF 6 2- and BF 4 - led to the formation of the hexanuclear circular helicate The use of bromide, a counteranion of intermediate size, resulted in a mixture of pentanuclear and hexanuclear circular helicates. These differences in the resultant products are due to the template effect of the anion during the formation of the helicate, and the process can be seen as a self-assembly of a receptor as a function of its substrate. In addition, it was demonstrated that the hexanuclear circular helicate can be quantitatively transformed into the pentanuclear structure by exchanging the anion SO 4 2- by Cl - Thus, the self-assembly process resulting in one or other structure formed from the same components is determined by the anion acting as a template.
It represents a procedure of selection from a virtual combinatorial library VCL 95 , consisting of all the possible complexes that can be generated from the available components. Thus, each member of the VCL represents the whole library, since it can be disassembled and reassembled into every other member in a dynamic combinatorial chemistry process DCC Recently, libraries of circular and double-stranded helicates have been provided in order to develop this subject In the same manner, complex 43 could be transformed into the corresponding circular helicate through heating in an acid medium These results are due to different thermodynamic and kinetic parameters of formation of the complexes from the metal ions and the bipyridine units.
In addition, formation of these circular structures can be considered sequential self-assembled processes , since the corresponding triple-stranded helicate is formed as an intermediate. Structural changes in the tris bipyridine 42 may produce different self-assembled circular structures. Thus, the modification of the CH 2 CH 2 bridges into CH 2 OCH 2 bridges yields a ligand able to form a tetranuclear circular helicate, under the same conditions described above, independent of the counteranion The features displayed by this self-assembled architecture can be related to the increased length of the ligand and its greater flexibility.
Many interesting circular architectures can be constructed with the use of a well-designed ligand and a suitable metal ion For example, Jones et al. The crystal structure of the complex revealed an anion encapsulated in its central cavity, as observed in complex The self-assembly of helical systems from achiral ligands and metal ions yield racemates containing the right-handed P and the left-handed M helicates Figure Since these complexes display special features, the isolation of helical architectures having high optical purity is of considerable interest.
One strategy for yielding chiral helicates involves their spontaneous resolution, which has been observed in the process of crystallization of the triple-stranded helicate 43 This result represents a successful application of molecular programming to the spontaneous but directed formation of a given supramolecular structure. Resolution techniques have also been developed in order to achieve optically pure helicates. The recent advances in resolution techniques of helical structures have been revised The enantiospecific synthesis of helicates is an interesting approach that has been used to achieve asymmetric induction.
One method involves the synthesis of enantiomerically pure ligands with chiral links between the bipyridine units. In this way, the optically pure chiral ligand 46 with S , S -configuration was synthesized Space-filling models and molecular-mechanics calculations suggested that steric effects may be responsible for the preferential formation of the right-handed P double helicate, from the two possible helical diastereomers for helicate 47 The induced sense of helicity by the presence of chiral centers in the ligand strands has been previously reported 66, and this highlights the importance of the ligand design in the organization of the binding centers during the self-assembly process aimed towards the formation of supramolecular species.
Other approaches have been studied in order to accomplish asymmetric induction utilizing an auxiliary template 81,, and the incorporation of a chiral substituent at the extremity of the ligand 67, A very interesting example of the complete stereospecific self-assembly of a circular helicate has been recently reported The reaction of a , a '-bis pinene-2,2'-bipyridyl -p-xylene chiral ligand 48 with AgPF 6 in a mixture of acetonitrile and chloroform led to the spontaneous formation of the circular single-stranded helicate 49 Figure Its structure was confirmed by X-ray crystallography and NMR and circular dichroism data demonstrated that the self-assembled structure is maintained in solution This may illustrate the countless possibilities in this area attainable through the suitable design of chiral ligands responsible for predetermining the chirality in self-assembled architectures As shown in the preceding sections, considerable work has been done in the area concerning metallohelicates, and their applications have also been recently reviewed The first visible possibility offered by this class of self-assembled structure is that specially designed ligands can be created in order to generate helical systems possessing endoreceptor properties.
The proper choice of a bridge connecting the binding elements in the ligand may lead to an informed system capable of yielding a cavity after the metal reading, which can act as an endoreceptor. Although this approach has been used independently by Beer , Harding 65a,b , and Nabeshima and co-workers , considerable effort has to be made to achieve expressive interactions between the receptor and anionic, cationic and neutral species.
These template-directed self-assembled architectures see Section 5. Furthermore, the design of metallohelicates with exoreceptor properties has been explored 45 and the potential of this field will be discussed later see Section 5. The use of double-stranded metallohelicates represents an interesting synthetic pathway towards molecules with non-trivial topology. Sauvage and co-workers have employed this strategy in the synthesis of interlocked catenanes and trefoil knots , which cannot be obtained in another way.
Electron-microscopy studies were performed on supramolecular liquid-crystalline polymers with right-handed helicity The use of different strategies to generate inorganic polymers and the study of their interesting properties have been successfully employed The design of helical architectures can be explored in the development of various supramolecular devices. Shanzer and co-workers have prepared ligands 50 and 51 , containing in their structure one 2,2'-bipyridine and one hydroxamate Chart 7 Recently, lipid-like ligands 52 and 53 were used in the assembly of metallohelicates in order to promote the occurrence of a liquid crystalline phase at room temperature The development of fluorescent sensors and switches represents a very important challenge in the comprehension of many chemical, biochemical and material science events 6,7, This rationalization was used by Piguet and co-workers in the conception of light-converting devices based on triple-stranded helicates with lanthanide metal ions 84, 85, Along with the fact that the helical structures are very beautiful molecular sculptures aesthetically created by means of self-assembly, these complexes may be able to perform innumerable functions.
Closed three-dimensional molecular cage-type structures that have the capability for encapsulating guest species have provided the chemical community with a continual source of fascination and study over the past three decades. A large variety of such molecules have currently been prepared, and which now encompass several classes of compounds. For example, cryptates which were developed for alkali metal ion and anion binding , multi-walled cyclophanes and carceplexes that imprison neutral molecular guests, sepulchrates and sarcophagines, into which are captured transition metal ions and inorganic clusters which enclose various cations and anions The importance of these compounds is mirrored by the diverse range of applications for which they have been used in materials science, medicine and chemical technology.
Additionally, the unusual properties of these systems provide interesting and challenging opportunities for the study of theoretical physicochemical issues However, the limited availability of many of these substances has in part posed a serious obstacle to their technological development and originated from the lengthy multistep reaction sequences and low overall yields often encountered during their synthesis Early investigations showed that it was possible to use metal ion-ligand interactions as a driving force for the generation of structural complexity at a molecular level.
This approach allowed direct access to topographically unusual metal ion containing entities such as helicates see preceding section , catenates , racks 7, , grids 7, and metallomacrocyclic receptors It was therefore decided to explore the possibility of using metal ion-mediated self-assembly as a design principle for the generation of molecular cages and cage-type receptors of controllable size and shape.
In addition, inorganic cage architectures which incorporate metal ions as integral structure generating units would be expected to exhibit novel and interesting physicochemical properties such as optical, magnetic, electrochemical and catalytic functions. As a first step we envisioned the self-assembly of the C 3 symmetric cage complex 54 Figure Most significantly, the resultant architecture would comprise two different ligand species and a single metal ion type and would therefore express a higher degree of structural informational complexity compared to the majority of preexisting self-assembled entities, which consist of a single ligand and metal ion species.
Such multi-ligand type architectures must be generated via a multicomponent self-assembly pathway in which the recognition, growth and termination events involve selective discrimination of hetero-ligand containing species along the reaction coordinate. Multicomponent self-assembly of cylindrical coordination architectures from five ligand units and six metal ions.
The 13 C NMR spectrum also showed the expected seventeen bands. Addition of a small quantity of 55 or 56 to a nitromethane solution of the product complex resulted in complication of the 1 H NMR spectrum with sharp peaks due to 54a still present as a major component. Thus 54a is undergoing slow exchange on the NMR timescale in nitromethane solution. The nature of cation 54a was confirmed by determination of its crystal structure a.
The structure possesses a C 2 axis passing through the middle of the central C-C bond of one of the qpy units; this distortion from ternary symmetry may be due to crystal packing.
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The overall twist of the whole structure results in a triple helical shape of the complex. Bond lengths and angles do not show any peculiarity. Complex 54a presents an internal cavity of cylindrical shape with a height of 7. Thus 54a represents a self-assembled molecular receptor for appropriately sized substrate species. The overall dimensions of the complex are The cation 54b possesses an internal cavity of 4. The guests are positioned in the same plane in such a way that almost all available space within the cavity is filled.
Each guest species also partly protrudes through each of the three cavity portals into the antechambers defined by the 56 ligand surfaces and the 55 phenyl rings. Interestingly, the 1 H NMR spectra of complexes 54a - b show the bands assignable to the ortho-phenyl ring protons of the 55 ligands to be considerably line broadened.
This phenomenon appears to linked to the presence of anions in the cage cavity. The formation of structures 54a-b represents a remarkable example of the spontaneous formation of a closed inorganic architecture through a process of multicomponent self-assembly from eleven particles belonging to two types of ligands and one type of metal ion. These results therefore successfully demonstrated the use of metal ion-mediated multicomponent self-assembly as a method of access to structurally complex molecular architecture, and represent a further step in the control of the self-organization of large and complex supramolecular structures through molecular programming.
The design and generation of elongated inorganic cylindrical cage architectures via metal ion-directed multicomponent self-assembly. Having established the success of the above design principle in constructing inorganic cages, an important further question concerned the possibility to engineer the size and shape of the internal void within cations such as 54a-b in a predictable and controllable way.
Such species would have the potential capacity for shape selective and multiple guest inclusion. Towards this goal, vertical elongation of the cage 54a was attempted by utilizing ligands structurally similar to 56 but incorporating bridging groups between the bipyridine subunits, and repeating the reaction conditions which were successful for the self-assembly of 54a-b.
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Attempted self-assembly of cylindrical cages with conformationally flexible ligand bridges. In all cases, no evidence for cage formation was observed even after prolonged reaction times and elevated temperatures. Interestingly, the cage complex 57 was isolated from the reaction of a 1. It could however be isolated pure in the solid state by slow diffusion of an excess of diisopropyl ether into the reaction mixture.
The identity of 57 was confirmed by X-ray crystallography The cation is shaped into a highly twisted helical cage in which two 55 ligands form respectively the top and bottom and three 58 ligands, the walls of the complex. Complex 57 is helically twisted to a greater degree than 54a. Inside the cage is a small cavity of dimensions 4. The contracted diameter of the cavity of 57 relative to 54a results directly from the steric volume imposed by the internally facing bridging ethylene groups of the three 58 ligands.
Self-assembly of elongated cylindrical cage architectures with rigid ligand bridges. Cages possessing cylindrical cavities were however successfully prepared by using ligands with rigidly preorganized bridges between the bipyridine subunits The 1 H and 13 C NMR spectra of 59 - 63 indicated the presence of a highly symmetrical species in solution with all ligands in a single magnetic and chemical environment.
In the complexes elongated by means of acetylene containing bridges 59 , 62 - 63 , the band corresponding to the ortho phenyl ring protons of 55 appeared as a sharp doublet. This is consistent with an unrestricted movement of anions through the larger portals in the walls of these complexes. The ES mass spectra showed bands assignable to the cylindrical complexes with successive loss of PF 6 - counterions. Thus, rigid preorganization of both reacting ligand species proved to be a successful design modification for generating cylindrical cages with a range of cavity sizes.
Complex 63 is therefore a truly nanoscopic cylinder which has been designed and generated through a multicomponent self-assembly strategy. Ligand selection in the self-assembly of hexacopper inorganic cages. In order to explore the degree to which recognition can operate within a complex mixture, an experiment was performed in which the correct stoichiometric combination of the ligand and metal ion components required to generate 54a , 60 and 61 , were allowed to stir in nitromethane for 72 hours.
Analysis of the resulting solution by 1 H NMR and ES mass spectroscopy showed that only three species were present in solution, i. Previous studies with helicate mixtures 44,63 showed that only products comprising identical ligand strands were formed through a process of self-recognition. In the above example many more particles are initially present within the reaction mixture and the products form only by recognition between ligands of different identity. This situation which is of a higher information content, may be termed as nonself-recognition , and bears analogy to biological phenomena as found for instance in the immune system.
The designed self-assembly of multicomponent and multicompartmental cylindrical nanoarchitectures. The successful generation of the hexanuclear cage complexes described above raised the question as to whether this process could give access to multicellular inorganic architectures that would present several internal cavities and might in addition incorporate selected substrates in the course of the assembly. The formation of supermolecular entities of this type would represent abiological analogues of numerous biological processes mediated by collective interactions and recognition events between large molecules.
In particular, it would amount to a self-compartmentalization process presenting analogies with that displayed by multicompartmental proteases Potential applications may also exist for example in materials science and nanotechnology, where the establishment of pathways for the controlled access to nano-sized chemical entities is of paramount interest.
Further experimental investigations successfully demonstrated that the generation of multicellular inorganic architectures was indeed possible. Thus, the bicompartmental 64 and tricompartmental 65 complex cations were generated in a single operation by self-assembly from the corresponding stoichiometric mixtures of ligand components of two different types and metal ions Figure In addition, X-ray structural determinations and 1 H NMR solution studies revealed that the multicellular architectures encapsulated anions in their cavities Self-assembly of multicompartmental nanoarchitectures.
Ellipsometry of Functional Organic Surfaces and Films. Karsten Hinrichs. Non-equilibrium Phenomena in Confined Soft Matter. Simone Napolitano. Joseph Woicik. Alex Frano. Low-Dimensional Functional Materials. Reinhold Egger. Physical Properties of Nanorods. Roman Krahne. Progress in Nanophotonics 2. Motoichi Ohtsu. Clusters in Nuclei, Volume 3. Christian Beck. Complex Plasmas. Michael Bonitz.
Inorganic Scintillators for Detector Systems. Paul Lecoq. Zhiming M. Soft Matter at Aqueous Interfaces. Peter Lang. Novel Functional Magnetic Materials. Arcady Zhukov. Youichi Murakami. Adsorption and Diffusion in Nanoporous Materials. Rolando M. Shinichiro Seki. New Trends in Atomic and Molecular Physics. Man Mohan. Topological Structures in Ferroic Materials. Jan Seidel.
Photoelectron Spectroscopy. Shigemasa Suga. Quantum Dot Devices. Electronic Structure of Metal Phthalocyanines on Ag Cornelius Krull. Laser Processing and Chemistry. Non-Centrosymmetric Superconductors. Ernst Bauer. Progress in Ultrafast Intense Laser Science. Kaoru Yamanouchi. Guido Langouche. Advanced Transmission Electron Microscopy.
Jian Min Zuo. Length-Scale Dependent Phonon Interactions. Subhash L. Andrey V. Polymer Materials. Kwang-Sup Lee. Surface Science Techniques. Gianangelo Bracco. Magnetic Fields in Diffuse Media.
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Alexander Lazarian. Photorefractive Optoelectronic Tweezers and Their Applications.
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