Metallocyclodextrins as building blocks in noncovalent assemblies of photoactive units for the study of photoinduced intercomponent processes

Cyclodextrin cups have been employed to build supramolecular systems consisting of metal and organic photoactive/redox-active components; the photoinduced communication between redox-active units assembled in water via noncovalent interactions is established. The functionalization of a beta-cyclodextrin with a terpyridine unit, ttp-beta-CD, is achieved by protection of all but one of the hydroxyl groups by methylation and attachment of the ttp unit on the free primary hydroxyl group. The metalloreceptors [(beta-CD-ttp)Ru(ttp)][PF(6)](2), [(beta-CD-ttp)Ru(tpy)][PF(6)](2), and [Ru(beta-CD-ttp)(2)][PF(6)](2) are synthesized and fully characterized. The [(beta-CD-ttp)Ru(ttp)][PF(6)](2) metalloreceptor exhibits luminescence in water, centered at 640 nm, from the (3)MLCT state with a lifetime of 1.9 ns and a quantum yield of Phi = 4.1 x 10(-)(5). Addition of redox-active quinone guests AQS, AQC, and BQ to an aqueous solution of [(beta-CD-ttp)Ru(ttp)](2+) results in quenching of the luminescence up to 40%, 20%, and 25%, respectively. Measurement of the binding strength indicates that, in saturation conditions, 85% for AQS and 77% for AQC are bound. The luminescence quenching is attributed to an intercomponent electron transfer from the appended ruthenium center to the quinone guest inside the cavity. Control experiments demonstrate no bimolecular quenching at these conditions. A photoactive osmium metalloguest, [Os(biptpy)(tpy)][PF(6)], is designed with a biphenyl hydrophobic tail for insertion in the cyclodextrin cavity. The complex is luminescent at room temperature with an emission band maximum at 730 nm and a lifetime of 116 ns. The osmium(III) species are formed for the study of photoinduced electron transfer upon their assembly with the ruthenium cyclodextrin, [(beta-CD-ttp)Ru(ttp)](2+). Time-resolved spectroscopy studies show a short component of 10 ps, attributed to electron transfer from Ru(II) to Os(III) giving an electron transfer rate 9.5 x 10(9) s(-)(1).     

Ruthenium and Osmium Podate Cyclodextrins with Dual-function Recognition Sites for Luminescent Sensing

A multifunctionalised podand cyclodextrin ligand, β-CD-(urebpy)⁷, with urea--bipyridine binding sites leads to ruthenium and osmium, {Ru[β-CD-(urebpy)⁷]}[PF⁶]² {Os[β-CD-(urebpy)⁷]}[PF⁶]², cyclodextrins. The bipyridine ligands are preorganised by the cyclodextrin cavity encapsulating the ruthenium and osmium core to give photoactive metallocyclodextrins. The podate cyclodextrin complexes show characteristic ruthenium and osmium tri-bipyridine luminescence. It is demonstrated that the ruthenium cyclodextrins participate in sensing schemes through both the cyclodextrin cavity and the urea cage at the bottom of the cyclodextrin rim. Luminescence quenching of the ruthenium emission is observed by addition of anthraquinone guests in the cyclodextrin cavity or addition of dihydrogen phosphate anion.     

Cyclodextrin phosphanes as first and second coordination sphere cavitands

The binding properties of two alpha-cyclodextrins, each containing two C(5)-linked "CH(2)PPh(2)" units, L 1 (A,D-substituted) and L 2 (A,C-substituted), have been investigated. Both ligands readily form transition-metal chelate complexes in which the metal centres are immobilised at the cavity entrance. Although diphosphane L 1 displays a marked tendency to behave only as a trans-spanning ligand, the ligand possesses a certain degree of flexibility, for example, allowing the stabilisation of a trigonal silver(I) complex in which the bite angle drops to 143 degrees. Another feature of L 1 concerns its ability to function as an hemilabile ligand. Together with four methoxy groups anchored onto the primary face, the two P(III) centres of L 1 form a circularly arranged P(2)O(4) 12-electron donor set able to complex an Ag(+) ion in a dynamic way, each of the four oxygen atoms coordinating successively to the silver ion. Furthermore, the particular structures of L 1 and L 2, characterised by the presence of P(III) units lying close to the cavity entrance, lead upon complexation to complexes whereby the first coordination sphere is partly entrapped in the cyclodextrin. Thus, when treated with metal chlorides, both ligands systematically produce complexes in which the Mbond;Cl unit is maintained inside the cyclodextrin through weak Cl.H-5 interactions. The chelate complex [Ag(L 1)]BF(4) reacts with acetonitrile in excess to afford a mixture of two equilibrating complexes, [Ag(acetonitrile)(L 1)]BF(4) and [Ag(acetonitrile)(2)(L 1)]BF(4), whose coordinated nitriles lie inside the cyclodextrin cavity. The inner-cavity ligands can be substituted by a benzonitrile molecule. The present study provides the first identification of an [Ag(acetonitrile)(2)(phosphane)(2)](+) ion. The unexpected stabilisation of this species probably rests on a cavity effect, the cyclodextrin walls favouring recombination of the complex after facile dissociation of the nitrile ligands.     

Perspectives in Supramolecular Chemistry—From Molecular Recognition towards Molecular Information Processing and Self-Organization

The selective binding of a substrate by a molecular receptor to form a supramolecular species involves molecular recognition which rests on the molecular information stored in the interacting species. The functions of supermolecules cover recognition, as well as catalysis and transport. In combination with polymolecular organization, they open ways towards molecular and supramolecular devices for information processing and signal generation. The development of such devices requires the design of molecular components performing a given function (e.g., photoactive, electroactive, ionoactive, thermoactive, or chemoactive) and suitable for assembly into an organized array. Light-conversion devices and charge-separation centers have been realized with photoactive cryptates formed by receptors containing photosensitive groups. Eleclroactive and ionoactive devices are required for carrying information via electronic and ionic signals. Redox-active polyolefinic chains, like the “caroviologens”, represent molecular wires for electron transfer through membranes. Push-pull polyolefins possess marked nonlinear optical properties. Tubular mesophases, formed by organized stacking of suitable macro-cyclic components, as well as “chundle”-type structures, based on bundles of chains grafted onto a macrocyclic support, represent approaches to ion channels. Lipophilic macrocyclic units form Langmuir-Blodgett films that may display molecular recognition at the air-water interface. Supramolecular chemistry has relied on more or less preorganized molecular receptors for effecting molecular recognition, catalysis, and transport processes. A step beyond preorganization consists in the design of systems undergoing self-organization, that is, systems capable of spontaneously generating a well-defined supramolecular architecture by self-assembling from their components under a given set of conditions. Several approaches to self-assembling systems have been pursued: the formation of helical metal complexes, the double-stranded helicates, which result from the spontaneous organization of two linear polybipyridine ligands into a double helix by binding of specific metal ions; the generation of mesophases and liquid crystalline polymers of supramolecular nature from complementary components, amounting to macroscopic expression of molecular recognition; the molecular-recognition-directed formation of ordered solid-state structures. Endowing photo-, electro-, and ionoactive components with recognition elements opens perspectives towards the design of programmed molecular and supramolecular systems capable of self-assembly into organized and functional supramolecular devices. Such systems may be able to perform highly selective operations of recognition, reaction, transfer, and structure generation for signal and information processing at the molecular and supramolecular levels.     

Linear polypseudorotaxanes possessing many metal centers constructed from inclusion complexes of alpha-, beta-, and gamma-cyclodextrins with 4,4'-dipyridine

Three cyclodextrin-based complexes, 1-3, bearing external coordination sites for metal cations were prepared in satisfactory yields (over 50%) by reactions of alpha-, beta-, and gamma-cyclodextrins with 4,4'-dipyridine in aqueous solutions. Subsequently, these inclusion complexes were further assembled to form linear polypseudorotaxanes 4-6 through the coordination linkage of Ni(II) or Cu(II) ions, and their assembly behaviors were comprehensively investigated in both solutions and the solid state by means of 1H NMR, FT-IR, UV-vis spectroscopy, conductivity titration, powder X-ray diffraction patterning, thermogravimetric and differential thermal analysis, scanning electron microscopy, scanning tunneling microscopy, and transmission electron microscopy. The results showed that these polypseudorotaxanes existed as individual linear arrays at a low concentration but tended to form polymeric rodlike fibers at a relatively high concentration. Significantly, the volume of the cyclodextrin cavity used not only determined the inclusion complexation stoichiometry between cyclodextrin and 4,4'-dipyridine but also predominated the morphology of resulting polypseudorotaxanes.     

Photoinduced processes in interlocked structures containing porphyrins

The recent literature on photoactive interlocked structures containing porphyrins is reviewed. Catenanes and rotaxanes studied both in the author's laboratory and by other groups, displaying either photoinduced energy or electron transfer processes are reported. In addition to porphyrins, the examined structures contain photo or electroactive components as C₆₀, paraquat, ferrocene, aromatic amines. Both metal catenanes/rotaxanes and free catenanes/rotaxanes are discussed and the differences in their behavior is outlined with respect to structural rigidity and electronic coupling properties. The role of different conformations and their effect on photophysical properties is examined. In spite of their uncommon topology, these arrays behave similarly to covalently or self-assembled photoactive multi-component architectures and display fast energy/electron transfer rates and high charge separation efficiency. A rationale for this behavior is provided.     

Application of Nanoparticles in Electrochemical Sensors and Biosensors

The unique chemical and physical properties of nanoparticles make them extremely suitable for designing new and improved sensing devices, especially electrochemical sensors and biosensors. Many kinds of nanoparticles, such as metal, oxide and semiconductor nanoparticles have been used for constructing electrochemical sensors and biosensors, and these nanoparticles play different roles in different sensing systems. The important functions provided by nanoparticles include the immobilization of biomolecules, the catalysis of electrochemical reactions, the enhancement of electron transfer between electrode surfaces and proteins, labeling of biomolecules and even acting as reactant. This minireview addresses recent advances in nanoparticle‐based electrochemical sensors and biosensors, and summarizes the main functions of nanoparticles in these sensor systems.     

Nanoparticles as structural and functional units in surface-confined architectures

The nanoscale engineering of functional chemical assemblies has attracted recent research effort to provide dense information storage, miniaturized sensors, efficient energy conversion, light-harvesting, and mechanical motion. Functional nanoparticles exhibiting unique photonic, electronic and catalytic properties provide invaluable building blocks for such nanoengineered architectures. Metal nanoparticle arrays crosslinked by molecular receptor units on electrodes act as selective sensing interfaces with controlled porosity and tunable sensitivity. Photosensitizer/electron-acceptor bridged arrays of Au-nanoparticles on conductive supports act as photoelectrochemically active electrodes. Semiconductor nanoparticle composites on surfaces act as efficient light collecting systems, and nanoengineered semiconductor 'core-shell' nanocrystal assemblies reveal enhanced photoelectrochemical performance due to effective charge separation. Layered metal and semiconductor nanoparticle arrays crosslinked by nucleic acids find applications in the optical, electronic and photoelectrochemical detection of DNA. Metal and semiconductor nanoparticles assembled on DNA templates may be used to generate complex electronic circuitry. Nanoparticles incorporated in hydrogel matrices yield new composite materials with novel magnetic, optical and electronic properties.     

Alkynylated phenazines: synthesis, characterization, and metal-binding properties of their bis-triazolyl cycloadducts

We have synthesized a series of ethynylated phenazines and their bis-triazolyl cycloadducts to serve as metal ion sensors. Binding of metal ions is achieved through coordination to the phenazine nitrogen atom and the triazole ring. To allow metal sensing in aqueous solution, the triazole units are substituted with water-soluble ethylene glycol chains. These phenazine cycloadducts exhibit a selective affinity for binding silver ions. Examination of the halogenated analogues reveals a lowering of the band gap and the corresponding bathochromic shifts in the absorption and emission spectra. The electron-withdrawing properties of these halogens also result in significantly decreased metal-binding activity of the phenazine cycloadducts.     

Design of Supramolecular Cyclodextrin Complex Sensors for Ion and Molecule Recognition in Water

The design and function of novel supramolecular fluoroionophore/cyclodextrin (CyD) complex sensors for ion and molecule recognition in water are reviewed. For the crown ether fluoroionophore/-CyD complex, the dimerization of the fluoroionophore inside the -CyD is found to be selectively promoted by alkali metal ion binding, thereby resulting in metal-ion-selective pyrene dimer emission in water. This supramolecular function is successfully utilized in the design of a podand fluoroionophore/-CyD complex for sensing toxic lead ion in water. The boronic acid fluoroionophore/-CyD complex binds sugars and produces increased fluorescence emission in water. The response mechanism appears to be due to the suppression of the photoinduced electron transfer (PET) from pyrene donor to trigonal phenylboronic acid acceptor. This is a novel emission function provided by the boronic acid fluoroionophore/-CyD complex sensors in water.     

Physics of artificial nano-structures on surfaces

Quantum electron transport through nano-structures such as metal atomic wires or molecular bridges is investigated with various theoretical approaches. The difference of the quantization feature between Na and Al atom wires is explained based on the eigenchannel analyses combined with the recursion-transfer matrix calculation. The eigenchannels are calculated self-consistently for Au atom wires at finite bias voltage and the nonlinear conductance is explored in relation to the offset energies of d band channels. As for molecular bridges, we find that a remarkable metalization is caused, if the coupling of the molecule with the metal electrode is enhanced. Internal current distribution within the molecular networks is discussed and exotic properties of the quantum transport is found. In particular, a strong induced loop current is revealed circulating the ring part of the molecule. The direction of the loop current is switched sharply when the electron incident energy sweeps a degenerate molecular level.     

New macrocyclic polyethers with remote binding sites

The synthesis and binding properties of new macrocyclic polyethers are described. These systems incorporate 2,2′-bipyridyl functions in such a fashion that binding of metal nuclei can occur at either the macro-cycle or the bipyridyl function. Evidence is presented that binding of alkali metals occurs at the crown ether cavity while binding of transition metals occurs at the bipyridyl function. Simultaneous binding of two different metals is interpreted in terms of electronic and allosteric effects.     

Single molecular multianalyte sensor: jewel pendant ligand

[structure: see text] The jewel pendant ligand has multiple chromogenic units combined in a single molecule with the dyes linked to a semiselective binding site by three heteroatoms (O, N, S) having different HSAB characteristics, to indicate diverse response to individual transition metal ions. Using a single-molecular multianalyte sensor, multiple analytes could be determined with a minimal sensing system.     

Molecular arrangement and assembly guided by hydrophobic cavities inside DNA

DNA is a unique yet useful material to organize nanoscale molecular arrays along the helix axis. In this study, we demonstrate a useful approach for creating molecular arrays inside a double helical DNA. Our approach is based on a host-guest system. Introducing abasic sites into DNA afforded a hydrophobic cavity that serves as a host. A planar aromatic molecule (cationic perylenediimide, PDI) was used as the guest molecule. In an aqueous solution, the PDI molecules tend to aggregate with themselves due to the strong hydrophobicity. In the presence of DNA with the cavity, the binding of the PDI was found to site-specifically occur in the hydrophobic cavity. The unique assembly and arrangement for more than two PDI molecules was achieved by controlling the sizes and positions of the cavities. Our approach would provide a simple and convenient way to construct one-dimensional aromatic arrays in DNA.     

Gating of single molecule transistors: combining field-effect and chemical control

Previously we have demonstrated that several structural features are crucial for the functionality of molecular field-effect transistors. The effect of additional structural aspects of molecular wires is explored. These include the type of, the thiol binding location on, and the chemical substitutions of a conjugated system. Pentacene, porphyrin, and the Tour-Reed devices are utilized as model systems. The thiol binding location is shown to have a varied effect on the transmission of a system depending on the molecular orbitals involved. Substitution by electron withdrawing and donating groups is illustrated to have a substantial effect on the transmission of single molecule devices. The substitution effect is either a simple energy shifting effect or a more complicated resonance effect, and can be used to effectively tune the electronic behavior of a single molecule field effect transistor.     

Fluorescent Cyclodextrin/Peptide Hybrids with a Novel Guest‐Responsive Chemosensor in the Peptide Side Chain

Peptides bearing β‐cyclodextrin and 4‐(N,N‐dimethylamino)benzoyl (DMAB) units in the peptide side chains were prepared as chemosensors for molecule detection. The DMAB unit was expected to be included into the cyclodextrin cavity intramolecularly. However, these peptides exhibited no twisted intramolecular charge transfer fluorescence and the normal fluorescence intensity decreased upon the addition of 1‐adamantanol as an exogenous guest, indicating that the DMAB units are shallowly included in the cyclodextrin cavities.     

Click To Bind: Metal Sensors

The copper‐catalyzed azide–alkyne “click” cycloaddition reaction is an efficient coupling reaction that results in the formation of a triazole ring. The wide range of applicable substrates for this reaction allows the construction of a variety of conjugated systems. The additional function of triazoles as metal‐ion ligands has led to the click reaction being used for the construction of optical sensors for metal ions. The triazoles are integral binding elements, which are formed in an efficient modular synthesis. Herein, we review recent examples of triazoles as a metal‐binding element in conjugated metal‐ion sensors.     

Dynamic chemical devices: modulation of photophysical properties by reversible, ion-triggered, and proton-fuelled nanomechanical shape-flipping molecular motions

The terpy-derived (terpy=terpyridine) ligand 1 has an extended W shape in which the two appended photoactive pyrenyl groups are held apart. On binding of a zinc(II) ion with a terpy group, ligand 1 is converted into complex 2 whereby it adopts a U shape, thus stacking the aromatic units. This structural modification leads to a very pronounced change in photophysical properties: from a highly fluorescent free ligand to a very weakly emitting complex. The W/U structural switching can be reversibly induced by the addition of a competitive tren ligand, which binds and releases a zinc(II) ion under protonation/deprotonation cycles, thus leading to oscillations in light emission. Therefore, the present system performs periodic modulation of optical output through a nanomechanical shape-flipping motion, triggered by metal ion binding and fuelled by acid-base neutralisation energy. Overall, it represents an ion-triggered opto-mechanical supramolecular device.     

Control over binding stoichiometry and specificity in the supramolecular immobilization of cytochrome c on a molecular printboard

Here, the stepwise assembly of an electroactive bionanostructure on a molecular printboard is described. The system consists of a cyclodextrin receptor monolayer (molecular printboard) on glass, a divalent linker, streptavidin (SAv), and biotinylated cytochrome c (cyt c). The divalent linker consists of a biotin moiety for binding to SAv and two adamantyl moieties for supramolecular host-guest interaction at the cyclodextrin molecular printboard. The binding of biotinylated cyt c onto a SAv layer bound to preadsorbed linker appeared to be highly specific. The coverages of cyt c as assessed by UV-vis spectroscopy and scanning electrochemical microscopy (SECM) appeared to be identical indicating that all cyt c units remained active. Moreover, the coverage values corresponded well with an estimate based on steric requirements, and the binding stoichiometry was therefore found to be by two biotin moieties of cyt c per one SAv molecule.     

Synthesis of multicomponent systems composed of one phthalocyanine and four terpyridine ligands

Two phthalocyanine-based multiple ligands were synthesized and characterized. Photochemical and electrochemical properties were measured for zinc(II) phthalocyanines covalently linked with four ruthenium(II) bisterpyridyl complexes. The absorption and electrochemical results are indicative of electronic interaction between two photoactive and redox-active components. Fluorescence spectroscopy of the five nuclear complexes provides evidence of an efficient photoinduced intramolecular energy transfer between the ruthenium-based metal-to-ligand charge-transfer (MLCT) chromophores and the zinc(II) phthalocyanine core. The absorption and fluorescence spectra of the phthalocyanine-based multiple ligands change dramatically as a result of the coordination of metal ions with peripheral terpyridine ligands. This change of fluorescence intensity upon addition of metal ions can apply to an output signal for metal ion sensing. The direct attachment of metal ion receptors with a zinc phthalocyanine core enhanced efficiency of the energy- and electron-transfer reaction from the core to the metal complexes.