Electron transfer and switching in rigid [2]rotaxanes adsorbed on TiO2 nanoparticles.

Bistable [2]rotaxanes have been attached through a bulky tripodal linker to the surface of titanium dioxide nanoparticles and studied by cyclic voltammetry and spectroelectrochemical methods. The axle component in the [2]rotaxane contains two viologen sites, V(1) and V(2), interconnected by a rigid terphenylene bridge. In their parent dication states, V(1)(2+) and V(2)(2+) can both accommodate a crown ether ring, C, but are not equivalent in terms of their affinity towards C and have different electrochemical reduction potentials. The geometry and size of the tripodal linker help to maintain a perpendicular [2]rotaxane orientation at the surface and to avoid unwanted side-to-side interactions. When the rigid [2]rotaxane or its corresponding axle are adsorbed on a TiO(2) nanoparticle, viologen V(2)(2+) is reduced at significantly more negative potentials (-0.3 V) than in flexible analogues that contain aliphatic bridges between V(1) and V(2). These overpotentials are analysed in terms of electron-transfer rates and a donor-bridge-acceptor (D-B-A) formalism, in which D is the doubly reduced viologen, V(1)(0), adjacent to the TiO(2) surface (TiO(2)-V(1)(0)), B is the terphenylene bridge and A is viologen V(2)(2+). We have also found that, in contrast with earlier findings in solution, no molecular shuttling occurs in rigid [2]rotaxane adsorbed at the surface. The observations were explained by the relative position of the viologen stations within the electrical double layer, screening of V(2)(2+) by the counterions and high capacity of the medium, which reduces the mobility of the crown ether. The results are useful in transposing of solution-based molecular switches to the interface or in the design and understanding of the properties of systems comprising electroactive and/or interlocked molecules adsorbed at the nanostructured TiO(2) surface.

Introducing negative charges into bis-p-phenylene crown ethers: a study of bipyridinium-based [2]pseudorotaxanes and [2]rotaxanes.

This paper describes novel host-guest systems comprising viologen cations (guests) and the derivatives of bis-para-phenylene-34-crown-10 (hosts) with anionic groups COO(-) or SO(3)(-). The structure of the resulting charge-compensated host-guest complexes, their association constants and their electrochemical behaviour have been studied. In the solid state, the viologen cations thread the negatively charged crown ethers forming electroneutral zwitterion-like [2]pseudorotaxane salts; in solution this threaded geometry is preserved. The association constants of [2]pseudorotaxane salts incorporating the 1,1'-diethylviologen moiety in solution are significantly higher than those of previously reported analogues. The extrapolated association free energies in non-aqueous media exceed -40 kJ mol(-1) at 25 degrees C. This significant increase of the interaction free energy makes these compounds stable even in aqueous solutions. The association constants of [2]pseudorotaxane salts incorporating sterically more hindered 1,1'-diethyl-3,3'-dimethylviologen moieties are significantly lower. Structurally related [2]rotaxane salts, in which the oppositely charged ionic components are mechanically interlocked, have been prepared in good yields. It has been shown that [2]rotaxane salts incorporating anti-isomers of bisfunctionalised crown ethers are cycloenantiomeric. In both [2]pseudorotaxane and [2]rotaxane salts, the electrostatic interactions between the viologen moieties and the negatively charged crown ethers lead to very significant negative shifts of viologen reduction potentials up to 450 mV. The findings of the present study are valuable for the design of nanoscale molecular electronic devices.

Quantitative conformational study of redox-active [2]rotaxanes, part 1: Methodology and application to a model [2]rotaxane.

This paper reports a novel methodology for the conformational analysis of [2]rotaxanes. It combines NMR spectroscopic (COSY, NOESY and the recently reported paramagnetic line-broadening and suppression technique) and electrochemical techniques to enable a quantitative analysis of the co-conformations of interlocked molecules and the conformations of their components. This methodology was used to study a model [2]rotaxane in solution. This [2]rotaxane consists of an axle that incorporates an electron-poor, doubly positively charged viologen that threads an electron-rich crown ether. It has been shown that the axle of the [2]rotaxane in its dicationic state adopts a folded conformation in solution and the crown ether is localised at the viologen moiety. Following a one-electron reduction of viologen, the paramagnetic radical cation of the [2]rotaxane retains its folded conformation in solution. The data also demonstrate that in the radical cation the crown ether remains localised at the viologen, despite its reduced affinity for the singly reduced viologen. The combined quantitative NMR spectroscopic and electrochemical characterisation of the electromechanical function of the model [2]rotaxane in solution provides an important reference point for the study of switching in structurally related bistable [2]rotaxanes, which is the subject of the second part of this work.

Quantitative conformational study of redox-active [2]rotaxanes, part 2: Switching in flexible and rigid bistable [2]rotaxanes.

Translational movement of the macrocycle in two structurally similar bistable [2]rotaxanes, which is induced by a four-step electrochemical process in solution, has been investigated by using a methodology developed in the preceding article (Chem. Eur. J. 2008, 14, 1107-1116). Both [2]rotaxanes contain a crown ether that can be accommodated by either of two interconnected viologen recognition sites. These sites are substantially different in terms of their affinity towards the crown ether and they possess considerably different electrochemical reduction potentials. The two [2]rotaxanes differ in the length and the rigidity of a bridge that links these sites. A combination of molecular mechanics modelling and NOE spectroscopy data provides information about the conformations of both [2]rotaxanes in the parent oxidation state when the crown ether exclusively populates the strong recognition site. To determine the population of the recognition sites at subsequent stages of reduction, a paramagnetic NMR technique and cyclic voltammetry were used. The key finding is that the flexibility of the connecting bridge element between the recognition sites interferes with shuttling of the crown ether in [2]rotaxanes. It can be demonstrated that the more flexible trimethylene bridge is folded, thus limiting the propensity of the crown ether to shuttle. Consequently, the crown ether populates the original site even in the second reduced state of the flexible [2]rotaxane. On the contrary, in the [2]rotaxane in which two viologen sites are connected by a larger and more rigid p-terphenylene bridge, the predominant location of the crown ether at the weak recognition site is achieved after just one single electron reduction.

A flexible method for the fabrication of gold nanostructures using oligonucleotide derivatives.

Several linear and branched DNA structures from 80-200 nm with a biotine molecule in the middle have been prepared. These structures have been decorated by addition of positively charged gold nanoparticles carrying 4-(dimethylamino)pyridine ligands. Streptavidin binds to the central biotine molecule introducing a 20 nm gap in the structure in which a biotinylated nanoparticle can be introduced. The simplest structure (80 nm, linear) is formed by 4 oligonucleotides. By changing some of these components changes on length, shape, and recognition system easily can be introduced.

A tripodal [2]rotaxane on the surface of gold.

Tripodal [2]rotaxane, 3, and the structurally related axle, 2, incorporating a viologen moiety, a crown ether, and three thiol anchoring groups have been synthesized. Analogous monopodal derivatives, 1, have also been prepared. Self-assembled monolayers of the above tripodal and monopodal systems on gold have been studied by cyclic voltammetry. It has been shown that a thiol anchoring group is required to attach the monopodal viologen 1 to the surface of gold and that the maximum surface coverage of 1 corresponds to 2.7 x 10(-10) mol.cm(-2). The adsorbed monopodal viologen 1 does not thread bis-p-phenylene-34-crown-10 ether, 6. However, the tripodal axle 2 adsorbed on the surface of gold threads the crown ether 6 to form a hetero [2]rotaxane. In the case of the tripodal axle 2, the surface coverage is 7 x 10(-11) mol.cm(-2), while for the tripodal [2]rotaxane 3 the surface coverage reaches 1.1 x 10(-10) mol.cm(-2).

Preparation and thermally promoted ripening of water-soluble gold nanoparticles stabilized by weakly physisorbed ligands.

Here, we report on a facile method for the preparation of an aqueous dispersion of 3.6 nm gold nanoparticles electrostatically stabilized by a weakly physisorbed ligand, namely, 4-(dimethylamino)pyridine (DMAP). The nature and extent of the interaction of this ligand with the surface of a gold nanoparticle has been examined. We also report on the thermally promoted ripening of these nanoparticles under mild conditions to yield a dispersion of 11.3 nm gold nanoparticles. The role of the weakly physisorbed DMAP ligand in facilitating thermally promoted ripening under mild conditions has also been examined.

DNA-templated assembly of nanoscale architectures for next-generation electronic devices.

We report the assembly and structural characterization of a Y-shaped DNA template incorporating a central biotin moiety. We also report that this template may be used assemble nanoscale architectures, which demonstrate the potential of this and related approaches to the fabrication of next-generation electronic devices. Of particular significance is the finding that it is possible to selectively metallize the above DNA template to obtain a three-electrode configuration. Also of particular significance is finding that a biotin modified nanoparticle will recognize and bind selectively the central biotin moiety of the same template, once functionalized by the protein streptavidin.

The oxidation-state dependent structural conformation and supramolecular function of a redox-active [2]rotaxane in solution.

A new NMR technique, namely paramagnetic suppression spectroscopy (PASSY), has been used to study the oxidation-state dependent structural conformation and supramolecular function of redox-active rotaxanes and catenanes in solution and at the surfaces of nanoparticles. Specifically, this technique has been used to study the structural conformation and supramolecular function in solution of a redox-active [2]rotaxane and the corresponding radical cation formed by a one-electron reduction. The findings of this and related studies provide important insights into the design of nanoscale devices based on rotaxanes and catenanes.

An experimental and theoretical study of the self-assembly of gold nanoparticles at the surface of functionalized multiwalled carbon nanotubes.

This paper reports the findings of a detailed study of the self-assembly of gold nanoparticles at the surface of carbon nanotubes (CNTs). The study included the development of a predictive model for the interactions (charge transfer, van der Waals, osmotic, elastic, nonelastic, and covalent) between tetraoctylammonium bromide-stabilized (TOAB) gold nanoparticles and alkyl- and alkylthiol-modified multiwalled carbon nanotubes (MWCNTs). It also included the measurement of the coverage of gold nanoparticles at the surface of the above MWCNTs as a function of increasing alkyl chain length. One key finding is that it is possible to predict with a high degree of accuracy using the above model the measured coverage of gold nanoparticles adsorbed, either noncovalently or covalently, at the surface of a MWCNT. Another key finding is that, as predicted, under well-defined conditions the measured coverage of nanoparticles is very sensitive to the nature of the modified CNT surface and the contiguous environment, providing valuable insights that will underpin the rational design of functional nanoscale devices assembled from nanoparticle and CNT building blocks.

Gold nanoparticle patterning of silicon wafers using chemical e-beam lithography.

This paper demonstrates a novel facile method for fabrication of patterned arrays of gold nanoparticles on Si/SiO2 by combining electron beam lithography and self-assembly techniques. Our strategy is to use direct-write electron beam patterning to convert nitro functionality in self-assembled monolayers of 3-(4-nitrophenoxy)-propyltrimethoxysilane to amino functionality, forming chemically well-defined surface architectures on the 100 nm scale. These nanopatterns are employed to guide the assembly of citrate-passivated gold nanoparticles according to their different affinities for amino and nitro groups. This kind of nanoparticle assembly offers an attractive new option for nanoparticle patterning a silicon surface, as relevant, for example, to biosensors, electronics, and optical devices.

DNA-templated assembly of nanoscale wires and protein-functionalized nanogap contacts.

The use of DNA to template the assembly of nanoscale wires and protein-functionalized nanogap contacts is described: Specifically, the use of DNA to template the assembly of gold nanowires between conventionally patterned gold contacts on a silicon wafer substrate. Also described is the use of DNA to template the assembly of protein-functionalized nanogap gold contacts on a silicon wafer substrate. Of particular significance is the finding that suitably modified gold nanoparticles recognize and bind selectively the protein-functionalized nanogap and are localized there.

Assembly of an electronically switchable rotaxane on the surface of a titanium dioxide nanoparticle.

This paper describes the self-assembly of a heterosupramolecular system consisting of a tripodal [2]rotaxane adsorbed at the surface of a titanium dioxide nanoparticle. The tripodal [2]rotaxane consists of a dumbbell-shaped molecule, incorporating two electron-poor viologens, threading an electron-rich crown ether. The [2]rotaxane also incorporates a bulky tripodal linker group at one end and a bulky stopper group at the other end. The [2]rotaxane is adsorbed, via the tripodal linker group, at the surface of a titanium dioxide nanoparticle. The structure and function of the resulting hetero[2]rotaxane have been studied in detail by (1)H NMR spectroscopy and cyclic voltammetry. A key finding is that it is possible to electronically address and switch the above hetero[2]rotaxane.

Self-assembly of a tripodal pseudorotaxane on the surface of a titanium dioxide nanoparticle.

This paper describes the self-assembly of a heterosupramolecular system consisting of a tripodal viologen, adsorbed at the surface of a titanium dioxide nanoparticle, that threads a crown ether to form a pseudorotaxane. The viologen, a 1,1'-disubstituted-4,4'-bipyridinium salt with a rigid tripodal anchor group, has been synthesized. This viologen is adsorbed at the surface of a titanium dioxide nanoparticle in solution. As intended, this tripodal viologen is both oriented normal to and displaced from the surface of the nanoparticle and threads a crown ether to form the heterosupramolecular complex. The threading of the crown ether by the tripodal viologen to form the above pseudorotaxane complex at the surface of a titanium dioxide nanoparticle has been studied by (1)H NMR, optical absorption spectroscopy, and cyclic voltammetry.

Synthesis of tripodal [2]rotaxanes: high concentration principlet.

Symmetrical and non-symmetrical tripodal [2]rotaxanes (1) incorporating 1,1'-disubstituted-4,4'-bipyridinium cations (2) and 34-crown-10 (3) have been prepared directly from 4,4'-bipyridine or from monocationic intermediates in high yields at room temperature and atmospheric pressure under conditions that permit the use of high reagent concentrations (0.1-0.2 M, 150-200 g(-1)).