Synthesis of a new photoactive nanovehicle: a nanoworm

A nanovehicle with a new photoactive moiety has been synthesized. The incorporation of the azobenzene chassis allows for potential wormlike movement accompanying the rolling behavior of the wheels. Two versions, the fullerene-wheeled and carborane-wheeled nanoworms, were synthesized to examine the solution-based photoisomerization yields of the chassis.     

Recent progress on nanovehicles

Nanovehicles are a new class of molecular machines consisting of a molecular scale chassis, axles, and wheels, that can roll across solid surfaces with structurally defined directions making them of interest to synthetic chemists, surface scientists, chemical engineers, and the general car enthusiast. In this tutorial review, following a brief introduction to the development of nanomachines, our recent progress on the nanovehicle project is presented including the design, synthesis, and testing of a series of nanocars, nanotrucks, and motorized nanocars.     

Synthesis by ring closing metathesis and properties of an electroactive calix[6]arene [2]catenane

Abstract

Tris-(N-phenylureido)-calix[6]arenes are heteroditopic non-symmetric molecular wheels that, in apolar media, bind viologen-based molecular axles in a pseudorotaxane-type fashion. Because of the precise kinetic requirements associated with the threading process, in apolar solvents, the dicationic portion of the axle enters the calixarene annulus exclusively from the upper rim. With the general aim to develop new prototypes of molecular devices and machines whose functions could be governed through a wider set of control elements, we envisaged that the unique properties of these calixarene wheels could be transferred to the synthesis of new catenanes for the construction of unidirectional rotary motors. Herein, we describe the synthesis of a tris(N-phenylureido)calix[6]arene-based catenane by applying the intramolecular ring-closing metathesis reaction for the catenation step on a pre-formed pseudorotaxane.     

Coordination polymers containing rotaxane linkers

A class of coordination polymers in which the linking ligands are mechanically interlocked rotaxane molecules is reviewed. To date, four different, axle - wheel templating motifs have been used to create the [2]pseudorotaxane linkers for these unique solid-state materials; (1) protonated diaminoalkane axles with cucurbit[6]uril wheels, (2) 1,2-bis(4,4'-bipyridinio)ethane axles with dibenzo[24]crown-8 wheels, (3) 2,6-naphthalene dicarboxylate axles with tetra-imidazolium macrocycle wheels and (4) a Cu(i) complex of a 1,10-phenanthroline containing dicarboxylate axle with a 1,10-phenanthroline containing crown ether wheel. The synthesis and solid state structure of each coordination polymer is described. The future directions of this area of research and some designs for the next generation of these compounds are discussed.     

Directional Shuttling of a Stimuli‐Responsive Cone‐Like Macrocycle on a Single‐State Symmetric Dumbbell Axle

Rotaxane‐based molecular shuttles are often operated using low‐symmetry axles and changing the states of the binding stations. A molecular shuttle capable of directional shuttling of an acid‐responsive cone‐like macrocycle on a single‐state symmetric dumbbell axle is now presented. The axle contains three binding stations: one symmetric di(quaternary ammonium) station and two nonsymmetric phenyl triazole stations arranged in opposite orientations. Upon addition of an acid, the protonated macrocycle shuttles from the di(quaternary ammonium) station to the phenyl triazole binding station closer to its butyl groups. This directional shuttling presumably originates from charge repulsion and an orientational binding preference between the cone‐like cavity and the nonsymmetric phenyl triazole station. This mechanism for achieving directional shuttling by manipulating only the wheels instead of the tracks is new for artificial molecular machines.     

Rheological complexity in simple chain models

Dynamical properties of short freely jointed and freely rotating chains are studied using molecular dynamics simulations. These results are combined with those of previous studies, and the degree of rheological complexity of the two models is assessed. New results are based on an improved analysis procedure of the rotational relaxation of the second Legendre polynomials of the end-to-end vector in terms of the Kohlrausch-Williams-Watts (KWW) function. Increased accuracy permits the variation of the KWW stretching exponent beta to be tracked over a wide range of state points. The smoothness of beta as a function of packing fraction eta is a testimony both to the accuracy of the analytical methods and the appropriateness of (eta(0)-eta) as a measure of the distance to the ideal glass transition at eta(0). Relatively direct comparison is made with experiment by viewing beta as a function of the KWW relaxation time tau(KWW). The simulation results are found to be typical of small molecular glass formers. Several manifestations of rheological complexity are considered. First, the proportionality of alpha-relaxation times is explored by the comparison of translational to rotational motion (i.e., the Debye-Stokes-Einstein relation), of motion on different length scales (i.e., the Stokes-Einstein relation), and of rotational motion at intermediate times to that at long time. Second, the range of time-temperature superposition master curve behavior is assessed. Third, the variation of beta across state points is tracked. Although no particulate model of a liquid is rigorously rheologically simple, we find freely jointed chains closely approximated this idealization, while freely rotating chains display distinctly complex dynamical features.     

Size Complementarity of Macrocyclic Cavities and Stoppers in Amide-Rotaxanes

New [2]rotaxanes were prepared by the threading and the slipping procedure, the latter having the advantage of not needing templating interactions. As a consequence, the first [2]rotaxane consisting of a tetraamide macrocycle and a pure hydrocarbon thread was synthesized (see 12a in Scheme 2). Sterically matching wheels and axles being the basic requirement of a successful slipping approach to rotaxanes, mono- and bishomologous wheels 5b , c with larger diameters than the parent 5a were synthesized and mechanically connected to amide axles 10a – c which were stoppered with blocking groups of different spatial demand (Scheme 1). The deslipping kinetics of the resulting rotaxanes 8a – c and 9a , b were measured and compared; it emerges that even slight increases in the wheel size require larger stoppers to stabilize the mechanical bond. Moreover, when the deslipping rate of 8a (amide wheel and amide axle) was determined in either DMF or THF, a strong dependence on the solvent polarity, which is caused by a differing extent of intramolecular H-bonds between the wheel and the axle, was observed. As expected, no such dependence was detected for rotaxane 12a (amide wheel and hydrocarbon axle) whose components cannot interact via H-bonds. The comparison of the sterically matching pairs of macrocycles and blocking groups, found by a systematic fitting based on the results of slipping and deslipping experiments, with other rotaxane types bearing similar stoppers allows conclusions concerning the relative cavity size of wheels of various structure.     

Rotaxanes as ligands: from molecules to materials

This tutorial review documents the discovery and application of the supramolecular template (1,2-bis(pyridinium)ethane) subset (24-crown-8) for preparing interlocked molecules. Focus is on the supramolecular chemistry of the pseudorotaxanes formed with various pyridinium axles and crown ether wheels and how this particular class of mechanically linked molecule has been (i) used to construct rudimentary molecular machines such as molecular shuttles and flip switches, (ii) used as ligands for coordination chemistry and (iii) used to create metal-organic framework (MORF) materials.     

Molecular dynamics integration and molecular vibrational theory. I. New symplectic integrators

New symplectic integrators have been developed by combining molecular dynamics integration with the standard theory of molecular vibrations to solve the Hamiltonian equations of motion. The presented integrators analytically resolve the internal high-frequency molecular vibrations by introducing a translating and rotating internal coordinate system of a molecule and calculating normal modes of an isolated molecule only. The translation and rotation of a molecule are treated as vibrational motions with the vibrational frequency zero. All types of motion are thus described in terms of the normal coordinates. The method's time reversibility requirement was used to determine the equations of motion for internal coordinate system of a molecule. The calculation of long-range forces is performed numerically within the generalized second-order leap-frog scheme, in the same way as in standard second-order symplectic methods. The new methods for integrating classical equations of motion using normal mode analysis allow us to use a long integration step and are applicable to any system of molecules with one equilibrium configuration.     

Pseudo-polyrotaxanes based on a protonated version of the 1,2-bis(4,4'-bipyridinium)ethane-24-crown-8 ether motif

Protonated 1,2-bis(4,4'-bipyridinium)ethane axles and dibenzo-24-crown-8 ether wheels thread to form [2]pseudorotaxanes which associate in the solid state to form pseudopolyrotaxanes by hydrogen bonding or pi-stacking.     

Simulation study of semi-crystalline polymer interphases

The study of structure and properties of semi-crystalline polymer inter-phases is important to explain and extend polymer applications. In this region, polymer chains exist in three distinct populations: tie chains that bridge the two crystals, chain folds and chain ends. The distribution of these populations influences the properties of the interphase. We have developed off-lattice Monte Carlo simulations of constrained interphases of semi-crystalline polymers which utilize robust off-lattice moves. A united atom model with polyethylene-like interactions and with freely rotating bonds is used to mimic the prototypical flexible chain structure. These simulations capture the limiting distributions of tight and loose chain folds and of tie chains within the metastable phase. The dissipation in order and density between the crystal and amorphous regions has been studied, and results for freely rotating chains indicate that the characteristic decay of anisotropy occurs in a length scale of ca. 10 Å. Simulation results for the effect of system size and molecular weight for freely rotating chains have also been investigated.     

Rotational relaxation in simple chain models

The rotational dynamics of chemically similar systems based on freely jointed and freely rotating chains are studied. The second Legendre polynomial of vectors along chain backbones is used to investigate the rotational dynamics at different length scales. In a previous study, it was demonstrated that the additional bond-angle constraint in the freely rotating case noticeably perturbs the character of the translational relaxation away from that of the freely jointed system. Here, it is shown that differences are also apparent in the two systems' rotational dynamics. The relaxation of the end-to-end vector is found to display a long time, single-exponential tail and a stretched exponential region at intermediate times. The stretching exponents beta are found to be 0.75+/-0.02 for the freely jointed case and 0.68+/-0.02 for the freely rotating case. For both system types, time-packing-fraction superposition is seen to hold on the end-to-end length scale. In addition, for both systems, the rotational relaxation times are shown to be proportional to the translational relaxation times, demonstrating that the Debye-Stokes-Einstein law holds. The second Legendre polynomial of the bond vector is used to probe relaxation behavior at short length scales. For the freely rotating case, the end-to-end relaxation times scale differently than the bond relaxation times, implying that the behavior is non-Stokes-Einstein, and that time-packing-fraction superposition does not hold across length scales for this system. For the freely jointed case, end-to-end relaxation times do scale with bond relaxation times, and both Stokes-Einstein and time-packing-fraction-across-length-scales superposition are obeyed.     

Persistence of short range order in the fluid phases of a mesogen copper complex studied by EPR

The synthesis and thermal characterization of a new copper complex, bis[N-(4-n-pentoxyphenyl)-4-n-decyloxysalicylaldiminate] Cu(II), having mesomorphic properties is given. The EPR spectrum of this material has been measured for its different phases as a function of temperature. The results are compared with those for two other Cu(II) compounds, bis[4-n-decyloxysalicylate] Cu[II] and bis[N-(n-pentyl)-4-n-decyloxysalicylaldiminate] Cu(II) which have a related molecular structure, but are not mesomorphic. The influence on the spectra of exchange interactions in all the phases, as well as the molecular motion in the fluid phases is discussed, and it is concluded that the exchange interactions are not destroyed by the molecular motion. These facts suggest that molecular motion in the high temperature phases takes places whilst keeping a short range order, resulting in a locally correlated motion.     

Energies conformationnelles des modeles des polyphenyleneoxydes

Conformational calculations made on dimers have shown that mono and disubstituted polyphenylene oxides must exhibit freely rotating chain statistics. Stable conformations are found to belong to two series according to the nature of the most energetical interactions. Moreover a quantitative analysis of the intramolecular mobility has revealed an anisotropy of motion depending on the substituent.     

Selective and Effective Binding of Pillar[5,6]arenes toward Secondary Ammonium Salts with a Weakly Coordinating Counteranion

The selective and effective binding of secondary ammoniums with a weakly coordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF) counteranion by per-ethylated pillar[5,6]arenes is reported. The construction of a first pillararene-based self-sorting system consisting of two wheels and two axles is also described.     

EPR studies on molecular orientation in a surface-stabilized paramagnetic liquid crystal cell

By EPR spectroscopy, we have developed a new method for determining the molecular orientation in a surface-stabilized liquid crystal (LC) cell, which includes a paramagnetic LC, (2S,5S)-2,5-dimethyl-2-heptyloxyphenyl-5-[4-(4-octyloxybenzenecarbonyloxy)phenyl]pyrrolidine-1-oxy (1), whose spin source is fixed in the rigid core. For each phase of racemic [(+/-)] and enantiomerically enriched [(S,S)] 1 in a surface-stabilized LC cell (4 microm thickness), the observed g-value profiles depending on the angle between the applied magnetic field and the cell plane were successfully simulated by the orientation models: (i) the LC molecule in the nematic (N) phase of (+/-)-1 freely rotates around the long axis, which is always parallel to the rubbing direction; (ii) the long axis of the freely rotating LC molecule in the chiral nematic (N*) phase of (S,S)-1 is always parallel to the cell plane but rotates in the plane to form a helical superstructure; and (iii) in the crystalline phase of (S,S)-1, the molecular long axis forms a helical superstructure similar to that of the N* phase, but the molecule is fixed around the long axis so that the NO bond lies in the cell plane. Fitting the temperature profile of the g-value in the N phase of (+/-)-1 by use of the Haller equation, we determined the molecular g-values along the molecular long axis (g(parallelM)) and short axis (g(perpendicularM)), which were successfully reproduced by the use of the set of principal g-values of a similar nitroxide with consideration of the structure of the LC molecule optimized by Molecular Mechanics 3 (MM3).     

Hydrodynamic interactions of evaporating droplet with droplet of nonvolatile substance

The hydrodynamic interactions of freely evaporating or growing droplet (suspended in gaseous medium) in the supersaturated vapor with the droplet of nonvolatile substance or spherical solid particle are theoretically studied with allowance for effects that are linear with respect to the Knudsen number. The process of interaction between the volatile droplet and the infinite plane surface of nonvolatile liquid or solid is considered as a limiting case. Numerical estimates of the velocities of the steady motion of evaporating droplets of water and castor oil are reported. For the droplet of water and spherical solid particle, the effect of the heat conductivity of the latter on the velocity of particle motion is considered. Analogous estimates are obtained for a water droplet that evaporates near the infinite solid surface of castor oil or solid. The effects of the droplet size and the heat conductivity of wall on the rate of the evaporation of water droplet are analyzed.     

Evidence for homo-conjugation between two revolving 14pi-electron systems in 10b-methyl-10c-[2-(10b,10c-dimethyl-10b,10c-dihydropyrenyl)]-10b,10c-dihydropyrene

Ring contraction followed by an elimination reaction on anti-9-methyl-18-trans-2-(10b,10c-dimethyl-10b,10c-dihydropyrenyl)-2,11-dithia[3,3]metacyclophane gave the desired compound 10b-methyl-10c-[2-(10b,10c-dimethyl-10b,10c-dihydropyrenyl)]-10b,10c-dihydropyrene. 1H NMR spectroscopic analysis indicated a ring current effect over a considerable distance from the macro-molecular plane of each of the aromatic rings with the two pi systems freely rotating. Bathochromic shifts and peak broadening in its electronic spectrum clearly supports the presence of through-space pi-pi interactions between the two aromatic rings. This should serve as a good model to verify homo-conjugation effect in such a novel system where the two pi systems are freely revolving.     

Evaluating New Chemistry to Drive Molecular Discovery: Fit for Purpose?

As our understanding of the impact of specific molecular properties on applications in discovery‐based disciplines improves, the extent to which published synthetic methods meet (or do not meet) desirable criteria is ever clearer. Herein, we show how the application of simple (and in many cases freely available) computational tools can be used to develop a semiquantitative understanding of the potential of new methods to support molecular discovery. This analysis can, among other things, inform the design of improved substrate scoping studies; direct the prioritization of specific exemplar structures for synthesis; and substantiate claims of potential future applications for new methods.     

Interfacial Molecular Imprinting in Nanoparticle-Stabilized Emulsions

A new interfacial nano and molecular imprinting approach is developed to prepare spherical molecularly imprinted polymers with well-controlled hierarchical structures. This method is based on Pickering emulsion polymerization using template-modified colloidal particles. The interfacial imprinting is carried out in particle-stabilized oil-in-water emulsions, where the molecular template is presented on the surface of silica nanoparticles during the polymerization of the monomer phase. After polymerization, the template-modified silica nanoparticles are removed from the new spherical particles to leave tiny indentations decorated with molecularly imprinted sites. The imprinted microspheres prepared using the new interfacial nano and molecular imprinting have very interesting features: a well-controlled hierarchical structure composed of large pores decorated with easily accessible molecular binding sites, group selectivity toward a series of chemicals having a common structural moiety (epitopes), and a hydrophilic surface that enables the MIPs to be used under aqueous conditions.