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David A Leigh

Publications and source records attributed to David A Leigh.

14 recordsLinked to original sources
PubMed2003

Electrochemically switchable hydrogen-bonded molecular shuttles.

A series of [2]rotaxanes containing succinamide and naphthalimide hydrogen-bonding stations for a benzylic amide macrocycle is described. Electrochemical reduction and oxidation of the naphthalimide group alters its ability to form hydrogen bonds to the macrocycle to such a degree that redox processes can be used to switch the relative macrocycle-binding affinities of the two stations in a rotaxane by over 8 orders of magnitude. The structure of the neutral [2]rotaxane in solution is established by (1)H NMR spectroscopy and shows that the macrocycle exhibits remarkable positional integrity for the succinamide station in a variety of solvents. Cyclic voltammetry experiments allow the simultaneous stimulation and observation of a redox-induced dynamic process in the rotaxane which is both reversible and cyclable. Model compounds in which various conformational and co-conformational changes are prohibited demonstrate unequivocally that the redox response is the result of shuttling of the macrocycle between the two stations. At room temperature in tetrahydrofuran the electrochemically induced movement of the macrocycle between the two stations takes approximately 50 micros.

Journal Article↗
PubMed2003

Unidirectional rotation in a mechanically interlocked molecular rotor.

Molecular motor proteins are ubiquitous in nature and have inspired attempts to create artificial machines that mimic their ability to produce controlled motion on the molecular level. A recent example of an artificial molecular rotor is a molecule undergoing a unidirectional 120 degrees intramolecular rotation around a single bond; another is a molecule capable of repetitive unimolecular rotation driven by multiple and successive isomerization of its central double bond. Here we show that sequential and unidirectional rotation can also be induced in mechanically interlocked assemblies comprised of one or two small rings moving around one larger ring. The small rings in these [2]- and [3]catenanes move in discrete steps between different binding sites located on the larger ring, with the movement driven by light, heat or chemical stimuli that change the relative affinity of the small rings for the different binding sites. We find that the small ring in the [2]catenane moves with high positional integrity but without control over its direction of motion, while the two rings in the [3]catenane mutually block each other's movement to ensure an overall stimuli-induced unidirectional motion around the larger ring.

Journal Article↗
PubMed2003

Hydrogen bond-assembled fullerene molecular shuttle.

A novel [2]rotaxane has been prepared in which fullerene C(60) behaves as both a stopper and a photoactive unit. The amphiphilic nature of the rotaxane thread can be used to shuttle the macrocycle from close to the fullerene spheroid (in nonpolar solvents) to far away (in polar solvents). The differing location of the macrocycle in dichloromethane and dimethyl sulfoxide gives rise to effects detectable by (1)H NMR and time-resolved spectroscopy.

Journal Article↗
PubMed2003

Mechanically linked polycarbonate.

The synthesis, by solid-state copolymerization, and characterization of the first polycatenanes based on a commercial polymer are reported. Various amounts of a benzylic amide [2]catenane, the corresponding macrocycle, and a rigid bisphenol fluorene derivative have been quantitatively and homogeneously incorporated into bisphenol A polycarbonate. The resulting copolymers were characterized by size exclusion chromatography coupled with viscosimetry, (1)H NMR, differential scanning calorimetry, and dynamic mechanical analysis. The unexpectedly small influence of [2]catenane incorporation on the glass transition temperature of the copolymers points to remarkable internal mobility of the catenane comonomer rings. A new relaxation linked to the flexible catenane units is also observed. The studies represent a detailed structural characterization of a polymer containing small amounts of mechanical linkages in its backbone and demonstrate that significant effects can be induced by doping conventional polymers with small percentages (2-6% of repeat units) of flexible catenanes.

Journal Article↗
PubMed2002

Photoisomerization of a rotaxane hydrogen bonding template: light-induced acceleration of a large amplitude rotational motion.

Establishing methods for controlling aspects of large amplitude submolecular movements is a prerequisite for the development of artificial devices that function through rotary motion at the molecular level. Here we demonstrate that the rate of rotation of the interlocked components of fumaramide-derived [2]rotaxanes can be accelerated, by >6 orders of magnitude, by isomerizing them to the corresponding maleamide [2]rotaxanes by using light.

Crystallography, X-Ray↗
PubMed2002

The effect of guest inclusion on the crystal packing of p-tert-butylcalix[4]arenes.

The effect of guest inclusion in the crystal structures of p-tert-butylcalix[4]arene complexes has been investigated through a combination of molecular-mechanics-based solid-state calculations and statistical analysis, with a procedure previously developed and used to study a variety of classes of organic compounds. The results indicate that the general trends in the behavior of calixarene crystals are very similar, irrespective of the presence or the absence of a guest encapsulated in the calixarene cavity, and are similar to those obtained for most other organics. Some differences arise only when a statistical analysis of several descriptors is performed. The investigation of the description of endo calix[4]arenes is extended by calculating the packing coefficient of the guest inside the calixarenes cavity, C(cavity), which shows that most of them are well accommodated inside the host and have coefficients that are similar to those found in the liquid phase. Further evaluation of the interaction energies between guest and host shows that the coefficients tend to be smaller than 30 kcal mol(-1). The combination of small C(cavity) and low interaction energies suggests that guest mobility in the solid could be rather common in endo complexes of calixarenes.

Binding Sites↗
PubMed2002

From reactants to products via simple hydrogen-bonding networks: Information transmission in chemical reactions.

The transmission of information is ubiquitous in nature and often occurs through supramolecular hydrogen bonding processes. Here we report that there is a remarkable correlation during synthesis between the efficiency of the hydrogen-bond-directed assembly of peptide-based [2]rotaxanes and the symmetry distortion of the macrocycle in the structure of the final product. It transpires that the ability of the flexible macrocycle-precursor to wrap around an unsymmetrical hydrogen bonding template affects both the reaction yield and a quantifiable measure of the symmetry distortion of the macrocycle in the product. When the yields of peptide rotaxane-forming reactions are high, so is the symmetry distortion in the macrocycle; when the yields are low, indicating a poor fit between the components, the macrocycle symmetry is relatively unaffected by the thread. Thus during a synthetic sequence, as in complex biological assembly processes, hydrogen bonding can code and transmit "information"--in this case a distortion from symmetry--between chemical entities by means of a supramolecularly driven multicomponent assembly process. If this phenomenon is general, it could have far reaching consequences for the use of supramolecular-directed reactions in organic chemistry.

Chemistry↗
PubMed2002

Switching "on" and "off" the expression of chirality in peptide rotaxanes.

The hydrogen-bond-directed synthesis, X-ray crystal structures, and optical properties of the first chiral peptide rotaxanes are reported. Collectively these systems provide the first examples of single molecular species where the expression of chirality in the form of a circular dichroism (CD) response can selectively be switched "on" or "off", and its magnitude altered, through controlling the interactions between mechanically interlocked submolecular components. The switching is achievable both thermally and through changes in the nature of the environment. Peptido[2]rotaxanes consisting of an intrinsically achiral benzylic amide macrocycle locked onto various chiral dipeptide (Gly-L-Ala, Gly-L-Leu, Gly-L-Met, Gly-L-Phe, and Gly-L-Pro) threads exhibit strong (10-20k deg cm(2) dmol(-1)) negative induced CD (theta;) values in nonpolar solvents (e.g. CHCl(3)), where the intramolecular hydrogen bonding between thread and macrocycle is maximized. In polar solvents (e.g., MeOH), where the intercomponent hydrogen bonding is weakened, or switched off completely, the elliptical polarization falls close to zero in some cases and can even be switched to large positive values in others. Importantly, the mechanism of generating the switchable CD response in the chiral peptide rotaxanes is also determined: a combination of semiempirical calculations and geometrical modeling using the continuous chirality measure (CCM) shows that the chirality is transmitted from the amino acid asymmetric center on the thread via the macrocycle to the C-terminal stopper of the rotaxane. This understanding could have important implications for other areas where chiral transmission from one chemical entity to another underpins a physical or chemical response, such as the seeding of supertwisted nematic liquid crystalline phases or asymmetric synthesis.

Circular Dichroism↗
PubMed2002

The effect of mechanical interlocking on crystal packing: predictions and testing.

The first statistical analyses of the X-ray crystal structures of mechanically interlocked molecular architectures, the first molecular mechanics-based solid-state calculations on such structures and atomic force microscopy (AFM) experiments are used in combination to predict and test which types of benzylic amide macrocycle-containing rotaxanes possess mobile components in the crystalline phase and thus could form the basis of solid-state devices that function through mechanical motion at the molecular level. The statistical studies and calculations show that crystals formed by rotaxanes possess similarities and unanticipated differences with respect to the crystal packing of noninterlocked molecules. Trends in the rotaxane series correlate quantities related to crystal packing, molecular size, stoichiometry, and H-bonding. In accordance with the findings of Gavezzotti et al. for conventional molecular architectures, a principal component analysis (PCA) showed that three vectors related to the size, packing parameters, and stoichiometry are sufficient to describe the crystal properties of benzylic amide macrocycle-containing rotaxanes. When hydrogen bond-related quantities are included in a second PCA, they combine with the size and the stoichiometry vectors but not with packing-related parameters, indicating that the intramolecular "saturation" of the H-bonds (between the interlocked components) takes precedence over crystal assembly (i.e., intermolecular packing) in these systems. However, cluster analyses also suggest a major role for the energy of interaction between the macrocycle and its crystal environment. The identification of such a "privileged" interaction is of fundamental importance to the development of rotaxanes with in-crystal mobility of one or more of their interlocked components, a prerequisite for the exploitation of molecular level mechanical motion in the solid state. The set of trends found, together with the calculated energies, was used to propose guidelines for which benzylic amide macrocycle-containing rotaxanes are best suited to become building blocks for systems with mobile submolecular units in the crystalline phase. An experimental test of the predictive power of such guidelines was carried out using AFM on a rotaxane and its thread, identified by the study as a promising candidate for solid-state mobility. Intuitively, the rotaxane should be less mobile in the solid state since it has multiple sets of both hydrogen bond donors and acceptors that can form strong inter- and intramolecular H-bonds. Conversely, the thread has no hydrogen bond donors and cannot form such bonds. The AFM experiments, however, confirm the statistical analysis prediction that the rotaxane is considerably more mobile in the solid than the thread.

Journal Article↗