A generic basis for some simple light-operated mechanical molecular machines

A novel type of mechanical switch is described in which light-induced translation of a macrocycle in a [2]rotaxane quenches anthracene fluorescence. Features of the system include the remarkable 200:1 difference in fluorescence intensity between the two positional states of the molecule ( approximately 85:1 between one isomer and the photostationary state). In principle the same concept could be used for mechanically switching virtually any property that can be influenced by functional group proximity effects.     

The mechanism of formation of amide-based interlocked compounds: prediction of a new rotaxane-forming motif

Molecular modeling of four different reagent systems shows that the (free) energies of supramolecular interactions in the gas phase and in solution can explain the different reaction products (i.e., various sized macrocycles, catenanes, and linear oligomers) that are formed in classic amide-catenane-forming reactions. Self-assembly of the catenanes requires the formation of ordered intertwined chains and is driven by bifurcated hydrogen bonds, with pi stacking only playing a lesser role. The understanding gained from the computational study was used to predict the possibility of a new rotaxane-forming system that does not permit catenane formation. The predictions were confirmed by the successful synthesis and characterization (including X-ray crystallography) of two novel rotaxanes.     

Shuttling through reversible covalent chemistry

The first stimuli-responsive molecular shuttle that functions through reversible C-C bond formation is reported.     

Artifacts in T(1rho)-weighted imaging: correction with a self-compensating spin-locking pulse

Significant artifacts arise in T(1rho)-weighted imaging when nutation angles suffer small deviations from their expected values. These artifacts vary with spin-locking time and amplitude, severely limiting attempts to perform quantitative imaging or measurement of T(1rho) relaxation times. A theoretical model explaining the origin of these artifacts is presented in the context of a T(1rho)-prepared fast spin-echo imaging sequence. Experimentally obtained artifacts are compared to those predicted by theory and related to B(1) inhomogeneity. Finally, a "self-compensating" spin-locking preparatory pulse cluster is presented, in which the second half of the spin-locking pulse is phase-shifted by 180 degrees. Use of this pulse sequence maintains relatively uniform signal intensity despite large variations in flip angle, greatly reducing artifacts in T(1rho)-weighted imaging.     

Temperature-dependent chemical shift and relaxation times of (23)Na in Na(4)HTm[DOTP

We describe the characterization of a (23)Na temperature-dependent chemical shift and relaxation rates in the complex, Na(4)HTm[DOTP]. This is the first characterization of a (23)Na temperature-dependent chemical shift in a nonmetallic sample. The (23)Na temperature-dependent chemical shift coefficient is approximately -0. 5 PPM/ degrees C for both an aqueous solution and a 6% agarose gel of this compound. This is 50 times the magnitude of the temperature-dependent chemical shift coefficient of water protons. The relaxation times, T(1), T(2f), and T(2s) increased by 0.1, 0.01, and 0.05 ms/ degrees C, respectively. Applications of these unique properties for designing an MRI technique for monitoring heat deposition in tissue and tissue phantoms are discussed.     

Miscibility studies of the smectic phases of trans,trans-4'-alkylbicyclohexyl-4-carbonitrile (CCH) liquid crystals

Complete temperature-composition phase diagrams for binary mixtures of 4'-ethyl-, 4'-propyl-, and 4'-butylbicyclohexyl-4-carbonitrile (CCH-2, CCH-3, and CCH-4, respectively) in each of the three possible combinations have been constructed from differential scanning calorimetry and thermal microscopy data. The highest temperature smectic phases of each of the pure mesogens are immiscible with one another, even though CCH-3 and CCH-4 have both been previously assigned the bilayer crystal-B structure on the basis of X-ray diffraction studies. The present studies indicate that the enantiotropic smectic phase of CCH-4 is slightly higher-ordered than is the monotropic smectic phase of CCH-3. The smectic phase of CCH-2, which previously has been found to be characterized by rhombohedral packing on the basis of X-ray diffraction data, is miscible with the second, previously uncharacterized (monotropic) smectic phase (S₂) of CCH-3. Photographs illustrating the subtle differences in the microscopic textures of these smectic phases are also presented.