Towards the molecular workshop: entropy-driven designer molecules, entropy activation, and nanomechanical devices

We introduce some basic concepts for designer molecules with functional units which are driven by entropic rather than energetic forces. This idea profits from the mechanically interlocked nature of topological molecules such as catenanes and rotaxanes, which allows for mobile elements whose accessible configuration space gives rise to entropic intramolecular forces. Such entropy-driven designer molecules open the possibility for externally controllable functional molecules and nanomechanical devices.Comment: 4 pages, 4 figure

Influence of the synthetic method on the properties of two-photon-sensitive mesoporous organosilica nanoparticles

International audienceHerein we report the modulation of the properties of mesoporous silica nanoparticles (NPs) via various synthetic approaches. Three types of elaborations were compared, one in aqueous media at 25 °C, and the other two at 80 °C in water or in a water–ethanol mixture. For all these methods, an alkoxysilylated two-photon photosensitizer (2PS) was co-condensed with tetraethylorthosilicate (TEOS) in the presence of cetyltrimethylammonium bromide (CTAB), leading to five two-photon-sensitive mesoporous silica (M2PS) NPs. The M2PS NP porous structure could be tuned from radial to worm-like and MCM-41 types of organization. Besides, the 2PS precursor spatial dispersion was found to be highly dependent on both the 2PS initial concentration and the elaboration process. As a result, two-photon properties were modulated by the choice of the synthesis, the best results being found in aqueous media at 25 or 80 °C. Finally, the M2PS NPs were used for in vitro two-photon imaging of cancer cells

Synthesis of Cyclen-Functionalized Ethenylene-Based Periodic Mesoporous Organosilica Nanoparticles and Metal-Ion Adsorption Studies

The preparation of two cyclens both possessing two triethoxysilyl groups through click chemistry is described. These two cyclens were incorporated into bis(triethoxysilyl)ethenylene-based periodic mesoporous organosilica nanoparticles (PMO NPs) at different proportions of bis(triethoxysilyl)ethenylene/cyclens (90/10, 75/25). The obtained nanorods were analyzed with different techniques and showed high specific surface areas at low proportion of cyclens. The nanorods containing free amino groups of cyclen were then used for Ni(II) and Co(II) removal from model solutions. The kinetics and isotherms of adsorption of Ni(II) and Co(II) were determined, and the materials showed high uptake of metals (up to 3.9 mmol . g(-1)). They demonstrated pronounced selectivity in separation of rare earth elements from late transition metals, e. g. Ni(II) and Co(II) by adsorption and even more so by controlled desorption

Bis{μ-1,3-bis­[(benzimidazol-1-yl)meth­yl]benzene-κ2 N 3:N 3′}bis­[dichloridozinc(II)] dimethyl­formamide disolvate

In the title compound, [Zn2Cl4(C22H18N4)2]·2C3H7NO, the 1,3-bis­[(benzimidazol-1-yl)meth­yl]benzene ligand bridges two ZnCl2 units, forming a centrosymmetric dinuclear mol­ecule. The ZnII atom shows a distorted tetra­hedral coordination within a Cl2N2 donor set

The Solution Chemistry of Cu2+–tren Complexes Revisited: Exploring the Role of Species That Are Not Trigonal Bipyramidal

Potentiometric and spectrophotometric titrations indicate that aqueous solutions that contain equimolar amounts of Cu2+ and tren contain the HCuL3+, CuL2+ and CuL(OH)+ species and that their relative concentrations depend on the pH of the solution. The stability constants and the UV/Vis and EPR spectra of the three species have been determined. The position of the absorption maximum clearly corresponds to a trigonal bipyramidal (tbp) geometry for CuL2+, whereas for HCuL3+ and CuL(OH)+ there are also bands that could correspond to square pyramidal (sp) complexes, but the EPR spectra indicate that only HCuL3+ can be considered to be sp. When any of these species is mixed with an excess of acid, an intermediate is formed within the mixing time of the stopped-flow technique. This intermediate undergoes complete decomposition in a second slower step. Interestingly, the spectrum of this intermediate is typical of sp geometry. Kinetic studies on complex formation in general indicate that complexation occurs in a single step, although under certain conditions an additional step has been observed that probably corresponds to the conversion of CuL2+ to HCuL3+, and the spectral changes indicate that the process involves structural reorganization from tbp to sp geometry. DFT and TDDFT calculations have been carried out for the three stable species, as well as for species in a higher protonation state. The results indicate that CuL2+ exists as a species with tetradentate tren and tbp geometry, although a wide range of distortions between the ideal tbp geometry and a geometry closer to sp is possible with a very modest energy cost. The energy change associated with hydrolysis of one of the Cu– N bonds to give a species with tridentate tren was found to be slightly higher than that previously found for a related ligand, which contains a substituent at one of the terminal amino groups. For CuL(OH)+, the calculations suggest that an equilibrium exists between species with essentially the same energy but different geometries, each one of the species is closer to one of the ideal tbp and sp limits. For HCuL3+, the relevance of the sp geometry was confirmed by the calculations

Organic nanofibers embedding stimuli-responsive threaded molecular components

While most of the studies on molecular machines have been performed in solution, interfacing these supramolecular systems with solid-state nanostructures and materials is very important in view of their utilization in sensing components working by chemical and photonic actuation. Host polymeric materials, and particularly polymer nanofibers, enable the manipulation of the functional molecules constituting molecular machines, and provide a way to induce and control the supramolecular organization. Here, we present electrospun nanocomposites embedding a self-assembling rotaxane-type system that is responsive to both optical (UV-visible light) and chemical (acid/base) stimuli. The system includes a molecular axle comprised of a dibenzylammonium recognition site and two azobenzene end groups, and a dibenzo[24]crown-8 molecular ring. The dethreading and rethreading of the molecular components in nanofibers induced by exposure to base and acid vapors, as well as the photoisomerization of the azobenzene end groups, occur in a similar manner to what observed in solution. Importantly, however, the nanoscale mechanical function following external chemical stimuli induces a measurable variation of the macroscopic mechanical properties of nanofibers aligned in arrays, whose Young's modulus is significantly enhanced upon dethreading of the axles from the rings. These composite nanosystems show therefore great potential for application in chemical sensors, photonic actuators and environmentally responsive materials.Comment: 39 pages, 16 figure

Donor-Acceptor Oligorotaxanes Made to Order

Five donor acceptor oligorotaxanes made up of dumbbells composed of tetraethylene glycol chains, interspersed with three and five 1,5-dioxynaphthalene units, and terminated by 2,6-diisopropylphenoxy stoppers, have been prepared by the threading of discrete numbers of cyclobis(paraquat-p-phenylene) rings, followed by a kinetically controlled stoppering protocol that relies on click chemistry. The well-known copper(I)-catalyzed alkyne-azide cycloaddition between azide functions placed at the ends of the polyether chains and alkyne-bearing stopper precursors was employed during the final kinetically controlled template-directed synthesis of the five oligorotaxanes, which were characterized subsequently by (1)H NMR spectroscopy at low temperature (233 K) in deuterated acetonitrile. The secondary structures, as well as the conformations, of the five oligorotaxanes were unraveled by spectroscopic comparison with the dumbbell and ring components. By focusing attention on the changes in chemical shifts of some key probe protons, obtained from a wide range of low-temperature spectra, a picture emerges of a high degree of folding within the thread protons of the dumbbells of four of the five oligorotaxanes-the fifth oligorotaxane represents a control compound in effect-brought about by a combination of C-H center dot center dot center dot O and pi-pi stacking interactions between the pi-electron-deficient bipyridinium units in the rings and the pi-electron-rich 1,5-dioxynaphthalene units and polyether chains in the dumbbells. The secondary structures of a foldamer-like nature have received further support from a solid-state superstructure of a related [3]pseudorotaxane and density functional calculations performed thereon.

Template-Directed Synthesis of Multiply Mechanically Interlocked Molecules Under Thermodynamic Control

The template-directed construction of crown-ether-like macrocycles around secondary dialkylammonium ions (R2NH2+) has been utilized for the expedient (one-pot) and highyielding synthesis of a diverse range of mechanically interlocked molecules. The clipping together of appropriately designed dialdehyde and diamine compounds around R2NH2+-containing dumbbell-shaped components proceeds through the formation, under thermodynamic control, of imine bonds. The reversible nature of this particular reaction confers the benefits of “errorchecking” and “proof-reading”, which one usually associates with supramolecular chemistry and strict self-assembly processes, upon these wholly molecular systems. Furthermore, these dynamic covalent syntheses exploit the efficient templating effects that the R2NH2+ ions exert on the macrocyclization of the matched dialdehyde and diamine fragments, resulting not only in rapid rates of reaction, but also affording near-quantitative conversion of starting materials into the desired interlocked products. Once assembled, these “dynamic” interlocked compounds can be “fixed” upon reduction of the reversible imine bonds (by using BH3·THF) to give kinetically stable species, a procedure that can be performed in the same reaction vessel as the inital thermodynamically controlled assembly. Isolation and purification of the mechanically interlocked products formed by using this protocol is relatively facile, as no column chromatography is required. Herein, we present the synthesis and characterization of 1) a [2]rotaxane, 2) a [3]rotaxane, 3) a branched [4]rotaxane, 4) a bis [2]rotaxane, and 5) a novel cyclic [4]rotaxane, demonstrating, in incrementally more complex systems, the efficacy of this one-pot strategy for the construction of interlocked molecules