[Show abstract][Hide abstract] ABSTRACT: Carrier proteins consume fuel in order to pump ions or molecules across cell membranes, creating concentration gradients. Their control over diffusion pathways, effected entirely through noncovalent bonding interactions, has inspired chemists to devise artificial systems that mimic their function. Here, we report a wholly artificial compound that acts on small molecules to create a gradient in their local concentration. It does so by using redox energy and precisely organized noncovalent bonding interactions to pump positively charged rings from solution and ensnare them around an oligomethylene chain, as part of a kinetically trapped entanglement. A redox-active viologen unit at the heart of a dumbbell-shaped molecular pump plays a dual role, first attracting and then repelling the rings during redox cycling, thereby enacting a flashing energy ratchet mechanism with a minimalistic design. Our artificial molecular pump performs work repetitively for two cycles of operation and drives rings away from equilibrium toward a higher local concentration.
[Show abstract][Hide abstract] ABSTRACT: We report a systematic investigation of size-complementary stoppering groups used to determine the kinetics of threading a cyclophane, namely cyclobis(paraquat-p-phenylene), onto a series of molecular dumbbells. We have identified a set of simple functionalized phenyl and biaryl groups that present activation energy barriers between 16.7 and 26.6 kcal mol-1 to threading the dumbbells. These will be employed as ‘steric speed bumps’ to modulate kinetics in artificial molecular pumps that operate based upon a delicate balance of noncovalent bonding interactions.
[Show abstract][Hide abstract] ABSTRACT: Recently, oxides of Ir(4+) have received renewed attention in the condensed matter physics community, as it has been reported that certain iridates have a strongly spin-orbital coupled (SOC) electronic state, Jeff = ½, that defines the electronic and magnetic properties. The canonical example is the Ruddlesden-Popper compound Sr2IrO4, which has been suggested as a potential route to a new class of high temperature superconductor due to the formal analogy between Jeff = ½ and the S = ½ state of the cuprate superconductors. The quest for other iridium oxides that present tests of the underlying SOC physics is underway. In this spirit, here we report the synthesis and physical properties of two new quaternary tetravalent iridates, MLa10Ir4O24 (M = Sr, Ba). The crystal structure of both compounds features isolated IrO6 octahedra in which the electronic configuration of Ir is d(5). Both compounds order antiferromagnetically despite the lack of obvious superexchange pathways, and resistivity measurement shows that SrLa10Ir4O24 is an insulator.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate strong iodine (I2) vapor adsorption using Mg/Al layered double hydroxide (MgAl-LDH) nanocomposites intercalated with polysulfide (Sx2–) groups (Sx-LDH, x = 2, 4, 6). The as-prepared LDH/polysulfide hybrid materials display highly efficient iodine capture resulting from the reducing property of the intercalated polysulfides. During adsorption, the I2 molecules are reduced to I3– anions by the intercalated [Sx]2– groups that simultaneously are oxidized to form S8. In addition to the chemical adsorption, additional molecular I2 is physically captured by the LDH composites. As a result of these parallel processes, and despite their very low BET surface areas, the iodine capture capacities of S2-LDH, S4-LDH, and S6-LDH are ∼1.32, 1.52, and 1.43 g/g, respectively, with a maximum adsorption of 152% (wt %). Thermogravimetric and differential thermal analysis (TG-DTA), energy dispersive X-ray spectroscopy (EDS), and temperature-variable powder X-ray diffraction (XRD) measurements show the resulting I3– ions that intercalated into the LDH gallery have high thermal stability (≥350 °C). The excellent iodine adsorption performance combined with the facile preparation points to the Sx-LDH systems as potential superior materials for adsorption of radioactive iodine, a waste product of the nuclear power industry.
Chemistry of Materials 12/2014; 26(24):7114-7123. DOI:10.1021/cm5036997 · 8.54 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cs2Hg3S4 was synthesized by slowly cooling a melted stoichiometric mixture of Hg and Cs2S4. Cs2Hg3S4 crystallizes in the Ibam spacegroup with a = 6.278(1) angstrom, b = 11.601(2) angstrom, and c = 14.431(3)angstrom; d(calc) = 6.29 g/cm(3). Its crystal structure consists of straight chains of [Hg3S4](n)(2n-) that engage in side-by-side weak bonding interactions forming layers and are charge balanced by Cs+ cations. The thermal stability of this compound was investigated with differential thermal analysis and temperature dependent in situ synchrotron powder diffraction. The thermal expansion coefficients of the a, b, and c axes were assessed at 1.56 x 10(-5), 2.79 x10(-5), and 3.04 x 10(-5) K-1, respectively. Large single-crystals up to similar to 5 cm in length and similar to 1 cm in diameter were grown using a vertical Bridgman method. Electrical conductivity and photoconductivity measurements on naturally cleaved crystals of Cs2Hg3S4 gave resistivity rho of >= 10(8) Omega.cm and carrier mobility-lifetime (mu tau) products of 4.2 x 10(-4) and 5.82 x 10(-5) cm(2) V-1 for electrons and holes, respectively. Cs2Hg3S4 is a semiconductor with a bandgap E-g similar to 2.8 eV and exhibits photoluminescence (PL) at low temperature. Electronic band structure calculations within the density functional theory (DFT) framework employing the nonlocal hybrid functional within Heyd-Scuseria-Ernzerhof (HSE) formalism indicate a direct bandgap of 2.81 eV at Gamma. The theoretical calculations show that the conduction band minimum has a highly dispersive and relatively isotropic mercury-based s-orbital-like character while the valence band maximum features a much less dispersive and more anisotropic sulfur orbital-based band.
Chemistry of Materials 12/2014; 27(1):370–378. DOI:10.1021/cm504089r · 8.54 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A new method is described to synthesize the semiconductor Cs2Hg6S7 and its alloy with Cd. Using the as-synthesized material, large single crystals have been grown by the Bridgman method under an improved set of crystal growth parameters. In addition, Cd alloying in the form of Cs(2)Hg(6)xCd(x)S(7) (x = 0.25, 0.5, 0.75, etc.) as well as doping with In, Cl was investigated and the influence on the electronic properties was studied. Cd alloying increases the band gap of Cs2Hg6S7 from 1.63 to 1.84 eV. Doping with In and Cl however creates electron carriers and changes p-type samples of Cs2Hg6S7 into n type. A 30-fold increase in the resistivity of the single crystals from 2 X 10(6) to 0.65 X 10(8) Omega cm has been achieved. The carrier mobility-lifetime product of the Cs(2)Hg6S7 crystals has been increased to 1.7 X 10(-3) cm2/V for electrons (mu t)(e) and 2.4 X 10(-3) cm2/V for holes (mu t)(h) (HgCl2 doped). The measured (mu t)e value is comparable to the commercial CdZnTe crystal while the (mu t)h is 10 times higher. Detection of Ag X-ray radiation is demonstrated using the as-grown Cs2Hg6S7 crystals.
[Show abstract][Hide abstract] ABSTRACT: A double solvent replacement method was employed for the synthesis of novel hybrid nanoflowers from boron nitride nanosheets (BNNSs) and the metal–organic framework (MOF) MIL-53 in aqueous solutions under hydrothermal treatments. The strong binding ability of aluminum ions onto the surface of BNNSs determines the 3D flowerlike architectures of the BNNSs/MOFs hybrid, and the BNNSs act as a structure-directing template. The BNNSs/MOFs showed an enhanced catalytic activity in the acetalization of benzaldehyde with methanol owing to the facilitated diffusion process in the hierarchical architectures.
[Show abstract][Hide abstract] ABSTRACT: A challenge in contemporary chemistry is the realization of artificial molecular machines that can perform work in solution on their environments. Here, we report on the design and production of a supramolecular flashing energy ratchet capable of processing chemical fuel generated by redox changes to drive a ring in one direction relative to a dumbbell toward an energetically uphill state. The kinetics of the reaction pathway juxtapose a low energy pseudorotaxane that forms under equilibrium conditions with a high energy, metastable pseudorotaxane which resides away from equilibrium.
Journal of the American Chemical Society 09/2014; 136(42). DOI:10.1021/ja508615f · 11.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report detailed studies showing that the novel layered polysulfide compounds S-x-LDH (S-x(2), polysulfides, x = 2, 4, 5; LDH, MgAl layered double hydroxides) can capture efficiently large quantities of mercury (Hg0) vapor. During the adsorption process, the interlayer polysulfides [S-x](2) react with Hg-0 through their SS bond to produce HgS. The structure of S-x-LDH before and after Hg-adsorption was characterized with X-ray diffraction, vibration spectroscopy, and scanning electron microscopy. The presence of adsorbed Hg was verified by weight gain, inductively coupled plasma atomic emission spectroscopy and X-ray photoelectron spectroscopy. Despite their relatively low surface areas, the S-2-LDH, S-4-LDH, and S-5-LDH samples show excellent Hg capture capacities of 4.9 x 10(5), 7.4 x 10(5), and 1.0 x 10(6) mu g/g, respectively, corresponding to 50-100% adsorption rates by weight, highlighting the potential of these materials in natural gas purification. The Hg-capture efficiency and mechanism in S-x-LDH are supported by control experiments with K2S4, S-8, LDH-NO3-CoS4, and MgAl-NO3-LDH.
Chemistry of Materials 09/2014; 26(17):5004-5011. DOI:10.1021/cm5020477 · 8.54 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Polysulfide [Sx]2− (x = 2, 4) species were intercalated into magnesium–aluminium layered double hydroxide (MgAl–LDH) by a [Sx]2−/NO3− anion-exchange reaction. The resulting Sx–LDH materials exhibit excellent affinity and selectivity for heavy metal ions such as Cu2+, Ag+ and Hg2+. For the highly toxic Hg2+, the distribution coefficient Kd values can reach 107 mL g−1. The Sx–LDH materials rapidly reduce the concentrations of Hg2+ and Ag+ ions in testing solutions from ppm levels to trace levels of ≤1 ppb. A larger series of metal ions were investigated and the selectivity order of Ni2+, Co2+ ≪ Zn2+, Pb2+ < Cd2+ < Cu2+, Ag+, Hg2+ was observed. The Sx–LDH materials show higher selectivity for Cu2+/Zn2 compared to Co2+/Ni2+, providing good separation for these transition metal ions. After ion capture, the LDH hybrid materials retained the original hexagonal prismatic shape and showed good stability under acidic conditions (pH 3). The adsorption process of the metals occurs via M–S bonding. The enhanced environmental stability of the [Sx]2− groups provided by the LDH protective space, the confinement effect offered by the LDH layers, along with the easy accessibility of polysulfide ions to metal ions enable high capture ability and excellent selectivity. The Sx–LDH materials are thus promising as superior sorbents for the decontamination of polluted water.
[Show abstract][Hide abstract] ABSTRACT: The ability to design and confer control over the kinetics of the processes involved in the mechanisms of artificial molecular machines is at the heart of the challenge to create ones that can carry out useful work on their environment, just as Nature is wont to do. As one of the more promising forerunners of prototypical artificial molecular machines, chemists have developed bistable redox-active donor-acceptor mechanically interlocked molecules (MIMs) over the past couple of decades. These bistable MIMs generally come in the form of rotaxanes, molecular compounds that constitute a ring mechanically interlocked around a dumbbell-shaped component, or catenanes, which are composed of two mechanically interlocked rings. As a result of their interlocked nature, bistable MIMs possess the inherent propensity to express controllable intramolecular, large-amplitude, and reversible motions in response to redox stimuli. In this Account, we rationalize the kinetic behavior in the ground state for a large assortment of these types of bistable MIMs, including both rotaxanes and catenanes. These structures have proven useful in a variety of applications ranging from drug delivery to molecular electronic devices. These bistable donorÀacceptor MIMs can switch between two different isomeric states. The favored isomer, known as the ground-state co-conformation (GSCC) is in equilibrium with the less favored metastable state co-conformation (MSCC). The forward (k f) and backward (k b) rate constants associated with this ground-state equilibrium are intimately connected to each other through the ground-state distribution constant, K GS . Knowing the rate constants that govern the kinetics and bring about the equilibration between the MSCC and GSCC, allows researchers to understand the operation of these bistable MIMs in a device setting and apply them toward the construction of artificial molecular machines. The three biggest influences on the ground-state rate constants arise from (i) ground-state effects, the energy required to breakup the noncovalent bonding interactions that stabilize either the GSCC or MSCC, (ii) spacer effects, where the structures overcome additional barriers, either steric or electrostatic or both, en route from one co-conformation to the other, and (iii) the physical environment of the bistable MIMs. By managing all three of these effects, chemists can vary these rate constants over many orders of magnitude. We also discuss progress toward achieving mechanostereoselective motion, a key principle in the design and realization of artificial molecular machines capable of doing work at the molecular level, by the strategic implementation of free energy barriers to intramolecular motion.
Accounts of Chemical Research 02/2014; 47(2):482-493. DOI:10.1021/ar400161z · 24.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The use of high-quality graphene as a local probe in combination with photoexcitation is described to establish a deep mechanistic understanding of charge generation/quenching processes underlying the graphene/environment interface. By combing a no-destructive bottom-up assembly technique with senstive graphene-based transistors, a bistable  rotaxane-graphene hybrid device is produced which exhibits a symmetric mirror-image photoswitching effect with logic capabilities.
[Show abstract][Hide abstract] ABSTRACT: Motor molecules present in nature convert energy inputs, such as a chemical fuel or incident photons of light, into directed motion and force biochemical systems away from thermal equilibrium. The ability, not only to control relative movements of components in molecules, but also and to drive their components preferentially in one direction relative to each other using versatile stimuli, is one of the keys to future technological applications. Herein, we describe a wholly synthetic, small-molecule system which, under the influence of chemical reagents, electrical potential, or visible light, undergoes unidirectional relative translational motion. Altering the redox state of a cyclobis(paraquat-p-phenylene) ring simultaneously (i) inverts the relative heights of kinetic barriers presented by the two termini - one a neutral 2-isopropylphenyl group and the other a positively charged 3,5-dimethylpyridinium unit - of a constitutionally asymmetric dumbbell, which can impair threading/dethreading of a pseudorotaxane, and (ii) controls the ring's affinity for a 1,5-dioxynaphthalene binding site located at the dumbbell's central core. The formation and subsequent dissociation of the pseudorotaxane by passage of the ring over the neutral and positively charged termini of the dumbbell component in one, and only one, direction relatively defined has been demonstrated by (i) spectroscopic (1H NMR and UV/vis) means and cyclic voltammetry, as well as with (ii) DFT calculations and by (iii) comparison with control compounds in the shape of constitutionally symmetrical pseudorotaxanes, one with two positively charged and the other with two neutral ends. Operation of the system relies solely on reversible, yet stable, noncovalent bonding interactions. Moreover, in the presence of a photosensitizer, visible light energy is the only fuel source that is needed to drive the unidirectional molecular translation, making it feasible to repeat the operation numerous times without the buildup of byproducts.
Journal of the American Chemical Society 10/2013; 135(49):18609-18620. DOI:10.1021/ja4094204 · 11.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: After the manner in which co-enzymes often participate in the binding of substrates in the active sites of enzymes, pillararene - a macrocycle containing five hydroquinone rings linked through their para positions by methylene bridges - modifies the binding properties of cucurbituril, such that the latter templates azide-alkyne cycloadditions that do not occur in the presence of only the cucurbituril - a macrocycle comprised of six glycoluril residues doubly linked through their nitrogen atoms to each other by methylene groups. Here, we describe how a combination of pillararene and cucurbituril interacts cooperatively with bipyridinium dications substituted on their nitrogen atoms with 2-azidoethyl- to 5-azidopentyl moieties to afford, as a result of orthogonal templation, two rotaxanes and one rotaxane in > 90% yields inside two hours at 55 oC in acetonitrile. Since the hydroxyl groups on pillararene and the carbonyl groups on cucurbituril form hydrogen bonds readily, these two macrocycles work together in a cooperative fashion to the extent that the four conformational isomers of pillararene can be trapped on the dumbbell components of the rotaxanes. In the case of the rotaxane, it is possible to isolate a compound containing two pillararene rings with local C5 symmetries. In addition to fixing the stereochemistries of the pillararene rings, the regiochemistries associated with the 1,3-dipolar cycloadditions have been extended in their constitutional scope. Under mild conditions, orthogonal recognition motifs have been shown to lead to templation with positive cooperativity that is fast and all but quantitative, as well as being green and efficient.
Journal of the American Chemical Society 09/2013; 135(45). DOI:10.1021/ja407229h · 11.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Flame-melting rapid-cooling reactions were used to synthesize a number of pure phases of the Cs2MIIMIV3Q8 family (MII = Mg, Zn, Cd, Hg; MIV = Ge, Sn; Q = S, Se, Te) whereas the more toxic members were synthesized using a traditional tube furnace synthesis. All Cs2MIIMIV3Q8 compounds presented here crystallize in the noncentrosymmetric space group P212121, except for Cs2ZnGe3S8, which crystallizes in the centrosymmetric space group P21/n. The structures contain chains of corner-sharing MIIQ4 and MIVQ4 tetrahedra linked by edge-sharing MIV2Q6 dimers to give a two-dimensional structure. All phases are structurally similar to the AMIIIMIVQ4 (A = alkali metal, Tl; MIII = Al, Ga, In; MIV = Si, Ge, Sn; Q = S, Se) phases; however, the members of this family have completely ordered MII and MIV sites as opposed to the occupational disorder of MIII and MIV over all tetrahedral sites present in AMIIIMIVQ4. The structural trends of the Cs2MIIMIV3Q8 family are discussed, along with a systematic study of their optical properties. Density functional theory (DFT) electronic structure calculations were performed using the projector augmented wave method to further investigate the trends in the band gaps of the Cs2MIIMIV3Se8 (MII = Mg, Zn; MIV = Ge, Sn) compounds. The experimental diffuse reflectance UV–vis spectroscopy results show that the Mg compounds have smaller band gaps than those containing Zn for both the Ge and the Sn families whereas the DFT calculations show the opposite trend. Cs2HgSn3Se8 was studied as a representative example of this family using differential thermal analysis and melts congruently at 595 °C. Crystal growth of this compound using the Bridgman method resulted in a polycrystalline ingot from which plate crystals 2 mm × 3 mm could be cleaved. The band gap of the compounds varies from a narrow 1.07 eV for Cs2ZnGe3Te8 to a wide 3.3 eV for Cs2ZnGe3S8 and Cs2CdGe3S8 making this family a potentially useful source of materials for a variety of electronic applications. Cs2HgSn3Se8 crystals exhibit photoconductivity response where the photoexcited electron and hole show mobility-lifetime products on the order of 3.69 × 10–5 cm2/V and (μτ)h = 7.78 × 10–5 cm2/V, respectively.