Hao Li

Argonne National Laboratory, Lemont, Illinois, United States

Are you Hao Li?

Claim your profile

Publications (44)366.37 Total impact

  • [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.
    Tetrahedron Letters 01/2015; DOI:10.1016/j.tetlet.2015.01.169 · 2.39 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
  • Source
  • [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.
    10/2014; 2(44). DOI:10.1039/C4TA04230A
  • Source
    [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 [2]pseudorotaxane that forms under equilibrium conditions with a high energy, metastable [2]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.
    06/2014; 2(26). DOI:10.1039/C4TA01203H
  • Source
    [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 [2]rotaxanes, molecular compounds that constitute a ring mechanically interlocked around a dumbbell-shaped component, or [2]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
  • Source
    [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 [2] rotaxane-graphene hybrid device is produced which exhibits a symmetric mirror-image photoswitching effect with logic capabilities.
    Advanced Materials 12/2013; 25(46). DOI:10.1002/adma.201302393 · 15.41 Impact Factor
  • Source
    [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 [2]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 [2]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 [2]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). 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, pillar[5]arene - a macrocycle containing five hydroquinone rings linked through their para positions by methylene bridges - modifies the binding properties of cucurbit[6]uril, such that the latter templates azide-alkyne cycloadditions that do not occur in the presence of only the cucurbit[6]uril - 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 pillar[5]arene and cucurbit[6]uril 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 [4]rotaxanes and one [5]rotaxane in > 90% yields inside two hours at 55 oC in acetonitrile. Since the hydroxyl groups on pillar[5]arene and the carbonyl groups on cucurbit[6]uril form hydrogen bonds readily, these two macrocycles work together in a cooperative fashion to the extent that the four conformational isomers of pillar[5]arene can be trapped on the dumbbell components of the [4]rotaxanes. In the case of the [5]rotaxane, it is possible to isolate a compound containing two pillar[5]arene rings with local C5 symmetries. In addition to fixing the stereochemistries of the pillar[5]arene 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.
    ChemInform 08/2013; 25(16):3344–3356. DOI:10.1021/cm401817r
  • [Show abstract] [Hide abstract]
    ABSTRACT: The chemical concept of lattice hybridization was applied to identify new chalcohalide compounds as candidates for X-ray and γ-ray detection. Per this approach, compound semiconductor materials with high density and wide band gaps can be produced that can absorb and detect hard radiation. Here, we show that the mixed chalcogenide–halide compound Tl6SI4 is a congruently melting, mechanically robust chalcohalide material with strong photoconductivity response and an impressive room-temperature figure of merit. Tl6SI4 crystallizes in the tetragonal P4/mnc space group, with a = 9.1758(13) Å, c = 9.5879(19) Å, V = 807.3(2) Å3, and a calculated density of 7.265 g·cm–3. The new material requires a more simplified crystal growth compared to the leading system Cd0.9Zn0.1Te, which is the benchmark room-temperature hard radiation detector material. We successfully synthesized Tl6SI4 crystals to produce detector-grade wafers with high resistivity values (1010 Ω·cm) and high-resolution detection of X-ray spectra from an Ag (22 keV) source.
    Chemistry of Materials 07/2013; 25(14):2868–2877. DOI:10.1021/cm401406j · 8.54 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Two new compounds CsCdInQ3 (Q = Se, Te) have been synthesized using a polychalcogenide flux. CsCdInQ3 (Q = Se, Te) crystals are promising candidates for X-ray and γ-ray detection. The compounds crystallize in the monoclinic C2/c space group with a layered structure, which is related to the CsInQ2 (Q = Se, Te) ternary compounds. The cell parameters are: a = 11.708(2) Å, b = 11.712(2) Å, c = 23.051(5) Å, β = 97.28(3)° for CsCdInSe3 and a = 12.523(3) Å, b = 12.517(3) Å, c = 24.441(5) Å, β = 97.38(3)° for CsCdInTe3. Both the Se and Te analogues are wide-band-gap semiconductors with optical band gaps of 2.4 and 1.78 eV for CsCdInSe3 and CsCdInTe3, respectively. High-purity polycrystalline raw material for crystal growth was synthesized by the vapor transfer method for CsCdInQ3. Large single crystals up to 1 cm have been grown using the vertical Bridgman method and exhibit photoconductive response. The electrical resistivity of the crystals is highly anisotropic. The electronic structure calculation within the density functional theory (DFT) framework indicates a small effective mass for the carriers. Photoconductivity measurements on the as grown CsCdInQ3 crystals gives high carrier mobility-lifetime (μτ) products comparable to other detector materials such as α-HgI2, PbI2, and CdxZn1–xTe (CZT).
    Chemistry of Materials 05/2013; 25(10):2089–2099. DOI:10.1021/cm400634v · 8.54 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Previous efforts to enhance thermoelectric performance have primarily focused on reduction in lattice thermal conductivity caused by broad-based phonon scattering across multiple length scales. Herein, we demonstrate a design strategy which provides for simultaneous improvement of electrical and thermal properties of p-type PbSe and leads to ZT ∼ 1.6 at 923 K, the highest ever reported for a tellurium-free chalcogenide. Our strategy goes beyond the recent ideas of reducing thermal conductivity by adding two key new theory-guided concepts in engineering, both electronic structure and band alignment across nanostructure-matrix interface. Utilizing density functional theory for calculations of valence band energy levels of nanoscale precipitates of CdS, CdSe, ZnS, and ZnSe, we infer favorable valence band alignments between PbSe and compositionally alloyed nanostructures of CdS1-xSex/ZnS1-xSex. Then by alloying Cd on the cation sublattice of PbSe, we tailor the electronic structure of its two valence bands (light hole L and heavy hole Σ) to move closer in energy, thereby enabling the enhancement of the Seebeck coefficients and the power factor.
    Journal of the American Chemical Society 05/2013; 135(19). DOI:10.1021/ja403134b · 11.44 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The new layered title compound is synthesized by solid state reaction of In, HgS, Cs2S, and sulfur (silica tube, 600 °C for 24 h, 800 °C for 8 h; 70% yield).
    ChemInform 02/2013; 44(9):no-no. DOI:10.1002/chin.201309007
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We describe in detail a strategy for creating foldamers in which interactions between mechanically interlocked components dictate the single-molecule assembly of a folded secondary structure. This unique folding motif is based on a flexible polyether dumbbell bearing 1,5-dioxynaphthalene (DNP) donors, which folds its way through a series of cyclobis(paraquat-p-phenylene) (CBPQT4+) acceptor rings in a serpentine fashion to enable extended donor-acceptor (D-A) stacking between DNP and the electron-poor 4,4'-bipyridinium (BIPY2+) units in CBPQT4+. These oligorotaxanes can be prepared in a wide range of sizes, with molecular weights up to >15000 Da, on account of novel one-pot reactions we developed to generate the necessary oligo-DNP precursors. The product distributions from the final kinetically-controlled stoppering reactions are highly biased towards oligorotaxanes in which approximately half of the DNP units are encircled by rings, a fact which can be rationalized if the dominant solution-state struc
    Chemical Science 02/2013; 4:1470-1483. DOI:10.1039/C3SC00015J · 8.60 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Most organic radicals possess short lifetimes and quickly undergo dimerization or oxidation. Here, we report on the synthesis by radical templation of a class of air- and water-stable organic radicals, trapped within a homo[2]catenane composed of two rigid and fixed cyclobis(paraquat-p-phenylene) rings. The highly energetic octacationic homo[2]catenane, which is capable of accepting up to eight electrons, can be configured reversibly, both chemically and electrochemically, between each one of six experimentally accessible redox states (0, 2+, 4+, 6+, 7+, and 8+) from within the total of nine states evaluated by quantum mechanical methods. All six of the observable redox states have been identified by electrochemical techniques, three (4+, 6+, and 7+) have been characterized by x-ray crystallography, four (4+, 6+, 7+, and 8+) by electron paramagnetic resonance spectroscopy, one (7+) by superconducting quantum interference device magnetometry, and one (8+) by nuclear magnetic resonance spectroscopy.
    Science 01/2013; 339(6118):429-433. DOI:10.1126/science.1228429 · 31.48 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: In one fell swoop, polyrotaxanes comprising up to 64 rings can be synthesized as a result of cucurbit[6]uril-templated 1,3-dipolar azide-alkyne cycloadditions accelerated in the presence of cyclodextrins as a consequence of self-sorting and positive cooperativity, brought about by hydrogen bonding. Mixing six components in one pot affords a hetero[4]rotaxane in one minute in quantitative yield.
    Angewandte Chemie International Edition 01/2013; 52(1). DOI:10.1002/anie.201205087 · 11.34 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A homologous series of [2]rotaxanes, in which cyclobis(paraquat-p-phenylene) (CBPQT4+) serves as the ring component, while the dumbbell components all contain single 4,4'-bipyridinium (BIPY2+) units centrally located in the midst of oligomethylene chains of varying lengths, have been synthesized by taking advantage of radical templation and copper-free azide-alkyne 1,3-dipolar cycloadditions in the formation of their stoppers. Cyclic voltammetry, UV/Vis spectroscopy and mass spectrometry reveal that the BIPY•+ radical cations in this series of [2]rotaxanes are stabilized against oxidation, both electrochemically and by atmospheric oxygen. The enforced proximity between the BIPY2+ units in the ring and dumbbell components gives rise to enhanced Coulombic repulsion, destabilizing the ground-state co-conformations of the fully oxidized forms of these [2]rotaxanes. The smallest [2]rotaxane with only three methylene groups on each side of its dumbbell component is found to exist under ambient conditions in a monoradical state, a situation which does not persist in acetonitrile solution at least in the case of its longer analogues. 1H NMR Spectroscopy reveals that the activation energy barriers to the shuttling of the CBPQT4+ rings over the BIPY2+ units in the dumbbells rise linearly with increasing oligomethylene chain lengths across the series of [2]rotaxanes. These findings provide a new way of producing highly stabilized BIPY•+ radical cations and open up more opportunities to use stable organic radicals as building blocks for the construction of paramagnetic materials and conductive molecular electronic devices.
    Journal of the American Chemical Society 11/2012; 135(1). DOI:10.1021/ja310060n · 11.44 Impact Factor

Publication Stats

527 Citations
366.37 Total Impact Points

Institutions

  • 2014
    • Argonne National Laboratory
      • Division of Materials Science
      Lemont, Illinois, United States
  • 2009–2014
    • Northwestern University
      • Department of Chemistry
      Evanston, Illinois, United States
  • 2012–2013
    • Northwest University
      Evanston, Illinois, United States