Hao Li

Northwestern University, Evanston, IL, United States

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Publications (25)279.28 Total impact

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    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; · 10.68 Impact Factor
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    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; · 10.68 Impact Factor
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    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 09/2013; · 14.83 Impact Factor
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    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; · 10.68 Impact Factor
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    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. · 8.31 Impact Factor
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    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. · 31.20 Impact Factor
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    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; · 10.68 Impact Factor
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    ABSTRACT: Semiconductor γ-ray detectors have broad applications, yet finding superior detector materials is a challenge because of its contradictory requirements. Here, we investigated a large set of native defects in Cs2Hg6S7 that has been suggested as a promising candidate for detector materials. Using first-principles calculations, we showed that S-vacancy and HgCs-antisite defect provide life-time limiting deep levels, and Cs-vacancy forms a shallow acceptor level, resulting in low resistivity. To decrease such detrimental effects, concentrations of defects and carriers were examined in various chemical environments, which reveal that carrier densities can be extremely reduced by adjusting Cs partial pressure.
    Applied Physics Letters 11/2012; 101(20). · 3.79 Impact Factor
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    ABSTRACT: The heavy element semiconductor compound Cs2Hg6S7 is of interest as a potential wide gap semiconductor for gamma ray detection. To determine electrically active defects and their energy levels, photoconductivity (PC) spectroscopy was carried out over the temperature range of 90-295 K. The low temperature spectrum exhibits photoconductive transitions at 1.495, 1.61, 1.66, and 1.68 eV. The optical transitions are tentatively attributed to defects with levels located at energies of 50, 70, 120, and 240 meV from the band edge. A superlinear dependence of photocurrent on illumination intensity is observed that is attributed to a two-center recombination process that involves shallow traps and recombination centers. Near band edge photoluminescence (PL) was observed over the temperature range of 24–80 K. The spectrum revealed three defect related emission bands located at 1.68, 1.66, and 1.62 eV, whose ionization energies are 57 meV, 78 meV, and 115 meV, respectively. From the temperature and excitation dependencies of the observed peak intensities and energies, the radiative recombination mechanisms of the bands were attributed to transitions involving excitons bound to neutral and ionized acceptors. Good agreement of the defect level energies determined by PL and PC were noted, indicating that they were of the same origin. The defects were tentatively attributed to metal vacancies that form shallow acceptor levels.
    Journal of Applied Physics 09/2012; 112(6). · 2.21 Impact Factor
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    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 09/2012; · 13.73 Impact Factor
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    ABSTRACT: Two redox-active bistable [2]catenanes composed of macrocyclic polyethers of different sizes incorporating both electron-rich 1,5-dioxynaphthalene (DNP) and electron-deficient 4,4'-bipyridinium (BIPY(2+)) units, interlocked mechanically with the tetracationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT(4+)), were obtained by donor-acceptor template-directed syntheses in a threading-followed-by-cyclization protocol employing Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloadditions in the final mechanical-bond forming steps. These bistable [2]catenanes exemplify a design strategy for achieving redox-active switching between two translational isomers, which are driven (i) by donor-acceptor interactions between the CBPQT(4+) ring and DNP, or (ii) radical-radical interactions between CBPQT(2(•+)) and BIPY(•+), respectively. The switching processes, as well as the nature of the donor-acceptor interactions in the ground states and the radical-radical interactions in the reduced states, were investigated by single-crystal X-ray crystallography, dynamic (1)H NMR spectroscopy, cyclic voltammetry, UV/vis spectroelectrochemistry, and electron paramagnetic resonance (EPR) spectroscopy. The crystal structure of one of the [2]catenanes in its trisradical tricationic redox state provides direct evidence for the radical-radical interactions which drive the switching processes for these types of mechanically interlocked molecules (MIMs). Variable-temperature (1)H NMR spectroscopy reveals a degenerate rotational motion of the BIPY(2+) units in the CBPQT(4+) ring for both of the two [2]catenanes, that is governed by a free energy barrier of 14.4 kcal mol(-1) for the larger catenane and 17.0 kcal mol(-1) for the smaller one. Cyclic voltammetry provides evidence for the reversibility of the switching processes which occurs following a three-electron reduction of the three BIPY(2+) units to their radical cationic forms. UV/vis spectroscopy confirms that the processes driving the switching are (i) of the donor-acceptor type, by the observation of a 530 nm charge-transfer band in the ground state, and (ii) of the radical-radical ilk in the switched state as indicated by an intense visible absorption (ca. 530 nm) and near-infrared (ca. 1100 nm) bands. EPR spectroscopic data reveal that, in the switched state, the interacting BIPY(•+) radical cations are in a fast exchange regime. In general, the findings lay the foundations for future investigations where this radical-radical recognition motif is harnessed in bistable redox-active MIMs in order to achieve close to homogeneous populations of co-conformations in both the ground and switched states.
    Journal of the American Chemical Society 07/2012; 134(28):11709-20. · 10.68 Impact Factor
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    ABSTRACT: It's just an illusion: Above a critical chain length, where oligomers contain five or more recognition units, apparently infinite donor-acceptor polypseudorotaxanes are formed in the solid state (see picture). X-ray crystallographic analyses of three different examples have shown that although the oligomeric chains are undoubtedly discrete and monodisperse, they nevertheless appear to be infinite in the crystal.
    Angewandte Chemie International Edition 06/2012; 51(29):7231-5. · 13.73 Impact Factor
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    ABSTRACT: CsSnI(3) is an unusual perovskite that undergoes complex displacive and reconstructive phase transitions and exhibits near-infrared emission at room temperature. Experimental and theoretical studies of CsSnI(3) have been limited by the lack of detailed crystal structure characterization and chemical instability. Here we describe the synthesis of pure polymorphic crystals, the preparation of large crack-/bubble-free ingots, the refined single-crystal structures, and temperature-dependent charge transport and optical properties of CsSnI(3), coupled with ab initio first-principles density functional theory (DFT) calculations. In situ temperature-dependent single-crystal and synchrotron powder X-ray diffraction studies reveal the origin of polymorphous phase transitions of CsSnI(3). The black orthorhombic form of CsSnI(3) demonstrates one of the largest volumetric thermal expansion coefficients for inorganic solids. Electrical conductivity, Hall effect, and thermopower measurements on it show p-type metallic behavior with low carrier density, despite the optical band gap of 1.3 eV. Hall effect measurements of the black orthorhombic perovskite phase of CsSnI(3) indicate that it is a p-type direct band gap semiconductor with carrier concentration at room temperature of ∼ 10(17) cm(-3) and a hole mobility of ∼585 cm(2) V(-1) s(-1). The hole mobility is one of the highest observed among p-type semiconductors with comparable band gaps. Its powders exhibit a strong room-temperature near-IR emission spectrum at 950 nm. Remarkably, the values of the electrical conductivity and photoluminescence intensity increase with heat treatment. The DFT calculations show that the screened-exchange local density approximation-derived band gap agrees well with the experimentally measured band gap. Calculations of the formation energy of defects strongly suggest that the electrical and light emission properties possibly result from Sn defects in the crystal structure, which arise intrinsically. Thus, although stoichiometric CsSnI(3) is a semiconductor, the material is prone to intrinsic defects associated with Sn vacancies. This creates highly mobile holes which cause the materials to appear metallic.
    Journal of the American Chemical Society 05/2012; 134(20):8579-87. · 10.68 Impact Factor
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    ABSTRACT: Something to remember: A rotaxane could be operated as a bistable memory element under kinetic control. Data was written on the molecule by an oxidation stimulus and locked by UV light irradiation. After the writing session, the oxidized species was reduced back to the original form without losing the written data for a few hours. Light irradiation also protects nonoxidized rotaxanes from accidental writing.
    Angewandte Chemie International Edition 01/2012; 51(7):1611-5. · 13.73 Impact Factor
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    ABSTRACT: The ability of the diradical dicationic cyclobis(paraquat-p-phenylene) (CBPQT(2(•+))) ring to form inclusion complexes with 1,1'-dialkyl-4,4'-bipyridinium radical cationic (BIPY(•+)) guests has been investigated mechanistically and quantitatively. Two BIPY(•+) radical cations, methyl viologen (MV(•+)) and a dibutynyl derivative (V(•+)), were investigated as guests for the CBPQT(2(•+)) ring. Both guests form trisradical complexes, namely, CBPQT(2(•+))⊂MV(•+) and CBPQT(2(•+))⊂V(•+), respectively. The structural details of the CBPQT(2(•+))⊂MV(•+) complex, which were ascertained by single-crystal X-ray crystallography, reveal that MV(•+) is located inside the cavity of the ring in a centrosymmetric fashion: the 1:1 complexes pack in continuous radical cation stacks. A similar solid-state packing was observed in the case of CBPQT(2(•+)) by itself. Quantum mechanical calculations agree well with the superstructure revealed by X-ray crystallography for CBPQT(2(•+))⊂MV(•+) and further suggest an electronic asymmetry in the SOMO caused by radical-pairing interactions. The electronic asymmetry is maintained in solution. The thermodynamic stability of the CBPQT(2(•+))⊂MV(•+) complex was probed by both isothermal titration calorimetry (ITC) and UV/vis spectroscopy, leading to binding constants of (5.0 ± 0.6) × 10(4) M(-1) and (7.9 ± 5.5) × 10(4) M(-1), respectively. The kinetics of association and dissociation were determined by stopped-flow spectroscopy, yielding a k(f) and k(b) of (2.1 ± 0.3) × 10(6) M(-1) s(-1) and 250 ± 50 s(-1), respectively. The electrochemical mechanistic details were studied by variable scan rate cyclic voltammetry (CV), and the experimental data were compared digitally with simulated data, modeled on the proposed mechanism using the thermodynamic and kinetic parameters obtained from ITC, UV/vis, and stopped-flow spectroscopy. In particular, the electrochemical mechanism of association/dissociation involves a bisradical tetracationic intermediate CBPQT((2+)(•+))⊂V(•+) inclusion complex; in the case of the V(•+) guest, the rate of disassociation (k(b) = 10 ± 2 s(-1)) was slow enough that it could be detected and quantified by variable scan rate CV. All the experimental observations lead to the speculation that the CBPQT((2+)(•+)) ring of the bisradical tetracation complex might possess the unique property of being able to recognize both BIPY(•+) radical cation and π-electron-rich guests simultaneously. The findings reported herein lay the foundation for future studies where this radical-radical recognition motif is harnessed particularly in the context of mechanically interlocked molecules and increases our fundamental understanding of BIPY(•+) radical-radical interactions in solution as well as in the solid-state.
    Journal of the American Chemical Society 12/2011; 134(6):3061-72. · 10.68 Impact Factor
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    ABSTRACT: In donor-acceptor mechanically interlocked molecules that exhibit bistability, the relative populations of the translational isomers--present, for example, in a bistable [2]rotaxane, as well as in a couple of bistable [2]catenanes of the donor-acceptor vintage--can be elucidated by slow scan rate cyclic voltammetry. The practice of transitioning from a fast scan rate regime to a slow one permits the measurement of an intermediate redox couple that is a function of the equilibrium that exists between the two translational isomers in the case of all three mechanically interlocked molecules investigated. These intermediate redox potentials can be used to calculate the ground-state distribution constants, K. Whereas, (i) in the case of the bistable [2]rotaxane, composed of a dumbbell component containing π-electron-rich tetrathiafulvalene and dioxynaphthalene recognition sites for the ring component (namely, a tetracationic cyclophane, containing two π-electron-deficient bipyridinium units), a value for K of 10 ± 2 is calculated, (ii) in the case of the two bistable [2]catenanes--one containing a crown ether with tetrathiafulvalene and dioxynaphthalene recognition sites for the tetracationic cyclophane, and the other, tetrathiafulvalene and butadiyne recognition sites--the values for K are orders (one and three, respectively) of magnitude greater. This observation, which has also been probed by theoretical calculations, supports the hypothesis that the extra stability of one translational isomer over the other is because of the influence of the enforced side-on donor-acceptor interactions brought about by both π-electron-rich recognition sites being part of a macrocyclic polyether.
    Proceedings of the National Academy of Sciences 11/2011; 108(51):20416-21. · 9.74 Impact Factor
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    ABSTRACT: Lead sulfide, a compound consisting of elements with high natural abundance, can be converted into an excellent thermoelectric material. We report extensive doping studies, which show that the power factor maximum for pure n-type PbS can be raised substantially to ~12 μW cm(-1) K(-2) at >723 K using 1.0 mol % PbCl(2) as the electron donor dopant. We also report that the lattice thermal conductivity of PbS can be greatly reduced by adding selected metal sulfide phases. The thermal conductivity at 723 K can be reduced by ~50%, 52%, 30%, and 42% through introduction of up to 5.0 mol % Bi(2)S(3), Sb(2)S(3), SrS, and CaS, respectively. These phases form as nanoscale precipitates in the PbS matrix, as confirmed by transmission electron microscopy (TEM), and the experimental results show that they cause huge phonon scattering. As a consequence of this nanostructuring, ZT values as high as 0.8 and 0.78 at 723 K can be obtained for nominal bulk PbS material. When processed with spark plasma sintering, PbS samples with 1.0 mol % Bi(2)S(3) dispersion phase and doped with 1.0 mol % PbCl(2) show even lower levels of lattice thermal conductivity and further enhanced ZT values of 1.1 at 923 K. The promising thermoelectric properties promote PbS as a robust alternative to PbTe and other thermoelectric materials.
    Journal of the American Chemical Society 11/2011; 133(50):20476-87. · 10.68 Impact Factor
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    ABSTRACT: A dynamic smart surface which was constructed by the self-assembly of an azobenzene-functionalised pillar[5]arene exhibits reversibly responsive morphologies towards UV and visible light as observed by TEM, SEM and AFM.
    Chemical Communications 09/2011; 47(41):11420-2. · 6.38 Impact Factor
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    Advanced Materials 08/2011; 23(36):4163-7. · 14.83 Impact Factor
  • Angewandte Chemie International Edition 07/2011; · 13.73 Impact Factor