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Wu J.,CAS Beijing National Laboratory for Molecular
Nature Nanotechnology | Year: 2017

High-mobility semiconducting ultrathin films form the basis of modern electronics, and may lead to the scalable fabrication of highly performing devices. Because the ultrathin limit cannot be reached for traditional semiconductors, identifying new two-dimensional materials with both high carrier mobility and a large electronic bandgap is a pivotal goal of fundamental research. However, air-stable ultrathin semiconducting materials with superior performances remain elusive at present. Here, we report ultrathin films of non-encapsulated layered Bi2O2Se, grown by chemical vapour deposition, which demonstrate excellent air stability and high-mobility semiconducting behaviour. We observe bandgap values of ∼0.8 eV, which are strongly dependent on the film thickness due to quantum-confinement effects. An ultrahigh Hall mobility value of >20,000 cm2 V-1 s-1 is measured in as-grown Bi2O2Se nanoflakes at low temperatures. This value is comparable to what is observed in graphene grown by chemical vapour deposition and at the LaAlO3–SrTiO3 interface, making the detection of Shubnikov–de Haas quantum oscillations possible. Top-gated field-effect transistors based on Bi2O2Se crystals down to the bilayer limit exhibit high Hall mobility values (up to 450 cm2 V-1 s-1), large current on/off ratios (>106) and near-ideal subthreshold swing values (∼65 mV dec1) at room temperature. Our results make Bi2O2Se a promising candidate for future high-speed and low-power electronic applications. © 2017 Nature Publishing Group

Jia Y.,CAS Beijing National Laboratory for Molecular | Li J.,CAS Beijing National Laboratory for Molecular
Chemical Reviews | Year: 2015

Researchers specifically provide information about covalent molecular assembly with the layer-by-layer (LbL) method through Schiff base interactions, including properties and advantages of Schiff base interactions, the micro- and nanostructures fabricated through Schiff base interactions, and the biomedical applications of the products fabricated through Schiff base interactions. Schiff base interactions are preferred as in situ Schiff base formation avoids the extra post-treatment processes to improve the stability of the multilayer and Schiff base reactions can be conducted both in aqueous and in organic solutions. This allows for incorporating materials that only dissolve or only can be used in nonaqueous solutions.

Li Y.,CAS Beijing National Laboratory for Molecular
Accounts of Chemical Research | Year: 2012

Bulk heterojunction (BHJ) polymer solar cells (PSCs) sandwich a blend layer of conjugated polymer donor and fullerene derivative acceptor between a transparent ITO positive electrode and a low work function metal negative electrode. In comparison with traditional inorganic semiconductor solar cells, PSCs offer a simpler device structure, easier fabrication, lower cost, and lighter weight, and these structures can be fabricated into flexible devices. But currently the power conversion efficiency (PCE) of the PSCs is not sufficient for future commercialization. The polymer donors and fullerene derivative acceptors are the key photovoltaic materials that will need to be optimized for high-performance PSCs.In this Account, I discuss the basic requirements and scientific issues in the molecular design of high efficiency photovoltaic molecules. I also summarize recent progress in electronic energy level engineering and absorption spectral broadening of the donor and acceptor photovoltaic materials by my research group and others. For high-efficiency conjugated polymer donors, key requirements are a narrower energy bandgap (E g) and broad absorption, relatively lower-lying HOMO (the highest occupied molecular orbital) level, and higher hole mobility. There are three strategies to meet these requirements: D-A copolymerization for narrower E g and lower-lying HOMO, substitution with electron-withdrawing groups for lower-lying HOMO, and two-dimensional conjugation for broad absorption and higher hole mobility. Moreover, better main chain planarity and less side chain steric hindrance could strengthen Ï€-Ï€ stacking and increase hole mobility. Furthermore, the molecular weight of the polymers also influences their photovoltaic performance. To produce high efficiency photovoltaic polymers, researchers should attempt to increase molecular weight while maintaining solubility. High-efficiency D-A copolymers have been obtained by using benzodithiophene (BDT), dithienosilole (DTS), or indacenodithiophene (IDT) donor unit and benzothiadiazole (BT), thienopyrrole-dione (TPD), or thiazolothiazole (TTz) acceptor units. The BDT unit with two thienyl conjugated side chains is a highly promising unit in constructing high-efficiency copolymer donor materials. The electron-withdrawing groups of ester, ketone, fluorine, or sulfonyl can effectively tune the HOMO energy levels downward.To improve the performance of fullerene derivative acceptors, researchers will need to strengthen absorption in the visible spectrum, upshift the LUMO (the lowest unoccupied molecular orbital) energy level, and increase the electron mobility. [6,6]-Phenyl-C 71-butyric acid methyl ester (PC 70BM) is superior to [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) because C 70 absorbs visible light more efficiently. Indene-C 60 bisadduct (ICBA) and Indene-C 70 bisadduct (IC 70BA) show 0.17 and 0.19 eV higher LUMO energy levels, respectively, than PCBM, due to the electron-rich character of indene and the effect of bisadduct. ICBA and IC 70BA are excellent acceptors for the P3HT-based PSCs. © 2012 American Chemical Society.

Li X.,CAS Beijing National Laboratory for Molecular | Gao X.,CAS Beijing National Laboratory for Molecular | Shi W.,CAS Beijing National Laboratory for Molecular | Ma H.,CAS Beijing National Laboratory for Molecular
Chemical Reviews | Year: 2014

The most often used strategies in designing small molecular chromogenic and fluorogenic probes for various analytes are studied. Most of the chromogenic and fluorogenic probes were constructed by using the fluorochromes such as anthracene, benzofurazan, BODIPY, coumarin, cyanine, naphthalene, quinoline, squaraine, and xanthene, and a lot of them exhibit fluorescence off-on or ratiometric (wavelength shifts) responses to an analyte via the photophysical processes like PET, ICT, or FRET. First, for practical applications of the probes in biochemical studies good water solubility is essential, because the usage of organic solvents usually destroys the normal function of biomolecules. Second, more probes with NIR features should be exploited. Unfortunately, most of the existing NIR fluorescent probes are derived from cyanine dyes, which suffer from poor stability. NIR chromogenic and fluorogenic probes with high stability and quantum yield as well as good water solubility are still expected.

Chen C.-F.,CAS Beijing National Laboratory for Molecular
Chemical Communications | Year: 2011

The development of new classes of macrocyclic hosts has always been one of the most important topics in supramolecular chemistry. During the past several years, based on the triptycene with unique three-dimensional rigid structure, several different kinds of novel triptycene-derived hosts including triptycene-derived cylindrical macrotricyclic polyether, triptycene-derived tris(crown ether)s, triptycene-derived molecular tweezers, triptycene-derived calixarenes, triptycene-derived heterocalixarenes, triptycene-derived tetralactam macrocycles and molecular cage have been designed and synthesized. Then, by exploring the applications of some of the triptycene derived hosts in molecular recognition and molecular assemblies, a series of new supramolecular systems with specific structures and properties have been developed. This feature article highlights our recent advances in the synthesis of triptycene-derived hosts and their applications in supramolecular chemistry. © 2011 The Royal Society of Chemistry.

A density functional theory study reveals that the dehydrogenation of ethanol catalyzed by an aliphatic PNP pincer ruthenium complex, (PNP)Ru(H)CO {1Ru, PNP = bis[2-(diisopropylphosphino)ethyl]amino}, proceeds via a self-promoted mechanism that features an ethanol molecule acting as a bridge to assist the transfer of a proton from ligand nitrogen to the metal center for the formation of H2. The very different catalytic properties between the aromatic and aliphatic pincer ligand in ruthenium complexes are analyzed. The potential of an iron analogue of 1Ru, (PNP)Fe(H)CO (1Fe), as a catalyst for the dehydrogenation of ethanol was evaluated computationally. The calculated total free energy barrier of ethanol dehydrogenation catalyzed by 1Fe is only 22.1 kcal/mol, which is even 0.7 kcal/mol lower than the calculated total free energy barrier of the reaction catalyzed by 1 Ru. Therefore, the potential of 1Fe as a low-cost and high-efficiency catalyst for the production of hydrogen from ethanol is promising. © 2013 American Chemical Society.

Liu W.,CAS Beijing National Laboratory for Molecular
Physics Reports | Year: 2014

A quantum mechanical equation Hψ=Eψ is composed of three components, viz., Hamiltonian H, wave function ψ, and property E(λ), each of which is confronted with fundamental issues in the relativistic regime, e.g.,(1) What is the most appropriate relativistic many-body Hamiltonian? How to solve the resulting equation? (2) How does the relativistic wave function behave at the coalescence of two electrons? How to do relativistic explicit correlation? (3) How to formulate relativistic properties properly?, to name just a few. It is shown here that the charge-conjugated contraction of Fermion operators, dictated by the charge conjugation symmetry, allows for a bottom-up construction of a relativistic Hamiltonian that is in line with the principles of quantum electrodynamics (QED). Various approximate but accurate forms of the Hamiltonian can be obtained based entirely on physical arguments. In particular, the exact two-component Hamiltonians can be formulated in a general way to cast electric and magnetic fields, as well as electron self-energy and vacuum polarization, into a unified framework. While such algebraic two-component Hamiltonians are incompatible with explicit correlation, four-component relativistic explicitly correlated approaches can indeed be made fully parallel to the nonrelativistic counterparts by virtue of the 'extended no-pair projection' and the coalescence conditions. These findings open up new avenues for future developments of relativistic molecular quantum mechanics. In particular, 'molecular QED' will soon become an active and exciting field. © 2013 Elsevier B.V.

Sun C.-L.,CAS Beijing National Laboratory for Molecular | Li B.-J.,CAS Beijing National Laboratory for Molecular | Shi Z.-J.,CAS Beijing National Laboratory for Molecular
Chemical Reviews | Year: 2011

A compiled analysis and discussion of advances in iron-catalyzed C-H transformations in the past decade is presented. For the oxidation of cyclohexane, simpler and more efficient system were reported by using catalytic amounts of commercial available iron salts in acetonitrile under mild conditions, and H2O2 as the oxidant. In 2008, Fu and co-workers reported the first example of iron-catalyzed amidation of C-H bonds and a new C-N formation process was demonstrated by Li in 2010. In 2008, Nakamura and co-workers developed the first iron-catalyzed direct arylation of aryl C-H bonds with in situ generated organozinc reagents from aryl Grignard reagents and ZnCl2 in the presence of a proper ligand. Among direct C-H transformations, general alkyl sp3 C-H faces the highest challenge due to its intrinsic inactivating features.

Yang X.,CAS Beijing National Laboratory for Molecular
ACS Catalysis | Year: 2014

A density functional theory study of the reaction mechanism of the production of H2 and CO2 from methanol and water catalyzed by an aliphatic PNP pincer ruthenium complex, (PNP)Ru(H)CO, reveals three interrelated catalytic cycles for the release of three H2 molecules: the dehydrogenation of methanol to formaldehyde, the coupling of formaldehyde and hydroxide for the formation of formic acid, and the dehydrogenation of formic acid. The formation of all three H2 molecules undergoes the same self-promoted mechanism that features a methanol or a water molecule acting as a bridge for the transfer of a ligand proton to the metal hydride in a key intermediate, trans-(HPNP)Ru(H)2CO. © 2014 American Chemical Society.

Xi Z.,CAS Beijing National Laboratory for Molecular
Accounts of Chemical Research | Year: 2010

The development of organometallic reagents remains one of the most important frontiers in synthetic chemistry. Commonly used organometallic reagents (such as RLi and RMgBr) are typically monometallic compounds, although they aggregate in many cases. When two carbon-metal bonds are in the same molecule in close proximity, however, these two carbon-metal moieties may exhibit novel reactivity. In this Account, we outline our work on new reactions and synthetic applications of the organo-dilithio reagents 1,4-dilithio-1,3- butadienes. The 1,4-dilithio-1,3-butadienes can be accessed readily in high efficiency with a wide variety of substitution patterns on the butadienyl skeleton. The configuration has been predicted and demonstrated to favor a double dilithium bridging structure in both solution and solid states. The two Li atoms are bridged by a butadiene moiety and are in close proximity. By taking advantage of this unique configuration, we have developed useful and interesting synthetic methodologies. Three types of reactions of 1,4-dilithio-1,3-butadienes, termed dilithio reagents here, have been developed and are discussed. An intramolecular reaction is introduced in the first section. The reaction is a result of the intracooperative effect among the two C-Li moieties, the butadienyl bridge, and the substituents. A useful transformation from silylated 1,4-dilithio-1,3-butadienes to α-lithio siloles is described. Second, we discuss an intermolecular reaction that results from the intercooperative effect of the two C-Li moieties toward substrates. As an example of the formation of functionalized cyclic dianions from the linear dianions of the dilithio reagents and organic substrates, we describe the isolation and structural characterization of a novel type of cyclic dianion; that is, fully substituted oxy-cyclopentadienyl dilithium formed via the reaction of dilithio reagents with CO. We also describe diverse reactions of dilithio reagents with nitriles to form substituted pyridines, tricyclic 1-bipyrrolines, and siloles, demonstrating the remarkable effect of substituents on the butadienyl skeleton. Third, we discuss transmetalation of dilithio reagents to generate other organo-dimetallic compounds. This section focuses on organo-dicopper compounds and their reactivity toward the synthesis of strained ring systems, such as semibullvalenes and twisted four-membered rings, with the metal-mediated C-C bond-forming approach. In addition to these three representative reactions, other useful applications are also briefly introduced. The dimetallic 1,4-dilithio-1,3-butadienes and their transmetalated derivatives provide unique synthetic organometallic reagents that are very different from monometallic reagents, both in terms of reactivity and synthetic application. These organo-dimetallic reagents provide access to interesting and useful compounds that are not available by other means. Moreover, given the possibilities afforded, the study of organo-dimetallic and organo-polymetallic compounds should yield further synthetic applications in the near future. © 2010 American Chemical Society.

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