Correlation between atomistic morphology and electron transport properties in defect-free and defected graphene nanoribbons: An interpretation through Clar sextet theory

Structural, electronic and transport properties of defect-free, defected and functionalized armchair and zig-zag graphene nanoribbons (GNRs) are investigated with density functional theory and non-equilibrium Green’s function calculations and rationalized in terms of Clar’s theory of the aromatic sextet. Calculations suggest a tight relationship between the transport properties of nanoribbons and the underlying bond patterning as described by valence bond and Clar sextet theory. Namely, armchair GNRs exhibit a strong dependence of the transport properties on the ribbon width, as a consequence of different valence bond representations. The occurrence of localized defects involving electron pairs does not significantly alter this behavior. Conversely, transport properties of zigzag GNRs are less affected by morphological details, such as width and occurrence of defects, as expected from the application of Clar’s theory. However, controlled edge functionalization and morphology modifications in zigzag GNRs can potentially lead to localization of aromatic sextets and, consequently, to strong changes in the transport properties. Our work indicates Clar sextet theory as a powerful and accurate tool to rationalize and predict the electronic and transport properties of complex carbon nanostructures based on GNRs. These principles can be extended to the design of novel systems with potential applications in nanoelectronics.     

TESTING THE Y–RULE IN CLAR THEORY

A recently proposed modification of the basic postulates of the Clar aromatic sextet theory, called the Y–rule, is tested on a suitably chosen homologous series of benzenoid hydrocarbons. For these benzenoid systems the original Clar theory predicts a significantly different distribution of π-electrons than the Y–rule. It was found that both the π-electron contents of rings as well as their energy effects agree with the predictions of the original Clar theory, failing to corroborate the Y–rule.     

ALGORITHM FOR SIMULTANEOUS CALCULATION OF KEKULÉ AND CLAR STRUCTURE COUNTS,AND CLAR NUMBER OF BENZENOID MOLECULES

The Chinese mathematicians Heping Zhang and Fuji Zhang conceived a combinatorial polynomial associated with benzenoid molecules. This “Zhang–Zhang polynomial” contains information on the Kekulé structure count (K), Clar structure count (C), Clar number (Cl), Hosoya-Yamaguchi sextet polynomial σ(B, x), and several other important characteristics of the underlying benzenoid molecule. We show how one can easily calculate the Zhang-Zhang polynomial, and thus arrive at an algorithm for simultaneous computation of K, C, Cl, and σ(B, x). Some applications of this algorithm are pointed out.     

π-ELECTRON CONTENTS OF RINGS IN THE DOUBLE-HEXAGONAL-CHAIN HOMOLOGOUS SERIES (PYRENE,ANTHANTHRENE AND OTHER ACENOACENES)

Abstract

Recently three methods for calculating the π-electron content of rings of benzenoid hydrocarbons were put forward: one based on the consideration of Kekuléstructural formulas, and the other two based on an analogous treatment of the Clar aromatic sextet formulas. These three methods are applied to the homologous series consisting of two condensed acene chains (whose first members are pyrene, anthanthrene, peri-naphthacenonaphthacene, …), leading to basically identical results. In contrast to acenes (in which the partition of π-electrons into rings is uniform), in the double-hexagonal-chain species the partition of π-electrons is highly non-uniform. The electron content monotonically decreases, in opposite directions, along the two acene chains, being maximal in the least annelated rings. Some other generally valid regularities in the π-electron properties of the double–hexagonal–chain benzenoids are also pointed out.     

A CLASSICAL VALENCE BOND VIEW OF CYLINDER-SHAPED AND FULLY BENZENOID-LIKE POLYARENES (POLYCYCLIC AROMATIC HYDROCARBONS) OF LARGE SIZE

This note first defines a class of cylindrical polyarenes polycyclic aromatic hydrocarbons (PAHs) C_{2(m+ 1)n}H_2n, for which the periphery is acenic. From this class the leapfrog algorithm derives the second class of cylindrical PAHs C_6nmH_4n, where the periphery is phenacenic. One further application of this algorithm to the second class leads to the third class of cylindrical PAHs C_12nmH_8n, where the less regular periphery has 2n capes (i.e., hexagons exposed on four adjacent sides to the external “sea” of space). Each molecule in the second and third classes is totally covered by aromatic sextets, that is, appears related to a fully benzenoid hydrocarbon (FBH). The Pauling bond order, and several new indices derived from the Pauling bond order and Clar sextet counts are calculated for the three classes with increasing numbers of carbons. This calculation suggests that a typical cylindrical PAH becomes more graphite-like than a typical flat FBH.     

Towards nano-organic chemistry: perspectives for a bottom-up approach to the synthesis of low-dimensional carbon nanostructures

Low-dimensional carbon nanostructures, such as nanotubes and graphenes, represent one of the most promising classes of materials, in view of their potential use in nanotechnology. However, their exploitation in applications is often hindered by difficulties in their synthesis and purification. Despite the huge efforts by the research community, the production of nanostructured carbon materials with controlled properties is still beyond reach. Nonetheless, this step is nowadays mandatory for significant progresses in the realization of advanced applications and devices based on low-dimensional carbon nanostructures. Although promising alternative routes for the fabrication of nanostructured carbon materials have recently been proposed, a comprehensive understanding of the key factors governing the bottom-up assembly of simple precursors to form complex systems with tailored properties is still at its early stages. In this paper, following a survey of recent experimental efforts in the bottom-up synthesis of carbon nanostructures, we attempt to clarify generalized criteria for the design of suitable precursors that can be used as building blocks in the production of complex systems based on sp(2) carbon atoms and discuss potential synthetic strategies. In particular, the approaches presented in this feature article are based on the application of concepts borrowed from traditional organic chemistry, such as valence-bond theory and Clar sextet theory, and on their extension to the case of complex carbon nanomaterials. We also present and discuss a validation of these approaches through first-principle calculations on prototypical systems. Detailed studies on the processes involved in the bottom-up fabrication of low-dimensional carbon nanostructures are expected to pave the way for the design and optimization of precursors and efficient synthetic routes, thus allowing the development of novel materials with controlled morphology and properties that can be used in technological applications.     

NOTE ON THE Y-RULE IN CLAR THEORY

The recently proposed Y-rule for constructing Clar-type aromatic sextet formulas for benzenoid molecules is examined, and the limits of its applicability are established. According to the Y-rule, the π-electron configuration of a benzenoid hydrocarbon is best represented by the Clar-type formula(s) in which all internal carbon atoms (the so-called Y-carbon atoms) are covered by aromatic sextets. The Y-rule is, by definition, applicable only to perifusenes (because catafusenes do not possess internal carbon atoms). We show that the rule is not applicable to all perifusenes, and that in some case it leads to numerous “best” Clar-type formulas.     

Free radicals trapped in polyethylene matrix: 1. Location of the alkyl radicals in irradiated polyethylene

Two types of alkyl radicals were found to be trapped in irradiated crystals grown from polyethylene solution. One of them corresponds to the broad sextet pattern of the e.s.r. spectrum and the other corresponds to the sharp sextet pattern. The free radicals attributed to the broad sextet began to disappear at a lower temperature than the temperature at which the free radicals attributed to the sharp sextet disappeared. When butadiene molecules were brought into contact with the specimen, the decay of the free radicals corresponding to the broad sextet was accelerated. When the specimen was subjected to fuming nitric acid treatment, no broad sextet was observed. The mat of the crystals, was aligned so that the c-axes of its crystallites were perpendicular to its surface. The broad sextet showed no anisotropy when the angle between the direction of applied magnetic field and that of the c-axis of the crystallite was varied. On the other hand, the sharp component of the spectrum showed apparent anisotropy. Consequently, it can be concluded that the broad component comes from the free radicals trapped in the lamellar surface and the sharp component is attributed to the free radicals trapped in the inner part of the crystallite. Hence, the locations of these two types of free radicals have been clarified with much more certainty than before.     

HEXAGONOIDAL PARTITIONING AND QUASI-SINGLE BONDS IN BENZENOID HYDROCARBONS

The concept of fully benzenoid hydrocarbons (molecular graphs; FBHs) is generalized. Each full-hexagon unit (aromatic sextet) in a FBH is replaced with a larger “hexagon-shaped” unit (subgraph; e.g., naphthalene, pyrene, coronene, or ovalene units). Such a molecular species may be called a “fully hexagonoid” hydrocarbon (FHHs). The Pauling bond order calculation suggests that a bond (edge) which interlinks one hexagon-shaped unit with another in the bottom of bays of FHH is quasi-single, if the two units are connected by more than two bonds. As a consequence, the hexagon-shaped units are delocalized locally in the hexagonoidal (graph-theoretical) partitioning.     

3,5-二甲基苯酚合成工艺改进

以乙酰乙酸乙酯和乙醛为起始原料，以碳酸钠作为碱，经过Robinson环合反应得到4-乙氧羰基-3,5-二甲基-2-环己烯-1-酮，收率为95 %。对碳酸钠体系催化的Robinson环合反应进行应用拓广，在该体系催化下，脂肪醛和芳香醛底物均可顺利转化为相应二取代环己烯酮产物。环合产物4-乙氧羰基-3,5-二甲基-2-环己烯-1-酮经皂化脱羧过程可定量转化为关键中间体3,5-二甲基-2-环己烯酮，收率为99 %。将3,5-二甲基-2-环己烯酮直接脱氢芳构化，得到了目标产物3,5-二甲基苯酚。采用1HNMR和13CNMR对产物及中间体结构进行了表征。本文建立的工艺条件下，3,5-二甲基苯酚的总收率为66 %。该工艺也应用于3-甲基-5-丙基苯酚的合成，具有一定的普适性。     

Strain-Free Total Resonant Sextet Benzenoid Hydrocarbons

The importance of strain-free (total) resonant sextet benzenoid hydrocarbons is pointed out. The first computer-aided generation and enumeration of such systems is reported, and the data represent substantial supplements to the previous numbers of strain-free resonant sextet benzenoid isomers. Supplementary depictions of some forms of the systems in question are also provided. Constant-isomer series are delineated for the resonant sextet benzenoids. The numbers associated with these series are found to match exactly the known numbers for corresponding constant-isomer series of strictly pericondensed benzenoids.     

Vapour liquid equilibrium with association in both phases: A model with the concentration-dependent liquid phase association constant applied to acetic acid—benzene, -toluene, -p- and -o-xylene

A model employing the correction factors of Marek and Standart, but using a concentration-dependent liquid phase association factor of Jenkins—Robinson, has been used to model vapour—liquid equilibrium data for mixtures of acetic acid with benzene, toluene, o-xylene and p-xylene. With the aim to use systems of acetic acid—benzene and acetic acid—toluene as the test mixtures for distillation columns, the examination of the systems of acetic acid—aromatic hydrocarbons was undertaken. The model promises to be useful in modelling isobaric and isothermal data of acetic acid—benzine, acetic acid—toluene, acetic acid-p-xylene and acetic acid-o-xylene systems. Deviation plots show that the isothermal and isobaric data are represented well.     

Poly(naphthylenethiophene)s and poly(naphthylenevinylenephenylenevinylene)s: effect of naphthalene positional isomers on the light emitting properties of their polymers

Poly(naphthylenethiophene)s and poly(naphthylenevinylene-phenylenevinylene)s prepared from 1,5-dibromo-3,7-di-t-butylnaphthalene showed a significant blue shift in their UV-vis and photoluminescence spectra compared to copolymers derived from the 1,4-naphthylene system, both in solution and in thin film. This is correlated to a change in conjugation effect of the two positional isomers of naphthalene in the polymer backbone, where in 1,4-conjugation an aromatic sextet remains intact in one of the naphthalene rings whereas 1,5-conjugation involves the loss of resonance energy in the entire naphthalene unit. The series of poly(naphthylenevinylene-phenylvinylene)s emitted blue-green light with very high fluorescence quantum yields (80-95%) in solution. A fabricated device showed external quantum efficiency estimated at 0.2-0.4%, with a relatively high turn-on voltage at ∼7.0 V.     

Phase behavior and moisture sorption of deliquescent powders

Deliquescence lowering was investigated in powder blends of different particle sizes. Deliquescent compounds, widely used in edible products, were used to prepare two model blends: fructose–citric acid anhydrous (M1) and sucrose–sodium chloride (M2). The relationship between the water activity and the composition of each system was measured and used to build phase diagrams, from which information about the eutonic composition (EC) was extracted. Data were evaluated using five models (Raoult’s law, and the Ross, Ferro Fontán–Chirife–Benmergui, Zdanovskii–Stokes–Robinson, and extended Zdanovskii–Stokes–Robinson equations), and the extended Zdanovskii–Stokes–Robinson equation provided the best fit. The effects of formulation, compound ratio, particle size, degree of compaction, and storage RH on moisture sorption profiles were determined and varied for M1 and M2. The EC was 33:67 wt% citric acid:fructose for M1 and 16:84 wt% NaCl:sucrose for M2. Blending powders created sufficient particle intimacy to enable deliquescence lowering in both systems (the mutual deliquescence RH was 44.1% for M1 and 63.6% for M2), and the eutonic composition was the most susceptible to moisture-induced changes. The results of this study show that powder behavior in the presence of moisture can be largely predicted from equilibrium phase diagrams.     

Biphasic Aqueous Organometallic Catalysis Promoted by Cyclodextrins: How to Design the Water‐Soluble Phenylphosphane to Avoid Interaction with Cyclodextrin

The ability of cyclodextrin to interact with meta‐trisulfonated triphenylphosphane derivatives bearing one or two methyl (or methoxy) groups on the aromatic ring has been investigated by NMR and UV‐vis spectroscopy. In the case of native β‐cyclodextrin (β‐CD), the presence of one methyl or methoxy group in the ortho‐position on each aromatic ring is necessary to hamper the formation of an inclusion complex between the β‐CD and meta‐trisulfonated triphenylphosphane derivatives. In the case of methylated β‐CD, the formation of an inclusion complex is only observed when the meta‐trisulfonated triphenylphosphane contains a methyl group in the para‐position. The poor affinity of methylated β‐CD towards modified trisulfonated triphenylphosphanes was attributed to the steric hindrance generated by the methyl groups on the CD secondary face. The absence or presence of an interaction between phosphanes and methylated β‐CD was also confirmed by catalytic experiments. Thus, the phosphanes that do not interact with the methylated CD were the most efficient mass‐transfer promoters in an aqueous biphasic palladium‐catalyzed Tsuji–Trost reaction.     

Synthesis and characterization of nanocrystallite ZnCrFeO4

Nanocrystallites of spinel ZnCrFeO₄ were synthesized through sol–gel method, and were characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM), SQUID magnetometer and Mössbauer spectrometer. It is found that the nanocrystallites are spherical and show antiferromagnetism with a higher T_N of 14 K for the sample with diameter of 14 nm and larger magnetic susceptibility than the bulk sample. These are associated with some A ions (Zn²⁺) and B ions (Fe³⁺, Cr³⁺) jumped to each other sites, which therefore enhances both the A–B interaction and B–B interaction. The Mössbauer spectrum of the nanomaterial is different from that of the bulk sample. Additional sextet appears in the Mössbauer spectrum of the nanomaterial except for a doublet. The appearance of the sextet suggests the magnetically ordered state between Fe³⁺ ions at A and B sites.     

DOUBLE FJORD MOTIF IN OVERCROWDED LARGE POLYCYCLIC AROMATIC HYDROCARBONS: THE CONFORMATIONAL SPACE OF HEXABENZO[a,cd,f,j,lm,o]PERYLENE

The overcrowded large polycyclic aromatic hydrocarbon (LPAH) hexabenzo[a,cd,f,j,lm,o]perylene (HBP) was subjected to a theoretical study. The ab-initio Hartree-Fock (HF) and the density functional theory (DFT) B3LYP methods were employed to calculate energies and geometries of the stationary point conformations of HBP. The global minimum was found to be the twisted t-D ₂ ; the local minimum anti-folded a-C _2h was 20.5 kJ/mol higher in energy (B3LYP/6-31G(d)). Thus, only t-D ₂ -HBP exists at room temperature in solution. The enantiomerization of HBP proceeds via a pathway that connects the chiral t-D ₂ to the achiral a-C _2h through a chiral twisted-folded transition state, tf-C ₁ . The energy barrier for enantiomerization was 134.4 kJ/mol. The central benzene ring F of t -D ₂ is highly distorted from planarity (torsion angles of 15.4° and ?30.5°), and has a remarkable resemblance to the twist angle of the central ene in the parent BAE bifluorenylidene (32°). Yet, nucleus-independent chemical shift (NICS) values, calculated at GIAO-B3LYP/6-31G(d)//B3LYP/6-31G(d) and the almost equal bond lengths (ca. 142 pm) of the central ring are indicative of the aromatic character of this ring. The NICS values are also indicative of the aromatic character of the peripheral rings and the central rings of the two minima and the transition state, consistent with the Clar picture.     

Response to pheromone traps and disruption of pheromone communication in the lesser peachtree borer and the peachtree borer (Lepidoptera: Sesiidae)

The sex pheromone communication of the lesser peachtree borer,Synanthedon pictipes (Grote and Robinson), and the peachtree borer,Sanninoidea exitiosa (Say), can be disrupted by permeation of the atmosphere with their respective sex pheromones, (E,Z)- and (Z,Z)-3,13-octadecadien-1-ol acetate. The two isomers seemed equally effective against both species. Disruption was greatest when the pheromone was evaporated from the tops of the peach trees; also, pheromone traps placed in the tree tops captured significantly more males than did traps placed lower in the trees. Neither the color nor the directional placement in a tree (NE, NW, SE, SW) of pheromone-baited traps influenced captures of male lesser peachtree borers.Mention of a commercial or proprietary product in this paper does not constitute a recommendation or an endorsement of that product by the USDA.     

Draw resonance involving rheological transitions

The periodic diameter variations in melt spinning known as draw resonance have been extensively studied, both theoretically and experimentally. The theory has evolved from analyses for isothermal, Newtonian spinning in which the critical draw ratio was found to be 20.21. Compared with this, recent theories have shown that elasticity and cooling are stabilizing (with certain qualifications on the range of variables) and that non-linear rheological response is destabilizing. The present work presents systematic data on two polymers, one that is always amorphous (polystyrene) and one that solidifies into a semi-crystalline state (polypropylene). Isothermal and non-isothermal data are available for the former; non-isothermal data, for the latter. The melt temperature, the cooling environment, the die length, the spinning length, the mass flow rate and the draw ratio were varied. Certain observations cannot be explained within the existing theoretical framework, specifically the destabilizing effect of certain cooling histories in the case of polystyrene and the destabilizing effect of short dies, presumably a die swell effect, in the case of polypropylene. Very highly oriented polystyrene can be produced under certain conditions when the fiber temperature is held above T_g.