Computational Study of the Stability of Nanotube Fragments

Extremely short (<1 nm) fragments of zig‐zag carbon nanotubes are studied with ab‐initio techniques to determine their geometric and electronic structure as well as their magnetic susceptibility. It is found that for lengths of a few carbon–carbon bonds, each fragment can be viewed as composed of crowns, that is, zig‐zag rings of carbon atoms along the circumference of the tube. In this case, two kinds of electronic structures are found, depending on whether the number of carbon atoms in each crown is even or odd. Systems comprising three or more crowns either have a high spin ground state or involve a charge transfer across the length of the fragment. Conjugation changes qualitatively when the length of the fragment approaches and surpasses its girth. Indications regarding the predicted chemical stability and electronic response are provided and interpreted in terms of current densities induced within each crown by a magnetic field along the tube axis.     

One‐Dimensional Arsenic Allotropes: Polymerization of Yellow Arsenic Inside Single‐Wall Carbon Nanotubes

The pnictogen nanomaterials, including phosphorene and arsenene, display remarkable electronic and chemical properties. Yet, the structural diversity of these main group elements is still poorly explored. Here we fill single‐wall carbon nanotubes with elemental arsenic from the vapor phase. Using electron microscopy, we find chains of highly reactive As₄ molecules as well as two new one‐dimensional allotropes of arsenic: a single‐stranded zig‐zag chain and a double‐stranded zig‐zag ladder. These linear structures are important intermediates between the gas‐phase clusters of arsenic and the extended sheets of arsenene. Raman spectroscopy indicates weak electronic interaction between the arsenic and the nanotubes which implies that the formation of the new allotropes is driven primarily by the geometry of the confinement. The relative stabilities of the new arsenic structures are estimated computationally. Band‐gap calculations predict that the insulating As₄ chains become semiconducting, once converted to the zig‐zag ladder, and form a fully metallic allotrope of arsenic as the zig‐zag chain.     

Dynamic Odd–Even Effect in Liquid n‐Alkanes near Their Melting Points

n‐Alkanes are the textbook examples of the odd–even effect: The difference in the periodic packing of odd‐ and even‐numbered n‐alkane solids results in odd–even variation of their melting points. However, in the liquid state, in which this packing difference is not obvious, it seems natural to assume that the odd–even effect does not exist, as supported by the monotonic dependence of the boiling points of n‐alkanes on the chain length. Herein, we report a surprising odd–even effect in the translational diffusional dynamic properties of n‐alkanes in their liquid states. To measure the dynamics of the molecules, we performed quasi‐elastic neutron scattering measurements near their melting points. We found that odd‐numbered n‐alkanes exhibit up to 30 times slower dynamics than even‐numbered n‐alkanes near their respective melting points. Our results suggest that, although n‐alkanes are the simplest hydrocarbons, their dynamic properties are extremely sensitive to the number of carbon atoms.     

Encapsulation and Polymerization of White Phosphorus Inside Single‐Wall Carbon Nanotubes

Elemental phosphorus displays an impressive number of allotropes with highly diverse chemical and physical properties. White phosphorus has now been filled into single‐wall carbon nanotubes (SWCNTs) from the liquid and thereby stabilized against the highly exothermic reaction with atmospheric oxygen. The encapsulated tetraphosphorus molecules were visualized with transmission electron microscopy, but found to convert readily into chain structures inside the SWCNT “nanoreactors”. The energies of the possible chain structures were determined computationally, highlighting a delicate balance between the extent of polymerization and the SWCNT diameter. Experimentally, a single‐stranded zig‐zag chain of phosphorus atoms was observed, which is the lowest energy structure at small confinement diameters. These one‐dimensional chains provide a glimpse into the very first steps of the transformation from white to red phosphorus.     

Closing the Nanographene Gap: Surface‐Assisted Synthesis of Peripentacene from 6,6′‐Bipentacene Precursors

The thermally induced cyclodehydrogenation reaction of 6,6′‐bipentacene precursors on Au(111) yields peripentacene stabilized by surface interactions with the underlying metallic substrate. STM and atomic‐resolution non‐contact AFM imaging reveal rectangular flakes of nanographene featuring parallel pairs of zig‐zag and armchair edges resulting from the lateral fusion of two pentacene subunits. The synthesis of a novel molecular precursor 6,6′‐bipentacene, itself a synthetic target of interest for optical and electronic applications, is also reported. The scalable synthetic strategy promises to afford access to a structurally diverse class of extended periacenes and related polycyclic aromatic hydrocarbons as advanced materials for electronic, spintronic, optical, and magnetic devices.     

Interaction of solitons on 2‐dimensional branched π‐electron surface of graphene ribbons

We have performed quantum‐mechanical study of charge distribution on the carbon atoms of two‐dimensional conjugated system of the model graphene ribbon. The study shows that charges in the quasi‐two dimensional conjugated system of the graphene ribbon are not localized on separate carbon atoms, but form one‐dimensional solitonic charge waves along zig‐zag sides. There are two solitonic charge waves in the dication or dication due to generation of own soliton by each charge; the first soliton in the acene chain is located on one side while the second one is positioned on the opposite side. The shapes of the solitonic waves in the graphene ribbon are similar to ones in one‐dimensional conjugated systems. Similarly, in the ionic two‐dimensional collective systems of π‐electrons two splitted solitonic levels are generated; lengthening of the chain leads to convergence of the levels. The widening of graphene ribbon (y‐expansion of two‐dimensional conjugated system is accompanied with recession of both solitons to outer sides, so amplitudes of the solitons on the inner sides regularly decrease under widening of the model graphene ribbon; the charges on the inner carbon atoms converge to zero.     

On‐Surface Self‐Assembly of a C3‐Symmetric π‐Conjugated Molecule Family Studied by STM: Two‐Dimensional Nanoporous Frameworks

The on‐surface self‐assembled behavior of four C ₃‐symmetric π‐conjugated planar molecules ( Tp , T12 , T18 , and Ex ) has been investigated. These molecules are excellent building blocks for the construction of noncovalent organic frameworks in the bulk phase. Their hydrogen‐bonded 2D on‐surface self‐assemblies are observed under STM at the solid/liquid interface; these structures are very different to those in the bulk crystal. Upon combining the results of STM measurements and DFT calculations, the formation mechanism of different assemblies is revealed; in particular, the critical role of hydrogen bonding in the assemblies. This research provides us with not only a deep insight into the self‐assembled behavior of these novel functional molecules, but also a convenient approach toward the construction of 2D multiporous networks.     

Oxygen‐Doped Zig‐Zag Molecular Ribbons

The synthesis of a zig‐zag oxygen‐doped molecular rhombic ribbon has been achieved. This includes oxidative C?C and C?O bond formations that allowed the stepwise elongation and planarization of an oxa‐congener of 2,7‐periacenoacene. X‐ray diffraction analysis corroborated the flat structure and the zig‐zag topology of the O‐doped edges. Photophysical and electrochemical investigations showed that the extension of the peri‐xanthenoxanthene (PXX) into the molecular ribbon induces a noticeable shrinking of the molecular band gap devised by a rising of the HOMO energy level, a desirable property for p‐type organic semiconductors.     

Synthesis and mesomorphic properties of chiral nematic liquid crystals based on cholesterol

A new series of mesogenic compounds having a cholesteryl moiety has been synthesised by condensing p-amino benzoate of cholesterol and methoxy substituted 4-n-alkoxy cinnamoyl chlorides and their liquid crystalline properties has been studied. All the members of the series are enantiotropic and exhibit chiral nematic (N*) mesophase. The plot of transition temperatures versus number of carbon atoms in the alkoxy chain exhibits odd–even effect and falling tendency for N*–isotropic transition temperatures. The compounds exhibit oily streak textures that on slight disturbance change to the plane textures and show iridescent colours. High anisotropy, linearity and intermolecular hydrogen bonding confer rich mesomorphic properties on the system. Intermolecular hydrogen bonding arising from amide linkage can lead to supramolecular motifs.     

Solvent‐Induced Structural Transition of Self‐Assembled Dipeptide: From Organogels to Microcrystals

Organogels that are self‐assembled from simple peptide molecules are an interesting class of nano‐ and mesoscale soft matter with simplicity and functionality. Investigating the precise roles of the organic solvents and their effects on stabilization of the formed organogel is an important topic for the development of low‐molecular‐weight gelators. We report the structural transition of an organogel self‐assembled from a single dipeptide building block, diphenylalanine (L ‐Phe‐L ‐Phe, FF), in toluene into a flower‐like microcrystal merely by introducing ethanol as a co‐solvent; this provides deeper insights into the phase transition between mesostable gels and thermodynamically stable microcrystals. Multiple characterization techniques were used to reveal the transitions. The results indicate that there are different molecular‐packing modes formed in the gels and in the microcrystals. Further studies show that the co‐solvent, ethanol, which has a higher polarity than toluene, might be involved in the formation of hydrogen bonds during molecular self‐assembly of the dipeptide in mixed solvents, thus leading to the transition of organogels into microcrystals. The structural transformation modulated by the co‐solvent might have a potential implication in controllable molecular self‐assembly.     

Chronological Change from Face‐On to Edge‐On Ordering of Zinc–Tetraphenylporphyrin at the Phenyloctane–Highly Oriented Pyrolytic Graphite Interface

The self‐assembled structure of alkoxy‐ and N‐alkylcarbamoyl‐substituted zinc–tetraphenylporphyrin at the liquid–highly oriented pyrolytic graphite (HOPG) interface was observed by using scanning tunneling microscopy. The alkoxy porphyrin showed a phase transition from face‐on to edge‐on ordering. The phase transition requires the close‐packed structure of alkoxy porphyrin. The chronological change of the ordering was traced to show the existence of several types of Ostwald ripening including two‐step phase transition from small edge‐on to face‐on and then further to edge‐on orderings. On the other hand, the N‐alkylcarbamoyl porphyrin showed persistent edge‐on ordering, and the ordering was analyzed by the Moiré pattern. Although the edge‐on ordering is observed only in the nonpolar solvent, the orderings have potential applications in the charge and energy transfer.     

A Dimeric Bis(melamine)‐Substituted Bispidine for Efficient Transmembrane H+/Cl− Cotransport

A 3,7‐diazabicyclo[3.3.1]nonane linking to two melamines is a unique transmembrane H⁺/Cl⁻ carrier. In the solid state, the V‐shaped compound forms a HCl‐bound zig‐zag network through cooperative protonation and hydrogen bond interactions. In the lipid membrane, the receptor forms a dimeric self‐assembly involving multiple H⁺ and Cl⁻ leading to the efficient transport of the acid. The pH‐dependent Cl⁻ efflux observed for the compound was rationalized based on a gradual protonation model that confers an active transmembrane carrier at physiological pH.     

Investigation on the Surface‐Confined Self‐Assembly Stabilized by Hydrogen Bonds of Urea and Amide Groups: Quantitative Analysis of Concentration Dependence of Surface Coverage

Formation of a hydrogen‐bond network via an amide group is a key driving force for the nucleation–elongation‐type self‐assembly that is often seen in biomolecules and artificial supramolecular assemblies. In this work, rod‐coil‐like aromatic compounds bearing an amide ( 1 a – 3 a ) or urea group ( 1 u – 3 u ) were synthesized, and their self‐assemblies on a 2‐D surface were investigated by scanning tunneling microscopy (STM). According to the quantitative analysis of the concentration dependence of the surface coverage, it was revealed that the strength of the hydrogen bond (i.e., amide or urea) and the number of non‐hydrogen atoms in a molecular component (i.e., size of core and length of alkyl side chain) play a primary role in determining the stabilization energy during nucleation and elongation processes of molecular ordering on the HOPG surface.     

High Resolution Scanning Tunneling Microscopy of a 1D Coordination Polymer with Imidazole‐Based N,N,O Ligands on HOPG

Novel κ³‐N,N,O ligands tend to form 1D coordination polymer strands. Deposition of 1D structures on highly oriented pyrolytic graphite (HOPG) was achieved from diluted solutions and polymer strands have been studied on HOPG by AFM/STM. Single strands were mapped by STM and their electronic properties were subsequently characterized by current imaging tunneling spectroscopy (CITS). Periodic density functional calculations simulating a polymer strand deposited on a HOPG surface are in agreement with the zig‐zag structure indicated by experimental findings. Both the observed periodicity and the Zn–Zn distances can be reproduced in the simulations. Van der Waals interactions were found to play a major role for the geometry of the isolated polymer strand, for the adsorption geometry on HOPG, as well as for the adsorption energy.     

Synthesis,Properties, and Remarkable 2 D Self‐Assembly at the Liquid/Solid Interface of a Series of Triskele‐Shaped 5,11,17‐Triazatrinaphthylenes (TrisK)

A series of 5,11,17‐triazatrinaphthylene (TrisK) derivatives, large disk‐like π‐conjugated molecules with C_3h symmetry, has been synthesised by following an optimised synthetic pathway. The synthesis was performed by a four‐step protocol based on the N‐arylation of 1,3,5‐tribromobenzene with appropriate anthranilate derivatives. This strategy permits the generation of either chlorinated ( TrisK‐Cl‐OCn ) or non‐chlorinated ( TrisK‐H‐OCn ) alkoxy‐substituted derivatives (OC_nH_2n+1 with n=3, 10, 12 and 16), thus providing additional versatility in the control of the structure–property relationships. The electronic properties of the various TrisK compounds have been characterised in solution by absorption and emission spectroscopies as well as cyclic voltammetry. The crystal structure of 2,8,14‐propyloxy‐5,11,17‐triazatrinaphthylene TrisK‐H‐OC3 has been determined by X‐ray diffraction analysis, which revealed the presence of stabilising weak intermolecular H bonds. Scanning tunnelling microscopy (STM) at the liquid/solid interface has revealed the remarkable 2D self‐assembling properties of the TrisK compounds. In particular, it has shown that TrisK‐H‐OC12 forms three concomitant self‐organised 2D phases with different row‐packing arrangements. This 2D polymorphism arises from slow ordering due to the presence of three long dodecyloxy chains on the molecular backbone. Individual molecules can be imaged with spectacular intramolecular resolution, thus providing the possibility of correlating the STM features with the calculated charge density distribution.     

BSSE‐free description of the formamide dimers

The different configurations (linear, zig‐zag, and cyclic) of formamide dimers have been studied at the level of both Hartree–Fock (HF) and second order Møller–Plesset perturbation theory (MP2). The widely used a posteriori Boys–Bernardi “counterpoise” (CP) correction scheme has been compared with our a priori methods utilizing the “chemical Hamiltonian approach” (CHA). The appropriate interaction energies have been calculated in six different basis sets (6‐31G, 6‐31G**, DZV, DZP, TZV, and cc‐pVDZ). © 2001 John Wiley & Sons, Inc. Int J Quant Chem, 2001     

Mechanochemical Encapsulation of Fullerenes in Peptidic Containers Prepared by Dynamic Chiral Self‐Sorting and Self‐Assembly

Molecular capsules composed of amino acid or peptide derivatives connected to resorcin[4]arene scaffolds through acylhydrazone linkers have been synthesized using dynamic covalent chemistry (DCC) and hydrogen‐bond‐based self‐assembly. The dynamic character of the linkers and the preference of the peptides towards self‐assembly into β‐barrel‐type motifs lead to the spontaneous amplification of formation of homochiral capsules from mixtures of different substrates. The capsules have cavities of around 800 ų and exhibit good kinetic stability. Although they retain their dynamic character, which allows processes such as chiral self‐sorting and chiral self‐assembly to operate with high fidelity, guest complexation is hindered in solution. However, the quantitative complexation of even very large guests, such as fullerene C₆₀ or C₇₀, is possible through the utilization of reversible covalent bonds or the application of mechanochemical methods. The NMR spectra show the influence of the chiral environment on the symmetry of the fullerene molecules, which results in the differentiation of diastereotopic carbon atoms for C₇₀, and the X‐ray structures provide unique information on the modes of peptide–fullerene interactions.     

Square Tiling by Square Macrocycles at the Liquid/Solid Interface: Co‐crystallisation with One‐ or Two‐Dimensional Order

We have systematically investigated the self‐assembled monolayers of seven bimolecular mixtures of square‐shaped pyridinophanes and cyclophanes bearing alkoxy or alkoxycarbonyl substituents in the presence of the tropylium ion as a marker of pyridinophanes at liquid/graphite interfaces by means of scanning tunnelling microscopy (STM). The purpose of this work was to elucidate the mixing behaviour of these macrocycles highlighting the formation of one‐ or two‐dimensionally ordered square tilings consisting of alternating alignments of different macrocycles as a result of attractive dipole–dipole or hydrogen‐bonding interactions; four co‐crystals differing in the dimensionality of the ordering of pyridinophane and cyclophane were observed. The different modes of interaction between the functional groups (ether or carbonyl group) in the side‐chains of the pyridinophanes and cyclophanes lead to the formation of co‐crystals with dimensionally different orderings of the two macrocycles. These observations revealed that a slight modification of the molecular structure may dramatically change the mixing behaviour and structures of the co‐crystals.     

5,12‐Diacetyl‐5,12‐dihydroquinoxalino[2,3‐b]quinoxalines: Solid‐State Fluorescence,AIE Properties,and Orbital Switching by Substituent Effect

The compound 5,12‐diacetyl‐5,12‐dihydroquinoxalino[2,3‐b]quinoxaline 1 a and its derivatives were prepared, and their solid‐ and solution‐state spectroscopic properties were studied; 1 a shows stronger fluorescence in solution than in the solid state due to aggregation caused by self‐quenching. Phenyl‐ or alkoxy‐substituted derivatives 1 b – d show solid‐state fluorescence with moderate quantum yields of about Φ=0.12–0.15, although the corresponding values are 0.01–0.07 in solution. The spectroscopic properties of alkoxy‐substituted derivatives were hardly changed compared to 1 a and 1 b , although 1 a and 1 b have similar absorption and fluorescence maxima in solution and in the solid state. DFT calculations indicate that orbital switching occurs between HOMO and HOMO‐1 and HOMO‐2 due to orbital interactions with introduced substituents. Crystal structure analysis revealed that the molecules have bent structures around tertiary nitrogen atoms and form a characteristic dimeric structure.     

Solvothermal Synthesis and Crystal Structures of Two Manganese Complexes [Mn(II)(acac−)2(4,4′‐bipy)]n (bipy=4,4′‐bipyridine) and [Mn(III)(acac−)3]·4CO(NH2)2

Two special manganese complexes [Mn(II)(acac^?)₂(4,4′‐bipy)]_n (bipy=4,4′‐bipyridine) (complex 1 ) and [Mn(III)(acac^?)₃]·4CO(NH₂)₂ (acacH=acetylacetone) (complex 2 ) were synthesized in the same strategy by solvothermal method. Single crystal X‐ray diffraction revealed the complex 1 consists of one‐dimensional infinite coordination chain, with the manganese centers bridged by 4,4′‐bipy. And free carbamides of complex 2 connect with each other through the hydrogen bonds to form a 14‐membered carbamide ring and a zig‐zag plane. Both enantiomers of Mn(III)(acac^?)₃ exist in the structure, forming a racemate. Furthermore, these enantiomers and those zig‐zag planes are linked with hydrogen bonds to form an unique spatial network.