A New Class of Boron Nanotube

The configurations, stability and electronic structures of a new class of boron sheet and related boron nanotubes are predicted within the framework of density functional theory. This boron sheet is sparser than those of recent proposals. Our theoretic results show that the stable boron sheet remains flat and is metallic. There are bands similar to the π‐bands in graphite near the Fermi level. Stable nanotubes with various diameters and chiral vectors can be rolled from the sheet. Within our study, only the thin (8, 0) nanotube with a band gap of 0.44 eV is semiconducting, while all the other thicker boron nanotubes are metallic, independent of their chirality. It indicates the possibility, in the design of nanodevices, to control the electronic transport properties of the boron nanotube through the diameter.     

Oxatriphyrins(2.1.1) Incorporating an ortho‐Phenylene Motif

An understanding of fundamental aspects of archetypal organic structural motifs remains a key issue faced by the experimental and theoretical chemists. Two possible bonding modes for a disubstituted benzene ring, that is a meta and para, determines the π delocalization for oligomeric structures. When the less abundant ortho‐substituted variant is introduced into a triphyrin(2.1.1) skeleton an aromatic molecule is obtained and the carbocyclic ring participates in the conjugation of the macrocycle. The two‐electron reduction and introduction of boron(III) changes the aromatic character and results in an anti‐aromatic structure which has been confirmed by single‐crystal analysis and supported by theoretical calculations.     

Aromaticity Effects on the Profiles of the Lowest Triplet‐State Potential‐Energy Surfaces for Rotation about the CC Bonds of Olefins with Five‐Membered Ring Substituents: An Example of the Impact of Baird’s Rule

A density functional theory study on olefins with five‐membered monocyclic 4n and 4n+2 π‐electron substituents (C₄H₃X; X=CH⁺, SiH⁺, BH, AlH, CH₂, SiH₂, O, S, NH, and CH^?) was performed to assess the connection between the degree of substituent (anti)aromaticity and the profile of the lowest triplet‐state (T₁) potential‐energy surface (PES) for twisting about olefinic C?C bonds. It exploited both Hückel’s rule on aromaticity in the closed‐shell singlet ground state (S₀) and Baird’s rule on aromaticity in the lowest ππ* excited triplet state. The compounds CH₂?CH(C₄H₃X) were categorized as set A and set B olefins depending on which carbon atom (C2 or C3) of the C₄H₃X ring is bonded to the olefin. The degree of substituent (anti)aromaticity goes from strongly S₀‐antiaromatic/T₁‐aromatic (C₅H₄⁺) to strongly S₀‐aromatic/T₁‐ antiaromatic (C₅H₄^?). Our hypothesis is that the shapes of the T₁ PESs, as given by the energy differences between planar and perpendicularly twisted olefin structures in T₁ [ΔE(T₁)], smoothly follow the changes in substituent (anti)aromaticity. Indeed, correlations between ΔE(T₁) and the (anti)aromaticity changes of the C₄H₃X groups, as measured by the zz‐tensor component of the nucleus‐independent chemical shift ΔNICS(T₁;1)_zz, are found both for sets A and B separately (linear fits; r²=0.949 and 0.851, respectively) and for the two sets combined (linear fit; r²=0.851). For sets A and B combined, strong correlations are also found between ΔE(T₁) and the degree of S₀ (anti)aromaticity as determined by NICS(S₀,1)_zz (sigmoidal fit; r²=0.963), as well as between the T₁ energies of the planar olefins and NICS(S₀,1)_zz (linear fit; r²=0.939). Thus, careful tuning of substituent (anti)aromaticity allows for design of small olefins with T₁ PESs suitable for adiabatic Z/E photoisomerization.     

Four Boron Atoms,Four Positive Charges,and Four Skeletal Electrons: A Fluorescent σ‐Aromatic Tetraborane(4)

Reaction of a ditriflatodiborane compound with the Lewis acids AlCl₃ or GaCl₃ leads to abstraction of the two triflate substituents and dimerization of the resulting dicationic diborane to give a σ‐aromatic tetracationic tetraborane with a planar, rhomboid B₄ core. The compound exhibits four skeletal σ‐electrons involved in two (3c,2e) bonds and represents the first stable fourfold base‐stabilized [B₄H₄]⁴⁺ analogue. The product is isolated from the reaction mixture in the form of bright orange crystals that display fluorescence. Further analysis shows that the new tetraborane(4) is stabilized in the solid state by the lattice energy. It exhibits an extremely high electron affinity and is only stable in solution after one‐electron reduction to the radical cation.     

Luminescent Di‐ and Trinuclear Boron Complexes Based on Aromatic Iminopyrrolyl Spacer Ligands: Synthesis,Characterization, and Application in OLEDs

New bis‐ and tris(iminopyrrole)‐functionalized linear (1,2‐(HNC₄H₃‐C(H)?N)₂‐C₆H₄ ( 2 ), 1,3‐(HNC₄H₃‐C(H)?N)₂‐C₆H₄ ( 3 ), 1,4‐(HNC₄H₃‐C(H)?N)₂‐C₆H₄ ( 4 ), 4,4′‐(HNC₄H₃‐C(H)?N)₂‐(C₆H₄‐C₆H₄) ( 5 ), 1,5‐(HNC₄H₃C‐(H)?N)₂‐C₁₀H₆ ( 6 ), 2,6‐(HNC₄H₃C‐(H)?N)₂‐C₁₀H₆ ( 7 ), 2,6‐(HNC₄H₃C‐(H)?N)₂‐C₁₄H₈ ( 8 )) and star‐shaped (1,3,5‐(HNC₄H₃‐C(H)?N‐1,4‐C₆H₄)₃‐C₆H₃ ( 9 )) π‐conjugated molecules were synthesized by the condensation reactions of 2‐formylpyrrole ( 1 ) with several aromatic di‐ and triamines. The corresponding linear diboron chelate complexes (Ph₂B[1,3‐bis(iminopyrrolyl)‐phenyl]BPh₂ ( 10 ), Ph₂B[1,4‐bis(iminopyrrolyl)‐phenyl]BPh₂ ( 11 ), Ph₂B[4,4′‐bis(iminopyrrolyl)‐biphenyl]BPh₂ ( 12 ), Ph₂B[1,5‐bis(iminopyrrolyl)‐naphthyl]BPh₂ ( 13 ), Ph₂B[2,6‐bis(iminopyrrolyl)‐naphthyl]BPh₂ ( 14 ), Ph₂B[2,6‐bis(iminopyrrolyl)‐anthracenyl]BPh₂ ( 15 )) and the star‐shaped triboron complex ([4′,4′′,4′′′‐tris(iminopyrrolyl)‐1,3,5‐triphenylbenzene](BPh₂)₃ ( 16 )) were obtained in moderate to good yields, by the treatment of 3 – 9 with B(C₆H₅)₃. The ligand precursors are non‐emissive, whereas most of their boron complexes are highly fluorescent; their emission color depends on the π‐conjugation length. The photophysical properties of the luminescent polyboron compounds were measured, showing good solution fluorescence quantum yields ranging from 0.15 to 0.69. DFT and time‐dependent DFT calculations confirmed that molecules 10 and 16 are blue emitters, because only one of the iminopyrrolyl groups becomes planar in the singlet excited state, whereas the second (and third) keeps the same geometry. Compound 13 , in which planarity is not achieved in any of the groups, is poorly emissive. In the other examples ( 11 , 12 , 14 , and 15 ), the LUMO is stabilized, narrowing the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO–LUMO), and the two iminopyrrolyl groups become planar, extending the size of the π‐system, to afford green to yellow emissions. Organic light‐emitting diodes (OLEDs) were fabricated by using the new polyboron complexes and their luminance was found to be in the order of 2400 cd m^?2, for single layer devices, increasing to 4400 cd m^?2 when a hole‐transporting layer is used.     

The Triboracyclopropenyl Dianion: The Lightest Possible Main‐Group‐Element Hückel π Aromatic

Hückel π aromaticity is typically a domain of carbon‐rich compounds. Only very few analogues with non‐carbon frameworks are currently known, all involving the heavier elements. The isolation of the triboracyclopropenyl dianion is presented, a boron‐based analogue of the cyclopropenyl cation, which belongs to the prototypical class of Hückel π aromatics. Reduction of Cl₂BNCy₂ by sodium metal produced [B₃(NCy₂)₃]^2?, which was isolated as its dimeric Na⁺ salt (Na₄[B₃(NCy₂)₃]₂?2 DME; 1 ) in 45 % yield and characterized by single‐crystal X‐ray diffraction. Cyclic voltammetry measurements established an extremely high oxidation potential for 1 (E_pc=?2.42 V), which was further confirmed by reactivity studies. The Hückel‐type π aromatic character of the [B₃(NCy₂)₃]^2? dianion was verified by various theoretical methods, which clearly indicated π aromaticity for the B₃ core of a similar magnitude to that in [C₃H₃]⁺ and benzene.     

A Metal‐Bridged Tricyclic Aromatic System: Synthesis of Osmium Polycyclic Aromatic Complexes

Aromaticity is one of the most important concepts in organic chemistry. A variety of metalla‐aromatic compounds have been recently prepared and in most of those examples, the metal participates only in a monocyclic ring. In contrast, metal‐bridged bicyclic aromatic molecules, in which a metal is shared between two aromatic rings, have been less developed. Herein, we report the first metal‐bridged tricyclic aromatic system, in which the metal center is shared by three aromatic five‐membered rings. These metalla‐aromatics are formed by reaction between osmapentalyne and arene nucleophiles. Experimental results and theoretical calculations reveal that the three five‐membered rings around the osmium center are aromatic. In addition, the broad absorption bands in the UV/Vis absorption spectra of these novel aromatic systems cover almost the entire visible region. This straightforward synthetic strategy may be extended to the synthesis of other metal‐bridged polycyclic aromatics.     

Origin of the Electron Transport Properties of Aromatic and Antiaromatic Single Molecule Circuits

Antiaromatic molecules have been predicted to exhibit increased electron transport properties when placed between two nanoelectrodes compared to their aromatic analogues. While some studies have demonstrated this relationship, others have found no substantial increase. We use atomistic simulations to establish a general relationship between the electronic spectra of aromatic, antiaromatic, and quinoidal molecules and illustrate its implications for electron transport. We compare the electronic properties of a series of aromatic-antiaromatic counterparts and show that antiaromaticity effectively p-dopes the aromatic electronic spectra. As a consequence, the conducting properties of aromatic-antiaromatic analogues are closely related. For similar attachment points to the electrodes, an interference feature is expected in the HOMO-LUMO gap of one whenever it is absent in the other one. We demonstrate how the relative conductance of aromatic-antiaromatic pairs can be tuned and even reversed through the choice of chemical linker groups. Our work provides a general picture relating connectivity, (anti)aromaticity, and quantum interference and establishes new design rules for single molecule circuits.     

Computational Studies on Non‐covalent Interactions of Carbon and Boron Fullerenes with Graphene

First‐principles DFT calculations are carried out to study the changes in structures and electronic properties of two‐dimensional single‐layer graphene in the presence of non‐covalent interactions induced by carbon and boron fullerenes (C₆₀, C₇₀, C₈₀ and B₈₀). Our study shows that larger carbon fullerene interacts more strongly than the smaller fullerene, and boron fullerene interacts more strongly than that of its carbon analogue with the same nuclearity. We find that van der Waals interactions play a major role in governing non‐covalent interactions between the adsorbed fullerenes and graphene. Moreover, a greater extent of van der Waals interactions found for the larger fullerenes, C₈₀ and B₈₀, relative to smaller C₆₀, and consequently, results in higher stabilisation. We find a small amount of electron transfer from graphene to fullerene, which gives rise to a hole‐doped material. We also find changes in the graphene electronic band structures in the presence of these surface‐decorated fullerenes. The Dirac cone picture, such as that found in pristine graphene, is significantly modified due to the re‐hybridisation of graphene carbon orbitals with fullerenes orbitals near the Fermi energy. However, all of the composites exhibit perfect conducting behaviour. The simulated absorption spectra for all of the graphene–fullerene hybrids do not exhibit a significant change in the absorption peak positions with respect to the pristine graphene absorption spectrum. Additionally, we find that the hole‐transfer integral between graphene and C₆₀ is larger than the electron‐transfer integrals and the extent of these transfer integrals can be significantly tuned by graphene edge functionalisation with carboxylic acid groups. Our understanding of the non‐covalent functionalisation of graphene with various fullerenes would promote experimentalists to explore these systems, for their possible applications in electronic and opto‐electronic devices.