Lipidic Carbo-benzenes: Molecular Probes of Magnetic Anisotropy and Stacking Properties of α-Graphyne

Exploring the enhancement of the thermoelectric properties of bilayer graphyne nanoribbons

Carbon materials are vital for sustainable energy applications based on abundant and non-toxic raw materials. In this scenario, carbon nanoribbons have superior thermoelectric properties in comparison with their 2D material counterparts, owing to their particular electronic and transport properties. Therefore, we explore the electronic and thermoelectric properties of bilayer α-graphyne nanoribbons (α-BGyNRs) by means of density functional theory, tight-binding, and the non-equilibrium Green's functions (NEGF) method. Our calculations indicate that Ab stacking is the most stable configuration regardless of the edge type. The band structure presents finite band gaps with different features for armchair and zigzag nanoribbons. Concerning the thermoelectric quantities, the Seebeck coefficient is highly sensitive to the width and edge type, while its room-temperature values can achieve a measurable mV K^-1 scale. The electric conductance is found to increase due to layering, thus enhancing the power factor for α-BGyNRs compared with single nanoribbons. These findings therefore indicate the possibility of engineering such systems for thermal nanodevices.

Carbo-mer of Barrelene: A Rigid 3D-Carbon-Expanded Molecular Barrel

After extensive studies of 1D and 2D skeletal carbo-mers based on C₈ π-conjugating dialkynylbutatriene units (DABs: ∼C≡C-(R)C=C=C=C(R)-C≡C∼) bridging sp or sp² centers in carbo-butene, carbo-xylylene or carbo-benzene derivatives, 3D versions are envisaged through carbo-barrelenes and partially reduced derivatives thereof where two or three DAB blades span a bridge between sp³ carbinol vertices or ether thereof. For R=Ph, stable representatives were synthesized through a pivotal [6]pericyclynedione, and extensively characterized by spectroscopic, electrochemical and crystallographic methods. Density functional theory calculations allow detailed analysis of structural and electronic features of the 7 Å high C₂₆ barrel-shaped molecules, and show that they can behave as cages for ionic species. Beyond aesthetical concerns, the results could open prospects of applications in host-guest supramolecular chemistry and single molecule charge transport.

Steric/π-Electronic Insulation of the carbo-Benzene Ring: Dramatic Effects of tert-Butyl versus Phenyl Crowns on Geometric, Chromophoric, Redox, and Magnetic Properties

Hexa-tert-butyl-carbo-benzene (C₁₈ tBu₆ ) and three phenylated counterparts (C₁₈ tBu_m Ph_6-m ; m=4, 2) have been synthesized. The peralkylated version (m=6) provides experimental access to intrinsic features of the insulated C₁₈ core independently from the influence of π-conjugated substituent. Over the series, structural, spectroscopic, and electrochemical properties are compared with those of the hexaphenylated reference (m=0). Anchoring tBu substituents at the C₁₈ macrocycle is shown to enhance stability and solubility, and to dramatically modify UV/Vis absorption and redox properties. Whereas all carbo-benzenes reported previously were obtained as dark-reddish/greenish solids, crystals and solutions of C₁₈ tBu₆ happen to be yellow (λ_max =379 vs. 472 nm for C₁₈ Ph₆ ). In comparison to C₁₈ Ph₆ , the reduction of C₁₈ tBu₆ remains reversible, but occurs at twice as high an absolute potential (E_1/2 =-1.36 vs. -0.72 V). Systematic XRD analyses and DFT calculations show that the C₁₈ ring symmetry is the nearest to D_6h for m=6, which indicates a maximum geometric aromaticity. According to calculated nucleus-independent chemical shifts (NICS), the macrocyclic magnetic aromaticity is also maximum for C₁₈ tBu₆ : NICS(0)=-17.2 ppm versus (-18.0±0.1) ppm for the theoretical references C₁₈ H₆ and C₁₈ F₆ , and -13.5 ppm for C₁₈ Ph₆ . Accurate correlations of NICS(0) with experimentally recorded or calculated maximum UV/Vis absorption wavelengths, λ_max , and chemical hardness, η=E_LUMO -E_HOMO , are evidenced.

Carbo-biphenyls and Carbo-terphenyls: Oligo(phenylene ethynylene) Ring Carbo-mers

Ring carbo-mers of oligo(phenylene ethynylene)s (OPEn, n=0-2), made of C₂ -catenated C₁₈ carbo-benzene rings, have been synthesized and characterized by NMR and UV-vis spectroscopy, crystallography and voltammetry. Analyses of crystal and DFT-optimized structures show that the C₁₈ rings preserve their individual aromatic character according to structural and magnetic criteria (NICS indices). Carbo-terphenyls (n=2) are reversibly reduced at ca. -0.42 V/SCE, i.e. 0.41 V more readily than the corresponding carbo-benzene (-0.83 V/SCE), thus revealing efficient inter-ring π-conjugation. An accurate linear fit of E_1/2^red1 vs. the DFT LUMO energy suggests a notably higher value (-0.30 V/SCE) for a carbo-quaterphenyl congener (n=3). Increase with n of the effective π-conjugation is also evidenced by a red shift of two of the three main visible light absorption bands, all being assigned to TDDFT-calculated excited states, one of them restricting to a HOMO→LUMO main one-electron transition.

Phonon thermal transport in a class of graphene allotropes from first principles

Utilizing first principle calculations combined with the phonon Boltzman transport equation (PBTE), we systematically investigate the phonon thermal transport properties of α, β and γ graphyne, a class of graphene allotropes.

Selective access to p-dialkyl-carbo-benzenes from a [6]pericyclynedione: the n-butyl nucleophile model for a metal switch study

The synthesis, spectroscopic properties, comparative electrochemical behavior in CHCl3 vs CH2Cl2, and X-ray crystal structure of p-di-n-butyl-tetraphenyl-carbo-benzene are described. The selectivity of preparation of the ultimate [6]pericyclynediol precursor has been examined by comparing the reactivity of the [6]pericyclynedione substrate with four n-Bu-MXn nucleophiles involving more or less halogenated metal centers MXn (0 ≤ n ≤ 2): Li, MgBr, MgCl, CeCl2/LiCl. The cerium reagent is found to be the most efficient, giving approximately twice the yield given by Grignard reagents in the target diadduct (90% vs 51%–53%). The dibutyl-carbo-benzene product happens to be soluble in both CHCl3 and CH2Cl2: cyclic voltammograms of either solution exhibit almost identical peak potentials with reduced reversibility of the redox processes in CHCl3.

Hexaaryl-carbo-benzenes revisited: a novel synthetic route, crystallographic data, and prospects of electrochemical behavior

Hexaphenyl-carbo-benzene and a substituted homologue were prepared via an efficient route, and characterized by crystallography and voltammetry with the view of studying charge transport properties within thin films.

3D and 2D supramolecular assemblies and thermotropic behaviour of a carbo-benzenic mesogen

A carbo-benzenic mesogen gives rise to a tubular-columnar discotic liquid crystal at 115 °C, the rectangular arrangement of which is identical in STM images on an HOPG surface.