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In this Perspective, we present an overview of recent fundamental studies on the nature of the interaction between individual metal atoms and metal clusters and the conjugated surfaces of graphene and carbon nanotube with a particular focus on the electronic structure and chemical bonding at the metal–graphene interface. We discuss the relevance of organometallic complexes of graphitic materials to the development of a fundamental understanding of these interactions and their application in atomtronics as atomic interconnects, high mobility organometallic transistor devices, high-frequency electronic devices, organometallic catalysis (hydrogen fuel generation by photocatalytic water splitting, fuel cells, hydrogenation), spintronics, memory devices, and the next generation energy devices. We touch on chemical vapor deposition (CVD) graphene grown on metals, the reactivity of its surface, and its use as a template for asymmetric graphene functionalization chemistry (ultrathin Janus discs). We highlight som...

We report the synthesis and properties of a series of spiro-bis(3,4,6,7-tetrachalcogenide-substituted-phenalenyl)boron salts and two of the corresponding tetrathioalkyl-substituted spiro-bis(phenalenyl)boron radicals [tetrathiomethyl (10) and tetrathioethyl (11)] in which all of the active positions of the phenalenyl (PLY) nucleus are functionalized. In the solid state, radicals 10 and 11 exist as a weak π-dimers due to the steric congestion of the thioalkyl groups in the superimposed PLY units. As a result, the spins are localized in the isolated (nonsuperimposed) PLY rings, and the structure, magnetic susceptibility measurements, and band structure calculations confirm that these PLY units are unable to undergo strong intermolecular interaction as a result of the orientation of the thioalkyl groups.

Encapsulation of sulfur in HiPCO-SWNTs leads to large changes in the Raman spectra with the appearance of new peaks at 319, 395, and 715 cm–1 which originate from the sulfur species within the SWNTs, while the high frequency SWNT bands (ν > 1200 cm–1) are decreased in intensity. The encapsulated species also shifts the near-IR interband electronic transitions to lower energy by more than 10%. These effects seem to originate with the van der Waals interaction of the confined sulfur species with the walls of the SWNTs which are not expected to be significant in the case of the previously studied large diameter SWNTs. We suggest that sulfur in the small diameter SWNTs exists as a helical polymeric sulfur chain that enters the SWNT interior in the form of S2 (3Σg–) molecules which undergo polymerization to linear diradicals. Fil: Li, Guanghui. University of California; Estados Unidos Fil: Fu, Chengyin. University of California; Estados Unidos Fil: Oviedo, María Belén. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina. University of California; Estados Unidos Fil: Chen, Mingguang. University of California; Estados Unidos Fil: Tian, Xiaojuan. University of California; Estados Unidos Fil: Bekyarova, Elena. University of California; Estados Unidos Fil: Itkis, Mikhail. University of California; Estados Unidos Fil: Wong, Bryan. University of California; Estados Unidos Fil: Guo, Juchen. University of California; Estados Unidos Fil: Haddon, Robert. University of California; Estados Unidos

We report the synthesis, crystallization, and solid-state characterization of the 3,7-ethoxy-substituted spirobiphenalenyl-boron neutral radical 22. The radical is distinguished by its low disproportionation energy and one-dimensional structure. We show that our strategy of substitution of OEt group at the active positions of the phenalenyl units changes the crystal packing from its previously known OMe analogue and the solid-state properties are dictated by the partial π-stack structure and the oxygen atoms at the 3,7-positions and can be best rationalized in terms of the resonating valence bond model. Magnetic susceptibility measurements show that in the solid state the radical remains paramagnetic but there is significant spin-spin interaction between the molecules. Band structure calculations reflect efficient overlap between the molecules along the π stack and show evidence of interactions between the spin-bearing oxygen atoms. The room temperature electrical conductivity (σ(RT)=2.0×10(-2) S cm(-1)) of 22 is higher than that observed in previously known one-dimensional phenalenyl radicals.

We report the preparation of the first benzannulated phenalenyl neutral radical conductor (18), and we show that the compound displays unprecedented solid state behavior: the structure is dominated by two sets of intermolecular interactions: (1) a pi-chain structure with superimposed pi-overlap of the benzannulated phenalenyls along [0 0 1], and (2) an interchain overlap involving a pair of carbon atoms (C4) along [0 1 0]. The pi-chain-type stacking motif is reminiscent of previously reported phenalenyl radicals and the room temperature structure (space group P2/c) together with the conductivity of sigma(RT) = 0.03 S/cm and the Pauli-like magnetic susceptibility are best described by the resonating valence bond (RVB) model. The interchain interaction is unstable with respect to the formation of a sigma-charge density wave (sigma-CDW) involving pairs of C4 carbon atoms between adjacent radicals and this phase is characterized by the P2(1)/c space group which involves a doubling of the unit cell along the [0 1 0] direction. The RVB and CDW phases compete for structural occupancy throughout the whole temperature range (15-293 K) with the RVB phase predominating at 15 and 293 K and the sigma-CDW phase achieving a maximum structural occupancy of about 60% at 150 K where it produces clearly discernible effects on the magnetism and conductivity.

We report the preparation, crystallization, and solid-state characterization of the first member of a new family of tris(1,9-disubstituted phenalenyl)silicon neutral radicals. In the solid state, the radical packs as weak partial pi-dimers with intermolecular carbon...carbon contacts that fall at the van der Waals atomic separation. Magnetic susceptibility measurements indicate approximately 0.7 Curie spins per molecule from room temperature down to 50 K, below which antiferromagnetic coupling becomes apparent; the compound has a room-temperature single-crystal conductivity of sigmaRT = 2.4 x 10(-6) S cm(-1).

We report the substitutional doping of solid-state spiro-bis(5-methyl-1,9-oxido-phenalenyl)boron radical ([2]2B) by co-crystallization of this radical with the corresponding spiro-bis(5-methyl-1,9-oxido-phenalenyl)beryllium compound ([2]2Be). The pure compounds crystallize in different space groups ([2]2B, P1̅, Z = 2; [2]2Be, P2₁/c, Z = 4) with distinct packing arrangements, yet we are able to isolate crystals of composition [2]2B(1-x)Be(x), where x = 0-0.59. The phase transition from the P1̅ to the P2₁/c space group occurs at x = 0.1, but the conductivities of the solid solutions are enhanced and the activation energies reduced for values of x = 0-0.25. The molecular packing is driven by the relative concentration of the spin-bearing ([2]2B) and spin-free ([2]2Be) molecules in the crystals, and the extended Hückel theory band structures show that the progressive incorporation of spin-free [2]2Be in the lattice of the [2]2B radical (overall bandwidth, W = 1.4 eV, in the pure compound) leads to very strong narrowing of the bandwidth, which reaches a minimum at [2]2Be (W = 0.3 eV). The results provide a graphic picture of the structural transformations undergone by the lattice, and at certain compositions we are able to identify distinct structures for the [2]2B and [2]2Be molecules in a single crystalline phase.

Poly(aminobenzene sulfonic acid) (PABS) and polyethylene glycol (PEG) were covalently attached to single-walled carbon nanotubes (SWNTs) to form water-soluble graft copolymers. Quantitative near-IR (NIR) spectroscopic studies of these SWNT graft copolymers indicate a water solubility of about 5 mg/mL, and atomic force microscopy studies show a fairly uniform length and diameter. On the basis of thermogravimetric analysis, the loading of SWNTs in the graft copolymers is estimated to be 30% for SWNT-PABS and 71% for SWNT-PEG. NIR spectroscopic studies of SWNT-PABS show that this graft copolymer has a ground state that is a hybrid of the electronic structures of the isolated PABS and SWNT macromolecules.

We report the preparation, crystallization, and solid-state characterization of the first members of a new family of spiro-bis-(1,9-diamino-substituted-phenalenyl)boron neutral radicals. The crystal structures show that the three radicals are monomeric and without close contacts in the crystal lattice. In all cases magnetic susceptibility measurements confirm the presence of free radicals with one unpaired spin per molecule. Two of the new radical compounds are among the most highly conducting neutral organic solids, with room-temperature conductivities reaching sigma(RT) = 4 x 10(-2) S/cm. The measured conductivities correlate with the closest intermolecular contacts in the solid state and with the calculated band dispersions, even though the bandwidths are much smaller than those found in other organic conductors.

We present a study on the electronic behavior of films of as-prepared and purified single-walled carbon nanotubes (SWNTs) and demonstrate the important role that chemical functionalization plays in modifying their electronic properties, which in turn throws further light on the mechanism of action of SWNT-based sensors. Films of electric arc SWNTs were prepared by spraying, and optical spectroscopy was used to measure the effective film thickness. The room-temperature conductivities (sigma(RT)) of thin films deposited from as-prepared and purified SWNTs are in the range sigma(RT) = 250-400 S/cm, and the nonmetallic temperature dependence of the conductivity indicates the presence of tunneling barriers, which dominate the film conductivity. Chemical functionalization of SWNTs with octadecylamine (ODA) and poly(m-aminobenzenesulfonic acid) (PABS) significantly decreases the conductivity; sigma(RT) = 3 and 0.3 S/cm for SWNT-ODA and SWNT-PABS, respectively.

We report a chemical processing technology that allows the continuous spinning of single-walled carbon nanotubes (SWNTs)-nylon 6 (PA6) fibers by the in-situ polymerization of caprolactam in the presence of SWNTs, which simultaneously optimizes the morphology of the composite. We show that caprolactam is an excellent solvent for carboxylic-acid-functionalized SWNTs (SWNT-COOH) and that this allows the efficient dispersal of the SWNTs and subsequent grafting of PA6 chains to the SWNTs through condensation reactions between the carboxylic-acid group on SWNT-COOH and the terminal amine group of PA6. The existence of a graft copolymer between the PA6 chains and the SWNTs is demonstrated by IR, TGA, and AFM studies, and we show that the solubility of the polymerized material in formic acid is controlled by the degree of graft copolymerization. The amount of grafted PA6 chains that are attached to the SWNTs can be adjusted by controlling the concentration of the initiator (6-aminocaproic acid). The process leads to a uniform dispersion of the SWNTs, and the presence of the graft copolymer increases the polymer/SWNT compatibility while strengthening the interfacial interaction between the nanotube and matrix. The Young's modulus, tensile strength, and thermal stability of the SWNT-reinforced composite fibers produced by this process are significantly improved.

The influence of Ni−Y catalyst composition on the bulk scale synthesis of single-walled carbon nanotubes (SWNTs) is studied using solution phase near-infrared spectroscopy. We found that a wide ran...

We report the preparation, crystallization, and solid-state characterization of a benzyl-substituted spirobiphenalenyl radical. The crystal structure shows that the radical is monomeric in the solid state, with the molecules packed in an unusual one-dimensional (1-D) fashion that we refer to as a pi-step stack. This particular mode of 1-D stacking is forced on the lattice arrangement by the presence of the orthogonal phenalenyl units that were specifically incorporated to prevent the crystallization of low-dimensional structures. The structure shows that this strategy is effective, and neighboring molecules in the stack can only interact via the overlap of one pair of active (spin-bearing) carbon atoms per phenalenyl unit, leading to the pi-step structure in which the remaining four active carbon atoms per phenalenyl unit do not interact with nearest neighbor molecules. The magnetic susceptibility data in the temperature range 4-360 K may be fit to an antiferromagnetic Heisenberg S = 1/2 linear chain model with intrachain spin coupling J = -52.3 cm(-1). Despite the uniform stacking, the material has a room temperature conductivity of 1.4 x 10(-3) S/cm and is best described as a Mott insulator.

New synthetic routes to 1,2,3-dithiazolo-1,2,3-dithiazolylium salts, based on double Herz condensations of N-alkylated 2,6-diaminopyridinium salts with sulfur monochloride, have been developed. The two prototypal 1,2,3-dithiazolo-1,2,3-dithiazolyl radicals HBPMe and HBPEt have been prepared and characterized in solution by cyclic voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-31G) gas-phase disproportionation enthalpies favor a low on-site Coulombic repulsion energy U in the solid state. The crystal structures of HBPR (R = Me, Et) have been determined by X-ray crystallography (at 293 K). Both consist of slipped pi-stacks of undimerized radicals, with many close intermolecular S- - -S contacts. Magnetic, conductivity, and optical measurements have been performed and the results interpreted in light of extended Hückel band calculations. The crystalline materials are paramagnetic above 100 K, with room-temperature conductivities sigma(RT) of 10(-5)-10(-6) S cm(-1); the slightly greater conductivity of the R = Et compound can be associated with a more well developed band structure. We suggest a Mott-Hubbard insulator ground state for these materials, with an on-site Coulomb repulsion energy U of about 1.0 eV.

We report the pressure dependence for the radial (omega R) and tangential (omega T) band frequencies in debundled single-walled carbon nanotubes (SWNTs) derived from laser-synthesized SWNT bundles. As previously described, a chemical procedure was used to prepare debundled SWNTs from as-prepared, large SWNT bundles. The normalized pressure coefficient for omega R in the debundled sample was compared with the corresponding value in the bundled sample to quantify the strength of van der Waals interactions between tubes in these nanotube materials. Furthermore, the pressure dependences for the radial (omega R) and tangential (omega T) band frequencies in debundled tubes were also compared with corresponding dependences predicted for isolated SWNTs, obtained with generalized tight binding molecular dynamic (GTBMD) simulations described in our previous work. The results presented here collectively suggest that the van der Waals interaction is still strong in the debundled sample studied here, which contained predominantly small bundles of SWNTs rather than isolated tubes.

Alkylation of the zwitterionic heterocycle 8-chloro-bis[1,2,3]dithiazolo[4,5-b:5',4'-e]pyridine (ClBP) with alkyl triflates affords 8-chloro-4-alkyl-4H-bis[1,2,3]dithiazolo[4,5-b:5',4'-e]pyridin-2-ium triflates [ClBPR][OTf] (R = Me, Et, Pr). Reduction of these salts with decamethylferrocene affords the corresponding ClBPR radicals as thermally stable crystalline solids. The radicals have been characterized in solution by cyclic voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-31G) gas-phase disproportionation enthalpies are consistent with a low on-site Coulombic barrier U to charge transfer in the solid state. The crystal structures of ClBPR (R = Me, Et, Pr) have been determined by X-ray crystallography (at 293 K). All three structures consist of slipped pi-stacks of undimerized radicals, with many close intermolecular S.S contacts. ClBPMe undergoes a phase transition at 93 K to a slightly modified slipped pi-stack arrangement, the structure of which has also been established crystallographically (at 25 K). Variable-temperature magnetic and conductivity measurements have been performed, and the results interpreted in light of extended Hückel band calculations. The room-temperature conductivities of ClBPR systems (sigma(RT) approximately 10(-)(5) to 10(-)(6) S cm(-)(1)), as well as the weak 1D ferromagnetism exhibited by ClBPMe, are interpreted in terms of weak intermolecular overlap along the pi-stacks. The latter is caused by slippage of the molecular plates, a feature necessitated by the steric size of the R and Cl groups on the pyridine ring.

The hydrofullerenes C(60)H(2) (1) and C(60)H(6) (2) have been prepared in (13)C-enriched form and 2D INADEQUATE NMR spectra were measured. These spectra have provided unambiguous (13)C assignments for 2, and nearly unambiguous assignments for 1. In both cases, the most downfield resonances are immediately adjacent to the sp(3) carbons, despite the fact that these carbons are the least pyramidalized carbons in the molecule. Typically, (13)C chemical shifts move downfield with increasing pyramidalization (THETA(p)), but in these systems there is no strong correlation between THETA(p) and delta. HF-GIAO calculations are able to predict the chemical shifts, but provide little chemical insight into the origin of these chemical shifts. London theory reveals a significant paramagnetic ring current in 1, a feature that helps explain the (1)H shifts in these compounds and may contribute to the (13)C chemical shifts as well.

Purification of single-walled carbon nanotubes by capillary electrophoresis (CE) is demonstrated. Real-time Raman spectroscopy of the separation process and single-wavelength UV/vis detection show the ability of CE to provide high-resolution separations of nanotube fractions with baseline separation. AFM images of collected fractions demonstrate that separations are based on tube length. The separation method is suggested to be based on alignment of the nanotubes along the separation field.

We report the crystallization of a subsitutionally doped organic conductor based on a host lattice composed of spiro-bis(phenalenyl)boron radicals. Co-crystallization of solutions of spiro-bis(9-oxidophenalenone)boron radical [PLY(O,O)]2B mixed with selected amounts of spiro-bis(9-oxidophenalenone)beryllium [PLY(O,O)]2Be leads to the formation of a series of solid-state solutions of composition [PLY(O,O)]2B(1-x)Be(x). The dopant molecules [PLY(O,O)]2Be serve to introduce holes into the lattice of spins provided by the [PLY(O,O)]2B radicals and lead to a systematic increase in the conductivity while decreasing the activation energy of the conduction process and leaving the solid-state structure relatively unperturbed. While the energies of the hole sites are expected to be high, the results are consistent with the interpretation of the electronic structure of [PLY(O,O)]2B in terms of the resonating valence bond model.

In this Perspective, we present an overview of recent fundamental studies on the nature of the interaction between individual metal atoms and metal clusters and the conjugated surfaces of graphene and carbon nanotube with a particular focus on the electronic structure and chemical bonding at the metal–graphene interface. We discuss the relevance of organometallic complexes of graphitic materials to the development of a fundamental understanding of these interactions and their application in atomtronics as atomic interconnects, high mobility organometallic transistor devices, high-frequency electronic devices, organometallic catalysis (hydrogen fuel generation by photocatalytic water splitting, fuel cells, hydrogenation), spintronics, memory devices, and the next generation energy devices. We touch on chemical vapor deposition (CVD) graphene grown on metals, the reactivity of its surface, and its use as a template for asymmetric graphene functionalization chemistry (ultrathin Janus discs). We highlight som...

We report the preparation, crystallization, and solid-state characterization of ethyl (3)- and butyl (4)-substituted spiro-biphenalenyl radicals. Both of these compounds are found to be conducting face-to-face pi-dimers in the solid state but with different room-temperature magnetic ground states. At room temperature, 4 exists as a diamagnetic pi-dimer (interplanar separation of approximately 3.1 A), whereas 3 is a paramagnetic pi-dimer (interplanar separation of approximately 3.3 A), and both compounds show phase transitions between the paramagnetic and diamagnetic forms. Electrical resistivity measurements of single crystals of 3 and 4 show that the transition from the high-temperature paramagnetic pi-dimer form to the low-temperature diamagnetic pi-dimer structure is accompanied by an increase in conductivity by about 2 orders of magnitude. This behavior is unprecedented and is very difficult to reconcile with the usual understanding of a Peierls dimerization, which inevitably leads to an insulating ground state. We tentatively assign the enhancement in the conductivity to a decrease in the on-site Coulombic correlation energy (U), as the dimers form a super-molecule with twice the amount of conjugation.

We report the preparation, crystallization, and solid-state characterization of a spiro-biphenalenyl radical. The crystal structure shows that the radical is monomeric in the solid state and withou...

Reductive coupling of 4,5-dichloro-1,2,3-dithiazolylium chloride yields trans-4,4‘-dichloro-1,1‘,2,2‘,3,3‘-tetrathiadiazafulvalene (DC-TAF), the first example of this heterofulvalene system. Ab initio molecular orbital (B3LYP/6-31G**) calculations on prototypal TAF confirm that the closed shell 1Ag state lies 22 kcal mol-1 below the 3Bu diradical triplet. Cyclic voltammetry on DC-TAF reveals two reversible oxidation waves at 0.80 and 1.25 V (in CH3CN, reference SCE). The ESR signal (g = 2.0117) of the radical cation [DC-TAF]+ (in SO2(l)) exhibits a five-line hyperfine coupling pattern with aN = 0.096 mT. DC-TAF forms a series of 1:1 radical ion salts [DC-TAF][X] by electrooxidation in the presence of tetrahedral counterions (X- = BF4-, ClO4-, FSO3-). The crystal structures of these salts are isomorphous, monoclinic space group P21/n, and consist of one-dimensional ladder-like arrays of [DC-TAF]+ radical cations bridged by S- - -S contacts ranging from 3.5 to 3.7 A. Variable-temperature conductivity and ma...

The synthesis and solid-state characterization of the heterocyclic π-radical 1,2,5-thiadiazolo[3,4-b]-1,2,3-dithiazolo[3,4-b]pyrazin-2-yl, 1,2,3-TDTA, is described. The ESR spectrum of 1,2,3-TDTA (in CH2Cl2, 293 K, g = 2.009) confirms a highly delocalized spin distribution, with observable hyperfine coupling to all five nitrogen atoms of the tricyclic molecule (aN = 0.514, 0.343, 0.109, 0.051, and 0.045 mT). While chemical and electrochemical oxidation (E1/2(ox) = 1.14 V vs SCE) of 1,2,3-TDTA requires relatively harsh conditions, reduction is extremely facile (E1/2(red) = 0.15 V vs SCE). More importantly both the observed cell potential Ecell and computed (MNDO) gas-phase enthalpy ΔHdisp for the disproportionation of this and other 1,2,3-dithiazolyls are significantly lower than those observed for their 1,3,2-isomers. Crystals of 1,2,3-TDTA are monoclinic P21/n, with a = 6.6749(16) A, b = 11.7178(14) A, c = 8.6148(14) A, β = 103.297(16)°, and Z = 4. The crystal structure consists of slipped stacks of head...

This article reports the first investigation into the regiochemistry of addition to the fulleroid C61H2 by Zn(Cu) reduction and hydroboration. Two major isomers of C61H4 are formed by the reduction with Zn(Cu) while only one major isomer is formed by hydroboration. The structures of the major isomers formed by reduction with Zn(Cu) were identified as 1,2-C61H4 and 3,4-C61H4. The 1,2-C61H4 isomer is the only dominant isomer formed by hydroboration with no indications of the 3,4-C61H4 isomer being formed. The regiochemistry observed in the formation of 1,2-C61H4 is the same regiochemistry seen in the further reactivity of azafulleroids (C60NR). Strain energies (calculated at the B3LYP-6-31G* level of theory) show that the relief of strain is greater for the hydrogenation of the fulleroid C61H2 than it is for the hydrogenation of C60 itself. This indicates that the twisted, anti-Bredt's rule, double bonds of the fulleroid are a source of greater localized strain than the pyramidalization of the carbons in th...

The reduction of 3He@C60 and 3He@C70 by lithium metal to give solutions of the corresponding hexaanions in THF-d8 has been achieved under gentle conditions, at subambient temperatures, without sonication, by capitalizing on the ability of corannulene (1) to serve as an efficient electron carrier between the lithium metal and the solid fullerenes, which are virtually insoluble in the reaction medium. The 3He inside the C60 hexaanion is found to be more strongly shielded (by nearly 20 ppm!) than any previously reported 3He in a fullerene [δ(3He@C606-) = −48.7 ppm, relative to dissolved 3He gas in the solution], whereas the 3He inside the C70 hexaanion is actually deshielded [δ(3He@C706-) = +8.3 ppm], resonating at nearly 15 ppm lower field than any previously reported 3He in a fullerene. These results stand in complete accord with earlier predictions that the magnetic properties of C60 and C70 would be altered dramatically, and in opposite directions, by reduction of the fullerenes to their hexaanions. The ...

Electroreduction of the 2,5-thiophene-bridged bis(1,2,3,5-diselenadiazolylium) salt [T-2,5-Se][SbF6]2 in acetonitrile, at a Pt wire and in the presence of iodine, affords a highly conductive ( sigma = 102 S cm-1 at 293 K) 1:1 charge-transfer (CT) salt [T-2,5-Se][I], the crystal structure of which has been determined by single-crystal X-ray diffraction. The crystals belong to the orthorhombic space group Fm2m, a = 3.544(2), b = 10.9808(16), c = 31.464(5) Å , V = 1224.5(7) Å 3. The structure consists of perfectly superimposed pi -stacks of molecular units bridged by columns of disordered iodines. This packing motif is similar to that of the related 1,3-benzene-bridged derivative [1,3-Se][I], but the lateral intermolecular Se···Se interactions linking adjacent pi -stacks are considerably shorter, indicative of a more isotropic electronic structure for [T-2,5-Se][I]. Magnetic susceptibility measurements on [T-2,5-Se][I] nonetheless indicate a phase transition to a diamagnetic state near 200 K, behaviour similar to that observed for [1,3-Se][I]. The electronic structures and transport properties of the two compounds are discussed in the light of extended Hückel band-structure calculations.Key words: diselenadiazolyl, diradical, charge-transfer salt, magnetic susceptibility, crystal structure.

Addition of benzyne to C70 results in four isomeric monoadducts (compounds 1a−d) in a 42:35:13:10 ratio as determined by 1H NMR. These compounds were separated by repeated passes through HPLC columns. The major isomer (1a) resulted from addition to the highly pyramidalized C1−C2 bond as shown by 13C NMR and UV/vis spectroscopy. The structure of the C1−C2 adduct (1a) was confirmed by X-ray crystallography. The second isomer (1b) was assigned as an adduct to the C5−C6 bond on the basis of 13C NMR and UV/vis spectroscopy. Compound 1c exhibited a 13C spectrum consistent with an adduct to the C7 and C8 positions. The presence of sp3 13C NMR resonances proved that the C7−C8 bond is still intact, making this the first identification of direct addition to a 5−6 ring fusion in a fullerene, and the first example of an adduct to a 5−6 ring fusion where the ring fusion bond remains intact. The fourth isomer (1d) displayed spectral data consistent with a compound with C1 symmetry and is assigned as an adduct to the C7...

Resonance stabilized bis-1,2,3-thiaselenazolyl radicals associate in the solid state to afford Se-Se sigma-bonded dimers.

The cycloaddition reactions and noncovalent π interactions of 2,3-dimethoxybutadiene (DMBD), 9-methylanthracene (MeA), tetracyanoethylene (TCNE), and maleic anhydride (MA) with graphene models have been investigated using density functional theory (DFT) calculations. Reaction enthalpies have been obtained to assess the reactivity and selectivity of covalent and noncovalent functionalization. Results indicate that graphene edges may be functionalized by the four reagents through cycloaddition reactions, while the interior regions cannot react. Noncovalent complexation is much more favorable than cycloaddition reactions on interior bonds of graphene. The relative reactivities of different sites in graphene are related to loss of aromaticity and can be predicted using Huckel molecular orbital (HMO) localization energy calculations.

Chemically modified single-walled carbon nanotubes (SWNTs) with varying degrees of functionalization were utilized for the fabrication of SWNT thin film catalyst support layers (CSLs) in polymer electrolyte membrane fuel cells (PEMFCs), which were suitable for benchmarking against the US DOE 2017 targets. Use of the optimum level of SWNT -COOH functionality allowed the construction of a prototype SWNT-based PEMFC with total Pt loading of 0.06 mg(Pt)/cm²--well below the value of 0.125 mg(Pt)/cm² set as the US DOE 2017 technical target for total Pt group metals (PGM) loading. This prototype PEMFC also approaches the technical target for the total Pt content per kW of power (0.125 g(PGM)/kW) at cell potential 0.65 V: a value of 0.15 g(Pt)/kW was achieved at 80°C/22 psig testing conditions, which was further reduced to 0.12 g(Pt)/kW at 35 psig back pressure.

Condensation of oxamidrazone with sulfur dichloride in acetonitrile affords 4,4‘-bis(1,2,3,5-dithiadiazolium) dichloride in moderate yield. Reduction of this salt with triphenylantimony yields the diradical 4,4‘-bis(1,2,3,5-dithiadiazolyl) [S2N2C−CN2S2], which has been isolated and characterized in the solid state as its dimer [S2N2C−CN2S2]2. The diradical is disjoint, and ab initio molecular orbital methods confirm a very small energy gap (

Single-walled carbon nanotubes (SWCNTs) have unique electronic, mechanical, and structural characteristics; consequently, promising applications derived from these materials, such as chemical sensors or nanometer-scale electronic devices, can be expected. Structurally altered nanotubes with appropriate addends should facilitate their use by improving solubility, processability, and ease of dispersion, as well as by providing sites for chemical attachment to surfaces and polymer matrices. Avexing problem is ascertaining the detailed structures of nanotube derivatives after their preparation. The characterization of functionalized SWCNTs is difficult; all experimental attempts to determine the precise location and mode of addition of newly attached groups have failed. SWCNT adducts with possible three-membered rings (3MRs) that result from oxygen, methylene, and NH additions are simple but very important side-wall functionalized derivatives. Oxidation reactions are used widely to purify nanotubes, and the electrical properties of carbon nanotubes are extremely sensitive to oxygen exposure. Methylene and NH adducts are the prototypes of the recently synthesized covalently bonded dichlorocarbene and nitrene adducts. The available theoretical studies on the structures of nanotube oxide and dichlorocarbene adducts that involve either armchair or zigzag tube models, employed rather unsatisfactory methodology (see below). Owing to the large size of nanotubes, carefully chosen truncated models, appropriate for the problem being investigated, are required. One approach uses small nanotube fragments to simulate a full nanotube, but carries out computations at a relatively high level. The other approach uses the ONIOM technique, which treats part of the system at a high theoretical level but the rest of the system at a lower level. This strategy allows larger systems to be simulated at a practical computational cost. Thus, a recent ONIOM(B3LYP/6-31G*:AM1) study employed a C16 fragment (Figure 1a) for the high-level computation to

The high-pressure, high-temperature {sup 3}He labeling procedure previously used with C{sub 60} and C{sub 70} was applied to mixtures of isomers of the higher fullerenes C{sub 76}, C{sub 78}, and C{sub 84}. Sufficient helium was introduced inside them to allow helium NMR spectra to be obtained. Each isomer yields a single, sharp, characteristic NMR peak resolved from the others. Our results show that there are more isomers of these fullerenes than previously reported. Three C{sub 78} isomers and three C{sub 84} isomers have been described, but we have shown that there are at least five C{sub 78} isomers and nine C{sub 84} isomers. 39 refs., 2 figs., 1 tab.

The zero-band-gap electronic structure of graphene enables it to function as either the diene or the dienophile in the Diels−Alder reaction, and this versatile synthetic method offers a powerful strategy for the reversible modification of the electronic properties of graphene under very mild conditions.

The reactions of N,N,N[prime]-tris(trimethylsilyl)formamidine with ECl[sub 2] (E = S, Se) afford 1,2,3,5-dithia- and 1,2,3,5-diselenadiazolium chloride, which can be reduced with triphenylantimony to the corresponding dithia- and diselenadiazolyl radicals [HCN[sub 2]E[sub 2]]. The solid state structure of the cofacial dimer [HCN[sub 2]S[sub 2]][sub 2] has been determined by X-ray diffraction; crystals of [HCN[sub 2]S[sub 2]][sub 2] belong to the monoclinic space group P2[sub 1]/n, with a = 6.833(6), b = 16.463(4), c = 19.161(4) A, [beta] = 93.57(4)[degrees], FW = 210.30, Z = 12. The crystal structure consists of stacked dimers, with three dimers in the asymmetric unit. Along the stacking axis the mean intradimer S---S contact is 3.11 A; the mean interdimer contact is 3.76 A. Ab initio calculations with split-valence and larger basis sets have been employed to evaluate the structures and energies of both the gas phase radicals and their dimers. Minor changes in geometry are predicted upon association of the monomers; this relative structural invariance is consistent with low binding energies, the best predictions (including zero point vibrational energy corrections) being ca. 4 kcal/mol (for E = S) and ca. 10 kcal/mol (for E = Se). 30 refs., 6 figs., 9 tabs.

We have investigated the solid-state electronic structure and properties of a phenalenyl-based butyl-substituted neutral radical, 3, that shows a hysteretic phase transition just above room temperature. We quantitatively analyzed the electron density distribution of this radical throughout both branches of the hysteretic phase transition using solid-state X-ray structures and found two distinct electronic states in the hysteresis loop that accompanies the phase transition. The bistability of the two electronic states was observed through a number of measurements, including IR transmittance spectra of single crystals in the vicinity of the phase transition. By comparing the changes in the crystal structures of 3 and the related ethyl-substituted radical 1 (which exhibits no hysteresis) at various temperatures, we show that the change in the interplanar π-π distance within dimers is the most important structural parameter in determining the physical properties of the radicals. The large change in the C-H···π interaction in 3 occurs in concert with the spin redistribution during the phase transition, but these factors are not responsible for the hysteresis effect. We suggest that the presence of a high-temperature state inside the hysteretic loop during the cooling cycle is due to thermodynamic stability, while the existence of the low-temperature state during the heating cycle is due to the presence of a large energy barrier between the two states (estimated to be greater than 100 kJ/mol) that results from the large-amplitude motion of the phenalenyl rings and the associated lattice reorganization energy that is required at the phase transition.

International audience; We report the effect of electrochemical oxidation in nitric acid on the electronic properties of epitaxial graphene (EG) grown on silicon carbide substrates; we demonstrate the availability of an additional reaction channel in EG, which is not present in graphite but which facilitates the introduction of the reaction medium into the graphene galleries during electro-oxidation. The device performance of the chemically processed graphene was studied by patterning the EG wafers with two geometrically identical macroscopic channels; the electro-oxidized channel showed a logarithmic increase of resistance with decreasing temperature, which is ascribed to the scattering of charge carriers in a two-dimensional electronic gas, rather than the presence of an energy gap at the Fermi level. Field-effect transistors were fabricated on the electro-oxidized and pristine graphene channels using single-walled carbon nanotube thin film top gate electrodes, thereby allowing the study of the effect of oxidative chemistry on the transistor performance of EG. The electro-oxidized channel showed higher values for the on-off ratio and the mobility of the graphene field-effect transistor, which we ascribe to the availability of high-quality internal graphene layers after electro-oxidation of the more defective top layers. Thus, the present oxidative process provides a clear contrast with previously demonstrated covalent chemistry in which sp3 hybridized carbon atoms are introduced into the graphitic transport layer of the lattice by carbon-carbon bond formation, thereby opening an energy gap.

The preparation and solid state characterization of the bifunctional radical furan-2,5-bis(1,2,3,5-dithiadiazolyl) 2,5-[(S2N2C)OC4H2(CN2S2)] and the related monofunctional radical 2-cyanofuran-5-(1,2,3,5-dithiadiazolyl) 2,5-[(S2N2C)OC4H2(CN)] are described. The crystal structure of 2,5-[(S2N2C)OC4H2(CN2S2)] is orthorhombic, space group Pna21, and consists of interleaved arrays of dimers, for which the mean interannular [Formula: see text] contact is 3.137 Å. The crystal structure of the monofunctional radical 2,5-[(S2N2C)OC4H2(CN)] is monoclinic, space group P21/n, and consists of a ribbon-like network of dimers (mean interannular [Formula: see text] interconnected by close head-to-tail [Formula: see text] contacts. The dimer units form stacks parallel to z, with a mean interdimer [Formula: see text] separation of 3.956 Å. The similarities and differences between these two crystal structures and those of related benzene-substituted systems are discussed. Keywords: dithiadiazolyl radicals, furan-based diradicals, cyanofuran-based radicals, radical dimers, crystal structures.

The authors have studied the doping of films of C{sub 60} and C{sub 70} by low-temperature alkali-metal vaporization. These films develop visible phase boundaries during the doping process. By using Raman, ESR, and microwave loss (superconductivity) spectroscopies to characterize the doped films in situ, the authors have established the compositions which occur under these conditions. The authors report the first preparation of a K{sub 3}C{sub 60} film in which the superconducting transition temperature approaches that of the bulk materials (T{sub c} = 19 K) and the first observation of superconductivity in an Rb{sub 3}C{sub 60} film (T{sub c} = 25 K). With C{sub 60} films, all of the alkali metals studied (A = Na, K, Rb, Cs) eventually form the insulating A{sub 6}C{sub 60} phase after prolonged doping. The A{sub 3}C{sub 60} phase occurs for A = Na, K, and Rb, but only the K and Rb compounds superconduct above 4 K. Large frequency shifts accompanying the superconducting transition indicate macroscopic microwave flux exclusion volumes and set critical field limits. In low magnetic fields, superconductivity is dominated by weak link behavior, resembling that of polycrystalline high-T{sub c} materials. Superconductivity was not observed with alkali-metal-doped C{sub 70} films. 43 refs., 6more » figs., 1 tab.« less

The solid-state characterization of a second or β-phase of the 1,3-phenylene-bridged diselenadiazolyl diradical 1,3-[(Se2N2C)C6H4(CN2Se2)] is reported. Crystals of this β-phase are monoclinic, space group P21/n, with a = 9.108 (6), b = 15.233 (13), c = 16.110 (5) Å, β = 103.37 (5)°, Z = 8. The crystal structure consists of chain-like arrays of discrete dimers (4 per unit cell), although one intradimer Se-Se linkage is notably longer (3.411 Å) than the other three (3.125, 3.196, 3.204 Å). The dimer units lie in chains that are linked by a complex three-dimensional array of Se-Se contacts. Variable-temperature single-crystal conductivity measurements on this phase indicate a band gap of 0.77 eV. Consistent with the conductivity measurements, extended Hückel band structure calculations suggest a relatively isotropic electronic structure.

We report an investigation of the nature and chemical functionalization of nitric acid treated single-walled carbon nanotubes (SWNTs). SWNTs washed with diluted sodium hydroxide solutions were characterized by near-IR, mid-IR, and Raman spectroscopy as well as TEM, and the remaining carboxylic acid content was determined to assess the effect of base washing on the removal of carboxylated carbon fractions, which are generated by the nitric acid treatment. It was found that even after exhaustive washing with aqueous base the purified SWNTs contain carboxylic acid groups in sufficient quantity to prepare high quality soluble SWNT materials by covalent functionalization with octadecylamine.

The addition of nitrophenyl groups to the surface of few-layer epitaxial graphene (EG) by the formation of covalent carbon−carbon bonds changed the electronic structure and transport properties of the EG from near-metallic to semiconducting.

The preparation and solid-state characterization of the 1,3-phenylene-bridged bis(dithiadiazolyl) and bis(diselenadiazolyl) diradicals 1,3-[(E2N2C)C6H4(CN2E2)] (E = S, Se) are reported. The isomorphous crystals of 1,3-[(E2N2C)C6H4(CN2E2)] so obtained are tetragonal, space group I41/a. Stacks of diradical molecules, linked vertically in a zigzag fashion through alternaie ends by long E- - -E contacts (mean 3.140/3.284 Å, for E = S/Se), are arranged in pinwheellike clusters about the 41 and ¯4 axes, producing complex patterns of interstack E- - -E contacts. Both compounds show the presence of spin defects in the lattice, and there is a very large enhancement in the paramagnetism of the sulfur compound at high temperatures. The selenium compound is a semiconductor, with a room temperature conductivity of 2 × 10-4 S cm-1. Solid-state NMR experiments find enhanced relaxation times, which have their origin in the presence of a mobile paramagnetic defect. Extended Hückel band structure calculations show the materials to be semiconductors, with band gaps of about 1.0/0.8 eV for E = S/Se. Although the compounds adopt a columnar structure, the calculations indicate significant interactions between the stacks and the materials exhibit well-developed three dimensionality. The enhanced paramagnetism in the sulfur compound is attributed to the presence of thermally generated phase kinks in the lattice, whereas the selenium compound is classified as an intrinsic semiconductor.

We report the development of an experimentally based structural analysis to examine the degree of localization of the spin and charge in the phenalenyl-based neutral radical molecular conductors--the results motivate a reinterpretation of the electronic structure of a number of the radicals that we have reported over the past 10 years. The analysis is based on the well-known relationship between bond order and bond length and makes use of the experimental bond distance deviations between the molecular structure of the radical and its corresponding cation. We determined the single crystal X-ray structure of the ethyl radical (1) at 11 temperatures between 90 K and room temperature so that we could follow the evolution of the structure and the electron density distribution through the magnetic phase transition that occurs in the vicinity of 140 K. We show that the enhanced conductivity in the dimeric ethyl (1) and butyl (3) radicals at the magnetic phase transition results from the development of a complex, but highly delocalized electronic structure and not to the formation of a diamagnetic pi-dimer. We find that the monomeric radicals 4, 12, and 13 have an asymmetric electron density distribution in the crystal lattice whereas radical 11 is the only monomeric radical which remains fully delocalized. The pi-chain radicals (7, 8, 14, and 15) retain the strongly delocalized electronic structures expected for a resonating valence bond ground-state structure.

A set of vectors reciprocal to those pointing along the internuclear axes from a conjugated carbon atom are introduced. The resultant of the reciprocal vectors is shown to define the π-orbital axis vector at a nonplanar conjugated atom. The quantities introduced in the transformation are sufficient to define all of the popular measures of pyramidalization in nonplanar conjugated organic molecules and to allow comparisons between the different schemes. The measures of nonplanarity are divided according to isotropy, and it is shown that there is a high degree of correlation between the different scales of pyramidalization, particularly in the case of the isotropic analyses

We provide definitive evidence for the mechanism of electronic detection of ammonia by monitoring in situ changes in the electrical resistance and optical spectra of films of poly(m-aminobenzenesulfonic acid)-functionalized SWNTs (SWNT-PABS). The increase of resistance during exposure to ammonia is associated with deprotonation of the PABS side chain that in turn induces electron transfer between the oligomer and the valence band of the semiconducting SWNTs. Near IR spectroscopy is used to demonstrate that the charge transfer is a weakly driven process, and this accounts for the high reversibility of the sensor. We show that the sensitivity of the chemiresistors increases as the film thickness is reduced to the percolation threshold and that the SWNT-PABS film thickness provides a simple means to enhance the electronic response.