and Applications of Fullerenes, Carbon Dots, Nanotubes, Graphene, Nanodiamonds, and Combined Superstructures Vasilios Georgakilas,† Jason A. Perman,‡ Jiri Tucek,‡ and Radek Zboril*,‡ †Material Science Department, University of Patras, 26504 Rio Patras, Greece ‡Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University in Olomouc, 17 listopadu 1192/12, 771 46 Olomouc, Czech Republic

Broad family of carbon nanoallotropes: classification, chemistry, and applications of fullerenes, carbon dots, nanotubes, graphene, nanodiamonds, and combined superstructures.

Thermal stability and flame retardancy of polyurethanes

A 2D metastable carbon allotrope, penta-graphene, composed entirely of carbon pentagons and resembling the Cairo pentagonal tiling, is proposed. State-of-the-art theoretical calculations confirm that the new carbon polymorph is not only dynamically and mechanically stable, but also can withstand temperatures as high as 1000 K. Due to its unique atomic configuration, penta-graphene has an unusual negative Poisson’s ratio and ultrahigh ideal strength that can even outperform graphene. Furthermore, unlike graphene that needs to be functionalized for opening a band gap, penta-graphene possesses an intrinsic quasi-direct band gap as large as 3.25 eV, close to that of ZnO and GaN. Equally important, penta-graphene can be exfoliated from T12-carbon. When rolled up, it can form pentagon-based nanotubes which are semiconducting, regardless of their chirality. When stacked in different patterns, stable 3D twin structures of T12-carbon are generated with band gaps even larger than that of T12-carbon. The versatility of penta-graphene and its derivatives are expected to have broad applications in nanoelectronics and nanomechanics.

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Penta-graphene: A new carbon allotrope

2.3. [4 + 4] Cycloadditions 1058 2.4. Photocycloadditions of Aromatic Compounds 1058 2.4.1. Benzene Derivatives 1058 2.4.2. Condensed Aromatic Compounds 1060 3. Photochemical Rearrangements 1061 4. Cyclizations 1064 4.1. Pericyclizations 1064 4.2. Norrish−Yang Reaction 1066 5. Photochemical Extrusion of Small Molecules 1067 6. Photochemical Electron Transfer 1068 6.1. Photochemical Electron-Transfer Reactions with a Catalytic Sensitizer 1068

Photochemical Reactions as Key Steps in Organic Synthesis

Photoelectron spectroscopy revealed that a 20-atom gold cluster has an extremely large energy gap, which is even greater than that of C60, and an electron affinity comparable with that of C60. This observation suggests that the Au20 cluster should be highly stable and chemically inert. Using relativistic density functional calculations, we found that Au20 possesses a tetrahedral structure, which is a fragment of the face-centered cubic lattice of bulk gold with a small structural relaxation. Au20 is thus a unique molecule with atomic packing similar to that of bulk gold but with very different properties.

http://cluster.physik.uni-freiburg.de/lehre/SS11/ueb11/Li2003b.pdf

Au20: A Tetrahedral Cluster

Fullerenes are graphitic cage structures incorporating exactly twelve pentagons. The smallest possible fullerene is thus C20, which consists solely of pentagons. But the extreme curvature and reactivity of this structure have led to doubts about its existence and stability. Although theoretical calculations have identified, besides this cage, a bowl and a monocyclic ring isomer as low-energy members of the C20 cluster family, only ring isomers of C20 have been observed so far. Here we show that the cage-structured fullerene C20 can be produced from its perhydrogenated form (dodecahedrane C20H20) by replacing the hydrogen atoms with relatively weakly bound bromine atoms, followed by gas-phase debromination. For comparison we have also produced the bowl isomer of C20 using the same procedure. We characterize the generated C20 clusters using mass-selective anion photoelectron spectroscopy; the observed electron affinities and vibrational structures of these two C20 isomers differ significantly from each other, as well as from those of the known monocyclic isomer. We expect that these unique C20 species will serve as a benchmark test for further theoretical studies.

Gas-phase production and photoelectron spectroscopy of the smallest fullerene, C20

Electron-impact ionization in a time-of-flight mass spectrometer of C20H0–3Br14–12 probes—secured from C20H20 dodecahedrane by a “brute-force” bromination protocol—provided bromine-free C20H0–2(3) anions in amounts that allowed the clean mass-separation of the hydrogen-free C20− ions and the photoelectron (PE) spectroscopic characterization as C20 fullerene (electron affinity (EA)=2.25±0.03 eV, vibrational progressions of 730±70). The extremely strained C20 fullerene ions surfaced as kinetically rather stable entities (lifetime of at least the total flight time of 0.4 ms); they only very sluggishly expel a C2 unit. The HOMO and LUMO are suggested to be almost degenerate (ΔE=0.27 eV). The assignment as a fullerene was corroborated by the PE characterization of the C20 bowl (EA=2.17±0.03 eV, vibrational progression of 2060±50 cm−1) analogously generated from C20H10 corannulene (C20H1–3Br9–8 samples) and comparably stable. Highly resolved low-temperature PE spectra of the known C20 ring (EA=2.49±0.03 eV, vibrational progressions 2022±45 and 455±30 cm−1), obtained from graphite, display an admixture of, most probably, a bicyclic isomer (EA=3.40±0.03 eV, vibrational progression 455±30 cm−1). The C20+(−) and C20H2+(−) cluster ions generated from polybrominated perylene (C20H0–2Br12–10) have (most probably) retained the planar perylene-type skeleton (sheet, EA=2.47±0.03 eV, vibrational progressions of 2089±30 and 492±30 cm−1 and EA=2.18±0.03 eV, vibrational progressions of 2105±30 and 468±30 cm−1).

C20 Carbon Clusters: Fullerene–Boat–Sheet Generation, Mass Selection, Photoelectron Characterization

A novel aerobic benzoate pathway has recently been discovered in various bacteria in which benzoate is first converted to benzoyl-CoA. The further downstream steps are associated with the gene products of the benzoate oxidation gene cluster (box) on the Azoarcus evansii chromosome. Benzoyl-CoA is oxidized to 2,3-dihydro-2,3-dihydroxybenzoyl-CoA (benzoyl-CoA dihydrodiol) by benzoyl-CoA oxygenase/reductase BoxBA in the presence of molecular oxygen. This study identified the next, ring cleaving step catalysed by BoxC. The boxC gene was expressed in a recombinant Escherichia coli strain as a fusion protein with maltose binding protein (BoxC(mal)) and the wild type as well as the recombinant proteins were purified and studied. BoxC catalyses the reaction 2,3-dihydro-2,3-dihydroxybenzoyl-CoA + H(2)O --> 3,4-dehydroadipyl-CoA semialdehyde + HCOOH. This is supported by the following results. Assays containing [ring-(13)C(6)]benzoyl-CoA, benzoyl-CoA oxygenase/reductase, BoxC(mal) protein, NADPH and semicarbazide were analysed directly by NMR spectroscopy and mass spectrometry. The products were identified as the semicarbazone of [2,3,4,5,6-(13)C(5)]3,4-dehydroadipyl-CoA semialdehyde; the missing one-carbon unit being formate. The same reaction mixture without semicarbazide yielded a mixture of the hydrate of [2,3,4,5,6-(13)C(5)]3,4-dehydroadipyl-CoA semialdehyde and [2,3,4,5,6-(13)C(5)]4,5-dehydroadipyl-CoA semialdehyde. BoxC, a 122 kDa homodimeric enzyme (61 kDa subunits), is termed benzoyl-CoA-dihydrodiol lyase. It contains domains characteristic for enoyl-CoA hydratases/isomerases, besides a large central domain with no significant similarity to sequences in the database. The purified protein did not require divalent metals, molecular oxygen or any cosubstrates or coenzymes for activity. The complex reaction is part of a widely distributed new principle of aerobic aromatic metabolism in which all intermediates are coenzyme A thioesters and the actual ring-cleavage reaction does not require molecular oxygen.

Aerobic benzoyl-CoA catabolic pathway in Azoarcus evansii: studies on the non-oxygenolytic ring cleavage enzyme.

Dodecahedranes from [1.1.1.1]Pagodanes

“One-pot” substitution of the twenty hydrogen atoms in pentagonal dodecahedrane (C20H20) by OH, F, Cl, and Br atoms is explored. Electrophilic insertion of oxygen atoms with DMDO and TFMDO as oxidizing reagents ended, far off the desired C20(OH)20, in complex polyol mixtures (up to C20H10(OH)10 decols, a trace of C20H(OH)19?). Perfluorination was successful in a NaF matrix but (nearly pure) C20F20 could be secured only in very low yield. “Brute-force” photochlorination (heat, light, pressure, time) provided a mixture of hydrogen-free, barely soluble C20Cl16 dienes in high yield and C20Cl20 as a trace component. Upon electron-impact ionization of the C20Cl16 material sequential loss of the chlorine atoms was the major fragmentation pathway furnishing, however, only minor amounts of chlorine-free C20+ ions. “Brute-force” photobrominations delivered an extremely complex mixture of polybromides with C20HBr13 trienes as the highest masses. The MS spectra exhibited exclusive loss of the Br substituents ending in rather intense singly, doubly, and triply charged C20H4–0+(2+)(3+) ions. The insoluble ∼C20HBr13 fraction (C20Br14 trienes as highest masses) obtained along a modified bromination protocol, ultimately allowed the neat mass selection of C20− ions. The C20Cl16 dienes and C20H0–3Br14–12 tri-/tetraenes, in spite of their very high olefinic pyramidalization, proved resistant to oxygen and dimerization (polymerization) but added CH2N2 smoothly. Dehalogenation of the respective cycloaddition products through electron-impact ionization resulted in C22–24H4–8+(2+) ions possibly constituting bis-/tris-/tetrakis-methano-C20 fullerenes or partly hydrogenated C22, C23, and C24 cages.

Jürgen Wörth

Papers

Gas-phase production and photoelectron spectroscopy of the smallest fullerene, C20

C20 Carbon Clusters: Fullerene–Boat–Sheet Generation, Mass Selection, Photoelectron Characterization

Aerobic benzoyl-CoA catabolic pathway in Azoarcus evansii: studies on the non-oxygenolytic ring cleavage enzyme.

Dodecahedranes from [1.1.1.1]Pagodanes

Towards Perfunctionalized Dodecahedranes—En Route to C20 Fullerene