Synthesis of polyyne molecules from hexane by irradiation of intense femtosecond laser pulses

Hydrogen-capped polyynes have been synthesized by femtosecond laser irradiation of hexane liquid. We used UV absorption to identify and high performance liquid chromatography to separate polyynes up to C₁₂H₂, and then confirmed with resonance Raman spectroscopy. The shortest hydrogen-terminated chain is found to be C₆H₂. A supplementary experiment on decane indicates that the formation via single step rearrangement of the parent molecule is not likely.     

Laser ablation of diamond particles suspended in ethanol: Effective formation of long polyynes

Polyynes were synthesized by laser ablation of diamond particles with various sizes suspended in ethanol. Chain length distributions and total yields of polyynes produced were compared with those produced from graphitized diamond particles and graphite particles. The relative amounts of long polyynes such as C₁₄H₂ and C₁₆H₂ produced from diamond particles were found to be larger than those from graphitized diamond particles and graphite particles. From the change of the chain length distribution with the laser irradiation time, it is concluded that the long polyynes are produced directly from diamond particles at the initial stage of ablation. Furthermore, the total yield of polyynes was found to increase with the size of diamond particles and decrease as their graphitization proceeds. Possible mechanisms of these results are discussed.     

Thermolysis of the polyyne C8H2 in hexane and methanol: Experimental and theoretical study

The mean lifetimes of polyyne C₈H₂ in hexane were determined at 50, 60, 80, and 100 °C and in methanol at 60 °C. The reactions are second order at all temperatures: ln k₂ = 20.5 ± 1.5-10303 ± 520T⁻¹ and the corresponding activation energy is 85.7 ± 6.3 kJ mol⁻¹ (7164 cm⁻¹). Extrapolation suggests that solutions at 1 mM concentration are significantly unstable at room temperature. Quantum chemical calculations show that polyynes C_mH₂ + C_nH₂ (m + n = 16) could be products, but these were not detected. Alternatively, C₁₆H₂ isomers could form. IR spectra of the solid residues from hexane and methanol solutions were obtained.     

Preparation of polyynes up to C22H2 by liquid-phase laser ablation and their immobilization into SiO2 gel

Long-chain polyynes up to C₂₂H₂ were prepared by the liquid-phase laser ablation of a graphite pellet. Short-chain polyynes (C₄H₂, C₆H₂, C₈H₂ and C₁₀H₂) prepared simultaneously were identified not by UV-Vis spectra but by LC/MS spectra because C₄H₂ and C₆H₂ have absorption peaks at short wavelengths below 200 nm. Polyynes could first be immobilized in a SiO₂ dried gel, which was confirmed by UV-Vis spectra of the gel.     

Photolytic hydrogenation of the polyynes C6H2, C8H2, C10H2, and C12H2 in n-hexane with 253.6 nm radiation

The polyynes C₆H₂, C₈H₂, C₁₀H₂, and C₁₂H₂ in n-hexane exposed to 253.6 nm radiation “disappear” much faster in air-free solutions than in solutions in equilibrium with air. These observations suggest that the disappearance is due to reaction of the polyyne molecules in the ground state with a photolytically produced chemical species which in solutions in equilibrium with air is overwhelmingly scavenged by dissolved O₂. Although no hydrogenation products were firmly established the most likely chemical is atomic hydrogen.     

Preparation of long-chain polyynes of C24H2 and C26H2 by liquid-phase laser ablation in decalin

The longest-chain polyyne that has ever been synthesized is C₂₄H₂ by Estmond et al. [1] using the complicated acetylene coupling method. On the other hand, in simple liquid-phase laser ablation method, we have recently obtained the longest-chain polyyne of C₂₂H₂. In the present study, we report the synthesis of long-chain polyynes of C₂₄H₂ and C₂₆H₂ from graphite pellets in solvent of decalin by liquid-phase laser ablation. It is believed that the termination reaction on both ends of these polyyne molecules hardly occurs in decalin with relatively less number of hydrogen atoms and these long-chain polyynes could be synthesized.     

Fabrication and laser behaviors of Nd:YAG ceramic microchips

Transparent Nd:YAG ceramic microchips were fabricated through the slip casting shaping directly from the slurry formed by the commercial Al₂O₃/Y₂O₃/Nd₂O₃ powders, and followed by the vacuum sintering procedure. Viscosity of the slurries, the phase evolution and the densification behavior were investigated. For the Al₂O₃/Y₂O₃/Nd₂O₃ compound slurries system, the optimal condition is 0.5 wt.% NH₄PAA dispersant and 30 wt.% solid loading at pH ≥ 8. The YAG phase started to form at 1250° C and pure YAG phase could be obtained at 1400° C. The optical in-line transmittance of the Nd:YAG ceramics with thickness of 2 mm was about 83.8% at 1064 nm and 82.5% at 400 nm, which hit the upper limit of the theoretically calculated value. For the 1.0 at.% Nd:YAG ceramic microchip, the slope efficiency was 43% for 1.0 at.% Nd:YAG ceramic pumped by 920 mW cw Ti:sapphire tunable laser, and the maximum laser output power 246 W was obtained for 2.0 at.% Nd:YAG ceramics pumped by 925 W LD.     

Production of natural graphite particles with high electrical conductivity by grinding in alcoholic vapors

Natural graphite particles with a high crystallinity, which were sieved to obtain particles less than 63 μm, were ground with a ball mill under a dry atmosphere and various alcoholic vapors such as i-C₃H₈OH, n-C₃H₈OH, C₂H₅OH, and CH₃OH. The size and flakiness of the ground products and the electrical conductivity of the films made from the ground products were experimentally examined. Grinding the particles under alcoholic vapors slowly reduced the particle size and was similar to grinding in dry air, but grinding in alcoholic vapors produced flakier products. The graphite films, which were composed of flakier particles and were ground in alcoholic vapor, displayed higher electrical conductivities than the feed graphite particles. The products ground in C₂H₅OH, i-C₃H₇O and n-C₃H₇OH vapors had 50% or less of the specific resistance of the feed particles.     

Magnetic Field Effect on Chemical Compositions of Magnetic Aerosol Particles Synthesized Photochemically from a Gaseous Mixture of Iron Pentacarbonyl and Trimethylsilyl Azide

Under intense UV light irradiation with an Nd:YAG laser at 355 nm, a gaseous mixture of iron pentacarbonyl (Fe(CO)₅) and trimethylsilyl azide (TMSAz) produced aerosol magnetic particles with diameters of 50–100 nm. Two photon absorption of TMSAz and Fe(CO)₅ took place efficiently to produce trimethylsilyl nitrene and iron carbonyls (Fe(CO) _n n = 1–4). In cases where a partial pressure of TMSAz was high, nucleation reactions of aerosol particles proceeded via chemical reactions between trimethylsilyl nitrene and two iron carbonyls, resulting in bridging between two iron carbonyls by silyl nitrene. In cases where a partial pressure of TMSAz was low, the nucleation reactions proceeded via chemical reactions between two iron carbonyls, resulting in the formation of di-iron carbonyls with bridging >C?O group. Both of these nucleation reactions took place concurrently during aerosol particle formation. From FT-IR spectral change of sedimentary particles produced under an external magnetic field of up to 5 T, it was found that application of an external magnetic field accelerated the latter chemical reaction to result in the appearance of bridging ν(>C?O) band in FT-IR spectrum of the magnetic particles.

Copyright 2013 American Association for Aerosol Research     

Multilayer YAG/Re:YAG/YAG laser ceramic prepared by tape casting and vacuum sintering method

This paper reports the use of tape casting and vacuum sintering process for the fabrication of optical grade YAG/Re:YAG/YAG (Re = Yb or Nd) composite laser ceramic. The influence of dispersant content on the rheological behavior of the slurry, the microstructure, the optical and laser performances, were studied. The experimental results showed that laser output at 1030 nm was generated for YAG/Yb:YAG/YAG with threshold absorbed pump power of 4.33 W and slope efficiency of about 12% when the transmission of output coupler (T_oc) was 2.3%. For YAG/Nd:YAG/YAG ceramic, 1064 nm laser output was obtained, and the slope efficiency increased from 30% to 38% while T_oc changed from 2.3% to 10%.     

Time of flight mass spectrometry of polyyne formation in the irradiation of liquid alkanes with femtosecond laser pulses

The production of polyyne molecules arising from the irradiation of liquid octane with femtosecond (fs) laser pulses has been studied using time of flight mass spectrometry (TOFMS). We attribute the primary formation mechanism for polyynes in solution to the creation of small polyyne and cumulene molecules formed in molecular dissociative ionization, that subsequently increase in length through addition reactions with other products C, C₂, C₂H, the ions of which are detected in the TOFMS of octane. Polyynes up to C₁₄H₂ have been identified and we propose that photo-dissociation limits the length of polyynes produced in solution.     

Preparation of polyynes by liquid-phase laser ablation using different irradiation target materials and solvents

Polyynes were prepared from pellets of a perylene derivative, 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), and graphite by liquid-phase laser ablation, using solvents of methanol, ethanol, 1-propanol, 1-butanol, t-butyl alcohol, n-hexane, and decalin. The amounts of polyynes produced from PTCDA strongly depended on the polarity of the solvent, while those from graphite did not. Polar functional groups of acid-anhydride side chains in PTCDA are believed to influence the amounts of C₂ radicals produced by laser ablation. The long-chain polyyne C₂₈H₂ was detected in a chromatogram of a solution obtained from a graphite pellet and solvent decalin.     

Synthesis of polyynes in a submerged electric arc in organic solvents

The products formed by the electric arc between graphite electrodes submerged in organic solvents like acetonitrile, n-hexane and methanol consist of a series of polyynes having the general chemical structure: H(CC)_mH (with m an integer 1,2,3,…m). The polyynes were separated through liquid chromatography (HPLC) and identified through their characteristic electronic absorption spectra recorded by a diode-array detector. When acetonitrile was arced at −40 °C, the entire polyynes series to m=9 (chain with 18 carbon atoms) were produced. Instead, at room temperature the entire polyynes series to m=8 (chain with 16 carbon atoms) were obtained. Similar results were obtained by arcing graphite electrodes in n-hexane and in methanol. The purest polyynes mixture is obtained in methanol while in n-hexane and in acetonitrile by-products are formed together with the polyynes series. In particular, in acetonitrile, a series of monocyanopolyynes was detected together with the normal polyynes series.The polyynes are formed by the vaporization of the elemental carbon from the graphite electrodes and dissolved in the solvent where the electrodes are submerged. The demonstration that the polyynes series H(CC)_mH is hydrogen-capped is based on several experimental data from electronic and FT-IR spectroscopy as well as their reactivity with a specific reagent for terminal acetylenes. Some chemical properties of the polyynes solutions (hydrogenation, oxidation) are also treated and discussed.     

Direct multipulse laser processing of titanium oxide–graphene oxide nanocomposite thin films

Nanocomposite thin films consisting of titanium oxide (TiO₂) nanoparticles (NPs) and graphene oxide (GO) platelets were deposited by a spin-coating technique. The obtained films were submitted to direct laser irradiation using a frequency quadrupled Nd:YAG (λ=266 nm, τ_FWHM≅3 ns, ν=10 Hz) laser source. The effect of the laser processing conditions, as laser fluence value and number of subsequent laser pulses incident onto the same target location, on the surface morphology, crystalline structure, and chemical composition of the TiO₂/GO nanocomposite thin films was systematically investigated. The laser fluence values were maintained below the vaporization threshold of the irradiated composite material. With the increase of the laser fluence and number of incident laser pulses melting and coalescence of the TiO₂ NPs into inter-connected aggregates as well as rippling of the GO platelets take place. The gradual reduction of GO platelets and the onset of anatase to rutile phase transition were observed at high laser fluence values.     

Computational study of linear carbon chains on gold and silver surfaces

Density-functional-theory calculations were carried out for hydrogen capped linear carbon chains, polyynes and cumulenes, adsorbed dissociatively on the (1 1 1) and (2 1 1) surfaces of gold and silver. In the studied adsorption reactions, carbon–hydrogen bonds are broken and covalent carbon–metal bonds are created. The adsorption of cumulenes is highly endothermic, whereas the adsorption of polyynes is near thermoneutral. Also, the hydrogenation of adsorbed polyynyl radicals (·C_nH) into adsorbed cumulene carbenes (:C_nH₂) was investigated, which was found to be exothermic on both metals. Vibrational calculations were conducted on the adsorption systems, and the results were compared with experimental surface enhanced Raman scattering spectra. An interpretation is proposed for the spectra of polyyne–silver solutions, and features of polyyne–gold spectra are predicted.     

C60-containing nanostructured polymeric materials with potential biomedical applications

Star-shaped polymers with a fullerene (C₆₀) core and an unexpanded structure were successfully prepared by reaction of C₆₀ with amino end-capped polyesters H_xC₆₀(NHPCL_n)_x and polyethers H_xC₆₀(NHPEG_n)_x, respectively. Upon irradiation of these C₆₀-derivatives, a large amount of singlet oxygen was released. Compared to previously synthesized star-shaped azafulleroids with an expanded structure, the photosensitivity of H_xC₆₀(NHPCL_n)_x is higher as assessed by a higher production of singlet oxygen. The cytotoxicity of the photoactive water-soluble H_xC₆₀(NHPEG_n)_x derivatives was tested against THP-1 cells and expressed in terms of cell viability. Moreover, they were processed as micro-/nanosized fibers by electrospinning, which however required the addition of poly(?-caprolactone) (PCL). The diameter distribution of the fibers was trimodal, where the fraction with the 270 nm average diameter was the major population. Because of their photoactivity, the herein reported star-shaped C₆₀-derivatives are promising candidates for photodynamic cancer therapy and treatment of multidrug resistant pathogens.     

Fabrication and characterization of fullerene-Nafion composite membranes

Fullerene-Nafion composite membranes have been fabricated through a new solution casting for the first time. The fullerenes used for the composites included C₆₀ and polyhydroxy fullerene (PHF), C₆₀(OH)_n (n ∼ 12). The dispersion of the fullerene in the composite membrane was much more refined with smaller agglomeration particles, relative to the previously prepared fullerene-Nafion composites in which the fullerene was introduced through doping. The miscibility of the hydrophobic fullerene, C₆₀, in the Nafion matrix was further improved by a new fullerene dispersant, poly[tri(ethylene oxide)benzyl]fullerene, C₆₀[CH₂C₆H₄(OCH₂CH₂O)₃OCH₃]_n (n ∼ 5), synthesized in this work. The solution-cast fullerene composites also demonstrated a significant improvement in the physical stability relative to the fullerene-doped Nafion composites through a better integration of the fullerene into the Nafion matrix. Furthermore, increased loadings of the fullerene in Nafion were made possible through the new solution-casting method, compared to the previous doping method. The water characteristics in the fullerene composites have been examined by TGA and ¹H pulse NMR measurements. The interactions between the fullerene and the Nafion have been studied through ATR FT-IR and molecular dynamics simulations which suggested PHF resides primarily in the hydrophobic domain of Nafion when the loading was low. The voltammetric measurements also have shown that the fullerene composites have the reduced limiting current density, compared to Nafion membranes without fullerenes.     

Direct synthesis of polyyne molecules in acetone by dissociation using femtosecond laser irradiation

Hydrogen-capped polyynes have been synthesized by 800 nm femtosecond photolysis of acetone molecules. Four bands appearing in the 1800-2200 cm⁻¹ range of surface enhanced Raman spectra show a mixture of polyynes with variable chain lengths in the irradiated acetone. The shortest hydrogen-terminated chain is found to be C₆H₂. The presence of charged carbon dimer, monomer , C⁺ and double charged C²⁺ has been confirmed by time-of-flight mass spectroscopy. These ions are generated as irradiation products of stepwise dissociation of acetone and further act as building blocks in the synthesis of polyynes.     

WB2 to WB3 phase change during reactive spark plasma sintering and pulsed laser ablation/deposition processes

Reactive spark plasma sintering (SPS) of WB₂/WB₃ ceramics from elements is studied; the sintering pressure dependence of the ratio of WB₃ to WB₂ in samples produced by SPS is discussed. Regardless of the sintering pressure, the obtained samples are very hard ~20 GPa. WB₃ superhard films prepared by pulsed laser deposition (PLD) from selected SPS targets are presented.WB3 coatings were prepared on Si (100) substrates using a nanosecond, Nd:YAG laser operating at a 355 nm wavelength. The phase analysis, crystallography, and orientations have been studied using X-ray diffraction (XRD). A WB₂ to WB₃ phase transformation from 8.2% WB₃ in a sintered target to 93.3% WB₃ in a deposited film was observed. Additionally, the surface of a SPS sintered WB_x target after the ablation process was examined. XRD studies show that already during the laser ablation there is a significant WB₂ to WB₃ phase transformation. Vickers hardness of sintered samples was measured in macro- and micro-scale, and PLD films in the nanoscale.     

Raman and surface-enhanced Raman scattering of a series of size-separated polyynes

Solutions of hydrogen-capped polyynes were prepared by laser ablation of graphite powder in n-hexane and subjected to size separation by high-performance liquid chromatography. Solutions of size-selected polyynes C_nH₂ (n = 8–16) were investigated by normal Raman (NR) and surface-enhanced Raman scattering (SERS) spectroscopy. A main band appearing in the 2000–2200 cm⁻¹ region of the NR spectra showed a systematic downward shift as the chain length increased. The observed NR bands were assigned to Raman-active CC stretching vibrational modes by comparison with calculations based on density functional theory. Raman bands observed in SERS spectra were very broad and located at frequencies lower than the NR bands. A systematic band shift with increasing chain length was also observed for one of the bands. This band was thus assigned to a counterpart of the strong band in the NR spectra. These results made it possible to assign the origins of previously reported SERS bands of mixed polyyne solutions.