Fractional Sublimation Research Articles

Unconventional, Gram-Scale Synthesis of a Molecular Dimer Organic Luminogen with Aggregation-Induced Emission.

Organic luminogens have been widely used in optoelectronic devices, bioimaging, and sensing. Conventionally, the synthesis of organic luminogens requires sophisticated, multistep design, reaction, and isolation procedures. Herein, the products of the melt-phase condensation of benzoguanamine (BG; 2,4-diamino-6-phenyl-1,3,5-triazine) at 370-410 °C display interesting reaction-condition-dependent luminescence properties, including photoluminescence (PL) at a variety of wavelengths in the visible spectrum and quantum efficiencies (PLQE) of up to 58% in the powder form. With a simple and straightforward solvent washing procedure, the prominent blue luminescent component BG dimer was obtained in gram scale with >93% purity (96.5% purity after fractional sublimation). The BG dimer exhibited distinct aggregation-induced emission (AIE) properties. PL measurements indicate that the electronically excited state of the BG dimer undergoes efficient intramolecular nonradiative deactivation in room-temperature solution, leading to a significantly reduced PLQE (<0.1%) in solution. These nonradiative processes are substantially inhibited when the dimer existed in the form of crystals, solid aggregates in solution or being fixed in a rigid polymer film, resulting in a significant increase in the PLQE and lifetime. This work not only provided a new understanding for PL properties of self-condensation luminescent products but also represented an unconventional strategy for large-scale preparation of organic luminogens with high purity.

Formation of Higher Silanes in Low-Temperature Silane (SiH4) Ices.

A novel approach for the synthesis and identification of higher silanes (SinH2n+2, where n ≤ 19) is presented. Thin films of (d4-)silane deposited onto a cold surface were exposed under ultra-high-vacuum conditions to energetic electrons and sampled on line and in situ via infrared and ultraviolet-visible spectroscopy. Gas phase products released by fractional sublimation in the warm-up phase after the irradiation were probed via a reflectron time-of-flight mass spectrometer coupled with a tunable vacuum ultraviolet photon ionization source. The formation mechanisms of (higher) silanes were investigated by irradiating codeposited 1:1 silane (SiH4)/d4-silane (SiD4) ices, suggesting that both radical-radical recombination and radical insertion pathways contribute to the formation of disilane along with higher silanes up to nonadecasilane (Si19H40).

PROBING THE CARBON-PHOSPHORUS BOND COUPLING IN LOW-TEMPERATURE PHOSPHINE (PH3)-METHANE (CH4) INTERSTELLAR ICE ANALOGUES.

Phosphine, which has now been confirmed around the carbon-rich star IRC+10216, provides the first example of a phosphorus-containing single bond in interstellar or circumstellar media. While four compounds containing both phosphorus and carbon have been discovered, none contain a carbon-phosphorus single bond. Here, we show that this moiety is plausible from the reaction of phosphine with methane in electron-irradiated interstellar ice analogues. Fractional sublimation allows for detection of individual products at distinct temperatures using reflectron time-of-flight mass spectrometry (ReTOF) coupled with vacuum ultraviolet photoionization. This method produced phosphanes and methylphosphanes as large as P8H10 and CH3P8H9, which demonstrates that a phosphorus-carbon bond can readily form and that methylphosphanes sublime at 12-17 K higher temperatures than the non-organic phosphanes. Also, irradiated ices of phosphine with deuterated-methane untangle the reaction pathways through which these methylphosphanes were formed and identified radical recombination to be preferred over carbene/phosphinidene insertion reactions. In addition, these ReTOF results confirm that CH3PH2 and CH6P2 can form via insertion of carbene and phosphinidene and that the methylenediphosphine (PH2CH2PH2) isomer forms in the ices, although methylphosphine (CH3P2H3) is likely the more abundant isomer and that phosphanes and organophosphanes preferentially fragment via the loss of a phosphino group when photoionized. While the formation of methylphosphine is overall endoergic, the intermediates produced by interactions with energetic electrons proceed toward methylphosphine favorably and barrierlessly and provide plausible mechanisms toward hitherto unidentified interstellar compounds.

Volatile Pd–Pb and Cu–Pb heterometallic complexes: structure, properties, and trans-to-cis isomerization under cocrystallization of Pd and Cu β-diketonates with Pb hexafluoroacetylacetonate

Preparation of volatile heterometallic precursors is a significant step on the way to advanced multicomponent materials. Study of molecular transformations in solution upon precursor synthesis is of importance to optimize the preparation of the stable solid product of desired composition. Two new volatile heterobimetallic complexes, cis-PdL2*Pb(hfa)2 and cis-CuL2*Pb(hfa)2, were obtained (L = 2-methoxy-2,6,6-trimethylheptane-3,5-dionate, hfa = 1,1,1,5,5,5-hexafluoropentane-2,4-dionate) under cocrystallization of trans-bis-beta-diketonates of Pd(II) and Cu(II) with Pb(hfa)2 from organic solvents. Crystals of these compounds are built of discrete bimetallic molecules where transition metal complex isomerized from trans-to-cis form. Complexation followed by isomerization was studied by solution NMR. The bimetallic molecular species were formed early in solution. Enthalpy and activation energy of isomerization were estimated to be 49 and 93 kJ mol−1, respectively. A new synthesis technique of Pd(II) beta-diketonates which is distinguished by simplicity and selectivity as well as the crystal structure of trans-PdL2 is described. Volatility of all obtained compounds was confirmed by thermogravimetric analysis and fractional sublimation in vacuum; Pd-containing heterobimetallic complex appeared to be more volatile than both the initial monometallic complexes and Cu-containing complex.

Synthesis, Characterization, and Theoretical Study of Stable Isomers of C70(CF3)n (n = 2, 4, 6, 8, 10)

Reaction of C70 with ten equivalents of silver(I) trifluoroacetate at 320-340 degrees C followed by fractional sublimation at 420-540 degrees C and HPLC processing led to the isolation of a single abundant isomer of C70(CF3)n for n = 2, 4, 6, and 10, and two abundant isomers of C70(CF3)8. These six compounds were characterized by using matrix-assisted laser desorption ionization (MALDI) mass spectrometry, 2D-COSY and/or 1D 19F NMR spectroscopy, and quantum-chemical calculations at the density functional theory (DFT) level. Some were also characterized by Raman spectroscopy. The addition patterns for the isolated compounds were unambiguously found to be C1-7,24-C70(CF3)2, C1-7,24,44,47-C70(CF3)4, C2-1,4,11,19,31,41-C70(CF3)6, Cs-1,4,11,19,31,41,51,64-C70(CF3)8, C2-1,4,11,19,31,41,51,60-C70(CF3)8, and C1-1,4,10,19,25,41,49,60,66,69-C70(CF3)10 (IUPAC numbering). Except for the last compound, which is identical to the recently reported, crystallographically characterized C70(CF3)10 derivative prepared by a different synthetic route, these compounds have not previously been shown to have the indicated addition patterns. The largest relative yield under an optimized set of reaction conditions was for the Cs isomer of C70(CF3)8 (ca. 30 mol % of the sublimed mixture of products based on HPLC integration). The results demonstrate that thermally stable C70(CF3)n isomers tend to have their CF3 groups arranged on isolated para-C6(CF3)2 hexagons and/or on a ribbon of edge-sharing meta- and/or para-C6(CF3)2 hexagons. For Cs- and C2-C70(CF3)8 and for C2-C70(CF3)6, the ribbons straddle the C70 equatorial belt; for C1-C70(CF3)4, the para-meta-para ribbon includes three polar hexagons; for C1-7,24-C70(CF3)2, the para-C6(CF3)2 hexagon includes one of the carbon atoms on a C70 polar pentagon. The 10.3-16.2 Hz 7JF,F NMR coupling constants for the end-of-ribbon CF3 groups, which are always para to their nearest-neighbor CF3 group, are consistent with through-space Fermi-contact interactions between the fluorine atoms of proximate, rapidly rotating CF3 groups.

Preparation and characterization of single-phase potassium-doped PTCDA thin films

Single-phase films with K/3,4:9,10-perylenetetracarboxylic dianhydride stoichiometry have been synthesized using the method of fractional sublimation in vacuum. This demonstrates that this method can be successfully applied to alkali metal-doped organic molecular crystals which is an important prerequisite for a number of spectroscopic studies of such systems where an in situ sample preparation is required.

Triamidoamine complexes of the actinides; from mixed-valence bimetallics to dinitrogen activation

Reduction of pale green {tris( tert-butyldimethylsilylamidoethyl)amino}chlorouranium(IV) [U(NN 3′)Cl] with potassium gives the purple mixed valence (III/IV) dimer [{U(NN 3′)} 2– μ–Cl]. Although the two metal atoms in this compound are indistinguishable by X-ray crystallography, other data are consistent with the presence of distinct tri- and tetravalent centres. Careful fractional sublimation of this species allows the `cracking' of the molecule into [U(NN 3′)Cl] and purple [U(NN 3′)]. This latter species reacts with dinitrogen gas to give reversible conversion to [{U(NN 3′)} 2( μ 2- η 2: η 2-N 2)], the first dinitrogen complex of an actinide. The molecular structure as determined by X-ray crystallography shows that the dinitrogen ligand is side-on bonded to the two uranium centres and that the N–N bond length is essentially the same as that in free dinitrogen. This, and variable temperature magnetic data are consistent with the formulation of this compound as a simple Lewis base adduct of uranium(III). The preference for side-on over end-on bonding is explained on the basis that the dinitrogen π p orbital is a better σ-donor than the σ p orbital to trivalent uranium.

Redox, Magnetic, and Structural Properties of 1,3,2-Dithiazolyl Radicals. A Case Study on the Ternary Heterocycle S3N5C4

The characterization of the heterocyclic radical 1,2,5-thiadiazolo[3,4-b]-1,3,2-dithiazolo[3,4-b]pyrazin-2-yl (TDP-DTA) is described. The compound is prepared by treatment of 5,6-dithiolo-1,2,5-thiadiazolo[3,4-b]pyrazine with S3N3Cl3 and purified by fractional sublimation in vacuo. The results of cyclic voltammetry and ESR analysis of TDP-DTA and related heterocyclic dithiazolyls indicate that the spin distributions and donor/acceptor properties of these radicals are extremely sensitive to the nature of the 4,5-substituents. The crystal structure of TDP-DTA has been determined at two temperatures. At 293 K, the crystals are triclinic, space group P1, with a = 4.4456(8), b = 8.407(2), c = 9.671(3) A, α = 71.34(2), β = 89.28(2), γ = 87.80(2)°, Z = 2 (for C4N5S3); at 150 K the crystals are triclinic, space group P1, with a = 7.489(7), b = 9.593(4), c = 10.759(6) A, α = 65.77(4), β = 74.10(6), γ = 74.64(6)°, Z = 2 (for C8N10S6). At 293 K the structure consists of ribbons of TDP-DTA radicals packed in a slip...

The Heterocyclic Diradical Benzo-1,2:4,5-bis(1,3,2-dithiazolyl). Electronic, Molecular and Solid State Structure

The preparation and purification of the heterocyclic diradical benzo-1,2:4,5-bis(1,3,2-dithiazolyl) (BBDTA) is described. Crystals of BBDTA, grown by fractional sublimation at 120-180 °C/10-3 Torr, are monoclinic, space group P21/c, with a ) 4.144(2), b ) 9.0344(13), and c ) 10.7424(16) A,‚ ) 91.11(3)°, and Z ) 2. The crystal structure consists of discrete, unassociated molecules of BBDTA. The molecules form slipped stacks along the x direction, with the mean molecular plane making an angle of 32.4° to the x axis. The interplanar separation between consecutive molecules along the stacking direction is 3.49 A. The ESR spectrum of BBDTA is solvent dependent, displaying signals attributable to partially associated materials ( not exchange coupled) and to free diradical (exchange coupled). Ab initio molecular orbital calculations suggest a small (ca. 0.5 kcal/mol) separation between the triplet and singlet diradical states. Cyclic voltammetry on BBDTA and the related benzo-1,3,2-dithiazolyl BDTA, coupled with gas phase photoelectron studies on BDTA, establish that both compounds are strong electron donors. Magnetic susceptibility measurements show that BBDTA is essentially diamagnetic up to room temperature; variable temperature single-crystal conductivity measurements provide a band gap of 0.22 eV. The transport properties are discussed in the light of extended Huckel band structure calculations, which suggest a highly three-dimensional electronic structure for bulk BBDTA in the solid state.

Properties of Pure Nitryl Bromide. Thermal Behavior, UV/Vis and FTIR Spectra, and Photoisomerization to trans-BrONO in an Argon Matrix

BrNO2 is produced by the heterogeneous reaction of ClNO2, highly diluted in He, with an aqueous bromide solution and subsequent trapping at −155 °C. The crude product is purified by trap to trap condensation and fractional sublimation in vacuo. At 300 K and a pressure of 1 mbar, a half-life of about 1 h is observed. The decomposition produces Br2 and NO2. The UV/vis and IR absorption cross sections of gaseous BrNO2 are determined in the ranges 185−600 nm and 2000−400 cm-1, respectively. Irradiation of matrix-isolated BrNO2 with visible light causes nearly quantitative isomerization to trans-BrONO, while ClNO2 is converted by UV light into a mixture of cis- and trans-ClONO. It is possible to assign all fundamentals of BrNO2 and trans-BrONO by comparison to the respective band position of the analogous molecules XNO2 and XONO (X = Cl, F).

Isolation of short-lived isotopes of lanthanum and indium by β-diketonates sublimation

Trace amounts of lanthanum hexafluoroacetylacetonate and indium acetylacetonate were isolated by combination of methods, which includes synthesis of volatile elements β-diketonates by nucleophilic ligands exchange and fractional sublimation. This method is useful for produce isotope generators.

Alkali-metal-fulleride phase equilibria.

The binary phase diagrams of the alkali-metal fullerides, ${\mathit{A}}_{\mathit{x}}$${\mathrm{C}}_{60}$, are discussed in light of recent experimental results concerning the ${\mathit{A}}_{1}$${\mathrm{C}}_{60}$ phases and their transformations. The ${\mathrm{Rb}}_{1}$${\mathrm{C}}_{60}$ and ${\mathrm{Cs}}_{1}$${\mathrm{C}}_{60}$ phases undergo diffusionless structural transformations upon cooling and their phase diagrams require solutions that differ from the eutectoid transformation of ${\mathrm{K}}_{1}$${\mathrm{C}}_{60}$. X-ray photoelectron spectroscopy has been used to characterize these transformations. Phase diagrams incorporating two invariant reactions are proposed. The equilibrium vapor pressure of ${\mathrm{C}}_{60}$ has been determined at lower temperatures than prevously examined and the pure ${\mathrm{C}}_{60}$ phase diagram has been assembled for a large range of temperature and pressure. The predicted effects of solid-solution formation on the fcc to simple cubic transition of \ensuremath{\alpha}-${\mathrm{C}}_{60}$ are discussed in terms of eutectoid and peritectoid phase-diagram topologies. High-temperature decomposition with fractional sublimation has been used to gain insight into the relative thermal stabilities of the various compounds. Using this information and data from the literature, a ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ phase diagram extending to vaporization temperatures is proposed.

Magnetische Suszeptibilität von Calcium, Strontium und Barium zwischen 295 und 3 K

The magnetic susceptibilities of calcium, strontium and barium (purified by fractional sublimation, purity at least 99.9%) have been determined in the temperature range 295-3 K. The samples are free from ferro- and paramagnetic impurities. The data of calcium are temperature independent between 295 and 45 K at 58(2) × 10 −6 cm 3 mol −1 and then increase to 63(2) × 10 −6 cm 3 mol −1 at 3.3 K. The susceptibility of strontium increases almost linearly from 98(2) × 10 −6 to 136(2) × 10 −6 cm 3 mol −1 in the temperature range 295-3.3 K. The data in the case of barium decrease linearly between 295 and 60 K from 31.0(5)× 10 −6 to 25.5(5) × 10 −6 cm 3 mol −1 before remaining constant down to 3 K.

The vibrational Raman spectra of purified solid films of C60 and C70

A technique to produce samples consisting primarily of C60 and C70 by fractional sublimation of carbon soot was found and used to produce solid films of these molecules. Film compositions were determined using a surface analytical mass spectrometric technique. Vibrational Raman spectra of the purified films were measured and vibrational lines of both C60 and C70 are identified. A C60 line at 273 cm−1 is observed, in agreement with theoretical predictions for the lowest frequency Hg “squashing” mode of Buckminsterfullerene. The two strongest C60 lines, found at 1469 and 497 cm−1, can consistently be assigned to the two totally symmetric Ag modes on the basis of their frequencies and measured depolarization ratios.

Separation of actinide and fission product elements by means of thermochromatography and fractional sublimation of their β-diketonates

The heterogeneous ligand exchange synthesis of volatile β-diketonates of some actinides /230Th,233U,239Pu,241Am/ and fission product elements /90Sr,90Y,113Sn,113mIn/ and their separation by means of thermochromatography and fractional sublimation are reported. The methods are efficient for the separation of elements of different oxidation states.

Convenient Synthesis of Methyl 1-Methyl-2,4-dibromo-5-imidazolecarboxylate

Three syntheses of methyl 1-methyl-2,4-dibromo-5-imidazolecarboxylate (8) are presented. One proceeds from sarcosine via ring closure, bromination, and desulfurization. The second uses N-methylimidazole, polybromination, and selective halogen-metal interchange. The third and most efficient and preparatively useful route begins with diaminomaleonitrile (13). Ring closure with triethyl orthoformate followed by methylation and hydrolysis affords 1-methyl-4,5-imidazoledicarboxylic acid (16). Regioselective decarboxylation followed by esterification yields methyl 1-methyl-5-imidazolecarboxylate (18). Subsequent dibromination gives the completely substituted imidazole 8. The primary purification in this sequence is fractional sublimation of 18 after the esterification step. An overall yield of 26% is achieved from diaminomaleonitrile (13) to methyl 1-methyl-2,4-dibromo-5-imidazolecarboxylate (8), which is a key intermediate for the synthesis of tricyclic imidazo cooked food mutagens.

Purification of Alkaline Earth Metals

The study demonstrates that alkaline earth metals can be sublimed or distilled to give metal of very high purity. The metallic impurities are the most easily separated although the other alkaline earth metals require a fractional sublimation. The interstitial elements 0, N and C can be reduced to very low levels if entrainment is prevented by subliming or distilling under a small torr pressure of an inert gas. Hydrogen transfers to the product in either distillation or sublimation and must be removed by vacuum traction from a Ta capsule. Alkaline earth metal purification by sublimation and distillation started in 1960 to supply pure Ca, Ba and Sr for phase equilibria studies and was expanded to provide high purity Ca for metallothermic preparation of very pure rare earth metals as described by Spedding et al.1 The objectives were to produce metals of the lowest possible interstitial solute concentration with a high yield and in a convenient form for handling in inert atmosphere gloveboxes. Enhance d process reliability with regard to product purity also eliminates expensive and time-consuming chemical analyses on each batch of metal.

Separation of elements by means of fractional sublimation of their \-diketonates in the vapours of \-diketones

A possibility to use exchange reaction of metal \-diketonates and the vapours of \-diketones for the separation of volatile \-diketonate complexes by the method of fractional sublimation has been studied. When a mixture of thenoyltrifluoroacetonates of95Zr−95Nb and Th−233Pa is treated with a carrier gas containing hexafluoroacetylacetone vapours, zirconium and thorium, unlike niobium and protactinium, form volatile hexafluoroacetyl-acetonates which sublimate quite completely at 115°C. Niobium and protactinium remain in the initial sample in the form of nonvolatile compounds.

Separation of cobalt and nickel by the fractional sublimation of their bis(acetylacetonates)

The quantitative separation of a mixture (1∶1) of cobalt and nickel has been carried out by vacuum sublimation of the adducts of acetylacetonates of these metals with water and 2,2′-dipyridyl with the compositions MA2·2H20 and MA2·dipy.

Composition of solid condensate on a cold surface from a mixed organic vapor.

A method to estimate the composition of solid condensate from a binary vapor in fractional sublimation accompanied by an entrainer gas is derived, including an overall coefficient for condensation of the individual components. The coefficient was empirically correlated with gas flow rates, surface temperatures of condenser, and condenser dimensions. Good agreement between the estimated and observed compositions of the condensates was obtained for two eutectic-forming organic mixtures and one solid solution organic mixture.