Dewar Benzene Research Articles

Cascade Photoreaction in Crystals: A Phase Change Caused by a Dewar Benzene Quantum Chain Triggers a Topochemical [2 + 2] Photodimerization.

Crystals undergoing tandem reactions where the first transformation enables the second one are rare. Using photoreactive Dewar benzene 3,4,5,6-tetramethyl-1,2-dicarboxylic diacid (DB) as a hydrogen-bonding template for the [2π+2π] photodimerization of trans-4,4'-bipyridyl-ethenes (BPE), we obtained crystals DB-BPE-NT with a DB:BPE = 2:1 stoichiometry with double bonds at a nonreactive distance of 5.957 Å. Exposure to UV light resulted in valence bond isomerization to Hückel benzene 3,4,5,6-tetramethyl-1,2-dicarboxylic acid (HB) by a quantum chain reaction that triggered the sought-after topochemical [2π+2π] photodimerization reaction. Notably, crystals HB:BPE-T with the Hückel benzene isomer and BPE adopted a 2:2 stoichiometry and displayed an efficient topochemically templated photodimerization reaction.

Reaction amplification with a gain: Triplet exciton–mediated quantum chain using mixed crystals with a tailor-made triplet sensitizer

Photochemical valence bond isomerization of a crystalline Dewar benzene (DB) diacid monoanion salt with an acetophenone-linked piperazinium cation that serves as an intramolecular triplet energy sensitizer (DB-AcPh-Pz) exhibits a quantum chain reaction with as many as 450 product molecules per photon absorbed (Φ ≈ 450). By contrast, isomorphous crystals of the DB diacid monosalt of an ethylbenzene-linked piperazinium (DB-EtPh-Pz) lacking a triplet sensitizer showed a less impressive quantum yield of ca. Φ ≈ 22. To establish the critical importance of a triplet excited state carrier in the adiabatic photochemical reaction we prepared mixed crystals with DB-AcPh-Pz as a dilute triplet sensitizer guest in crystals of DB-EtPh-Pz. As expected from their high structural similarities, solid solutions were easily formed with the triplet sensitizer salt in the range of 0.1 to 10%. Experiments carried out under conditions where light is absorbed by the triplet sensitizer-linked DB-AcPh-Pz can be used to initiate a triplet state adiabatic reaction from 3DB-AcPh-Pz to 3HB*-AcPh-Pz, which can serve as a chain carrier and transfer energy to an unreacted DB-EtPh-Pz where exciton delocalization in the crystalline solid solution can help carry out an efficient energy transfer and enable a quantum chain employing the photoproduct as a triplet chain carrier. Excitation of mixed crystals with as little as 0.1% triplet sensitizer resulted in an extraordinarily high quantum yield Φ ≈ 517.

Mechanical and thermal properties of W-20Cu alloy for packaging HgCdTe IRFPA detector at cryogenic temperatures

Tungsten-copper alloy, as an electronic packaging material with excellent comprehensive performance, is widely used in the cooled packaging dewar structure of HgCdTe infrared focal plane detector. Thermodynamic properties (Coefficient of Thermal Expansion (CET), Thermal Conductivity (TC), Specific Heat Capacity (SHC)), mechanical properties and microstructure of W-20 wt% Cu(W-20Cu) are investigated in the range of 300 K ∼ 4.2 K, respectively. The quasi-static tensile tests are carried out on an electronic universal testing machine to obtain the typical stress–strain curves and mechanical properties of W-20Cu. The yield strength (YS), ultimate tensile strength (UTS), elongation and shrinkage of the specimens are measured. Besides, the Scanning Electronic Microscopy (SEM) are applied to observe the microstructure of W-20Cu alloy. The results demonstrate that W-20Cu alloy has outstanding thermal properties of high strength and hardness, high specific heat and low heat capacity and expansion coefficient. Tensile properties of the alloy significantly improve at extremely low temperature, and compared with 300 K, the YS and UTS of W-20Cu increase by 30.68 % and 11.27 % at 77 K. At 4.2 K, the YS increase by 44.49 %, and the UTS increase by 17.20 %, while the plasticity and fracture toughness have a sharp reduction from 300 K to 4.2 K. All specimens exhibit brittle fracture characteristics. With the increase of W crystals cleavage ratio and W-W continuity, the microstructure continuity and uniformity are improved. The number of plastic dimples of Cu phase decreases and the dimples become shallow slightly.

Toward a Formyl-to-Phenyl Conversion: An Unexpected Photochemical Fulvene Rearrangement.

Toward a conversion of aldehydes into arenes, we designed a sequence involving the initial reaction of an aldehyde to give a fulvene, followed by photochemical and platinum-catalyzed rearrangements into a Dewar benzene derivative, which finally isomerizes into the targeted arene. While computational studies support the plausibility of this route, we found that fulvene irradiation resulted in an unexpected isomerization into a spiro[2.4]heptadiene. This unusual photorearrangement has been investigated mechanistically and provides access to a variety of spiro[2.4]heptadienes with different substituents.

Quantum chain amplification in nanocrystalline Dewar benzenes by intramolecular sensitization.

Quantum chain reactions are characterized by the formation of several photoproducts per photon absorbed (ΦQC > 1) and constitute a promising signal amplification mechanism. The triplet-sensitized isomerization of Dewar benzene is known to undergo quantum chain reactions characterized by an adiabatic valence-bond isomerization to the excited state of Hückel benzene, which is able to transfer its triplet energy to a new ground state Dewar benzene that reacts to continue the chain. Given that diffusion-mediated energy transfer is the chain-limiting event in solution, we demonstrate here that reactions in crystals are significantly more efficient by taking advantage of energy transfer by a presumed exciton delocalization mechanism. Using Dewar benzenes with covalently attached, high energy triplet sensitizers we have demonstrated the efficiency of the solid state by the amplification of a quantum yield of ca. ΦQC ≈ 76 in acetonitrile solution to as much as ca. ΦQC ≈ 100-120 in submicron size specimens prepared by the re-precipitation method, and up to ca. ΦQC ≈ 300 with microcrystalline powders suspended in water.

Metal sandwich and ion complexes in cyclacene nanobelts

Cyclacene carbon nanobelts represent the last unsynthesized building blocks of carbon nanotubes and are the shortest possible hydrogen capped nanotubes of zigzag type. Recent theoretical predictions indicate that stable cyclacene isomers containing Dewar benzene derived structures may exist. The properties of the Hückel-[n]cyclacene nanobelt complexes containing alkali metal and halide ions (n = 5-8), with zero to two Dewar benzenoid structures present, and transition metal complexes (n = 6, 8) with zero and two Dewar structures present, have been calculated using density functional theory. The halides, alkali metals, and transition metals were initially placed centrally within the belt cavities, and binding interactions have been observed for both positive and negative ions. The most stable isomeric forms are found to change for several of the belt sizes, depending on which ions are present. Internal transition metal sandwich complexes with η6-haptic bonds are also indicated to form within the cyclacene belts containing two Dewar benzenoids.

Light-Induced Reactions within Poly(4-vinyl pyridine)/Pyridine Gels: The 1,6-Polyazaacetylene Oligomers Formation.

Cyclic 6-membered aromatic compounds such as benzene and azabenzenes (pyridine, pyridazine, and pyrazine) are known to be light-sensitive, affording, in particular, the Dewar benzene type of intermediates. Pyridine is known to provide the only Dewar pyridine intermediate that undergoes reversible ring-opening. We found that irradiation of photosensitive gels prepared from poly(4-vinyl pyridine) and pyridine at 254 or 312 nm leads to pyridine ring-opening and subsequent formation of 5-amino-2,4-pentadienals. We show that this light-induced process is only partially reversible, and that the photogenerated aminoaldehyde and aminoaldehyde-pending groups undergo self-condensation to produce cross-linked, conjugated oligomers that absorb light in the visible spectrum up to the near-infrared range. Such a sequence of chemical reactions results in the formation of gel with two distinct morphologies: spheres and fiber-like matrices. To gain deeper insight into this process, we prepared poly(4-vinyl pyridine) with low molecular weight (about 2000 g/mol) and monitored the respective changes in absorption, fluorescence, 1H-NMR spectra, and electrical conductivity. The conductivity of the polymer gel upon irradiation changes from ionic to electronic, indicative of a conjugated molecular wire behavior. Quantum mechanical calculations confirmed the feasibility of the proposed polycondensation process. This new polyacetylene analog has potential in thermal energy-harvesting and sensor applications.

A Crystalline B4N2 Dewar Benzene as a Building Block for Conjugated B,N-Chains.

Dewar benzene, one of the isolable valence isomers of C6H6, has been extensively studied since its first synthesis in 1962. By contrast, the chemistry of inorganic congeners of Dewar benzene, which can be formally gained by replacing the skeletal carbon atoms with heteroatoms, has been less developed despite their peculiar structural and electronic features. Among them, the extant B,N-Dewar benzenes are limited to the B3N3 system. Herein, we report the development of the first example of an isolable B4N2 Dewar benzene, 3. As predicted by DFT calculations, a judicious selection of the substituents allows synthesizing 3. Single-crystal X-ray analysis, NMR, and computational studies confirmed that 3 possesses a high-lying B(sp3)-B(sp3) σ-bond at the bridgehead position. Reactions with ethylene and phenylacetylene proceeded smoothly under mild conditions, affording the fused B4C4N2 ring systems (4 and 5). Structural characterization as well as DFT calculations revealed that compounds 4 and 5 involve a rigid and conjugated (BN)4 tetraene scaffold. Formation of 4 and 5 demonstrates that 3 may serve as a building block for the construction of conjugated B,N-chains.

UV irradiation of benzene in N2 matrix: A relevant study for titan's chemistry

We report in this contribution the first photochemical study of benzene trapped in solid nitrogen at low temperature (16 K) and irradiated with a mercury lamp (λ > 230 nm). We identify three benzene isomers (fulvene, Dewar benzene and benzvalene) and two radicals (phenyl and benzyne) as photo-dissociation products, along with acetylene derivatives and substituted benzene. We present for the first time the infrared spectrum of fulvene trapped in benzene ice, which is obtained after nitrogen sublimation. This solid-state characterization could be particularly useful for a better understanding of the photochemical fate of benzene ices recently detected in Titan's atmosphere.

Development and testing of a cryogenic induction motor with a novel Dewar structure

A Dewar is the key structure of motors with high temperature superconducting (HTS) armature windings. Traditional Dewars use total immersion cooling, which can cause additional mechanical losses and challenges for rotary sealing. To avoid these problems, a novel Dewar structure was developed with partial immersion cooling. A 15 kW cryogenic induction motor with the designed Dewar was then developed. Its stator core and the coil were cooled with liquid nitrogen through partial immersion cooling. We studied the thermal insulation performance of the Dewar, and the running performance of the cryogenic induction motor. The temperature characteristics of the stator core were measured by PT-100 temperature sensors. Results show that the stable temperature of the stator is 76.5 K. Under no-load conditions, the slip rate of the motor is 0.02 with the no-load current being 30 A at 350 V of the stator voltage. The no-load loss power at low temperature is much larger than that at room temperature. These results suggest that the partial immersion cooling is effective, which validates the reliability of the designed Dewar structure, and that running performance of the motor designed is desirable.

Theoretical Study of the Si/C Mixed Benzenes and Their Major Valence Isomers

Changes in the structures and relative stabilities of benzene and the major valence isomers fulvene, Dewar benzene, benzvalene, and prismane on the substitution of the skeletal carbons with silicon...

Inorganic Benzene Valence Isomers.

It has been computationally proposed that there are more than 200 possible isomeric structures for the C6H6 molecule, among which benzene (CH)6 represents the most thermodynamically stable molecule and its chemistry has been extensively developed. Three benzene valence isomers (CR)6, namely, prismane, Dewar benzene, and benzvalene, have also been synthesized to date. Incorporation of heteroatoms in the skeletal frameworks allows access to inorganic analogues of benzene and its valence isomers, which have attracted considerable attention due to their peculiar structural and electronic features. Experimentally, more than 30 examples of inorganic benzene and its valence isomers have been developed and characterized so far. In this contribution, their synthesis and characterization, as well as their stability and chemical properties, are discussed.

A systematic study of stereochemical effects in homologous poly(alkenamer)s: Dewar benzene versus norbornene

AbstractTwo diastereomeric derivatives of norbornene, dimethyl (1R,2R,3S,4S)‐bicyclo[2.2.1]hept‐5‐ene‐2,3‐dicarboxylate and dimethyl (1R,2S,3S,4S)‐bicyclo[2.2.1]hept‐5‐ene‐2,3‐dicarboxylate, were synthesized and polymerized using ring‐opening metathesis polymerization (ROMP). For comparative purposes, diastereomeric derivatives of Dewar benzene, dimethyl (1R,2S,3R,4S)‐bicyclo[2.2.0]hex‐5‐ene‐2,3‐dicarboxylate and dimethyl (1R,2S,3S,4S)‐bicyclo[2.2.0]hex‐5‐ene‐2,3‐dicarboxylate, were also synthesized and polymerized using ROMP. The polymerization reactions proceeded in a controlled manner as evidenced in part by linear relationships between the monomer‐to‐catalyst feed ratios and the molecular weights of the polymer products. Chain extension experiments were also conducted which facilitated the formation of block copolymers. Although the poly(norbornene) derivatives exhibited glass transition temperatures that were dependent on their monomer stereochemistry (cis: 115°C vs. trans: 125°C), more pronounced differences were observed upon analysis of the polymers derived from Dewar benzene (cis: 70°C vs. trans: 95°C). Likewise, microphase separation was observed in block copolymers that were prepared using the diastereomeric monomers derived from Dewar benzene but not in block copolymers of the norbornene‐based diastereomers. The differential thermal properties were attributed to the relative monomer sizes as reducing the distances between the polymer backbones and the pendant stereocenters appeared to enhance the thermal effects.

Dewar Benzenoids Discovered In Carbon Nanobelts.

The synthesis of cyclacene nanobelts remains an elusive goal dating back over 60 years. These molecules represent the last unsynthesized building block of carbon nanotubes and may be useful both as seed molecules for the preparation of structurally well-defined carbon nanotubes and for understanding the behavior and formation of zigzag nanotubes more broadly. Here we report the discovery that isomers containing two Dewar benzenoid rings are the preferred form for several sizes of cyclacene. The predicted lower polyradical character and higher singlet-triplet stability that these isomers possess compared with their pure benzenoid counterparts suggest that they may be more stable synthetic targets than the structures that have previously been identified. Our findings should facilitate the exploration of new routes to cyclacene synthesis through Dewar benzene chemistry.

Hydrogenated Poly(Dewar benzene): A Compact Cyclic Olefin Polymer with Enhanced Thermomechanical Properties

Dihydro Dewar benzene(bicyclo[2.2.0]hex-2-ene) was synthesized and polymerized using the Grubbs third generation catalyst. The corresponding ring-opening metathesis polymerization proceeded in a co...

Quantum Chemical Exploration of Intermolecular Reactions of Acetylene.

Quantum chemical explorations of potential energy surfaces showed that acetylene produces various products starting from molecular arrays in short distances of 1.3-2.5 Å. Arrays of (C2 H2 )2 gave cyclobutadiene, tetrahedrane, and acetylene dimers. Arrays of (C2 H2 )3 gave benzene, prismane, benzvalene, Dewar benzene, and acetylene trimers. Arrays of (C2 H2 )4 gave cubane, cyclooctatetranene, and acetylene tetramers. Different forms of initial arrays yielded different sets of products; a parallel array of two monomers gave cyclobutadiene, whereas a cross array gave tetrahedrane. Initial molecular arrays with unusually close contacts were estimated to require local forces of 1-9 × 10-8 N. © 2019 Wiley Periodicals, Inc.

Unveiling a Masked Polymer of Dewar Benzene Reveals trans-Poly(acetylene)

A dibromo derivative of Dewar benzene, trans-5,6-dibromobicyclo[2.2.0]hex-1-ene, was polymerized using ring-opening metathesis polymerization (ROMP). The reaction proceeded in a controlled manner as changing the initial monomer-to-catalyst ratio afforded monodispersed polymers with tunable molecular weights and growing polymer chains were extended upon subsequent exposure to additional monomer. Treatment of the halogenated polymers with an alkyllithium reagent resulted in elimination followed by isomerization to afford trans-poly(acetylene). Based on a series of mechanistic and spectroscopic studies, the transformation was proposed to proceed through a cyclobutenyl intermediate that undergoes rearrangement. The methodology was found to be versatile as triblock copolymers containing the halogenated homopolymer were prepared and converted to their poly(acetylene)-containing derivatives. The polymers were characterized using gel permeation chromatography as well as a range of spectroscopic (NMR, FT-IR, UV–vi...

Revisiting the von Neumann–Wigner noncrossing rule and validity of a dynamic correlation diagram method

The noncrossing rule for potential energy surfaces can be applied only, as originally postulated by von Neumann and Wigner, to slowly occurring changes; it has, however, over many years, been widely used to rationalize fast chemical reactions. Taking the conversion of Dewar benzene to benzene as an example, we demonstrate a reaction that has a timescale for which crossings are allowed. Since it is now established that elementary chemical reactions proceed over ca. 10–100[Formula: see text]fs, as revealed experimentally by Zewail, the noncrossing rule cannot any longer be said to be valid for most chemical reactions. We further demonstrate that the mechanism of the chemiluminescent conversion of Dewar benzene to benzene is explained by an electronic state diagram derived using a dynamic correlation diagram method which allows crossings, whereas the reaction is not explained by a conventional approach, applying the noncrossing rule using a static correlation diagram method.

Mechanical Switching of Aromaticity and Homoaromaticity in Molecular Optical Force Sensors for Polymers.

The sensing of mechanical stress in polymers is indispensable for investigating the origin and propagation of cracks that lead to material failure and for designing mechanically responsive polymers. Here the unique approaches of using the force-induced switching of aromaticity and homoaromaticity in molecular optical force sensors for the real-time measurement of mechanical forces acting in stretched polymers are suggested. The mechanical switching of aromaticity in Dewar benzene is an irreversible event, whereas the degree of π-orbital overlap in homoaromatic compounds like homotropylium can be adjusted progressively over a wide range of forces. Using computational methods, it is demonstrated that both approaches lead to significant changes in the visible part of the UV/Vis spectra of the force sensors upon application of weak forces (pN-nN). Polymers that incorporate such molecular force sensors therefore change their color well before material failure occurs.

Polymer Valence Isomerism: Poly(Dewar-o-xylylene)s

Poly( o-xylylene) (POX) has long been a challenging synthetic target despite its simple structure and potentially useful physical properties. In this report, we demonstrate a valence isomer strategy that leads to the formation of high molecular weight POX via an intermediate polymer of a unique structure, namely poly(Dewar- o-xylylene) (PDOX). We show that the free radical polymerization of highly strained Dewar- o-xylylene (DOX) monomer afforded PDOX, a material with a high density of Dewar benzene units in the backbone through ring-retaining propagation. The thermal- and photoinduced isomerizations of PDOX to produce POX were investigated. This chemistry yields POXs that are difficult to obtain using traditional methods. Moreover, it also provides a potential entry into new reconfigurable materials featuring highly efficient postpolymerization main chain structural transformations.