Novel discotic liquid crystal oligomers: 1,3,5-triazine-based triphenylene dimer and trimer with wide mesophase

Series of novel 1,3,5-triazine-based triphenylene oligomers 7, 10 and 12 with large bridging polyaromatic core were designed and synthesised by simple procedures in high yields. Their structures were confirmed by FTIR, ¹H NMR, ESI-MS and elemental analyses. Their mesomorphic behaviours were studied by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. 1,3,5-triazine-based triphenylene monomer 7 has no mesomorphic property, but 1,3,5-triazine-based triphenylene dimer 10 and 1,3,5-triazine-based triphenylene trimer 12 possess excellent mesomorphic properties. The mesomorphic temperature range of compound 12 was as wide as 180°C. These studies indicated that the mesomorphic properties were determined by the numbers of triphenylene units. More units of triphenylene in the oligomers resulted in better mesomorphic properties.     

Protecting and Leaving Functions of Trimethylsilyl Groups in Trimethylsilylated Silicates for the Synthesis of Alkoxysiloxane Oligomers

The concept of protecting groups and leaving groups in organic synthesis was applied to the synthesis of siloxane‐based molecules. Alkoxy‐functionalized siloxane oligomers composed of SiO₄, RSiO₃, or R₂SiO₂ units were chosen as targets (R: functional groups, such as Me and Ph). Herein we describe a novel synthesis of alkoxysiloxane oligomers based on the substitution reaction of trimethylsilyl (TMS) groups with alkoxysilyl groups. Oligosiloxanes possessing TMS groups were reacted with alkoxychlorosilane in the presence of BiCl₃ as a catalyst. TMS groups were substituted with alkoxysilyl groups, leading to the synthesis of alkoxysiloxane oligomers. Siloxane oligomers composed of RSiO₃ and R₂SiO₂ units were synthesized more efficiently than those composed of SiO₄ units, suggesting that the steric hindrance around the TMS groups of the oligosiloxanes makes a difference in the degree of substitution. This reaction uses TMS groups as both protecting and leaving groups for SiOH/SiO⁻ groups.     

From Nongelating to Gelating: Synthesis and Structural Self‐Assembling Property Relationships of a Homologous Series of Oligo(amide–triazole)s

A homologous series of oligo(amide–triazole)s (OAT) [ OAT‐CO₂H‐2 n and OAT‐COPrg‐(2 n +1) ] with an increasing number of primary amide (CONH) and triazole hydrogen‐bonding functionalities was prepared by an iterative synthetic procedure. It was found that their self‐assembly and thermoreversible gelation strength had a strong correlation to the number of hydrogen‐bonding moieties in the oligomers. There also existed a threshold value of the number of CONH units, above which all the oligomers became organogelators. Hence, oligomers with ≤4 CONH units are devoid of intermolecular hydrogen bonding and also non‐organogelating, whereas those that contain >4 CONH units show intermolecular association and organogelating properties. For the organogelators, the T_gel value increases monotonically with increasing number of CONH units. On the basis of FTIR measurements, both the CONH and triazole C? H groups were involved in the hydrogen‐bonding process. A mixed xerogel that consisted of a 1:1 weight ratio of two oligomers of different lengths ( OAT‐CO₂H‐6 and OAT‐CO₂H‐12 ) was found to show microphase segregation according to differential scanning calorimetry, thus indicating that oligomers that bear a different number of hydrogen‐bonding units exhibited self‐sorting to maximize the extent of intermolecular hydrogen bonding in the xerogel state.     

A novel way to study the initial stages of soap-free emulsion polymerizations: The intercalation of polystyrene oligomers into hydrotalcite

The dominant species in the early stages of an emulsifier-free emulsion polymerization of styrene has been found to be an oligomer of two to three monomer units using a novel trapping technique. This involved the intercalation of charged primary oligomers between the layers of a hydrotalcite, [Mg₄Al₂(OH)₁₂]²⁺[A]^2- (where A = dianion). Hydrotalcites are an important class of lamellar, inorganic compounds whose interlayer spacing can be mod-ified by anion exchange. Our approach first involved preparing a hydrotalcite precursor in which the layers were propped apart by an organic dianion (terephthalate = TA). This material was then used to capture the negatively charged polystyrene oligomers from the emulsion polymerization reaction mixture. We found that TA was rapidly ion-exchanged for the charged oligomers. The resulting pillared hydrotalcite material was characterized using XRD and SEC. We found that the interlayer spacing between the hydroxide layers increased to 23.2 Å on exposure to the emulsion reaction mixture. This represents an interlayer expansion of 18.3 Å (after subtraction of the hydroxide layer contribution), which is cnsistent with intercalation of oligomers with two to three monomer units arranged in a bilayer. This size estimate was confirmed by the results of size exclusion chromatography. © 1995 John Wiley & Sons, Inc.     

Two new 1D cadmium(II) azido complexes: catina-Poly[di-μ1,1-azido(2,2′-bipyridyl)cadmium(II)] and a new polymorph of catina-Poly[di-μ1,1-azido (2-acetylpyridine)cadmium(II)]

Two new cadmium(II) azido complexes, [Cd(2,2′-bipy)(N₃)₂]_n (1) and [Cd(2-acpy)(N₃)₂]_n (2) (2,2′-bipy?=?2,2′-bipyridyl and 2-acpy?=?2-acetylpyridine), have been synthesized and structurally characterized by single-crystal X-ray diffraction methods. The coordination environment of the central cadmium atom is distorted octahedral (MN₆) in 1 and (MN₅O) in 2, with one-dimensional chains formed through Cd₂N₂ units and alternatively chelating N,N′-bipyridyl or N,O-2-acetylpyridine groups. The central Cd(II) ion is coordinated to two nitrogen atoms of a chelating bipyridyl group or one nitrogen atom and one acetyl oxygen of 2-acetylpyridine and four nitrogen atoms of four different end-on bridging (μ_1,1-N₃) groups. Chains in the c direction in 1 are stabilized in b direction by π–π interactions involving the aromatic rings of bipyridyl ligands. IR and NMR spectra of the two complexes are reported.     

Molecular-mass-distribution of oligomers in the products of tetrafluoroethylene telomerization in various solvents

The possibility of using differential thermogravimetry (DTG) for determination of the molecularmass-distribution (MMD) of oligomers in polymerization products has been experimentally and theoretically substantiated. The technique is based on the assumptions that the increase in the boiling temperature of oligomers with the increasing chain length is described by the additivity rule and the shape of DTG curves obeys the equation of mass transfer in the stationary convective flow created by evaporation. The results have been confirmed by gel-permeation chromatography and ¹⁹F NMR data. It has been shown that the chain length of oligomers produced via tetrafluoroethylene (TFE) telomerization in carbon tetrachloride, chloroform, acetone, and ethyl acetate reaches about 20–25 C₂F₄ units at an initial TFE concentration of C ₀ ≈ 0.5 mol/l. The MMD includes a low-molecular-mass component (n ≤ 5) in an amount that decreases with increasing C ₀ and a high-molecular-mass component, whose maximum shifts toward higher values of n with an increase in C ₀. The bimodality of MMD is explained in terms of decrease in the propagation rate constant with the accumulation of long oligomers.     

The synthesis of diene and of vinyl copolymers containing predetermined quantities of polynuclear hydrocarbon or heterocyclic adducts-III. Copolymers formed from polynuclear hydrocarbons and alkyldihalides

The reaction in tetrahydrofuran between the disodium salt of anthracene (A) and alkyl dihalides (RX₂) results in oligomeric products with a repeat unit -[A-R]-. The same reaction with the dilithium derivative is complicated by the high reactivity of this species toward solvent; oligomers with -(CH₂)₄OH end groups are isolated. The xylylene dihalides undergo metallation and Wurtz coupling, and they produce low yields of oligomers rich in xylylene units.The reaction with dilithium acenaphthylene and alkyl dihalides gives a high yield of copolymer but there is evidence of crosslinking. Although addition occurs principally across the 1.2 position, some 1,5 addition is believed to cause this crosslinking. With dilithium phenanthrene the reaction is mainly one of electron transfer; the oligomers produced in low yield are low in phenanthrene adducts.     

Thermodynamic Control of Intramolecular Singlet Fission and Exciton Transport in Linear Tetracene Oligomers

We newly synthesized a series of homo- and hetero-tetracene (Tc) oligomers to propose a molecular design strategy for the efficient exciton transport in linear oligomers by promoting correlated triplet pair (TT) dissociation and controlling sequential exciton trapping process of individual doubled triplet excitons (T+T) by intramolecular singlet fission. First, entropic gain effects on the number of Tc units are examined by comparing Tc-homo-oligomers [(Tc)_n: n=2, 4, 6]. Then, a comparison of (Tc)_n and Tc-hetero-oligomer [TcF₃-(Tc)₄-TcF₃] reveals the vibronic coupling effect for entropic gain. Observed entropic effects on the T+T formation indicated that the exciton migration is rationalized by number of possible TT states increased both by increasing the number of Tc units and by the vibronic levels at the terminal TcF₃ units. Finally, we successfully observed high-yield exciton trapping process (trapped triplet yield: Φ_TrT=176 %).     

A Polymer Coated Silica Gel for High-Performance Liquid Chromatography

Abstract

The treatment of microparticulate silica gel with Carbowax 20M results in a stable and reproducible support which greatly reduces the irreversible adsorption effects observed for polyoxyethylene compounds on untreated silica gel. The selectivity of this coated support, likely a result of selective deactivation, permits the rapid separation of oligomers differing by ≤ 3% in molecular weight. The advantages and disadvantages of this support and a C₁₈ reversed-phase support for the separation and analysis of polyoxyethylene oligomers and their adducts are illustrated and discussed.     

A Hexameric Peptide Barrel as Building Block of Amyloid‐β Protofibrils

Oligomeric and protofibrillar aggregates formed by the amyloid‐β peptide (Aβ) are believed to be involved in the pathology of Alzheimer’s disease. Central to Alzheimer pathology is also the fact that the longer Aβ₄₂ peptide is more prone to aggregation than the more prevalent Aβ₄₀. Detailed structural studies of Aβ oligomers and protofibrils have been impeded by aggregate heterogeneity and instability. We previously engineered a variant of Aβ that forms stable protofibrils and here we use solid‐state NMR spectroscopy and molecular modeling to derive a structural model of these. NMR data are consistent with packing of residues 16 to 42 of Aβ protomers into hexameric barrel‐like oligomers within the protofibril. The core of the oligomers consists of all residues of the central and C‐terminal hydrophobic regions of Aβ, and hairpin loops extend from the core. The model accounts for why Aβ₄₂ forms oligomers and protofibrils more easily than Aβ₄₀.     

Anionic Oligomerization of Methyl Methacrylate by Alkali Metal Alkoxides

Abstract

The kinetics of the oligomerization of methyl methacrylate (MMA) by methoxide/methanol solutions was studied using gas chromatography techniques. The effects of the type of the alkali metal, [CH OH]/[monomer] ratio, solvent, and initiator concentration were investigated. The rate of conversion using different alkali metal alkoxides was in the order CH₃OLi < CH₃ONa < CH₃OK, but no oligomers higher than the addition product, RO—MMA (n=1), could be obtained by CH₃OLL DP_n decreased with increasing the [CH₃OH]/[monomer] ratio and with lowering of the initiator concentration. Using DMSO as solvent increased the yields of the higher oligomers. The formation of n=1 was reversible, contrary to the formation of the higher oligomers. Some of the rate constants of the various steps of the oligomerization were estimated by taking into account the reversibility of the initiation reaction and assuming steady-state conditions in the concentration of the various anions present in the system.     

Novel Synthesis of a Macromonomer Having Organosilyl and Amino Groups

Abstract

Reaction between dimethyldivinylsilane (1) and N,N′-diethyl-N-lithio-ethylenediamine (2a) in the presence of N,N′-diethylethylenediamine (3a) in THF at 20°C gave a monoadduct, 3,3-dimethyl-6-ethyl-3-sila-6,9-diaza-1-undecene (4a). An anionic self-polyaddition reaction of 4a in the presence of lithium diisopropylamide (LDA) proceeded to form oligomers. Each of the oligomers thus obtained was found to carry a polymerizable vinylsilane moiety at the oligomer chain terminal. As a result, a new type of macromonomer having alternating repeating units of ethylenediamine and organosilyl groups was synthesized. Acid-base titration showed the macromonomer to have unique characteristics on protonation of diamine moieties. Anionic polyaddition reactions between 1 and N-lithio-piperazine (2b) in the presence of piperazine (3b) also gave a macromonomer consisting of alternating repeating units of piperazine and organosilyl groups (4b). Radical copolymerizations of styrene with 5b gave comblike graft copolymers.     

Solubility of CO2 in CO2-philic oligomers; COSMOtherm predictions and experimental results

We report the solubility of carbon dioxide in four physical solvents and compare our data to predicted phase behavior using the conductor-like screening model for real solvents (COSMO-RS) formalism. The solubility data are presented in pressure-composition (Px) diagrams as well as Henry's law coefficients on a wt% basis at 298.15 K. The oligomers presented in this study are poly ethylene glycol di-methyl ether (PEGDME), perfluoro polyether (PFPE), poly di-methyl siloxane (PDMS), and poly propylene glycol di-methyl ether (PPGDME), which is a new solvent designed for this application by our group. These oligomers had 2–5 repeat units. We assess these four oligomers for capturing CO₂ from high-pressure streams. The COSMO-RS formalism is able to qualitatively and to some extent quantitatively describe solubilities of CO₂ in each of the oligomers.     

(Li6Si5)2–5: The Smallest Cluster-Assembled Materials Based on Aromatic Si56− Rings

Extensive explorations of their potential energy surfaces, combined with high-level quantum chemical computations, strikingly show that the lowest energy structures of the (Li₆Si₅)_2–5 systems consist of 2–5 Si₅⁶⁻ aromatic units, surrounded by Li⁺ counterions, respectively. These viable gas-phase compounds are the pioneering reported cases of oligomers made by planar aromatic silicon rings. Based on the key evidence that these oligomers are energetically favored, and that their silicon rings aromaticity is thoroughly preserved, the Li₆Si₅ cluster is proposed as a potential assembly unit to build silicon-lithium nanostructures, thus opening new paths to design Zintl compounds at the nanoscale level.     

Quantum-chemical study of the structure of copper(I) acetate and trifluoroacetate oligomers

The results of B3LYP quantum-chemical calculations of the equilibrium structures of [(CX₃COOCu)₂]₃, [(CX₃COOCu)₂]₂, and (CX₃COOCu)₂ oligomers (X = H, F) using the cc-pVTZ correlation-consistent basis for C, O, and F atoms and the Stuttgart 1997 RSC basis and relativistic effective core potential for Cu(I) atoms are presented. The differences in the structures of the free dimer and dimer units in oligomers were studied. The hexamer structure was chosen as the model of a fragment of the crystalline phase. Good agreement was obtained between the experimental and calculated differences between the geometrical parameters of the structures in the “gas phase-crystal” and “acetate-trifluoroacetate” series. Based on the calculated data, the increase in the Cu(I)-Cu(I) bond length in the silver acetate crystal compared with the gas phase can be explained by the effect of the neighboring dimer units of the polymer ribbon, while the increase in the Cu(I)-Cu(I) bond length in gaseous trifluoroacetate compared with acetate, by the acceptor effect of fluorine atoms.     

Synthesis and Structure of Phosphorus-Containing Complexes [Ph4P]

Complexes [Ph₄P] ₂ ⁺ [Hg₄I₁₀]²⁻ (I) and [[Ph₄P] ₂ ⁺ [BiI₅(Me₂S=O)]²⁻ (II) are synthesized by the reactions of tetraphenylphosphonium Ph₄PI with mercury diiodide in acetone and with bismuth triiodide in dimethyl sulfoxide, respectively. According to X-ray diffraction analysis, structural units of these complexes are tetraphenylphosphonium cations and tetra- and mononuclear anions, respectively. The phosphorus atoms in the tetraphenylphosphonium cations have a distorted tetrahedral coordination. In the central fragment of the centrosymmetric anion [Hg₄I₁₀]²⁻, the distances between the terminal mercury atoms and iodine atoms are 3.503(2) Å. The mercury atoms in the central and terminal fragments of compound I have distorted tetrahedral and trigonal coordinations, respectively. The bismuth atom in the mononuclear octahedral anion of complex II contains a dimethyl sulfoxide molecule along with five iodine atoms in the coordination sphere. __________ Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 10, 2005, pp. 791–795. Original Russian Text Copyright ? 2005 by Sharutin, Egorova, Sharutina, Dorofeeva, Molokov, Fukin.     

Theoretical study on nonlinear optical properties of metalloporphyrin using elongation method

A quantum-chemical analysis of the central metal effect on the (hyper)polarizabilities of meso-meso-linked metalloporphyrin (MP) oligomers was carried out using elongation finite-field (ELG-FF) method. It is found that meso-meso-linked MPs exhibit evident evolution of the third-order nonlinear optical (NLO) response (γ) along with an increasing number of porphyrin units N. The order of γ values is as following: γ_Mg > γ_Zn > γ_Ni. In contrast to the polarizability, the γ values of meso-meso-linked MPs are sensitive to the metals, that is, the nature of the metal can influence the third-order NLO response of MPs. However, the band structures for three MPs are similar to each other, and the differences on the band gaps of three MPs are very small. The local density of states (LDOSs) shows that the central metal gives the significant contributions for unoccupied bands in meso-meso-linked MPs.     

Anionic polymerization of 2,7‐di‐t‐butyldibenzofulvene: Synthesis,structure, and photophysical properties of the oligomers with a π‐stacked conformation

2,7‐Di‐t‐butyldibenzofulvene (tBu₂DBF), a bulky dibenzofulvene derivative, was polymerized using n‐butyllithium as initiator in tetrahydrofuran at ?78 °C and in toluene at 0 °C. tBu₂DBF afforded mainly oligomers up to trimer even at [monomer]₀/[initiator]₀ = 20 ([monomer]₀ = 0.2 M) at ?78 °C and 0 °C, indicating that this monomer is much less reactive than dibenzofulvene (DBF), its parent compound. The reaction at the same [monomer]₀/[initiator]₀ ratio at an elevated [monomer]₀ gave a small amount of insoluble polymer. The oligomers indicated a hypochromic effect in the absorption spectra and only monomer emission in the fluorescent spectra. The conformation of the trimer and the dimer was examined by means of NOESY NMR spectra and semiempirical calculations. In the trimer conformation, the fluorene moieties of the central and the initiation‐side monomeric units were found to be closely stacked on top of each other, while the termination‐side monomeric unit appeared to be in a faster conformational dynamics compared with the other monomeric units. Although the dimer seemed to have a relatively flexible conformation, a π‐stacked structure appeared to be involved in the conformational dynamics to show hypochromicity. The results of this study suggest that the reported intramolecular excited dimer (excimer) formation of the poly‐ and oligo(DBF)s [J Am Chem Soc 2003, 125, 15474] is based on a slight, local conformational change upon photo absorption, leading to a closer π‐stacked alignment of two neighboring fluorene units than that in the ground state. Such a local conformational transition may be difficult for the tBu₂DBF trimer because of steric repulsion of the t‐butyl groups. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 561–572, 2006     

Rationalizing chain microstructure in the polyα-olefins synthesized by cationic AlCl3/H2O catalytic system

In this study, three middle range α-olefin monomers including 1-hexene, 1-octene, and 1-decene were oligomerized using conventional AlCl₃/H₂O catalytic system. Molecular weight and microstructure of the oligomers were analyzed by GPC and ¹HNMR, respectively. By ¹HNMR spectra, both internal (CHR=CHR′ and CHR=CR′R′′) and external (CH₂=CR′R′′) olefins containing di and tri-substituted C=C bonds were detected. After successful oligomerization, synthesized polyα-olefins underwent hydrogenation process using Pd(0)-Hal catalyst to yield synthetic oils of PHex, POct, and PDec, respectively and then completion of the hydrogenation was confirmed by ¹HNMR spectroscopy. The microstructure of the synthesized oligomers was rationalized using the ratio under the peak of CH?+?CH₂/CH₃ hydrogens (S1/S2) in ¹HNMR spectra and the degree of oligomerization obtained from M_n. According to the results, the best match between theoretical and real S1/S2 is obtained when considering double bond isomerization in the synthesized PAOs. By knowing PAO molecular weight, a relationship between monomer type and S1/S2 in the PAO homopolymers was detected. Our suggested methodology can be generalized to the unknown PAO homopolymers to unravel their monomer type by simple ¹HNMR and GPC analyses.     

Self-activated DNA cleavage of a water-soluble mononuclear Cu(II) complex with polyquinolinyl ligand

The synthesis and characterization of a water-soluble mononuclear Cu(II) complex, [CuLCl₂]·2CH₃CH₂OH, where L = bis(2-quinolinyl methyl)benzyl-amine has been reported. L is a tridentate polyquinolinyl ligand, coordinated to Cu(II) via NNN donors. The central copper ion of 1 has N₃Cl₂ donor set in a distorted trigonal-bipyramidal geometry. The dimer existing in the solid state resulted from hydrogen bonds and π–π accumulation between two mononuclear units. The interaction of 1 with CT-DNA has been explored by absorption and emission titration methods, revealing partial intercalation between 1 and CT-DNA. Moreover, 1 could make pBR322 plasmid DNA cleaved by a self-activated oxidative process; hydroxyl radical and singlet oxygen may be the main reactive oxygen species species in the process. Complex 1 may quench the intrinsic fluorescence of bovine serum albumin in a static quenching process, which has been investigated by UV–visible and fluorescence spectroscopic methods. 1 also demonstrates potent cytotoxity against Hela cells with IC₅₀ value of 2.84 μM, which shows it to be a potential candidate as an anticancer metal-based drug.