Search

Single-molecule surface-enhanced Raman spectroscopy (SERS) offers new opportunities for exploring the complex chemical and biological processes that cannot be easily probed using ensemble techniques. However, the ability to place the single molecule of interest reliably within a hot spot, to enable its analysis at the single-molecule level, remains challenging. Here we describe a novel strategy for locating and securing a single target analyte in a SERS hot spot at a plasmonic nanojunction. The "smart" hot spot was generated by employing a thiol-functionalized cucurbit[6]uril (CB[6]) as a molecular spacer linking a silver nanoparticle to a metal substrate. This approach also permits one to study molecules chemically reluctant to enter the hot spot, by conjugating them to a moiety, such as spermine, that has a high affinity for CB[6]. The hot spot can accommodate at most a few, and often only a single, analyte molecule. Bianalyte experiments revealed that one can reproducibly treat the SERS substrate such that 96% of the hot spots contain a single analyte molecule. Furthermore, by utilizing a series of molecules each consisting of spermine bound to perylene bisimide, a bright SERS molecule, with polymethylene linkers of varying lengths, the SERS intensity as a function of distance from the center of the hot spot could be measured. The SERS enhancement was found to decrease as 1 over the square of the distance from the center of the hot spot, and the single-molecule SERS cross sections were found to increase with AgNP diameter. [ABSTRACT FROM AUTHOR]

A C-pyrenyl poly(methylenephosphine) (Mn = 4800, Đ = 1.56) was synthesized from the anionic polymerization of a P-mesityl phosphaalkene. The phosphine oxide polymer exhibits blue fluorescence in solution (λmax 379, 400 nm, λonset 560 nm; Φ = 0.05) and blue-green fluorescence in the solid state at room temperature (λmax 460 nm, Φ = 0.07). In contrast, unoxidized polymer displays only a weak emission in solution and no emission in the solid state. Fluorescence studies of partially oxidized polymers (50, 75, 83% oxidation) indicate that “turn on” emission occurs only at very high degrees of oxidation (near 100%). We conclude that even a small number of unoxidized phosphine moieties within the partially oxidized polymer intramolecularly quench the fluorescence. [ABSTRACT FROM AUTHOR]

Supramolecular semicrystalline hydrogels are soft functional materials consisting of water-swollen hydrophilic polymer chains interconnected by hydrophobic segments forming lamellar crystals. Although such hydrogels with high crystallinity are mechanically strong, with elastic moduli and tensile strength of 80-300 MPa and 4-7 MPa, respectively, they are brittle and rupture at a stretch of less than 20% without yielding. Here, we report that the incorporation of a small amount of a weak hydrophobe into semicrystalline hydrogels significantly increases their toughness and stretchability without losing their high modulus and high strength. We design a highly entangled physical network based on poly(N,N-dimethylacrylamide) (PDMA) chains containing n-octadecyl acrylate (C18A) and lauryl methacrylate (C12M) segments with side chain lengths of 18 and 12 carbons, respectively. By including 0.1-0.4 mol % C12M into the PDMA backbone containing 30 mol % C18A segments, we were able to create more ordered and thinner lamellar crystals with a layered structure. Simultaneously, a brittle-to-ductile transition was observed due to the appearance of necking behavior leading to 10-fold increase of toughness. The significant toughness improvement upon incorporation of C12M into the semicrystalline hydrogels could be explained with the appearance of active tie molecules under external force interconnecting the lamellar clusters. The hydrogels also exhibit reversible tensile deformation induced by heating above the melting temperature of crystalline domains. [ABSTRACT FROM AUTHOR]

Hyperjapones A-E (1-5), novel terpenoid polymethylated acylphloroglucinols (TPAPs) with unusual architectures, were characterized from Hypericum japonicum. Their structures and absolute configurations were determined by comprehensive spectroscopic data and X-ray diffractions. Compound 1 was obtained as a racemic mixture and was separated by a column coated with cellulose tris(4-methylbenzoate) after attempts with various chiral materials. Compounds 1, 2, and 4 exhibited moderate antitumor activities in vitro. [ABSTRACT FROM AUTHOR]

This is the first report on the folding phenomenon of pyridazinone and triazinone dimers (1-5) bridged through an alkyl chain. Among them, direct evidence of conformational preference of 1, 2, and 5 was obtained through single crystal X-ray structures. However, prevalence of folded conformations was analyzed through theoretical investigation. [ABSTRACT FROM AUTHOR]

Poly(trimethylene terephthalate)(PTT) nanocomposites with twodifferent clays were fabricated via a novel two-step process. Structure,morphology, and cold crystallization of these nanocomposites werestudied using differential scanning calorimetry (DSC), Fourier transforminfrared (FTIR) spectroscopy, and X-ray diffraction (XRD). XRD resultsreveals that PTT-15A nanocomposites were fully exfoliated, whereasPTT-30B nanocomposites were partially exfoliated. Non-isothermal coldcrystallization temperature and the crystallinity of non-isothermallycold-crystallized PTT-15A nanocomposites increased with increasing15A content. The crystallinity of isothermally cold-crystallized PTT-15Ananocomposites was shown to increase with crystallization temperatureand the clay content, but no clear trend in crystallinity was observedfor PTT-30B nanocomposites. The combination of FTIR and DSC has beenused to determine the conformational changes during isothermal coldcrystallization. The results show that the crystalline gauche andamorphous gauche conformations increased and the amorphous trans conformationdecreased with crystallization temperature and clay content for PTT-15Ananocomposites, whereas no trend was observed for PTT-30B nanocomposites. [ABSTRACT FROM AUTHOR]

Thepresent work demonstrates the solvation dynamics and rotationalrelaxation of Coumarin 153 (C-153) in the micelles of a series ofcationic gemini surfactants, 12-s-12, 2Br–containing a hydrophobic polymethylene spacer with s= 3, 4, 6, 8, 12. Steady-state and time-correlated single-photoncounting (TCSPC) fluorescence spectroscopic techniques have been usedto carry out this study. Steady-state and TCSPC fluorescence datasuggest that C-153 molecules are located at the Stern layer of micelles.While probe molecules feel more or less the same micropolarity inthe micellar phase, the microviscosity of micelles decreases withspacer chain length. Solvation dynamics at the Stern layer is bimodalin nature with fast solvation as a major component. Counter ions andwater molecules bonded with the polar headgroups of surfactant moleculesare responsible for the slow component. Average solvation time increaseswith spacer chain length because of the increased degree of counterion dissociation. Some water molecules are involved in the solvationof counter ions themselves, resulting in the decrease in “free”water molecules to be available for the solvation of C-153. The hydrophobicspacer chain also has an effect on increasing the solvation time withincreasing chain length. The average rotational relaxation time forC-153 decreases with spacer chain length with a rapid decrease at s> 4. The anisotropy decay of C-153 in micelles is biexponentialin nature. The slow rotational relaxation is due to the lateral diffusionof C-153 in micelles. Lateral diffusion is much faster than the rotationalmotion of a micelle as a whole. The rotational motion of the micelleas a whole becomes faster with the decreasing size of micelles. [ABSTRACT FROM AUTHOR]

A seriesof syndiotatic P5CPnpolymethylenes wasprepared with 4-(trans-4-pentylcyclohexyl)phenoxy moieties linkedto each backbone carbon atom via an alkyloxycarbonyl spacer and witheven numbers of alkyl carbons nranging from 2 to14, and their thermotropic behaviors were investigated. The P5CPn, except P5CP2, formed smectic phases in which the rod-likepolymethylene backbones were arranged in rectangular lattices, andthe side-chain mesogens were aggregated into layers parallel to theshorter sides of the rectangular lattices. The packing of the mesogenschanged with decreasing temperature from smectic C-like to smecticI-like (SmI-like) for n= 4–8 and from smecticA-like to SmI-like for n= 10–14. In the SmI-likephases, each mesogen along the main-chain axis was connected to everysixth backbone carbon atom, revealing a correlation between the packingof the mesogens and the main-chain conformation of a 3/2 helix. Conversely,P5CP2 formed a smectic phase with the main chains arranged in rowsand with the mesogens barely aggregated into layers due to the shortspacers. [ABSTRACT FROM AUTHOR]

Herein,we report a cost-effective and easy synthetic procedurefor the fabrication of a graphene–single-walled carbon nanotube–poly(3-methylthiophene)ternary nanocomposite for high-performance supercapacitor electrodes.The possible interactions of both graphene (Gr) and single-walledcarbon nanotubes (SWCNTs)with poly(3-methylthiophene) (PMT) were characterized by Fourier transforminfrared, UV–visible, and Raman spectroscopies. A morphologicalstudy confirmed the formation of a bridge between PMT-coated SWCNTsand Gr layers. The ternary nanocomposite showed superior electricalconductivity of 4.68 S/cm at room temperature and also exhibited nonlinearcurrent–voltage characteristics. The ternary nanocompositeachieved a maximum specific capacitance of 561 F/g at a 5 mV/s scanrate. Both a high energy density and a high power density were obtainedfor the ternary nanocomposite. Here, both the Gr and SWCNTs take partin the increment of the electrochemical properties. A higher thermalstability was also observed for the ternary nanocomposite. Based onits outstanding properties, the ternary nanocomposite is a potentialcandidate for a high-performance supercapacitor electrode. [ABSTRACT FROM AUTHOR]

Specific functional-group or domain interactions of fully deuterium-exchanged, carbonyl-13C-labeled benzophenone and different types of natural organic matter (NOM) were investigated through two-dimensional 1H-13C heteronuclear correlation NMR spectroscopy. The sorbents included Beulah-Zap lignite, type II kerogen (IL-6), Pahokee peat, Amherst humic acid, and a polystyrene-poly(vinylmethyl ether) (PS-PVME) blend. PSPVME consists of PS and PVME chains that are mixed on a scale of <5 nm. The NOM sorbents all consist predominantly of a mixed aromatic-alkyl or aromatic-O-alkyl matrix that is homogeneous on the 3 nm scale, as evidenced by fast equilibration of aromatic and alkyl 1H magnetization. In addition, Beulah lignite and IL-6 kerogen exhibit small fractions of distinct polymethylene (CH2)n domains, and Pahokee peat contains significant fractions of polar and nonpolar alkyl domains. Benzophenone-(13C=O)-d10 shows proximity to both aromatic rings and alkyl segments in all samples but preferentially interacts with aromatic rings in PSPVME and Beulah lignite, possibly due to π-π electron donor-acceptor interactions. The data for IL-6 kerogen are also compatible with preferential location of benzophenone near the alkyl-substituted edges of aromatic rings, while in Pahokee peat, clear signatures of benzophenone affinity to both aromatic-rich and nonpolar alkyl domains have been detected. Amherst humic acid shows evidence of some affinity to polar alkyl segments but which is weaker than that to aromatic rings. Our results indicate that specific interactions of the sorbate and the presence of domains in the sorbent influence the magnitude and selectivity of sorption. [ABSTRACT FROM AUTHOR]

O-Acylcholines (OACs), which are true cationiclipids due to the quaternary ammonium functionality in the headgroup,exhibit interesting biological activities and medicinal properties.In the present study, a homologous series of OACs with even chainlengths (n= 12–20) have been synthesized,and their thermotropic and chaotropic phase transitions have beencharacterized. The role of various anions (Cl–,Br–, I–, NO3–, SO42–, ClO3–, ClO4–) on the phase behavior of O-stearoylcholine was investigated by calorimetric, spectroscopic,and turbidimetric approaches. The results obtained revealed that inaqueous dispersion O-stearoylcholine undergoes acooperative phase transition from a gel phase to a micellar structureand that the transition temperature increases when the counterionsare changed in the Hofmeister series. Single-crystal X-ray diffractionstudies showed that O-stearoylcholine iodide formsan interdigitated bilayer structure, with the polymethylene chainadopting an all-trans conformation. The Hofmeister effect and phasebehavior were explained using the concepts of matching wateraffinities, water penetration into the bilayer, and electrostatic repulsion. It was also observedthat one counterion per molecule is sufficient to strongly modulatethe phase properties of the lipid/surfactant. [ABSTRACT FROM AUTHOR]

The crystal polymorphism of isotacticpoly(5-methyl-hexene-1) (P5MHex1)is reexamined on the basis of new data: electron diffraction of solution-and thin film-grown single crystals, fiber patterns from stroked films.Preliminary crystal structures are determined by conformational analysisand packing energy minimization. A crystal structure initially describedas orthorhombic (Corradini, P.; Eur. Polym. J.1970, 6, 1201) is actuallya frustrated, chiral phase with a trigonal unit-cell (a= b= 17.62 Å, c= 6.33 Å,γ = 120°, space group P31or P32). Analogies between the structures of isotacticP5MHex1 and poly(5-methyl-heptene-1) reported by Corradini et al.suggest that the latter polymer has also a frustrated structure. Thefrustrated character is lost upon heating. The high temperature formis best described as a one chain, trigonal unit-cell with probablysignificant side chain mobility (a=b= 10.17 Å, c= 6.33 Å, γ = 120°,space group P31or P32). Interconversion on heating and cooling between the frustratedand the one-chain unit-cells is probable. An original crystal modificationformed during synthesis of P5MHex1 but not previously analyzed isobtained as single crystals in dilute solution. The monoclinic cell(a= 20.48 Å, b= 16.94 Å, c= 6.33 Å, but α = β = γ = 90°)houses four 3-fold helices (two pairs of enantiomorphous chains).This metastable form converts by melting-recrystallization to thechiral, frustrated phase. On standing at room temperature, a structuralrearrangement takes place in the frustrated form, and a packing oflayers similar to the monoclinic phase develops. The rich polymorphismof P5MHex1 illustrates some of the many packing modes available to3-fold helices of polymers with long, flexible side chains. [ABSTRACT FROM AUTHOR]

The size-dependent reactivity of cobalt tropocoronands [TC-n,n]2- is manifest in the NO chemistry of the cobalt(III) nitrite complexes [Co(η2-NO2)(TC-n,n)] (n = 4--6), the synthesis and characterization of which are reported for the first time. Complete conversion of [Co(η2-NO2)(TC-4,4)] to the cobalt mononitrosyl [Co(NO)(TC-4,4)] occurs upon exposure to NO(g). In contrast, addition of NO(g) to [Co(η2-NO2)(TC-5,5)] generates both cobalt mono- and dinitrosyl adducts, and addition of nitric oxide to [Co(η/2-NO2)(TC-6,6)] converts this complex to the dicobalt tetranitrosyl species [Co2(NO)4(TC-6,6)]. In the latter complex, two tetrahedral cobalt dinitrosyl units are bound to the aminotroponeiminate poles of the [TC-6,6]2- hgand. These results significantly broaden the chemistry of cobalt tropocoronands with nitric oxide and the nitrite anion. [ABSTRACT FROM AUTHOR]

Secondary ion mass spectrometry (SIMS) is a very surface sensitive analysis technique with low detection limits. The main drawback of SIMS is its inherent incapability of providing quantitative information about sample compositions due to the frequent occurrence of ionization- and sputter-induced matrix effects. Metal-assisted SIMS (MetA-SIMS) is an experimental approach that consists in covering an organic sample with a minute amount of a noble metal prior to a static SIMS analysis, the main objective being an increase of the characteristic secondary ion intensities. We show in this article that MetA-SIMS is also a simple and efficient tool for reducing matrix effects in a set of polymer blend samples containing different relative concentrations polystyrene (PS) and poly(methylene methacrylate) (PMMA). These findings can be explained by diffusion processes leading to a sample surface configuration consisting of individual polymer chains embedded in a common Ag matrix. [ABSTRACT FROM AUTHOR]

A systematic study has been carried out to investigatethe roleof the side chains in lowering the reversible thermochromic transitiontemperature of polydiacetylenes. The type of interacting groups alsoproved to be crucial. Polymers with hydrogen-bonded side chains haveconsistently behaved differently as compared to polymers with lessspecific interactions associated with metallic ions. The most strikingfactor is the orderliness of the polymethylene portion of the sidechains. The length of the side chains is critical in determining thetransition temperature. In some cases, the packing order of the sidechains is of sufficient order to crystallize. When high order exists,a reversible thermochromic transition is difficult to achieve. Conversely,if the side chains are disordered at any temperature, the transitiontemperatures are low and the transition is always reversible. Thisis the case for side chains stabilized by the presence of metallicions. It is apparent that the presence of long-range nonspecific electrostaticinteractions is important in determining reversible thermochromictransitions. These structural features are most easily observed usinginfrared spectroscopy and Raman scattering. Supporting evidence havebeen obtained using thermal analysis and X-ray diffraction. [ABSTRACT FROM AUTHOR]

Interaction of a few dansyl–naphthalimide conjugates 1a–elinked through polymethylene spacer groups with various metal ions was investigated through absorption, fluorescence, NMR, isothermal calorimetric (ITC), and laser flash photolysis techniques. The characteristic feature of these dyads is that they exhibit competing singlet–singlet energy transfer (SSET) and photoinduced electron transfer (PET) processes, both of which decrease with the increase in spacer length. Depending on the spacer group, these dyads interact selectively with divalent Cu2橷 Zn2誉, as compared to other mono- and divalent metal ions. Jobs plot analysis showed that these dyads form 2:3 complexes with Cu2誉, while 1:1 complexes were observed with Zn2誉. The association constants for the Zn2橷 Cu2犉ꦕ were determined and are found to be in the order 103–105M–1. Irrespective of the length of the spacer group, these dyads interestingly act as fluorescence ratiometric molecular probes for Cu2誉 by altering the emission intensity of both dansyl and naphthalimide chromophores. In contrast, only the fluorescence intensity of the naphthalimide chromophore of the lower homologues (n= 1–3) was altered by Zn2誉. 1H NMR and ITC measurements confirmed the involvement of both sulfonamide and dimethylamine groups in the complexation with Cu2誉, while only the latter group was involved with Zn2誉. Laser excitation of the dyads 1a–eshowed formation of a transient absorption which can be attributed to the radical cation of the naphthalimide chromophore, whereas only the triplet excited state of the dyads 1a–ewas observed in the presence of Cu2誉. Uniquely, the complexation of 1a–ewith Cu2誉 affects both PET and SSET processes, while only the PET process was partially inhibited by Zn2誉 in the lower homologues (n= 1–3) and the higher homologues exhibited negligible changes in their emission properties. Our results demonstrate that the spacer length dependent variations of the photophysical properties of these novel conjugates not only enable the selective detection of Cu2橷 Zn2誉 but also aid in discriminating these two biologically important metal ions. [ABSTRACT FROM AUTHOR]

We have investigated the impedance behavior of immersed superhydrophobic (SH) polymethylene surfaces by tailoring the surface tension of the contacting liquid phase to gradually transition the surface from the Cassie to the Wenzel state. Control over the surface tension is accomplished by varying the ethanol content of the aqueous phase. To establish the mechanism of the transition, we imaged the interface of the film and identified three distinct events of this process: a nucleation event at low concentrations of ethanol in which small areas beneath the liquid phase transition into the Wenzel state, a propagation event characterized by the enlargement of the Wenzel domains and the lateral displacement of air, and a final event at higher concentrations of ethanol in which the thin air layer at the interface morphs into isolated pockets of air. Using this visualization of the transition, we characterized the Cassie and the Wenzel states by measuring the impedance at a frequency of 1 kHz for an initially SH film that changes its wetting behavior upon addition of ethanol. Establishment of the Cassie and Wenzel state conditions was based on concepts of electrochemical impedance spectroscopy (EIS) and quantitatively validated using both the Helmholtz theory and the analytical description of the electrochemical system in terms of the circuit model of a metal surface covered by a polymer film. Finally, we apply this strategy to determine the possibility for SH polymethylene (PM) films to reversibly transition between the Cassie and the Wenzel states. Results show that after rinsing and drying at ambient conditions for 24 h, the film recovers the SH state, suggesting the applicability of these SH films in outdoor environments with occasional periodic submersion in water. [ABSTRACT FROM AUTHOR]

We herein describe the preparation and characterization of a phase-separated fluorinated polymethylsilsesquioxane (PMSQ), which may be used as an antireflective coating. The results of FTIR analysis showed that when PMSQ is synthesized from methyltrimethoxysilane (MTMS), it exists mostly in the form of a cage structure. Its reflectivity of normally incident light (R) may be reduced from 3.9 to 0.9% by grafting 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FTES) by means of a sol–gel process. In the study described herein, the minimum reflectivity of PMSQ was lowered to less than 1% by coating it with a single layer of fluorinated polymethylsilsesquioxane, through phase segregation of PMSQ and FTES that took place during film formation. The RI of PMSQ at 550 nm was also reduced from 1.51 to 1.42 by heating at 80 °C for 30 min, a temperature suitable for substrates, such as plastics, that have low heat resistance. Wetting and adhesion to substrates were both improved by the additional grafting of tetraethoxysilane (TEOS), again by means of a sol–gel process. Results of solid-state 29Si NMR and GPC showed increases in T3, Q3, Q4, and the molecular weight, which signal the effective grafting of both the TEOS and the FTES on the PMSQ. Results from energy-dispersive X-ray spectroscopy (EDX) show that the quantity of fluorine atoms at the surface of the PMSQ film increased from 0 to 22%, providing the evidence of the phase segregation within the PMSQ film. When FTES is used, we found an increase in the water contact angle from 92° to 108°, which indicated that the hydrophobicity at the PMSQ film surface increased by increasing the FTES content from 0 to 100 wt %, using the weight of MTMS as 100%. [ABSTRACT FROM AUTHOR]

Tetranuclear cubane-type rare-earth methylidene complexes consisting of four "Cp′LnCH2" units, [Cp′Ln(μ3-CH2)]4 (4-Ln; Ln = Tm, Lu; Cp′ = C5Me4SiMe3), have been obtained for the first time through CH4 elimination from the well-defined polymethyl complexes [Cp′Ln(μ2-CH3)2]3 (2-Ln) or mixed methyl/methylidene precursors such as [Cp′3Ln3(μ2-Me)3(μ3-Me)(μ3-CH2)] (3-Ln). The reaction of the methylidene complex 4-Lu with benzophenone leads to C-O bond cleavage and C-C bond formation to give the cubane-type oxo complex [Cp′Lu(μ3-O)]4 and CH2-CPh2, while the methyl/methylidene complex 3-Tm undergoes sequential methylidene addition to the C-O group and ortho C-H activation of the two phenyl groups of benzophenone to afford the bis(benzo-1,2-diyl)ethoxy-chelated trinuclear complex [Cp′3Tmμ(μ2-Me)3{(C6H4)2C(O)Me}] (6-Tm). [ABSTRACT FROM AUTHOR]

Preordering behavior of poly(trimethylene terephthalate) (PTT) upon cold crystallization was revealed via in situ simultaneous small/wide-angle X-ray scattering and differential scanning calorimetry (SAXS/WAXS/DSC) as well as time-resolved Fourier transform infrared (FTIR) spectroscopy. In the induction stage prior to the appearance of WAXS crystalline peaks, changes in the low-q(0.005−0.03 Å−1) range of SAXS profiles suggested the formation of mesomorphic domains (with a linear dimension ∼18 nm). Parallel DSC and FTIR results indicated respectively that the preordering involved a trace amount of exothermic heat prior to discernible trans-to-gaucheconformation change. Subsequently observed were the growth of these mesomorphic domains to a size ≈50 nm and the accompanying emergence of nanocrystalline grains (of dimensions of several nanometers) via a more localized nucleation-and-growth route. The final ripened structure hence comprised a percolated network of mesomorphic domains filled with dispersed nanocrystallites. Hierarchical evolutions of the large mesomorphic domains and the small nanocrystallites with crystallization time were correlated with WAXS-determined crystallinity development and DSC-revealed endothermic events. Transitions from the amorphous to mesomorphic domains and the mesomorphic to crystalline domains are illustrated quantitatively. The formation of large mesomorphic domains in the induction stage of crystallization and their influences on the subsequent crystallization are discussed in terms of liquid−liquid phase separation and gel formation. [ABSTRACT FROM AUTHOR]

Bolaamphiphiles with l-glutamic acid headgroups and hybrid linkers, each composed of two rigid benzene rings and different polymethylene units, were designed, and morphological controls of the hierarchical self-assemblies were realized via changing solution pH and application to solid surfaces. At a low pH of 3, bolaamphiphiles formed hydrogels with water and molecules with short and long spacers formed nanofibers and helical nanoribbon−nanotubes, respectively. In a pH 12 aqueous solution, vesicles were observed from cryo-TEM measurements for amphiphiles with short spacers that could transfer to huge vesicles when cast onto a mica surface. Amphiphiles with longer spacers self-assembled into nanoparticles in a pH 12 aqueous solution while micellar fibers were formed on a mica surface. Those assemblies were characterized with UV−vis, CD, and FT-IR spectroscopy and AFM and TEM observations. With molecular structure modification and the fine tuning of conditions, morphology transitions between various nanostructures were obtained from the self-assembled bolaamphiphiles. The environmental pH can induce different interaction modes between the headgroups, and at high pH, there are strong interactions between molecular assemblies and the mica surface. It is suggested that the active headgroups, rigid necks, and flexible linkers with different lengths render molecules with diverse aggregation morphologies. [ABSTRACT FROM AUTHOR]

The isothermal melt crystallization of poly(trimethylene terephthalate) (PTT) has been investigated using differential scanning calorimetry (DSC), optical microscopy, and Fourier transform infrared (FTIR) spectroscopy. Triple-melting endotherms were dominant at crystallization temperatures (Tc) below 195 °C, while double-melting endotherms were dominant at Tcabove 195 °C. These multiple-melting behaviors were attributed to melting and recrystallization as well as thermal stability of the crystallites. The difference in crystallinity observed between melt-crystallized PTT films before and after cooling to room temperature was attributed to lamellar thickening. The kinetics of isothermal melt crystallization of PTT was analyzed using an Avrami equation, and the rate constant (k) and t1/2increased with increasing Tc. An Avrami exponent n= 2−3 was obtained for the PTT melt crystallized from 150 to 205 °C, suggesting two-dimensional to three-dimensional growth with heterogeneous nucleation and was confirmed by polarized optical microscopy. The bands at 1358 and 976 cm−1associated with gauche and trans conformations of −OCH2CH2CH2O− were used to determine gauche and trans conformations assuming a two-phase conformational model. The crystalline and the amorphous gauche conformations were separated by combining them with DSC crystallinity. It was shown that the crystalline gauche conformation increased at the expense of the amorphous trans conformation. No significant change in the amorphous gauche conformation was found. On the other hand, the crystalline and amorphous gauche conformations were found to increase with the annealing temperature (Ta) at the expense of the trans conformations during the annealing of PTT. The difference between melt crystallization and annealing was also discussed. [ABSTRACT FROM AUTHOR]

A wide variety of different linear, diquaternary alkylammonium ions have been used as supplementary crystallization structure-directing agents (SDAs) in the synthesis of UZM-12, a high-silica version of zeolite erionite, via a charge density mismatch (CDM) approach. When tetraethylammonium is used as a CDM SDA, the crystallization of UZM-12 was found to be critically dependent not only on the type of alkali metal cations added as another crystallization SDA to the synthesis mixture, but also on the size of the groups on the diquaternary ammonium ion employed and the length of its central polymethylene chain that are closely related to the dimensions of cylindrical 23-hedral [4126586] eri cages in this small- pore zeolite. 27Al MO MAS NMR measurements reveal a preferential location of Al on the high-multiplicity site over the lower-multiplicity site of the UZM-12 framework. The catalytic results from the methanol-to- olef in reaction over a series of H-UZM-12 zeolites with similar acidic properties but different crystallite sizes (100-2500 nm in length) demonstrate that the nanocrystallinity (probably the ≤100 nm range) may have a detrimental effect on the activity and stability for this reaction, probably due to the fast buildup of large coke molecules on the external surface of zeolite crystallites that inhibits the methanol diffusion to intrazeolitic acid sites, rendering them ultimately inaccessible for catalysis. [ABSTRACT FROM AUTHOR]

Superhydrophobic polymethylene (PM) films provide remarkable protection against the transport of aqueous redox probes as a result of entrapped air at the water/polymer interface. The wetting properties and the topography for a set of superhydrophobic (SH) and nonsuperhydrophobic (NSH) PM films were established to compare their interfacial behavior using electrochemical impedance spectroscopy (EIS). EIS results show that SH PM films exhibit resistances against ion transfer that are ∼3 orders of magnitude higher than those of NSH PM films. Rationalization of these results in the context of the Helmholtz theory reveals that the imaginary impedance or inverse capacitance of SH PM films exhibits positive deviations from that predicted by Helmholtz theory for smooth PM films. Here, we model the capacitance behavior of a SH film as a PM/air composite that acts as a circuit of capacitors. The resulting mathematical model of this analysis enables correlation of the effective dielectric properties of the film (deffective, εeffective) to measurable properties such as thickness (dfilm) and the dielectric constant (εPM) of the PM film. Finally, a sensitivity analysis shows that the limited contact area between the aqueous solution and PM in SH films is the primary reason for the enhancement in the barrier properties of the film. [ABSTRACT FROM AUTHOR]

We report a new surface–initiated polymerization strategy that yields superhydrophobic polym– ethylene (PM) films from initially smooth substrates of gold and silicon. The films are prepared by assembling a vinyl–terminated self–assembled monolayer, followed by exposure of the surface to a 0.1 M solution of borane, and polymerizing from the borane sites upon exposure to a solution of diazomethane at –17 °C. Surface–initiated polymethylenation (SIPM) presents rapid growth in relation to other surface–initiated reactions, producing PM films thicker than 500 nm after 2 min of reaction and 3 μm after 24 h of reaction. AFM and SEM images show the presence of micro– and nanoscale features that enable the entrapment of air when exposed to water. Consistent with this result, these films exhibit advancing water contact angles greater than 160°, dramatically different than 103° measured for smooth PM films, and hysteresis values ranging from 2° to 40°, depending on the substrate and polymerization time. The superhydrophobic character of the films results in the entrapment of air at the polymer/solution interface to provide remarkable resistances greater than 1010 Ω ∎cm2 against the transport of aqueous redox probes and cause the film to behave as a "perfect" capacitor. [ABSTRACT FROM AUTHOR]

New binary ligands consisting of 2,2':6',2″-terpyridine and 2,2'-bipyridine were synthesized through the linkage of various lengths of the polymethylene chain (1a-1d; n=3-6, where n is the number of CH2 units). Their Co(II) complexes, 2a-2d (Cl complexes) and 2a'-2d' (H2O complexes), were synthesized. They exclusively formed mononuclear complexes, which could be oxidized to Co(III) complexes in aqueous solutions after prolonged exposure to the air. The single crystals of 2b and Co(III) complex 2c″ were obtained, and their crystal structures were determined. Cyclic voltammograms of the Co(II) complexes showed the redox processes of the terpyridine, Co(II)/Co(I), and Co(III)/Co(II). The redox waves of Co(II)/Co(I) in 2a and 2a' were slightly different from those of other complexes because of the short polymethylene chain. [ABSTRACT FROM AUTHOR]

Chimeric dyads 1a and 1b based on dansyl and naphthalimide units linked through the polymethylene group were synthesized, and their photophysical and interactions with various metal ions were investigated under different conditions. These dyads showed dual emission centered at around 375 and 525 nm, respectively, due to the locally excited state of the naphthalimide chromophore and energy- transfer-mediated emission (FRET) from the dansyl moiety. When titrated with various metal ions, these systems exhibited unusual selectivity for Cu2+ ions as compared to Na+, Li+, K+, Zn2+, Pb2+, Hg2+, Co2+, Fe2+, Cd2+, Mg2+, and Ba2+ ions and signaled the binding event through inhibition of FRET mediated emission at 525 nm with concurrentenhancement in the emission intensity of the naphthalimide chromophore at 375 nm. The uniqueness of these dyads is that they form stable 2:1 stoichiometric complexes involving sulfonamide functionality and act as visual fluorescence ratiometric probes for the selective recognition of Cu2+ ions. [ABSTRACT FROM AUTHOR]

The temperature-dependent self-assembly and the mixing behavior of symmetrical single-chain bolaamphiphiles with different polymethylene chain lengths and different headgroup structures were investigated in water by differential scanning calorimetry (DSC), cryo transmission electron microscopy (cryo-TEM), and small angle neutron scattering (SANS). The even-numbered polymethylene-1,ω-bis(phosphocholines) (PC-C n-PC) are known to form nanofibers composed of stretched molecules with an all- transalkyl chain conformation (Drescher, S.; Meister, A.; Blume, A.; Karlsson, G.; Almgren, M.; Dobner, B. Chem.Eur. J.2007, 13, 5300−5307). The odd-numbered analogues were synthesized to study a possible even−odd effect of these bolaamphiphiles during their aggregation in water. In addition to these bolaamphiphiles with phosphocholine headgroups, a new series of polymethylene-1,ω-bis(phosphodimethylethanolamines) (Me 2PE-C n-Me 2PE) with smaller headgroup sizes was synthesized. These bolaamphiphiles show an additional fiber−fiber transition when the alkyl chain length exceeds 26 carbon atoms. The mixing behavior of both types of bolaamphiphiles indicates that differences in the alkyl chain length up to six carbon atoms are tolerated within the fiber structure. The mixing of two Me 2PE-C n-Me 2PE or PC-C n-PC type bolaamphiphiles with different alkyl chain lengths offers the possibility to adjust the temperature, where the cross-linking of the fibers is disrupted and where the fibers break apart. As a consequence, temperature switchable hydrogels are obtained that can be fine-tuned for drug delivery applications. The comparison with dotriacontane-1,32-diyl-bis[2-(methylammonio)-ethylphosphate] (MePE-C32-MePE), a new bolaamphiphile with even smaller phosphomonomethylammonio headgroups, illustrates the importance of the headgroup size for the aggregation behavior. This bolaamphiphile self-assembles exclusively into lamellar structures, and this aggregate type persists in mixtures with the fiber forming Me 2PE-C32-Me 2PE. [ABSTRACT FROM AUTHOR]

Results of an investigation, aimed at gaining information about the factors governing the efficiencies of single electron transfer (SET)-promoted photocyclization reactions of linked acceptor-polydonor systems, are described. One set of substrates used in this effort includes α-trimethylsilyl ether terminated, polymethylene- and polyethylenoxy-tethered phthalimides and 2,3-naphthalimides. Photocyclization reactions of the polyethylenoxy-Iinked phthalimides and naphthalimides were observed to take place in higher chemical yields and with larger quantum efficiencies than those of analogs containing polymethylene tethers of near equal length. These findings show that the rates of formation of 1,ω-zwitterionic biradicals that serve as key intermediates in the photocyclization processes are enhanced in substances that contain oxygen donor sites in the chain. The findings suggest that these donor sites facilitate both initial SET to acceptor excited states and ensuing intrachain SET, resulting in migration of the cation radical center to the terminal α-trimethylsilyl ether position. In addition, an inverse relationship was observed between the quantum yields of photocyclization reactions of the tethered phthalimides and naphthalimides and the length of the polyethylenoxy chain. Finally, the roles played by chain type and length in governing photoreaction efficiencies were investigated by using intramolecular competition in photoreactions of polyethylenoxy and polymethylene bis-tethered phthalimides. Mechanistic interpretations and synthetic consequences of the observations made in this study are discussed. [ABSTRACT FROM AUTHOR]

We have achieved a growth of highly oriented crystalline pentacene thin films, with preferred a−bin-plane orientation with respect to the rubbing direction of a rubbed polymethylene surface. The polymethylene thin film, generated on a gold surface by gold-catalyzed decomposition of diazomethane, was annealed and gently rubbed in a fixed direction by a flannelette cloth to serve as an alignment layer during the deposition of pentacene molecules. Various surface analysis techniques, including reflection absorption IR spectroscopy (RAIRS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, grazing incidence X-ray diffraction (GIXD), and atomic force microscopy were used to elucidate the structural details of the polymethylene and the pentacene thin films deposited on it. Two crystalline morphologies of pentacene thin film were observed:  the minor one of rod-like molecular crystals having their long axes of the crystals perpendicular to the rubbing direction, and the dominant one of platelet-like and layered crystals having the molecular axes stand near vertical to the surface. Moreover, GIXD revealed that the rubbing on polymethylene indeed induced a preferential azimuthal alignment of pentacene crystallites. The deposition of pentacene at 25 °C led to a twin growth of crystallites with the 110 direction predominately aligned perpendicular to the rubbing direction. In contrast, the pentacene deposition at 50 °C produced twinned crystallites of lower twin angle and the 120 direction aligned parallel to the rubbing direction. [ABSTRACT FROM AUTHOR]

Two mixtures of unequal chain length n-alkanes in which one component is deuterated have been investigated by infrared spectroscopy as they demixed. The measurements followed the band shapes of the scissors vibrations as a function of time. The band envelopes are analyzed as composites of a number of reference mixtures of known concentration. The unequal-chain mixtures separate into phases that slowly change their composition toward pure alkane phases. The method of analysis, which reveals local concentrations, should be generally applicable to polymethylene systems. [ABSTRACT FROM AUTHOR]

Interaction of -cyclodextrin (-CD) with a few novel electron donor acceptor dyads 1a−cand 2a−c,having aryl and flexible methylene spacer groups, has been investigated through photophysical, chiroptical, electrochemical, NMR, and microscopic techniques. Dyads 1aand 1c, with p-tolyl and biphenyl spacer groups, respectively, exhibited significantly decreased fluorescence quantum yields and lifetimes in the presence of -CD, while negligible changes were observed for dyad 1bwith an o-tolyl spacer. In contrast, spacer-length-dependent significant enhancement in fluorescence quantum yields and lifetimes was observed for dyads 2a−c, with flexible polymethylene (n1, 3, 11) spacer groups. Association constants of -CD encapsulated complexes have been determined and the contrast behavior observed in these systems is explained through an electron transfer (kET) mechanism based on calculated favorable change in free energy (GET−1.27 eV) and the redox species characterized through laser flash photolysis studies. Rates of kEThave been estimated and are found to increase ca. 2-fold in the case of dyads 1aand 1cwhen encapsulated in -CD, while significantly decreased kETvalues were observed for the dyads 2a−cwith flexible spacer (ca. 9-fold for 2c). As characterized through cyclic voltammetry, 2D NMR correlated (COSY) and nuclear Overhauser enhancement (NOESY) spectroscopy, and laser flash photolysis studies, the -CD encapsulation of dyads with aliphatic spacer groups leads to the conformational unfolding of a sandwich type of structure, whereas dyads with rigid aryl spacer groups undergo unusual planarization as compared to the uncomplexed dyads, resulting in enhanced electron-transfer reaction between the donor and acceptor moieties. [ABSTRACT FROM AUTHOR]

We have investigated the effects of film composition and thickness on the rate of pH-induced response of a copolymer film containing predominately polymethylene with randomly distributed carboxylic acid side groups (denoted as PM-CO2H). These responsive films are prepared directly onto a gold electrode surface by surface-catalyzed polymerization and subsequent hydrolysis. We measured electrochemical impedance at fixed frequency (100 Hz) to monitor the barrier properties of the polymer film during a step change in pH. At a 1−3% molar acid content, the copolymer films exhibit a 2 order of magnitude change in impedance at 100 Hz when the contacting solution pH changes from 11 to 4 (or 4 to 11). For all films, the rate of protonation is slower than that of ionization, consistent with a more gradual transfer of protons through an increasingly hydrophobic film at the outermost nanometers during the protonation step. Increased acid content within the film accelerates both the rate of protonation and ionization. Thinner films (50 nm) with the same acid content show faster response rate in both directions, since water and ions have a shorter transfer path. A large and reversible pH response was obtained for all films studied, but selection of appropriate film composition and thickness can greatly influence the rate of response. [ABSTRACT FROM AUTHOR]

The surface characteristics of thin films of poly(tetrafluoroethylene) (PTFE) melts were investigated by molecular dynamics simulations, employing the recent explicit atomic force field, including partial charges, which was fine-tuned to the experimental PVT data, the chain conformation and the crystal structure (Macromolecules2003, 36, 5331). Surface tension was calculated from the virial equation of the pressure tensor, taking into full account of the long-range correction terms, in good agreement with experimental data. Compared with polymethylenes of the same chain length, PTFEs have larger vacuum/liquid interface thicknesses concomitant with lower surface tensions. In the surface region, the chain conformations remain unperturbed, but the chain backbone segments tend to be oriented parallel to the surface whereas the chain-end segments tend to be perpendicular to the surface. As compared with polymethylenes, the orientation of PTFE segments in the surface is found to persist deeper into the film due to the larger intermolecular orientational correlation length in PTFE melts than in polymethylene melts. [ABSTRACT FROM AUTHOR]

Molecular dynamics is used for the simulation of silicon-containing polymers with promising membrane material properties. An atomistic force field is developed for the description of bond bending, torsional angle variation, and nonbonded intra- and intermolecular interactions. Detailed ab initio quantum mechanics calculations on corresponding monomers that appeared recently in the literature are used for the parametrization of the bonded and nonbonded local intramolecular force field. For the intermolecular and nonbonded nonlocal intramolecular interactions, parameters are obtained from accurate force fields proposed in the literature for similar compounds. The force field is used subsequently for the calculation of thermodynamic, structure, and dynamic properties of two homopolymers, namely, poly(dimethylsilamethylene) and poly(dimethylsilatrimethylene), and their alternating copolymer. A wide range of temperatures and pressures is examined. Polymer systems of different molecular weights are simulated. Experimental data available for these polymers are very limited. In all cases, simulation results are in good agreement with these data. Furthermore, simulation results agree very well with empirical macroscopic correlations used widely for rubbery polymers for the properties under consideration. [ABSTRACT FROM AUTHOR]

A short and efficient synthesis of (+)-spirolaxine methyl ether, a metabolite of the fungus Sporotrichum laxum with inhibitory activity against Helicobacter pylon, is described. The synthesis has been carried out by a Prins cyclization, to obtain the [6,5]-spiroketal system, and a Wadsworth-Emmons condensation, applied for the installation of the polymethylene chain on the phthalide moiety. [ABSTRACT FROM AUTHOR]

An efficient and straightforward two-step approach toward N(3),N′(3)-polymethylene-bis-hydantoins was developed. As a first step, a pyroglutamate is reacted with a diisocyanate to produce a bis-carbamoyllactam. The second step is a double-ring transformation by treatment of this bis-carbamoyllactam with KOtBu in ethanol. In this fashion N(3),N′(3)-polymethylene-bis-hydantoins are produced in two quantitative steps and under very mild conditions. When properly derivatized, these compounds can be converted to their macrocyclic derivatives upon treatment with 5 mol % of second-generation Grubbs' catalyst. These macrocyclic derivatives are so far not described in the literature. It was proven that exclusively (E)- isomers are formed. [ABSTRACT FROM AUTHOR]

Computational studies of three different reaction types involving hydrocarbons (homolytic C-C bond breaking of alkanes, progressive insertions of triplet methylene into C-H bonds of ethane, and [2+2] cyclizations of methyl-substituted alkenes to form polymethylcyclobutanes) show that the B3LYP model consistently underestimates the reaction energy, even when extremely large basis sets are employed. The error is systematic and cumulative, such that the reaction energies of reactions involving hydrocarbons with more than 4–6 C-C bonds are predicted quite poorly. Energies are underestimated for slightly and highly methyl-substituted cyclic and acyclic hydrocarbons, so the errors do not arise from structural issues such as steric repulsion or ring strain energy. We trace the error associated with the B3LYP approach to its consistent underestimation of the C-C bond energy. Other DFT models show this problem to lesser extents, while the MP2 method avoids it. As a consequence, we discourage the use of the B3LYP model for reaction energy calculations for organic compounds containing more than four carbon atoms. We advocate use of a collection of pure and hybrid DFT models (and ab initio models where possible) to provide computational ‘error bars’. [ABSTRACT FROM AUTHOR]

A flipping motion rapidly inverts the bent structure of uranyl-salophen compounds and, consequently, causes fast enantiomerization of nonsymmetrically substituted derivatives. This process has been previously slowed by introducing bulky substituents in the imine bond region. Since the resulting complexes dissociate upon chromatographic treatment, an alternative approach to the design and synthesis of robust, nonflipping uranyl-salophen compounds is here described. Such an approach is based on the idea that the flipping motion would be blocked by connecting the para positions with respect to the phenoxide oxygens by means of polymethylene bridges of suitable length. Analysis of a number of uranyl-salophen compounds by molecular mechanics, while showing that bulky substituents in the imine bond region cause severe distortions of the ligand backbone, suggested that the best chain lengths are those that fit the gap between the phenoxide rings without altering the natural geometry of the parent uranyl-salophen compound. Calculations showed that such chains are those composed of 12 and 13 methylene units. Accordingly, chiral uranyl-salophen macrocycles bridged with 12- and 13-methylene chains were synthesized in fairly good yields and resolved by chiral HPLC. [ABSTRACT FROM AUTHOR]

We report the preparation of a new class of copolymer films that consist of polymethylene (PM) with a low percentage (&tild;1 -4%) of randomly distributed ethyl ester side groups, consistent with poly- (methylene-co-ethyl acetate). The films are prepared through a surface-catalyzed polymerization on gold surfaces upon exposure to a dilute solution of diazomethane (DM) and ethyl diazoacetate (EDA) in ether at 0 °C. While EDA alone does not polymerize at gold surfaces but DM does decompose on gold to grow PM films, the combined presence of EDA and DM results in dramatic enhancements in film growth and promotes an alternative mechanism for propagation as compared with that for the PM homopolymerization. The rate of copolymer film growth is constant over a &tild;24 h period, consistent with a controlled polymerization in which chain terminations are minimized. Carefully controlled experiments indicate that chain propagation does not occur at the outer film-solvent interface, but more likely, at the film-metal interface, suggesting a catalyzed insertion mechanism that extends the chain and pushes the outer chain termini further away from the metal-polymer interface. The results also suggest that adsorbed intermediates of EDA function as co-catalysts to promote the propagation reaction. Of particular importance for materials modification is that the ester side chains of these copolymer films can be hydrolyzed to carboxylate groups that exhibit pH-dependent wettability. [ABSTRACT FROM AUTHOR]

A series of 3,3′-polymethylene-bridged bi[1,8]naphthyridine (binap) ligands, 3a-c, are complexed with Ru(II) to afford [Ru(tpy)(3a-c)(H2O)]2+ where an uncomplexed nitrogen on 3a-c is situated so it can form a H-bond with the coordinated water. An additional complex involving [Ru(4′-NMe2tpy)(3b)(H2O)]2+ is also prepared. X-ray analyses of the [Ru(tpy)(3a,c)(H2O)]2+ complexes indicate well-organized H-bonds even when the binap is nonplanar. In an attempt to realize photooxidation, the effects of light, varying potential, and pH were examined. A Pourbaix diagram indicated that the oxidation potential decreased by ∼0.5 V in the pH range of 1.9–11.6. The lowest-energy electronic absorption for the binap complexes involves the metal-to-ligand charge transfer to the binap ligand and is sensitive to ligand planarity. The absorbance shifted to a lower energy as the auxiliary ligand became a better donor (4′-NMe2tpy) or as the water was deprotonated. Acetonitrile was found to displace water most easily for the complex of 3c, where the ligand is the least planar. Despite promising features, photooxidation of the bound water was not observed. [ABSTRACT FROM AUTHOR]