Dipolar Groups Research Articles

Dipolar C[triple-bond]N...C[triple-bond]N interactions in organic crystal structures: database analysis and calculation of interaction energies

The Cambridge Structural Database (CSD) has been used to study nonbonded interactions between dipolar cyano groups. The analysis shows that C[triple-bond]N...C[triple-bond]N interactions form in an analogous manner to those involving carbonyl groups, and with the same interaction motifs: a dominant antiparallel dimer (57.5%) together with smaller populations of perpendicular (19.4%) and sheared parallel (23.0%) motifs. Ab initio calculations using intermolecular perturbation theory (IMPT) show an attractive C[triple-bond]N...C[triple-bond]N interaction in the dominant antiparallel dimer, with E(t) = -20.0 kJ mol(-1) at d(C...N) = 3.30 A and with the motif having a shear angle close to 102 degrees . The antiparallel C[triple-bond]N...C[triple-bond]N interaction is therefore slightly weaker than the analogous C=O...C=O dimer (-23.5 kJ mol(-1)), but both interactions have attractive energies similar to that of a medium-strength hydrogen bond and, where sterically favoured, they are important in the stabilization of extended crystal structures.

Dielectric Properties of Carbon Black–Epoxy Resin Composites Studied with Impedance Spectroscopy*

Dielectric properties of carbon black–epoxy resin composites were studied in the frequency range between 100 Hz and 15 MHz and over a temperature range 23–98°C. The results show that the composites exhibit, below the critical concentration, a dielectric relaxation phenomenon due to dipolar groups of epoxy resin and to interfacial polarization. This relaxation phenomenon was modeled by the Cole–Cole model.

Dipolar Ordering in the Ripple Phases of Molecular-Scale Models of Lipid Membranes

Symmetric and asymmetric ripple phases have been observed to form in molecular dynamics simulations of a simple molecular-scale lipid model. The lipid model consists of an dipolar head group and an ellipsoidal tail. Within the limits of this model, an explanation for generalized membrane curvature is a simple mismatch in the size of the heads with the width of the molecular bodies. The persistence of a bilayer structure requires strong attractive forces between the head groups. One feature of this model is that an energetically favorable orientational ordering of the dipoles can be achieved by out-of-plane membrane corrugation. The corrugation of the surface stabilizes the long range orientational ordering for the dipoles in the head groups which then adopt a bulk anti-ferroelectric state. We observe a common feature of the corrugated dipolar membranes: the wave vectors for the surface ripples are always found to be perpendicular to the dipole director axis.

Synthesis and characterization of a polyfluorene containing carbazole and oxadiazole dipolar pendent groups and its application to electroluminescent devices

AbstractWe have synthesized a blue‐light‐emitting polyfluorene (PF) derivative (PF‐CBZ‐OXD) that presents bulky hole‐transporting carbazole and electron‐transporting oxadiazole pendent groups functionalized at the C‐9 positions of alternating fluorene units. The results from photoluminescence and electrochemical measurements indicate that both the side chains and the PF main chain retain their own electronic characteristics in the copolymer. An electroluminescent device incorporating this polymer as the emitting layer was turned on at 4.5 V; it exhibited a stable blue emission with a maximum external quantum efficiency of 1.1%. Moreover, we doped PF‐CBZ‐OXD and its analogue PF‐TPA‐OXD with a red‐light‐emitting iridium phosphor for use as components of phosphorescent red‐light emitters to investigate the effect of the host's HOMO energy level on the degree of charge trapping and on the electrophosphorescent efficiency. We found that spectral overlap and individual energy level matching between the host and guest were both crucial features affecting the performance of the electroluminescence devices. Atomic force microscopy measurements indicated that the dipolar nature of PF‐CBZ‐OXD, in contrast to the general nonpolarity of polydialkylfluorenes, provided a stabilizing environment that allowed homogeneous dispersion of the polar iridium triplet dopant. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2925–2937, 2007

Polyfluorene presenting dipolar pendent groups and its application to electroluminescent devices

AbstractWe have synthesized a blue‐light‐emitting polyfluorene derivative (PF‐TPAOXD) that presents sterically hindered, dipolar pendent groups functionalized at the C‐9 positions of alternating fluorene units. The incorporation of the dipolar side chains, each comprising an electron‐rich triphenylamine group and an electron‐deficient oxadiazole group connected through a π‐conjugated bridge, endows the resultant polymer with higher highest occupied molecular orbital and lower lowest unoccupied molecular orbital energy levels, which, consequently, lead to an increase in both hole and electron affinities. An electroluminescent device incorporating this polymer as the emitting layer exhibited a stable blue emission with a maximum brightness of 2080 cd/m2 at 12 V and a maximum external quantum efficiency of 1.4% at a brightness of 137 cd/m2. Furthermore, atomic force microscopy measurements indicated that the dipolar nature of PF‐TPAOXD, in contrast to the general nonpolarity of polydialkylfluorenes, provided a stabilizing environment allowing the polar organometallic triplet dopant to be dispersed homogeneously. We also fabricated an electrophosphorescent device incorporating PF‐TPAOXD as the host material doped with a red‐emitting osmium complex to realize red electroluminescence with Commission Internationale de l'Eclairage color coordinates of (0.66, 0.34). The resulting device exhibited a maximum external quantum efficiency of 7.3% at a brightness of 1747 cd/m2 and a maximum brightness of 7244 cd/m2. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2073–2084, 2007

Polar Group Enhanced Gas-Phase Acidities of Carboxylic Acids: An Investigation of Intramolecular Electrostatic Interaction

We studied the effects of polar groups on the gas-phase acidities of carboxylic acids experimentally and computationally. In this connection, the gas-phase acidities (DeltaH(acid), the enthalpy of deprotonation, and DeltaG(acid), the deprotonation free energy) of borane-complexed methylaminoacetic acid ((CH(3))2N(BH(3))CH(2)CO(2)H) and methylthioacetic acid (CH(3)S(BH(3))CH(2)CO(2)H) were measured using the kinetic method in a flowing afterglow-triple quadrupole mass spectrometer. The values of DeltaH(acid) and DeltaG(acid) of (CH(3))2N(BH(3))CH(2)CO(2)H were determined to be 328.8 +/- 1.9 and 322.1 +/- 1.9 kcal/mol, and those of CH(3)S(BH(3))CH(2)CO(2)H were determined to be 325.8 +/- 1.9 and 319.2 +/- 1.9 kcal/mol, respectively. The theoretical enthalpies of deprotonation of (CH(3))2N(BH(3))CH(2)CO(2)H (329.2 kcal/mol) and CH(3)S(BH(3))CH(2)CO(2)H (325.5 kcal/mol) were calculated at the B3LYP/6-31+G(d) level of theory. The calculated enthalpies of deprotonation of N-oxide-acetic acid (CH(3)NOCH(2)CO(2)H, 329.4 kcal/mol) and S-oxide-acetic acid (CH(3)SOCH(2)CO(2)H, 328.6 kcal/mol) are comparable to the experimental results for borane-complexed methylamino- and methylthioacetic acids. The enthalpy of deprotonation of sulfone-acetic acid (CH(3)SO2CH(2)CO(2)H, 326.1 kcal/mol) is about 2 kcal/mol lower than the S-oxide-acetic acid. The calculated enthalpy of deprotonation of sulfoniumacetic acid, (CH(3))2S+CH(2)CO(2)H, is 243.0 kcal/mol. Compared to the corresponding reference molecules, CH(3)NHCH(2)CO(2)H and CH(3)SCH(2)CO(2)H, the dipolar group and the monopolar group substituted carboxylic acids are stronger acids by 11-14 and 97 kcal/mol, respectively. We correlated the changes of the acidity upon a polar group substitution to the electrostatic free energy within the carboxylate anion. The acidity enhancements in polar group substituted carboxylic acids are the results of the favorable electrostatic interactions between the polar group and the developing charge at the carboxyl group.

Relationship between Enhanced Positronium Formation and Enhanced Positron Trapping in Polymers at Low Temperature

Positron annihilation lifetime spectroscopy is used for investigation of low-density polyethylene and ethylene-methyl methacrylate copolymers of 1.45, 3.0, and 5.4 mole% of methyl methacrylate. The lifetime spectra are collected at 30 K, one by one, as a function of elapsed time. In the computer analysis a new theoretical model is developed, which enables separating the annihilation from positron free state, its trapped state and bound state in positronium. The positron trapping rate μ and the enhanced positronium formation rate κ are determined. The calculated values of μ and κ turned out to be linearly correlated. This correlation presumably originates from an influence of trapped electrons on the trapping of positrons. The dependences of κ on measurement time are determined for low-density polyethylene and ethylene-methyl methacrylate of different methyl methacrylate content. A theoretical model describing quantitatively the dependences is proposed. The model considers the processes of electron–ion recombination, electron trapping, and electron scavenging by dipolar carbonyl groups supplied by methyl methacrylate additives.

Structural characterization, thermal and electric properties of imidazolium bromoantimonate(III): [formula omitted

Tris(imidazolium) nonabromodiantimonate(III), [ C 3 H 5 N 2 ] 3 [ Sb 2 Br 9 ] , is trimorphic. Its crystal structure has been determined at 100 K (form I) and 293 K (form II) by X-ray single crystal diffraction in, respectively, the monoclinic space groups, P 2 1 / c (form I) and P 2 1 / n (form II). In both structures, the anionic sublattice forms corrugated two-dimensional layers in the bc plane. In forms I and II there are, respectively, three and two crystallographically independent imidazolium cations. Two types of cations are present in the structures: the one that occupies cavities within the polyanions layer appears to be ordered in the lowest temperature form I and disordered in form II. The second type of imidazolium cations placed between the layers is ordered over the studied temperature region. The temperature dependence of the lattice parameters has been determined between 100 and 280 K. DSC studies indicate a presence of two reversible phase transitions: continuous at 237 K ( I → II ) and discontinuous at 373/351 K (heating–cooling) from form II to form III. The phase transition II → III is accompanied by a huge entropy transition ( Δ S II → III ) equal to ca. 28 Jmol - 1 K - 1 , which suggests an order (form II)–disorder (form III) transition mechanism. The dielectric relaxation process was found to appear in a low frequency region over the form I with an activation energy ca. 16.5 kJ/mol. A polydispersive character of the dielectric dispersion indicates a presence of complex molecular motions of dipolar groups in the title compound. The explanation of mechanism of the I → II phase transition in [ C 3 H 5 N 2 ] 3 [ Sb 2 Br 9 ] is proposed.

Recent developments of polar piezoelectric polymers

Following the extensive studies on the piezoelectric, pyroelectric and ferroelectric properties of polyvinylidene fluoride and its copolymers, a vast number of polymers were investigated for these properties. Piezoelectric thin films of polyurea were prepared by vapor deposition polymerization in vacuum and stable up to 200 degC. Piezoelectric odd nylon was characterized by the field induced rotation of amide dipoles in the molecular sheet crystalline structure. Completely amorphous vinylidene cyanide copolymers produced a remanent polarization proportional to the large dielectric relaxation strength during poling. Drawn films of polylactic acid exhibited the large shear piezoelectricity due to the crystalline structure of helical molecules, however, a remnant polarization was also produced by poling at high temperatures. Various polymers containing dipolar groups of carbonyl, urethane, urea and thiourea bonds were poled and their ferroelectric hysteresis was observed. Application of magnetic field as high as 10 Tesla to the liquid crystalline state of helical molecules of poly-y-benzyl-L-glutamate during film formation resulted in a very high shear piezoelectric constant 26 pC/N. Unprecedented high piezoelectric constant such as 1000 pC/N was found in the highly hydrated cornea membrane consisting of collagen fibrils

Phase transitions and electric properties of imidazolium chlorobismuthate(III): [C 3H 5N 2] 6[Bi 4Cl 18

[C 3H 5N 2] 6[Bi 4Cl 18] crystallizes at room temperature in the monoclinic space group; C2/m. It undergoes two solid–solid phase transitions of first-order type at 221/218 K (heating–cooling) from phase I to II and at 195/145 K from phase II to III. The dielectric relaxation process was found to appear in a kilohertz frequency region over the phase III with the activation energy c.a. 14 kJ/mol. A polydispersive character of the dielectric dispersion indicates a presence of complex molecular motions of dipolar groups in the crystal. The mechanism of the phase transitions in [C 3H 5N 2] 6[Bi 4Cl 18] is proposed.

Crystal structure and phase transitions in dipropylammonium hexachloroantimonate(V): [N(C 3H 7) 2H 2][SbCl 6

The crystal structure of dipropylammonium hexachloroantimonate(V), [N(C 3H 7) 2H 2][SbCl 6] (DPACA) has been determined by means of X-ray diffraction at 290 K. The compound crystallizes in the monoclinic space group: P2 1 / n. The crystal undergoes three structural phase transitions: two first-order type at 388/376 K (heating-cooling) from phase (I) to (II) and at 157/144 K from phase (III) to (IV), and one second-order type at 210 K from phase (II) to (III). The dielectric relaxation process has been found to appear in a kilohertz frequency region over the phases (II) and (III) with the activation energy ca. 28 kJ mol −1. A polydispersive character of the dielectric dispersion indicates a presence of complex molecular motions of dipolar groups in the crystal. The mechanism of the phase transitions in DPACA is proposed.

HRTEM microstructures of PAN precursor fibers

Ultrathin sections prepared from both transverse and longitudinal PAN precursor fibers were examined by high-resolution transmission electron microscopy. Ordered and amorphous phases coexist in these sections, providing verification for the two-phase structure of PAN fibers, however, onion-like spheres and crystallites predominate in the transverse sections, and highly oriented structures are dominant in the longitudinal sections. The formation of the onion-like spheres can be ascribed to intramolecular and intermolecular dipolar nitrile group interactions. The larger area of oriented structures in the longitudinal sections is due to the coalescence of several ribbons with the approximative orientations. Some defects, such as pores and voids, have also been observed. These examinations provide not only some direct structural evidence but also some information for optimizing spinning technology and for deliberately controlling the microstructures to obtain PAN precursor fibers applicable to fabricating high strength carbon fibers.

On the Rigidity of Polynorbornenes with Dipolar Pendant Groups

A range of polynorbornenes (PNBs) with fused dipolar pendant groups at C-5,6 positions was synthesized by ring-opening metathesis polymerization catalyzed by a ruthenium carbene complex (Grubbs I). Photophysical studies, EFISH measurements, and atomic force microscopy images have been used to investigate the structures and morphology of these polymers. These results suggest that the polymers may adopt rigid rod-like structures. The presence of the double bonds in PNBs appeared to be indispensable for the rigidity of the polymers. Interaction between unsaturated pendant groups may result in coherent alignment leading to a rod-like structure.

Dispersive charge transport due to strong charge–dipole interactions of cyano-group in the cyano-carbazole based molecular glass

Using the time of flight (ToF) method, we investigated the bipolar charge transport for two glass-forming molecules containing carbazole and cyano-carbazole moiety. The enhanced electron mobility was observed in the cyano-carbazole compound. From the numerical method based the Laplace formalism, the distribution of hole trapping energy was obtained for the carbazole compound. This result was compared with the exponential distribution extracted from dispersion parameter for the cyano-carbazole material. Considering charge–dipole interactions as a reason for the disordered trapping mechanism, we discussed dispersive charge transport induced by a strong dipolar (i.e. cyano) group by comparing the distributions of hole trapping sites for two compounds.

Dielectric relaxation behavior of poly(methyl methacrylate) under high‐pressure carbon dioxide

AbstractAn in situ dielectric measurement for atactic poly(methyl methacrylate) (at‐PMMA) was performed under high‐pressure CO2 under various pressures and temperatures. The at‐PMMA has the acetate side group with a large dipole moment. In the glassy state, a local relaxation process (β‐process) can be observed using dielectric measurement. In the rubbery state, the micro‐Brownian motion of main chain (α‐process) occurs, and the β‐process changes into αβ‐process coordinated with the α‐process. The dielectric loss (ε″) spectrum of at‐PMMA in the glassy state is asymmetric because of the density fluctuation for the amorphous structure. The loss peak frequency shifted to higher frequencies, and the relaxation strength increased with increasing CO2 pressure. In the glassy state, the shape of ε″ spectrum became more symmetric with increasing CO2 pressure. These show that the molecular mobility enhanced by the plasticization effect of CO2 allows the dipolar side groups in the high‐density region to contribute to the relaxation process. We also found that the apparent activation energy decreased under high‐pressure CO2. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2951–2962, 2005

Phase separation in randomly charged polystyrene sulphonate ionomer solutions

The dynamics of randomly charged polystyrene caesium–sulfonate ionomers in semi-dilute solutions were studied using a combination of dynamic light scattering (DLS), small angle neutron scattering (SANS), and bulk rheology. The samples were studied in toluene solutions where the aggregation of the dipolar groups is favoured. Evidence of aggregation in dilute solution is found using DLS and SANS with both the hydrodynamic and static radius of gyration indicating that there is a contraction of the chains due to intra-chain attractive forces. SANS experiments demonstrate the evolution of the aggregates into a network structure as a function of polymer concentration. The association process is caused by the dipolar attraction between the charged groups and introduces two static correlation lengths in the mesh structure of the network; the standard semi-dilute mesh size ( ξ=1.12 c −0.72±0.03) and an inhomogeneity length ( Ξ=24 c 0.58±0.05) due to micro-phase separation. The scaling of the amplitudes of the correlation lengths I 1(0)∼ c −0.33±0.07 and I 2(0)∼ c 2.0±0.4 are consistent with good solvent conditions and micro-phase separation, respectively. An imposed shear causes the break up of the micro-phase separated micellar system with a characteristic yield stress for the Bingham step-like shear thinning.

Dielectric studies of three epoxy resin systems during microwave cure

Dielectric properties of three curing epoxy resin systems at an industrial microwave frequency, 2.45 GHz, were measured over a temperature range lower than the cure temperature. Extent of cure, which is determined by DSC, is used to describe the progress of the polymerization. It has been found that, normally, the real and the imaginary part of the complex dielectric constant increased with temperature and decreased during microwave cure. The changes of the dielectric properties during the reaction are related to the decreasing number of the dipolar groups in the reactants and the increasing viscosity. The Davidson–Cole model can be used to describe the experimental data. The Zong model is applicable to polymeric materials at high microwave frequencies and can be used to calculate the parameters of the Davidson–Cole model. The epoxy resins exhibit one γ relaxation, which can be described by the Arrhenius rate law. The evolutions of the parameters in the models are discussed.

Dielectric properties of an epoxy-amine system at a high microwave frequency

The dielectric properties of a curing diglycidyl ether of bisphenol A (DGEBA)/Jeffamine D-230 system have been studied over the temperature range of 20–90°C at 2.45 GHz. It was found that, generally, both the dielectric constant and the dielectric loss factor of the system increased with temperature and decreased as the reaction proceeded. The epoxy resins at different extents of cure exhibited the γ relaxation, which can be described by the Arrhenius Rate Law. The relaxation is attributed to the motions of the dipolar groups associated with the reactants. The Davidson-Cole model can represent the temperature dependencies of the dielectric properties. The nature of the information yielded by dielectrometry on the dynamics of the system is discussed. The evolution of the parameters of the models during the polymerization was mainly affected by the decreasing number of the dipolar groups involved in the reaction and increasing medium viscosity. POLYM. ENG. SCI. 45:1576–1580, 2005. © 2005 Society of Plastics Engineers

Lamination Method for the Study of Interfaces in Polymeric Thin Film Transistors

A method for the fabrication of polymeric thin-film transistors (TFTs) by lamination is described. Poly(dimethylsiloxane) stamps were used to delaminate thin films of semiconducting polymers from silicon wafers coated with a self-assembled monolayer (SAM) formed from octyltrichlorosilane. These supported films were laminated onto electrode structures to form coplanar TFTs. The fabrication process was used to make TFTs with poly(3-hexylthiophene), P3HT, and poly[5,5'-bis(3-dodecyl-2-thienyl)-2,2'-bithiophene], PQT-12. TFTs, where these polymers were laminated onto gate dielectrics coated with SAMs from octyltrichlorosilane, had effective field-effect mobilities of 0.03 and 0.005 cm2/(V s), respectively. TFTs where PQT-12 was laminated onto gate dielectrics that were not coated with a SAM also had mobility of 0.03 cm2/(V s). In contrast, TFTs fabricated by spin-coating PQT-12 onto the same structure had mobilities ranging from 10-3 to 10-4 cm2/(V s). These results suggest that the lower mobilities of polymer TFTs made with hydrophilic gate dielectrics are caused by molecular ordering in the semiconducting film rather than electronic effects of dipolar groups at the interface.

Enthalpies of formation of monoderivatives of hydrocarbons: Interaction of polar groups with an alkyl group.

Energies of hydrocarbon monoderivatives CH(3)X, C(2)H(5)X, n-C(4)H(9)X, and n-C(5)H(11)X with 16 different substituents X were calculated at the levels B3LYP/6-311+G(d,p) and B3LYP/AUG-cc-pVTZ//B3LYP/6-311+G(d,p). The results were used to test the validity of the additive rule that has served commonly for estimating the enthalpies of formation Delta(f)H(T). The exact additivity corresponds to zero reaction energy DeltaE of the isodesmic reaction, in which the substituent X is transferred from one alkyl group R to another. Additivity is approximately fulfilled for butyl and pentyl derivatives with the differences less than 0.3 kJ mol(-1) (except charged groups X). Methyl derivatives deviated from the additive rule up to 22 kJ mol(-1) for dipolar groups X and 45 kJ mol(-1) for charged group, in agreement with the available experiments and with the anticipation of all suggested empirical schemes. In addition, smaller deviations of ethyl derivatives (3 or 20 kJ mol(-1), respectively) were observed here for the first time. There is no correlation between the deviations of methyl and ethyl derivatives; they are also not related to steric effects, and only partly to polarization. Deviations of methyl derivatives are proportional to the electronegativity of the first atom of the substituent; even when the definition of electronegativity is somewhat questionable, one can say in any case that it is controlled by the first atom.