Alkyl Chain Layers Research Articles

Investigating hydrophobic environment in alkyl-group-functionalized silica particle with various chain lengths using absorption microspectroscopy.

A well-known solvatochromic dye, Reichardt's dye (R-dye), was used to evaluate the hydrophobicity of alkyl-group-functionalized silica particles (ASPs) with different chain lengths. The absorption spectra of R-dye were measured in a single ASP in a mixed solution of water and an organic solvent (methanol (MeOH), ethanol (EtOH), acetonitrile (ACN), tetrahydrofuran (THF), or N,N-dimethylformamide (DMF)) using absorption microspectroscopy. The polarity parameter in the ASPs (ET), determined by the absorption maximum, was observed to be smaller than those in bulk solutions, indicating that R-dye was present in a more hydrophobic environment. In EtOH, THF, and DMF, R-dye was distributed within the alkyl chain layer including the organic solvent. An increase in the organic solvent content of the bulk solution led to a higher organic solvent concentration in the alkyl chain layer, resulting in a decrease in ET. In MeOH and ACN, the R-dye was distributed within the alkyl chain layer and concentrated phase. Moreover, with the increase in the organic molecule content, the distribution of R-dye in the concentrated phase became dominant in MeOH and ACN system, leading to an increase in the ET value. The findings presented in this paper are expected to attract the attention of a wide range of researchers in chromatography.

Effect of the methacrylic group on the degradation of polar achiral composites after successive poling cycles and appearance of antiferroelectric behaviour

We are studying a polar system composed of a polymer and its monomer in a 2:1 ratio. Although both components are non-chiral, their mixture shows a high spontaneous polarization due to hydrogen bond structuration. The original system shows a decrease in the spontaneous polarization through successive poling cycles. We replace the methacrylic moiety on two monomers with different aliphatic tail length (octyloxy/dodecyloxy) for a non-polymerizable isobutyric group. We study their mesomorphism as pure molecules and forming part of the composites by differential scanning calorimetry, polarizing optical microscopy and powder X-ray diffraction. We compare how the successive poling cycles affects the polar response as pyroelectric signal (γ) and loop hysteresis of these new polymer-isobutyric monomer composites vs their conventional counterpart polymer-methacrylic monomer composites. We found a decrease in the pyroelectric response with successive poling cycling in all composites. Regarding the structure of the composites obtained with the new saturated monomers, they exhibited polar behaviour. Surprisingly, for the isobutyric monomer with a dodecyloxy tail, the composite shown antiferroelectric behaviour unlike the ferroelectric methacrylic counterpart. The new phenomena must be due to steric hindrance caused by the bulky isobutyric group, forcing the monomers to lie between the sidechain of the polymer with the bulkier isobutyric group at the alkyl chain layer interface rather than near the polymeric main chain.

Mechanistic Analysis of Solid-State Colorimetric Switching: Monoalkoxynaphthalene-Naphthalimide Donor-Acceptor Dyads.

There is growing interest in creating solids that are responsive to various stimuli. Herein we report the first molecular-level mechanistic picture of the thermochromic polymorphic transition in a series of MAN-NI dyad crystals that turn from orange to yellow upon heating with minimal changes to the microscopic morphology following the transition. Detailed structural analyses revealed that the dyads assemble to create an alternating bilayer type structure, with horizontal alternating alkyl and stacked aromatic layers in both the orange and yellow forms. The observed dynamic behavior in the solid state moves as a yellow wavefront through the orange crystal. The overall process is critically dependent on a complex interplay between the layered structure of the starting crystal, the thermodynamics of the two differently colored forms, and similar densities of the two polymorphs. Upon heating, the orange form alkyl chain layers become disordered, allowing for some lateral diffusion of dyads within their own layer. Moving to either adjacent stack in the same layer allows a dyad to exchange a head-to-head stacking geometry (orange) for a head-to-tail stacking geometry (yellow). This transition is unique in that it involves a nucleation and growth mechanism that converts to a faster cooperative wavefront mechanism during the transition. The fastest moving of the wavefronts have an approximately 38° angle with respect to the long axis of the crystal, corresponding to a nonconventional C-H···O hydrogen bond network of dyad molecules in adjacent stacks that enables a transition with cooperative character to proceed within layers of orange crystals. The orange-to-yellow transition is triggered at a temperature that is very close to the temperature at which the orange and yellow forms exchange as the more stable, while being lower than the melting temperature of the original orange, or final yellow, solids.

Trap-state suppression and band-like transport in bilayer-type organic semiconductor ultrathin single crystals

Fundamental study of the carrier transport in both the channel layer and the electrode-channel contact of organic semiconductor crystals is indispensable for achieving high-performance organic field-effect transistors. In this paper, we report the temperature-dependent carrier transport of high-mobility organic semiconductors called $2\text{--}\text{decyl}\text{--}7\text{-phenyl-}[1]\text{benzothieno}[3,2\text{--}b][1]\mathrm{benzothiophene}$ (Ph-BTBT-C10) and $3\text{-decyl-}9\text{-phenyl-}[1]\text{benzothieno}[3,2\text{--}b]\text{naphtho}[2,3\text{--}b]\text{thiophene}$ (Ph-BTNT-C10). The use of single-crystal films with controlled bilayer-number thickness enabled the simultaneous study of intralayer and interlayer transport at cryogenic temperatures. Four-probe measurement of two-bilayer- and three-bilayer-thick films of Ph-BTBT-C10 suggests that the access resistance is dominated by tunneling transport across the insulating alkyl-chain layers. Single-crystal thin-film transistors of these materials showed band-like carrier transport down to 80 K and the carrier mobility of Ph-BTBT-C10 reached $34\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$. Detailed analysis of the low-temperature characteristics revealed small activation energy of approximately 5 meV and a sharp distribution of band tail states. These findings suggest that high crystallinity owing to the bilayer-type crystal structure effectively suppresses the localization of gate-induced carriers.

Effects of tunneling-based access resistance in layered single-crystalline organic transistors

Abstract

Adsorption at the Water/Hydrophobe Interface versus Partition into the Interior of the Hydrophobe: Quantitative Evaluation of the Solute Retention Selectivity at the Water/Hydrocarbon Interface

Molecular level understanding of the chemistry at the water/hydrophobe interface is crucial to chemical separation processes in aqueous media, such as reversed-phase liquid chromatography and solid-phase extraction. However, direct observation of the behavior of molecules and ions at the interface in reversed-phase separation systems still remains a major challenge, and probing techniques that provide the spatial information of the distribution of molecules and ions are required. In this paper, the molecular distribution between the aqueous solution and alkyl-bonded silica particles is studied by surface-bubble-modulated liquid chromatography (SBMLC). We determine the distribution coefficients of various organic compounds referring to their accumulations onto the water/alkyl chain interface and into the alkyl chain layer from the bulk water by SBMLC. The bulk water-to-alkyl chain layer distribution coefficient is corrected for the contribution of the end-capped silica surface to the solute retention using...

Tunneling and Origin of Large Access Resistance in Layered-Crystal Organic Transistors

Very high layered crystallinity is achieved in some alkyl-substituted organic semiconductors, allowing high-performance organic thin-film transistors (OTFTs). The authors observe that nonlinear device characteristics depend on the number of layers, for OTFTs based on large-area single-crystalline films of Ph-BTBT-Cn. This nonlinearity is caused by tunneling-based access resistance across layers of alkyl chains, which is the origin of the wide device-to-device variation in mobility seen in single-crystalline OTFTs. With this understanding, since these insulating chains can be tuned by length, they are a point for device optimization.

Mimicking the behaviour of rigid rod molecules. Smectic H liquid crystals from amphiphilic quaternary ammonium salts

ABSTRACTThe effect of secondary hydrogen-bonding interactions on the crystalline and liquid crystalline phases of quaternary ammonium salts functionalised with a carboxylic group attached at the polar head through a decyl spacer of a homologous series of N-alkyl-N-carboxydecyl-N,N-dimethylammonium bromides was investigated by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction. The low-temperature crystal phases were found to have a lamellar structure in which the ammonium bromide groups are arranged within the layers in two distinct planes, alternately separated by single layers of alkyl chains and double layers of carboxydecyl chains coupled through the carboxyl end groups. At higher temperatures, although these molecules were made from soft flexible chains, smectic H mesophases were identified. The smectic layers were found to be formed by the same two ionic planes alternately separated by the alkyl and carboxydecyl sub-layers. The smectic structure was compared with the three-dimensional positional order observed in the smectic T phase of dihydroxyl functionalised quaternary ammonium salts already described in the literature.

Molecular Requirements for Printable Organic Semiconductors in 7-Alky1-2-phenyl[1]benzothieno[3,2-b][1]benzothiophenes (Ph-BTBT-Cn’s)

Here we discuss requirements for high performance and solution processable organic semiconductors, by presenting a systematic investigation of 7-alkyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophenes (Ph-BTBT-C n ’s). We found that the solubility and thermal properties of Ph-BTBT-C n ’s depend systematically on the substituted alkyl-chain length n. The observed features are well understood in terms of the change of molecular packing motif with n: The compounds with n ≤ 4 do not form independent alkyl chain layers, whereas those with n ≥ 5 form isolated alkyl chain layers. The latter compounds afford a series of isomorphous bilayer-type crystal structures that form two-dimensional carrier transport layers within the crystals. We also show that the Ph-BTBT-C10 afford high performance single-crystalline field-effect transistors the mobility of which reaches as high as 15.9 cm2/Vs. These results demonstrate a crucial role of the substituted alkyl chain length for obtaining high performance organic semiconductors and field-effect transistors.

Ionic liquid crystals based on viologen dimers: tuning the mesomorphism by varying the conformational freedom of the ionic layer

ABSTRACTWe investigated the liquid crystal behaviour of newly synthesised bistriflimide salts of symmetric viologen dimers. A smectic A phase was observed for intermediate spacer lengths and for relatively long lateral alkyl chains. The systems were characterised by thermal analysis, polarised optical microscopy, X-ray scattering and solid-state NMR. An intermediate ordered smectic phase was also exhibited by the compounds (except for systems with very short lateral chains) consisting of molten layers of alkyl chains and partially ordered ionic layers. These results, relating to the mesomorphic behaviour of viologen salts, are qualitatively compared to those of the more common imidazolium salts, highlighting the importance of the conformational degrees of freedom of the anions and of the cationic core. It appears that fine tuning of the conformational degrees of freedom of the ionic layer is an important component of mesophase stabilisation.

Self-Assembled Monolayers Generated from Unsymmetrical Partially Fluorinated Spiroalkanedithiols.

Self-assembled monolayers (SAMs) were prepared on gold substrates from an unsymmetrical partially fluorinated spiroalkanedithiol adsorbate with the specific structure of [CH3(CH2)7][CF3(CF2)7(CH2)8]C[CH2SH]2 (SADT) and compared to SAMs formed from the semifluorinated monothiol F8H10SH [CF3(CF2)7(CH2)10SH] of analogous chain length and n-octadecanethiol. The adsorbate with two alkyl chains, one terminally fluorinated and the other nonfluorinated, was designed to form monolayers in which the bulky helical fluorocarbon segments assemble on top of an underlying layer of well-packed trans-extended alkyl chains. Different combinations of deposition solvents and temperatures were used to produce the bidentate SAMs. Characterization of the resulting monolayers revealed that SAMs formed in DMF at room temperature allow complete binding of the sulfur headgroups to the surface and exhibit higher conformational order than those produced using alternative solvent/temperature combinations. The reduced film thicknesses and enhanced wettability of the SADT SAMs, as compared to the SAMs generated from F8H10SH, suggest loose packing and an increase in the tilt of the terminal fluorocarbon chain segments. Nevertheless, the density of the underlying hydrocarbon chains of the SADT SAMs was higher than that of the F8H10SH SAMs, owing to the double-chained structure of the new adsorbate. The conformational orders of the SAM systems were observed to decrease as follows: C18SH > F8H10SH > SADT. However, the SAMs formed from this new double-chained bidentate adsorbate in DMF expose a fluorinated interface with a relatively low surface roughness, as determined by contact-angle hysteresis.

Elucidation of Softening Mechanism in Rinse Cycle Fabric Softeners. Part 1: Effect of Hydrogen Bonding

Most softening agents, such as rinse cycle fabric softeners, used by consumers at home contain cationic surfactants that have two long alkyl chains as their main component. The softening mechanism on fibers, especially cotton, has not yet been scientifically established, despite the market prevalence of fabric softeners for decades. One explanation for the softening effect is that the friction between fibers is reduced. According to this explanation, the fiber surfaces are coated by layers of alkyl chains. Because of the low coefficient of friction between alkyl chain layers of low surface energy, the fibers easily slide against one another yielding softer cotton clothing. However, no direct scientific evidence exists to prove the validity of this explanation. The softening mechanism of cotton yarn is discussed in this paper. Bending force values of cotton yarn treated with several concentrations of softener are measured by bend testing, and cotton and polyester yarns are compared. Results indicate that increases in cotton yarn hardness after natural drying are caused by cross-linking among inner fibers aided by bound water. This type of bound water has been known to exist even after 2 days of drying at 25 °C and 60 % relative humidity. Yarn dried in vacuo is soft, similar to that treated with softener. Thus, some of the softening effect caused by fabric softeners on cotton can be attributed to the prevention of cross-linking by bound water between cotton fibers.

Ionic liquids containing cationic SNS-pincer palladium(II) complexes: Effects of ancillary ligands on thermal properties and solvent polarities

Ionic liquids comprising cationic PdII complexes with SNS-pincer ligands ([Pd(LSNS)Me]+ ([1]+) and [Pd(LSNS)Cl]+ ([2]+); LSNS = 2,6-bis(n-butylthiomethyl)pyridine) and bis(trifluoromethanesulfonyl)amide (Tf2N) or tetrafluoroborate (BF4) anions were prepared, and the effect of ancillary ligands on their thermal properties and solvent polarities were investigated. [1]Tf2N and [2]Tf2N were liquids and exhibited glass transitions at −47 °C and −23 °C, and their solvent polarity parameters (ENT) were 0.51 and 0.46, respectively. [1]BF4 and [2]BF4 were solids with melting points of 73.2 °C and 141.5 °C, respectively. X-ray structure determination of [2]BF4 revealed a structure with alternating ionic and non-polar alkyl chain layers.

Template‐Guided Ionic Self‐Assembled Molecular Materials and Thin Films with Nanoscopic Order

AbstractIonic self‐assembly (ISA) is a proven method that exploits non‐covalent interactions to generate supramolecular materials. Here, we have expanded the scope of this approach fabricating thin films with nanoscopic order maintained over centimeters. Cationic layers of benzalkonium surfactants form a lamellar template. The template is able to host layers of negatively charged polyaromatic functional units, here demonstrated with β‐naphthol‐derived azo‐dyes. We show that a variety of these functional building blocks can be incorporated in the lamellar template through ISA. Sixteen different materials were produced, characterized, and processed into thin films, with lamellar order perpendicular to the substrate. Thus, a design concept is demonstrated in which diverse functional motifs can be isolated and ordered in a 2D lattice between layers of alkyl chains in bulk and in thin films, in which the molecular order is maintained and aligned to the substrate.

Characterization of ion-sensitive extended-gate field effect transistor coated with functional self-assembled monolayer

Extended-gate field effect transistors (FETs) were characterized by modifying self-assembled monolayers (SAMs) of alkanethiols with functional groups (–CH3, –COOH, –NH2, and –OH) on the gate surface. The SAMs with a polar group such as –COOH or –NH2 on the Au gate surface of the FET showed potential responses in the pH range of 5 to 10. In particular, the carboxylated SAM-coated gate FET showed higher sensitivities of 42 to 56 mV/pH, which are close to those of a Nernstian response. Moreover, the pH dependence of the carboxylated SAM-coated gate FET was maintained even in a solution with a higher salt concentration of 500 mM, which was used as the measurement solution. The effect of the ion strength in the solution on the pH response using the SAM-coated gate FET should be considered for the change in the concentration of hydrogen ions within the Debye length, which can be explained using the site-binding model for the functional groups at the terminus of the alkyl chains. In this case, the Debye length is assumed to be the distance from the packed alkyl chain layer of SAMs, which are hydrophobic. From these results, the construction of the SAMs with various functional groups on extended-gate FETs enables the FETs to be applied in ion-sensitive FET biosensors depending on the chemical characteristics of biological targets.

Interlayer Resistance and Edge-Specific Charging in Layered Molecular Crystals Revealed by Kelvin-Probe Force Microscopy

Organic field-effect transistors (OFETs) having an active channel of solution-processed 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8–BTBT) were investigated by Kelvin-probe force microscopy (KFM). We found step-like potential distributions in a channel region, suggesting that the interlayer resistance between the conjugated BTBT core layers is quite high and each conjugated layer is electrically isolated from one another by insulating alkyl chain layers. We also found a noticeable positive charging in the channel region especially at the step edges after the device operation. The observed charging was explained by long-lived positive charges on the trap sites, and the trap density at the step edge was estimated to be on the order of 1011 cm–2. The KFM measurements suggest that the device performance of the staggered C8–BTBT OFETs could deteriorate due to the considerably high access resistance, which stems from the high interlayer resistance and/or by the site-specific charge trapping at the contact/semiconductor interface which originates from step edge structures.

Bifunctional alkyl chain barriers for efficient perovskite solar cells.

Perovskite solar cells as a hot research topic show the necessity of controlling the interface. In this work, an insulating alkyl chain layer is self-assembled at the perovskite/hole transport material interface, which successfully exhibits a dual function: blocking electron recombination and resisting moisture at the same time. Improved solar energy conversion efficiency and stability of the device are both achieved.

Supramolecular [60]fullerene liquid crystals formed by self-organized two-dimensional crystals.

Fullerene-based liquid crystalline materials have both the excellent optical and electrical properties of fullerene and the self-organization and external-field-responsive properties of liquid crystals (LCs). Herein, we demonstrate a new family of thermotropic [60]fullerene supramolecular LCs with hierarchical structures. The [60]fullerene dyads undergo self-organization driven by π-π interactions to form triple-layer two-dimensional (2D) fullerene crystals sandwiched between layers of alkyl chains. The lamellar packing of 2D crystals gives rise to the formation of supramolecular LCs. This design strategy should be applicable to other molecules and lead to an enlarged family of 2D crystals and supramolecular liquid crystals.

Supramolecular [60]Fullerene Liquid Crystals Formed By Self‐Organized Two‐Dimensional Crystals

AbstractFullerene‐based liquid crystalline materials have both the excellent optical and electrical properties of fullerene and the self‐organization and external‐field‐responsive properties of liquid crystals (LCs). Herein, we demonstrate a new family of thermotropic [60]fullerene supramolecular LCs with hierarchical structures. The [60]fullerene dyads undergo self‐organization driven by π–π interactions to form triple‐layer two‐dimensional (2D) fullerene crystals sandwiched between layers of alkyl chains. The lamellar packing of 2D crystals gives rise to the formation of supramolecular LCs. This design strategy should be applicable to other molecules and lead to an enlarged family of 2D crystals and supramolecular liquid crystals.

Self-Assembly and Near Perfect Macroscopic Alignment of Fluorescent Triangulenium Salt in Spin-Cast Thin Films on PTFE

Highly fluorescent, discotic trioxatriangulenium dyes were aligned by simple spin-casting on substrates with friction transferred PTFE layers. The fluorescent crystalline thin films show near perfect macroscopic alignment on centimeter large areas directly from spin-casting. Gracing Incidence X-ray Diffraction (GIXD) unambiguously allowed the determination of a long-range order unit cell as well as its orientation with respect to the PTFE fibers. Further analysis of the X-ray data, in conjunction with polarized absorption spectroscopy, suggest a lamellar packing model with alternating layers of alkyl chains and ionic dyes oriented parallel to the substrate. This structure results in a highly anisotropic electrostatic potential around the cationic chromophore, causing significant shifts in energy and orientation of the optical transitions. Thus, the optical properties of the material are, to a large extent, controlled by the position of the otherwise inert PF6(-) counterions. The bright fluorescence from the films is also polarized parallel to the PTFE alignment layer. Doping of the thin films with fluorescent energy acceptor traps shows that efficient exciton migration takes place in the thin films. The excellent exciton transfer capabilities, in conjunction with the perfect alignment, might be of interest in future applications in solar energy harvesting or as thin film sensors.