Interdigitation Of Alkyl Chains Research Articles

Aqueous solutions of hydroxyl-functionalized ionic liquids: Molecular dynamics studies

A series of aqueous solutions of 1-(n-hydroxyalkyl)-3-(n-hydroxyalkyl) imidazolium bromide ([HOCnCmOHIm][Br], with n and m = 2, 6,10 and 14) were studied by atomistic molecular dynamics simulations. Structural properties were characterized by the radial distribution functions between different pairs, angular distributions and aggregation numbers. Dynamics of the system has been investigated by computing the diffusion of the ions and molecules. Structures of the aggregates formed depend upon the length of the hydroxyalkyl chains. The long-distance spatial correlations observed in solutions with cations having long chain substituent are arising due to the formation of intercalated structures. A thin film like structure is formed in solutions having longer hydroxyalkyl chains, with the structure stabilized by the dispersion interactions between the interdigitated alkyl chains and the hydrogen bond formation between the hydroxyl group of a cation with head group of a different cation. Anions are dispersed near the surface of the film.

Molecular orientation and anisotropic charge transport in the large area thin films of regioregular Poly(3-alkylthiophenes) fabricated by ribbon-shaped FTM

Abstract Introduction of alkyl chains is a plausible approach to impart the facile solution processability in regioregular (poly-3-alkylthiophenes) (RR-P3ATS) but drastic fall in the conductivity as a function of increasing alkyl chain length restricted the versatile use of its hexyl substituted derivative (P3HT). In this work, fabrication of large area oriented thin films of RR-P3ATs with varying alkyl chain lengths was successfully demonstrated using ribbon shaped floating film transfer method (FTM). Observed decrease in the molecular orientation with increasing alkyl chain length was explained by enhanced alkyl chain inter-digitation as evidenced by trends of d-spacing estimated from the out-of-plane XRD results. In-plane GIXD revealed edge-on orientation in the thin films of all of the P3ATs prepared by ribbon-shaped FTM, which is highly desired for planer charge carrier transport. Parallel oriented thin film of longest alkyl chain (octadecyl) substituted P3AT poly (3-octadecylthiophene) exhibited a field effect transistor (FET) mobility of 1.9 × 10−2 cm2/V, which is about 4–5 orders of magnitude higher as compared its previously reported values estimated by time-of-flight and FET methods. Thus, fabrication of thin films by FTM leads to versatile choice of RR-P3ATs for solution processing for FETs using common organic solvents without having any detrimental impact of alkyl chain length on the charge carrier mobility.

Structural Understanding, Photoswitchability, and Supergelation of a New Class of Four Ring-Based Bent-Shaped Liquid Crystal.

Herein, we report a new type of azobenzene-based unsymmetrical bent-core molecules exhibiting photoswitchability in the liquid crystalline state, solid state, and solution state and in mixture upon UV irradiation and intense visible light. The compounds exhibited solid-state photochromism upon exposure to UV light, whereas in liquid crystalline state, reversible phase transitions were observed via both UV irradiation and intense visible light exposure. Crystal structure analysis reveals the basic structural understanding such as nonplanar bent molecular shape, antiparallel arrangement of the polar bent molecules, intra- and intermolecular hydrogen bonding, and different π-π interactions and interdigitation of long alkyl chains. The compounds are also found to act as supergelator toward various organic solvents. Hence, this is an excellent example of such potential bent-shaped liquid crystals that promise an immense perspective for device applications such as optical storage, molecular switches, etc.

Hexagonal Molecular Tiling by Hexagonal Macrocycles at the Liquid/Solid Interface: Structural Effects on Packing Geometry.

We present here hexagonal tiling using hexagonal phenylene-ethynylene and phenylene-butadiynylene macrocycles attached by alkyl ester groups, PEM-C6 and PBM-C8, respectively, or triethylene glycol ester groups, PEM-TEG and PBM-TEG, respectively, at each vertex of the macrocyclic periphery at the liquid/solid interface. In this study, we focused on the effects of macrocyclic core size and the chemical properties of side chains attached to macrocyclic cores as well as solute concentrations on the hexagonal geometry of self-assembled monolayers. STM observations at the 1,2,4-trichrolobenzene/graphite interface revealed that PEM-C6 formed a honeycomb structure by van der Waals interactions between the interdigitated alkyl chains. However, upon increasing solute concentration, it changed to more dense hexagonal structure (tentatively called loose hexagonal structure I). In contrast, PBM-C8 formed loose hexagonal structure II of a slightly different packing mode at low concentration, while at high concentration it formed a high-density hexagonal structure in which alkyl chains are not adsorbed on the surface (dense hexagonal structure). In the dense hexagonal structure, macrocyclic cores are linked by hydrogen bonds between the ester carbonyl oxygen and the aromatic hydrogen atoms of the neighboring macrocycles. The packing geometries of loose hexagonal structures of PEM-C6 and PBM-C8 are different due to the different distance between the attachment of the alkyl ester groups which are located in confined space. On the other hand, PEM-TEG and PBM-TEG formed dense hexagonal structures, similar to PBM-C8 at high concentration, with their TEG units not adsorbed on the surface.

Self-assembled 2D patterns of structural isomers in isobutenyl compounds revealed by STM at solid/liquid interface

The 2D patterns of structural isomers in isobutenyl compounds possessing long alkyl tails were systematically investigated by using scanning tunneling microscopy at solid/liquid interface. The alkyl chains with carbon numbers of 18 and 19 were linked with either amide or ester bonds at different positions of naphthalene unit. In addition to the 2,3-substituted isobutenyl compounds of our previous studies, the 2,6- and heterogeneously substituted 2,3–2,6-isobutenyl compounds were investigated in the present study. In the 2,6-substituted amide compounds (2,6-NCn), the intermolecular hydrogen bonds lead to the columnar structure without the interdigitation of alkyl chains, whereas 2,6-substituted ester compounds (2,6-OCn) showed the 2D structure with interdigitated alkyl chains. In the 2,3–2,6-substituted compounds containing amide linked alkyl chains (2,3-2,6-NCn), the alkyl chain unit exhibited a herring bone-like arrangement possibly due to the defined structural framework based on a intramolecular hydrogen bond. In contrast, flexibility of 2,3-2,6-OCn allowed to form the 2D structure with parallel alignment of the naphthalene unit and interdigitation of alkyl chains. The rearranged structural isomers of isobutenyl compounds could be obtained by the thermal reaction of tandem Claisen rearrangement (TCR). The different orientations of naphthalene units in the 2,6-NCnTCR were found depending on the alkyl chain length in order to avoide the steric repulsion between alkyl chains, namely odd-even effect of alkyl chains was emerged. In the case of 2,6-OCnTCR, odd-even effect was projected as presence or absence of alkyl chain interdigitation. The alkyl chain length had no influence on the 2D structures of 2,3-2,6-NCnTCR, whereas 2D structures of 2,3-2,6-OCnTCR showed slight difference in the orientation of naphthalene units depending on the alkyl chain length. These results suggest that a wide variety of 2D structures can be formed by structural isomers with different alkyl chain length, and that the TCR can be used for tuning the emergence of odd-even effect.

Solid-liquid equilibrium and heat capacity trend in the alkylimidazolium PF6 series

The heat capacity and thermal behavior trend along the 1-alkyl-3-methylimidazolium hexafluorophosphate [CnC1im][PF6] (with n=2–10, 12) ionic liquids series, is used to explore the effect of the alkyl chain length in the nanostructuration. The heat capacities of the studied ILs were measured with an uncertainty better than ±0.15% and are in excellent agreement with the available data in the literature. An odd-even effect for the specific and volumic heat capacities of the [CnC1im][PF6] series was found. The observed odd-even effect in the liquid heat capacity was rationalized considering the preferential orientation of the terminal CH3 group. The higher specific/volumic heat capacities shown for the [C6C1im][PF6] and [C8C1im][PF6] are an indication of an additional conformational disorder increase in the liquid phase that could be related with a weaker alkyl chain interdigitation capability of the even number chain ILs. The melting temperatures and consequent ∆slHmo and ∆slSmo trend along the alkyl series present a V-shape profile that is explained based on the analysis of the balance between the initial decrease of the electrostatic interaction potential and the increase of the van der Waals interactions with the increasing size of the alkyl side chain of the cation. The inhibition of crystallization for intermediate alkyl chain size (from [C5C1im][PF6] to [C8C1im][PF6]) seems to arise from the overlapping of the hypothetical cold crystallization temperature by the melting temperature. Above the critical alkyl size, CAS, a regular increase in the entropy and enthalpy profiles presents a similar shape than the observed in other alkane series and is a strong support of the intensification of the ILs nanostructuration.

The Design of Radical Stacks: Nitronyl-Nitroxide-Substituted Heteropentacenes.

The first alkyl chain‐anchored heteropentacene, dithieno[2,3‐d;2′,3′‐d′]benzo‐[1,2‐b;3,4‐b′]dithiophene (DTmBDT), mono‐ or disubstituted with a nitronyl nitroxide group has been prepared through a cross‐coupling synthetic procedure of the corresponding dibromo‐derivative (Br2‐DTmBDT) with a nitronyl nitroxide‐2‐ide gold(I) complex. The synthesized nitroxides possess high kinetic stability, which allowed us to investigate their structure and thermal, optical, electrochemical, and magnetic properties. Single‐crystal X‐ray diffraction of both mono‐ and diradicals revealed that the nitronyl nitroxide group lies almost in the same plane as the nearest side thiophene ring. Such arrangement favors formation of edge‐to‐edge dimers, which then form close π‐stacks surrounded by interdigitating alkyl chains. Before melting, these nitronyl nitroxide radical substituted molecules undergo at least two different phase transitions (PTs): for the monoradical, PTs are reversible, accompanied by hysteresis, and occur near 13 and 83 °C; the diradical upon heating shows a reversible PT with hysteresis in the temperature range 2–11 °C and an irreversible PT near 135 °C. PTs of this type are absent in Br2‐DTmBDT. Therefore, the step‐by‐step substitution of bromine atoms by nitronyl nitroxide groups changes the structural organization of DTmBDT and induces the emergence of PTs. This knowledge may facilitate crystal engineering of π‐stacked paramagnets and related molecular spin devices.

Interplay of Orientation and Blending: Synergistic Enhancement of Field Effect Mobility in Thiophene-Based Conjugated Polymers

Trade-off between mechanical flexibility due to amorphicity and highly facile charge transport emanating from molecular crystallinity demands the orientation of conjugated polymers (CPs) for their utilization as active semiconducting material for flexible organic electronics. We have already demonstrated that it is rather easy to orient nonregiocontrolled poly(3-hexylthiophenes) (NR-P3HT) as compared to their highly regioregular counterparts due to very high alkyl chain interdigitation. To provide an amicable solution, efforts have been directed to orient blends of two CPs such as NR-P3HT (amorphous and flexible) and poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) (crystalline and facile charge transport) using a solution-based procedure floating film and transfer method (FTM). FTM-processed thin films of this blend system exhibited very high field effect transistor (FET) mobility reaching up to 0.1 cm2/V s, which is much higher than the corresponding individual CPs. In spite of onl...

Host-Guest Chemistry in Integrated Porous Space Formed by Molecular Self-Assembly at Liquid-Solid Interfaces.

Host-guest chemistry in two-dimensional (2D) space, that is, physisorbed monolayers of a single atom or a single molecular thickness on surfaces, has become a subject of intense current interest because of perspectives for various applications in molecular-scale electronics, selective sensors, and tailored catalysis. Scanning tunneling microscopy has been used as a powerful tool for the visualization of molecules in real space on a conducting substrate surface. For more than a decade, we have been investigating the self-assembly of a series of triangle-shaped phenylene-ethynylene macrocycles called dehydrobenzo[12]annulenes (DBAs). These molecules are substituted with six alkyl chains and are capable of forming hexagonal porous 2D molecular networks via van der Waals interactions between interdigitated alkyl chains at the interface of organic solvents and graphite. The dimension of the nanoporous space or nanowell formed by the self-assembly of DBAs can be controlled from 1.6 to 4.7 nm by simply changing the alkyl chain length from C6 to C20. Single molecules as well as homoclusters and heteroclusters are capable of coadsorbing within the host matrix using shape- and size-complementarity principles. Moreover, on the basis of the versatility of the DBA molecules that allows chemical modification of the alkyl chain terminals, we were able to decorate the interior space of the nanoporous networks with functional groups such as azobenzenedicarboxylic acid for photoresponsive guest adsorption/desorption or fluoroalkanes and tetraethylene glycol groups for selective guest binding by electrostatic interactions and zinc-porphyrin units for complexation with a guest by charge-transfer interactions. In this Feature Article, we describe the general aspects of molecular self-assembly at liquid/solid interfaces, followed by the formation of programmed porous molecular networks using rationally designed molecular building blocks. We focus on our own work involving host-guest chemistry in integrated nanoporous space that is modified for specific purposes.

Surprising Effects upon Inserting Benzene Units into a Quaterthiophene‐Based D‐A Polymer–Improving Non‐Fullerene Organic Solar Cells via Donor Polymer Design

Benzene units are inserted into the backbone of a quaterthiophene-based polymer named PffBT4T, and the resulting polymer, PffBT4T-B, exhibits remarkably tight alkyl chain interdigitation, which can expel the ITIC-Th molecules from the polymer domains thus forming more pure and crystalline ITIC-Th domains. As a result, PffBT4T-B-based non-fullerene organic solar cells achieve a high power conversion efficiency of 9.4%.

Adaptive Building Blocks Consisting of Rigid Triangular Core and Flexible Alkoxy Chains for Self-Assembly at Liquid/Solid Interfaces

Abstract Supramolecular self-assembly in two-dimensional (2D) spaces on solid surfaces is the subject of intense current interest because of perspectives for various applications in nanoscience and nanotechnology. At the liquid/graphite interface, we found by means of scanning tunneling microscopy molecules with a rigid triangular core, a twelve-membered phenylene-ethynylene macrocycle called dehydrobenzo[12]annulene (DBA), substituted by six flexible alkoxy chains self-assembled to form hexagonal porous 2D molecular networks via van der Waals interactions between interdigitated alkyl chains as the directional intermolecular linkages. Factors that affect the formation of the porous 2D molecular networks including alkyl chain length, solvent, solute concentration, and temperature were elucidated through a systematic study. Because DBA molecules are versatile for chemical modification, they turned out to be highly adaptive for on-surface supramolecular chemistry with respect to (i) pore size control by changing the chain length, (ii) study of parity effect due to even or odd number chains, (iii) generation of supramolecular chirality on surfaces by introducing stereocenters, (iv) chemical modification of the pore interior for selective co-adsorption of guest molecules by introducing functional groups. Additionally, formation of superlattice structures on surfaces was incidentally observed by mixing DBAs of different alkoxy chain parity or by addition of guest molecules via an induced-fit mechanism. These results made significant contribution to advancement of supramolecular chemistry in 2D space.

Soluble Flavanthrone Derivatives: Synthesis, Characterization, and Application to Organic Light-Emitting Diodes.

Simple modification of benzo[h]benz[5,6]acridino[2,1,9,8-klmna]acridine-8,16-dione, an old and almost-forgotten vat dye, by reduction of its carbonyl groups and subsequent O-alkylation, yields solution-processable, electroactive, conjugated compounds of the periazaacene type, suitable for the use in organic electronics. Their electrochemically determined ionization potential and electron affinity of about 5.2 and -3.2 eV, respectively, are essentially independent of the length of the alkoxyl substituent and in good agreement with DFT calculations. The crystal structure of 8,16-dioctyloxybenzo[h]benz[5,6]acridino[2,1,9,8-klmna]acridine (FC-8), the most promising compound, was solved. It crystallizes in space group P1‾ and forms π-stacked columns held together in the 3D structure by dispersion forces, mainly between interdigitated alkyl chains. Molecules of FC-8 have a strong tendency to self-organize in monolayers deposited on a highly oriented pyrolytic graphite surface, as observed by STM. 8,16-Dialkoxybenzo[h]benz[5,6]acridino[2,1,9,8-klmna]acridines are highly luminescent, and all have photoluminescence quantum yields of about 80 %. They show efficient electroluminescence, and can be used as guest molecules with a 4,4'-bis(N-carbazolyl)-1,1'-biphenyl host in guest/host-type organic light-emitting diodes. The best fabricated diodes showed a luminance of about 1900 cd m(-12) , a luminance efficiency of about 3 cd A(-1) , and external quantum efficiencies exceeding 0.9 %.

Fabrication of chiral networks for a tri-substituted anthraquinone derivative using molecular self-assembly.

Chiral structures are recorded, with the adsorption of an achiral anthraquinone derivative and co-adsorption of achiral solvent on an achiral surface. Dimer, trimer and tetramer aggregations are observed while only the tetramer-dimer combination constructs the whole monolayer, and the formation mechanism is explained from the thermodynamic and kinetic viewpoints.

Crystal structure of 11-[4-(hex-yloxy)phen-yl]-1,2,4-triazolo[4,3-a][1,10]phenanthroline.

The title compound, C25H24N4O, was prepared from 2-chloro­phenanthroline and hexyl­oxyphenyl­tetra­zole. The main difference between the two independent mol­ecules (A and B) in the asymmetric unit is the orientation of the all-anti-configured hex­yloxy chain: in A the C—O—C—C torsion angle is 175.9 (2)° whereas it is −88.3 (3)° in B. The benzene substitution in the bay of the triazolophenanthroline results in a helical distorsion of the heterocyclic core, the dihedral angles between the mean planes formed by quinoline and benzotriazole ring systems are 13.73 (9) for mol­ecule A and 14.87 (8)° for B. The dihedral angles between the triazole ring and the attached benzene ring are 45.87 (15) in A and 53.93 (14)° in B. The angular annulation of four rings and the benzene substituent results in a helical distortion of the aromatic framework. The crystal is formed from layers composed of centrosymmetric pairs of A 2, B 2 mol­ecules with inter­digitating alkyl chains.

Monolayer effect of a gemini surfactant with a rigid biphenyl spacer on its self-crystallization at the air/liquid interface

A gemini surfactant with a biphenyl spacer can spontaneously generate crystals at the air/solution interface. X-ray crystallography reveals that surfactant molecules exhibit an almost fully extended conformation with interdigitating alkyl chains, together with an approximate co-planarity of two C—C—C planes in two alkyl chains of one gemini molecule, and a prominent dihedral angle between the benzene rings and C—C—C planes of the alkyl chains. Infrared reflection–absorption spectroscopy shows that the gemini surfactant was stretched at the air/water interface, with the hydrocarbon chains oriented at a tilt angle of ∼75° with respect to the surface normal. In particular, the biphenyl group is more or less perpendicular to the water surface, and the C—C—C plane of the alkyl chain tends to be parallel to the water surface. Both results point out a remarkable similarity in the molecular conformation between the crystal and the monolayer. Meanwhile, dynamic light scattering and transmission electron microscopy results indicate that the crystallization of such gemini surfactants at the interface is contrary to the crystallization behavior in the bulk phase, meaning that the surfactant solution can only form a supersaturated solution as it is cooled, though the crystallization temperature of 296 K is lower than the Krafft temperature (∼303 K). Therefore, our findings indicate that the Gibbs monolayer of the gemini surfactant plays a critical role in its interfacial crystallization. Additionally, multiple weak intermolecular interactions, involving van der Waals interaction, π–π stacking and cationic–π interactions, as well as the hydrophobic effect during the aggregation of the gemini molecule in solution, are responsible for the formation of the interfacial crystal.

Influence of the Counteranion on the Ability of 1-Dodecyl-3-methyltriazolium Ionic Liquids to Form Mesophases

The influence of the counteranion on the ability of the mesogenic cation 1-methyl-3-dodecyl-triazolium to form mesophases is explored. To that avail, salts of the cation with anions of different size, shape, and hydrogen bonding capability such as Cl–, Br–, I–, I3–, PF6–, and Tf2N– [bis(trifluorosulfonyl)amide] were synthesized and characterized. The crystal structures of the bromide, the iodide, and the triiodide reveal that the cations form bilayers with cations oriented in opposite directions featuring interdigitated alkyl tails. Within the layers, the cations are separated by anions. The rod-shaped triiodide anion forces the triazolium cation to align with it in this crystal structure but due to its space requirement reduces the alkyl chain interdigitation which prevents the formation of a mesophase. Rather the compound transforms directly from a crystalline solid to an (ionic) liquid like the analogous bis(trifluorosulfonyl)amide. In contrast, the simple halides and the hexafluorophosphate form liquid crystalline phases. Their clearing points shift with increasing anion radius to lower temperatures.

Structure of tetraalkylammonium ionic liquids in the interlayer of modified montmorillonite

We perform molecular dynamics simulations of tetraalkylammonium ionic liquids confined in the interlayer of montmorillonite (MMT). We study the structure and energetics of the systems, which consist of cations with two different alkyl chain lengths and several ionic liquid concentrations. The results we obtained for the structure, namely the presence of a strong layering in all systems and the formation of nonpolar domains with interdigitated alkyl chains in some cases, are largely consistent with previous surface force balance experiments performed on similar systems. Finally, we show that swelling of the organo-modified MMT by a large amount of ionic liquid seems energetically favorable in all cases.

The Unusual Self-Organization of Dialkyldithiophosphinic Acid Self-Assembled Monolayers on Ultrasmooth Gold

We report the formation and characterization of self-assembled monolayers (SAMs) of dialkyldithiophosphinic acid adsorbates [CH3(CH2)n]2P(S)SH (R2DTPA) (n = 5, 9, 11, 13, 15) on ultrasmooth gold substrates prepared by the template stripping method. The SAMs were characterized using X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, contact angle measurements, lateral force microscopy, and electrochemical impedance spectroscopy. The data show these SAMs exhibit an unusual trend in alkyl chain crystallinity; SAMs formed from adsorbates with short alkyl chains (n = 5) are ordered and crystalline, and the alkyl groups become increasingly disordered and liquidlike as the number of methylene units is increased. This trend is the opposite of the typical behavior exhibited by n-alkanethiolate SAMs, in which the alkyl layer becomes more crystalline and ordered as the alkyl chain length is increased. We discuss four factors that operate together to determine how R2DTPA self-organize within SAMs on TS gold: (i) adsorbate-substrate interactions; (ii) gold substrate morphology; (iii) lateral van der Waals interactions between alkyl groups; and (iv) steric demands of the alkyl groups. We also present a model for the structures of these SAMs on the basis of consideration of the data and the structural parameters of a model (n)Bu2DTPA adsorbate. In this model, interdigitation of short alkyl chains stabilizes a trans-extended, crystalline arrangement and produces an ordered alkyl layer. As the alkyl chain length is increased, the increased steric demands of the alkyl groups lead to liquidlike, disorganized alkyl layers.

Porous molecular networks formed by the self-assembly of positively-charged trigonal building blocks at the liquid/solid interfaces.

Tris-(2-hydroxybenzylidene)triaminoguanidinium salts having six alkyl chains with proper spacing served as new molecular building blocks for the formation of porous honeycomb networks by van der Waals interaction between interdigitated alkyl chains at the liquid/graphite interfaces.

Adlayer structures of thiophene and pyrrole derivatives on Au(1 1 1) probed by scanning tunneling microscopy

The adsorption structures of three thiophene derivatives and one pyrrole derivative on Au(111) have been investigated by scanning tunneling microscopy (STM) and cyclic voltammetry in 0.1 M HClO4. The structural details and differences of four monolayers are examined by STM. All the four monomers arrange into ordered adlayers at E=0.55V. Two kinds of alkyl substituted thiophenes, 3-hexylthiophene (HT) and 3-decylthiophene (DT), show similar adsorption behavior. Molecules form dimer basic units through S–S bonding and arrange into the “U-shaped like” patterns. 2-((6-Bromohexyloxy)methyl)-2,3-dihydrothieno[3,4-b][1,4]dioxine (BDD) molecules with an 3,4-ethylenedioxythiophene (EDOT) as molecular skeleton assemble into liner structure with tetramers as building unit. 1-Octadecylpyrrole (ODP) with alkylated pyrrole ring form a lamella structure on Au(111) with interdigitated alkyl chains. The flat-lying configuration for the four monomers is beneficial to enhance the molecule–substrate interaction, which plays an important role to stabilize the organic thin film.