Terminal Alkyl Chain Length Research Articles

Structure–property relationship in charge transporting behaviour of room temperature liquid crystalline perylenebisimides

A homologous series of pentadecyl phenol functionalized perylenebisimide (PBI) terminated with trialkoxy gallate esters was synthesized, where the terminal alkyl chain length was varied from n = 4 to 12 (PBI-En). The thermotropic liquid crystalline (LC) characteristics of the molecules were analyzed using differential scanning calorimetry (DSC), polarized light microscopy (PLM) combined with variable temperature wide angle X-ray diffraction (WXRD) techniques. A clear odd–even oscillation was observed in the melting as well as isotropization enthalpies as a function of alkyl spacer length in the terminal gallate unit, with the even spacers exhibiting higher values. The higher members of the series with n > 8 exhibited thermotropic liquid crystalline textures in the PLM, which remained stable until room temperature. The nature of the LC phase was identified to be columnar rectangular and columnar hexagonal based on detailed analysis of the WXRD pattern recorded in the LC phase. The WXRD pattern of the room temperature LC frozen samples indicated a nearly constant intra columnar stack distance of ∼3.7 A for all the members. The space-charge-limited current (SCLC) values of the LC frozen sample films were analyzed for dependence of the bulk mobility estimate on the nature of the LC phase. The columnar hexagonal phase exhibited a mobility value one order (10−3 cm2 V−1 s−1) higher than that of crystalline (10−4 cm2 V−1 s−1) and two orders higher than that of columnar rectangular phase (10−5 cm2 V−1 s−1), indicating a strong dependence of packing on bulk mobility.

The effect of the alkyl chain length on the mesomorphic properties of new lactic acid derivatives

New series of lactic acid derivatives with alkyl terminal chain have been synthesised and their mesomorphic properties studied. We have varied the length of chiral and non-chiral terminal alkyl chains and found that prolonging both chains has a strong effect on the SmA*–SmC* phase transition. Most of the new materials exhibit only paraelectric SmA* phase; for homologues with a longer non-chiral chain (m ≥ 10), the ferroelectric (SmC*) phase appears below the SmA* on cooling and persists down to a room temperature. The role of the chiral terminal chain in the molecule is quite opposite – only its short length supports the existence of ferroelectric phase. Additionally, a hexatic phase appeared below the SmA*–SmC* phase sequence for several homologues at low temperatures. All materials have been studied using standard experimental techniques (differential scanning calorimetry (DSC), texture observations, polarisation and tilt angle measurements, etc.). Liquid crystalline properties of new materials have been compared with the previously prepared and studied lactic acid derivatives.

3-Hydroxycinnamic acid – a new central core for the design of bent-shaped liquid crystals

The synthesis and mesomorphic properties of a new series of bent-shaped liquid crystals based on 3-hydroxycinnamic acid are presented. Prepared compounds differ in the orientation of an ester linking group in one of the side arms. Additionally, the mesomorphic properties have been tuned by employing either an alkoxy or carboxylic terminal group and by varying the length of terminal alkyl chains. Texture observation and calorimetric studies, as well as electro-optics, X-ray and dielectric measurements have been performed. Influence of the ester linking group orientation and the length of the terminal alkyl chain on mesomorphic behaviour has been established. Out of 12 studied compounds only one formed lamellar phases, namely the B2–B5 phase sequence, while all other exhibited columnar phases, either B1 or B1Rev type, depending on the direction of the ester group. Structural properties of the columnar phases have been studied in detail using X-ray diffraction techniques for pure materials as well as for binary mixtures of compounds forming the B1 and B1Rev phase. All results are discussed with respect to the molecular structure.

Biaxial Order Parameter in an Achiral Bent–Core Smectic Liquid Crystal

The important parameter of the mesogens which governs nearly all fascinating physical properties is its order parameter. In the biaxial liquid crystals (LCs), there are two order parameters: primary (long axis) and secondary (short axis). The secondary orientational order parameter (η) of an achiral bent core (BC) LC material with terminal alkyl chain length 8 was estimated from the temperature dependent field induced biaxiality measurements in SmAPA phase for the first time. The Haller's extrapolation technique is used for the estimation of η as it can directly apply to biaxiality measurements rather than to refractive indices.

Synthesis and characterization of six-arm star-shaped liquid crystalline cyclotriphosphazenes

The synthesis and mesomorphic properties of a new class of liquid crystalline cyclotriphosphazenes with rigid cyanostilbene groups and flexible alkoxy groups are reported. The thermal behaviors of cyclotriphosphazene derivatives 5a–5d are studied by the means of differential scanning calorimetry, polarizing optical microscopy. Nematic phases were observed in compounds 5a–5d both on heating and cooling. The clear temperatures of compounds 5a–5d slightly increased with increasing length of terminal alkyl chain. Compounds 5a–5d showed different mesogenic region from minimum 5 °C to maximum 142 °C.

Helical twisting properties of new chiral dopants with double (S)-1,2-propanediol units for nematic liquid crystals

A novel series of chiral dopants synthesized from (S)-1,2-propanediol and mesogenic carboxylic acids were characterized by FT-IR, 1H NMR, elemental analysis and their helical twisting properties were investigated by doping the chiral dopants into a nematic liquid crystal host (SLC-1717). The results show that, the helical pitch of N*-LC mixture exhibited a terminal alkyl chain length dependence and the molecular twisting power β also exhibited a temperature dependence (increasing β with increasing temperature).

Direct electrochemistry of hemoglobin adsorbed on self-assembled monolayers with different head groups or chain length

The electrochemical behaviors of hemoglobin (Hb) were studied by adsorbing Hb on the gold electrodes modified with self-assembled monolayers (SAMs) of different terminal groups and alkyl chain length. Specifically, through adsorbing the Hb molecules onto the SAMs of 3-mercaptopropanoic acid (MPA), 1-propanethiol (PT) and cysteamine hydrochloride (cys), the influences of the terminal groups of alkanethiols on the electron transfer of Hb were examined. A quasi-reversible redox process was observed when Hb was adsorbed on the MPA-modified electrode. However, an irreversible reduction process and no redox response were shown when Hb molecules were adsorbed on the SAMs of PT and cys-modified electrodes, respectively. The dependence of the direct electrochemical response of Hb on the alkyl chain length of alkanethiols (n) was further investigated on the SAMs of HS(CH(2))(n)COOH (n=1, 2, 5 and 10). When n was 1, Hb showed a weak current response. When n were 2 and 5, quasi-reversible redox processes with nearly similar electron transfer rate constants (k(s)) of 0.49 and 0.47 s(-1) were obtained, respectively. Increasing the number of methylene groups within carboxyl alkanethiol to 10 resulted in a significant decrease of the electron transfer rate and current response of Hb. The observations indicated that the SAM of MPA can provide suitable properties to keep Hb in a favorable adsorption state for direct electron transfer (DET). Furthermore, the direct electrochemistry of the Hb adsorbed on the SAM of MPA was studied in detail.

Influence of terminal alkyl chain length on helical twisting property of chiral 1,2-propanediol derivatives

In this study, a novel series of chiral 1,2-propanediol derivatives with different terminal alkyl chain length were synthesized and characterized by FT-IR, 1H NMR and DSC. After doped into a nematic liquid crystal host, all the chiral dopants induced chiral nematic liquid crystals exhibiting a helix inversion with temperature variation. The results indicate that terminal alkyl chain length has a prominent influence on helical twisting property of the chiral dopants. With increasing the terminal alkyl chain length, the molecular twisting power increases, the helix inversion temperatures varies slightly while the phase transition temperature decreases. An odd–even effect may exist in the molecular twisting power of the chiral dopants.

Fast Switching in the Vicinity of Smectic-to-Nematic Phase Transition in High Birefringence Isothiocyanate Mixtures

Two series of high birefringence liquid crystal compounds, isothiocyanato phenyl-tolane and terphenyl, and mixtures were prepared and their physical properties evaluated. Depending on the terminal alkyl chain length, different mesomorphic sequences were observed. In the vicinity of nematic-to-smectic transition, the visco-elastic coefficient decreases, instead of increase, as the temperature decreases. We investigated the possibility of extending the observed phenomenon from single compounds to mixtures for practical applications. This behavior is useful for improving the response time of a liquid crystal device, if the operating temperature can be accurately controlled.

Study on the helical twisting behaviour of chiral 1,2-propanediol derivatives inducing chiral nematic liquid crystals with a helix inversion and controllable pitch variation

In this study, a novel series of chiral dopants were synthesised via the esterification reaction between optically active (S)-1,2-propanediol and mesogenic carboxylic acids. The phenomenon of a helix inversion with temperature variation could be observed in the chiral nematic liquid crystals (N*-LCs) by doping the chiral dopants into a nematic liquid crystal host. The results show that the inversion temperature varies only slightly with terminal alkyl chain length, but can be influenced by the structural rigidity and molecular shape of the chiral dopants, while the molecular twisting power has an apparent dependence on terminal alkyl chain length and the number of chiral centres. Depending on the composition proportion of binary chiral mixtures prepared by the synthesised chiral dopants and the commercially-available chiral compounds, induced N*-LCs with controllable pitch variation could be easily achieved.

Biomimetic Self‐Assembly of Tetrapeptides into Fibrillar Networks and Organogels

AbstractThe self‐assembly features of a family of tetrapeptides with a silk‐inspired structure are presented. An exhaustive study of the influence of the terminal alkyl chain length in this process is undertaken. Scanning electron microscopy (SEM), wide‐angle X‐ray diffraction (WAXD), FTIR spectroscopy, and circular dichroism were used for structural analysis. These compounds, as in the natural model, self‐assemble into antiparallel β‐sheet structures that further organize to form fibrillar aggregates. Furthermore, some of them arecapable of forming a crowded network that entraps thesolvent leading to physical gels with different microscopic morphologies. A model for the assembly process is proposed.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Fast switching near a smectic‐to‐nematic phase transition

We have designed, synthesised and evaluated the physical properties of high‐birefringence (Δn) isothiocyanato phenyl‐tolane and terphenyl liquid crystals. These compounds exhibit a Δn∼0.36–0.52 at room temperature and wavelength λ = 633 nm. A different mesomorphic sequence was observed depending on the terminal alkyl chain length. Compounds with a longer chain show smectic and nematic phases, whereas short‐chain homologues only exhibit a nematic phase. In the vicinity of the nematic‐to‐smectic transition, the viscoelastic coefficient decreases, instead of increases, as the temperature decreases. The observed change is as large as 75% for the 4′‐(3,5‐difluoro‐4‐isothiocyanatophenylethynyl)‐4‐pentylbiphenyl compound studied. This behaviour is useful for improving the response time of a liquid crystal device, if the operating temperature can be controlled accurately.

Synthesis and study of azo dye compounds: variation of mesogenic behaviour with length of alkyl chains

The mesogenic behaviours of two azo dye liquid crystals, 1 and 4, differ because of the different length of terminal alkyl chains; azo dye 1 shows a SmB phase, azo dye 4 shows nematic and SmA phases during thermal processing. The molecular stackings of these two azo dye liquid crystals are compared on the basis of crystallographic studies to explain such unexpected mesogenic behaviour.

Liquid crystalline properties of salicylaldimine-based dimers: influence of terminal alkyl chain length and central part

The synthesis and thermotropic properties of four homologous series of salicylaldimine-based dimer liquid crystals are reported. Two 4-(4-alkoxy-2-hydroxybenzylideneamino)benzoyloxy groups are connected to a central part consisting of a 1,3-phenylene, 1,5-pentylene, 2,2-dimethyl-1,5-pentylene or 3,3-dimethyl-1,5-pentylene unit. The terminal alkoxy chains have been varied from 4 to 16 carbon atoms in length. All the compounds exhibit liquid crystalline phases whose behaviour depends on the nature of the central part and the length of the alkoxy terminal chains. All compounds of the series with the central phenyl part exhibit enantiotropic B-phases, and the sequence B6–B1–B2 on increasing terminal chain length was observed. Replacement of the phenyl group with a pentyl central group partly suppresses the formation of B-phases. The longer homologues of this series show the B1 phase, while the shorter exhibit an intercalated SmCc mesophase. The introduction of methyl substituents to the pentyl spacer causes the melting points to fall dramatically and the formation of B-phases is totally suppressed. The compounds with the long tails show intercalated SmAc phases and those with short tails show intercalated SmCc phases.

Synthesis and Thermal Properties of Twin Dimers Containing Cyanostilbene Derivatives as a Mesogenic Group

We synthesized twin dimers (TDs) with different linking groups between a mesogenic group and a flexible spacer, different spacer length, and different terminal alkyl chain length to clarify the effect of these groups on the mesomorphic properties of the TDs. A remarkable odd–even effect on the phase transition temperatures and nematic–isotrophic phase transition entropy changes was observed for the TDs with different spacer length. Liquid crystallinity of TDs with ester linking group was superior to that of TDs with ether linking group. Length of terminal group played an important role in exhibiting a smectic phase for TDs with ester linking group.

Mesomorphic Properties of 3-Pyridyl 4-(4-Alkoxybenzyleneamino) benzoates and Isomeric Compounds

The nematic-isotropic transition temperature of 3-pyridyl 4-(4-propoxybenzylideneamino) benzoate is higher by 13°C than that of the phenyl analogue and the smectic A-nematic/isotropic transition is also promoted by the replacement of the terminal moiety as much as 23°C. The enhancement of both the nematic and smectic thermal stabilities decreases as the terminal alkyl chain length increases. Similar changes in the mesomorphic properties by the replacement are recorded for N-[4-(4-alkoxy-benzoyloxy) benzylidene] anilines and other isomeric compounds.

The effect of alkyl chain length and terminal group chemistry on the attachment and growth of murine 3T3 fibroblasts and primary human osteoblasts on self-assembled monolayers of alkanethiols on gold

Murine 3T3 fibroblasts and primary human osteoblasts have been cultured on self-assembled monolayers (SAMs) formed by the adsorption of alkanethiols of varying terminal group and alkyl chain length on gold. It has been found that the interactions of both cell types with SAMs are influenced by surprisingly subtle changes in adsorbate structure. Both the nature of the adsorbate terminal group and the length of the alkyl chain have a significant influence on cellular attachment and growth. For both cell types, attachment and growth on the carboxylic acid terminated SAMs was extensive and was accompanied by a high degree of cell spreading. In contrast, attachment and growth on long-chain methyl terminated SAMs were poor. Attachment and growth on short-chain methyl terminated SAMs were intermediate, with reasonable numbers of attached cells being observed and some degree of cell spreading. Sharply contrasting behaviour was observed for the two cell types when cultured on hydroxy terminated SAMs. It was found that fibroblasts attached poorly to these surfaces, while osteoblasts attached and grew nearly as successfully on short-chain hydroxy terminated SAMs as on carboxylic acid terminated SAMs. Osteoblasts attached less well to long-chain hydroxy terminated SAMs, but better than to methyl terminated SAMs.

Stabilization of the Sc Phase in Mixtures of Laterally Aryl Substituted Mesogens

Laterally 4-nitrobenzyloxycarbonyl substituted three-ring compounds preferably exhibit smectic A phases. However, induction of smectic C phases can be observed in binary mixtures with mesogens having a similar molecular shape but not a strongly polar group at the laterally positioned phenyl ring. The scale of phase induction depends on the length of terminal alkyl chains, type and position of substituents as well as the length of the flexible spacer linking the lateral aryl group to the basic mesogen. X-ray investigations of selected mixtures verify that the structure of induced smectic C phases is intercalated comparable to orthogonal phases of lateral aryl substituted mesogens.

Smectogenic Copper(II) Complexes of N-Salicylideneaniline Derivatives. A Comparative Study of Homologous Series Carrying Alkoxy and/or Alkanoyloxy Substituents

A series of bis[4-alkoxy-N-(4-ethoxyphenyl)salicylaldiminato]copper(II) complexes have been prepared and shown to possess smectic A phases with isotropization temperatures ranging from 157 to 182 °C over butoxy to octadecyloxy homologues. Corresponding series of the ligands have also been isolated and characterized for their mostly nematogenic properties. Analogous series of bis[4-alkoxy-N-(4-acetoxyphenyl)salicylaldiminato]copper(II) complexes and their ligands have also proved to be mesomorphic; both of the phase type and the thermal stability are similar to those of the N-(4-ethoxyphenyl) counterparts. On the other hand, a series of bis[4-alkanoyloxy-N-(4-ethoxyphenyl)salicylaldiminato]copper(II) complexes, in which terminal alkyl groups are attached to the 4-oxysalicylidene groups via carbonyl groups, have been shown to melt at high temperatures (184−228 °C) and not mesogenic at all, while the phase behavior of the ligand series is very similar to that of the above 4-alkoxy series. An X-ray powder diffraction study of the two series of metallosmectic phases has led to a common structural model, in which layers of bis[N-phenylsalicylaldiminato]copper(II) cores are intervened by fully interdigitated alkyl chains. The thermodynamic parameters for the first series have been analyzed for their dependence on the terminal alkyl chain length and partial ordering of short chains is suggested.

Thermotropic properties of monosubstituted ferrocene derivatives bearing bidentateN-benzoyl-N′-arylthiourea ligands—novel building blocks for heterometallic liquid crystal systems

Ferrocene-based derivatives such as 4-{3-[4-(octyloxy)benzoyl]thioureido}phenyl 4-ferrocenylbenzoate and other higher homologues (n=12,16 and 18; n=length of alkoxy chain) were prepared by reacting 4-alkoxybenzoyl isothiocyanates with the corresponding amines containing the ferrocenyl moiety. Their mesomorphic properties were investigated by means of polarized optical microscopy and differential scanning calorimetry. All the compounds exhibit enantiotropic nematic phases and the nematic range increases with increasing terminal alkyl chain length. On cooling, the nematic phase persists below 0 °C in the first three compounds and in the case of n=18, a phase transformation, possibly to the SCphase, around 72 °C during cooling was observed. In all cases, a glass transition was observed around Tg=18–35 °C, which is remarkable for low molecular mass calamitic metallomesogen systems.