Smectic Polymorphism Research Articles

Synthesis and liquid crystalline behavior of three-arm urethane derivatives bearing protruded alkyoxylbiphenyls

Here, we describe an approach to synthesis of three-arm urethane derivatives bearing alkyloxybiphenyl units. Four new families of oligomers derived from 1,6-hexamethylene diisocyanate and their di- or trimers (uretdione, biuret or isocyanurate) and ω-hydroxyalkylated biphenyls were prepared. Their mesomorphic and optical properties were investigated by differential scanning calorimetry and POM. Mesomorphic properties were substantially impacted by the structure of the central core spacer (urethane, uretdione, biuret, or isocyanurate). The synthesized compounds shown the rich smectic polymorphism form from one to three different smectic phase. Stabilization of the LC state of synthesized oligomers was attributed to the H-bonding interactions confirmed by FT-IR spectra. The part of H-bonded urethane units is maximal for compounds with star-shaped central core.

SYNTHESIS AND MESOGENIC PROPERTIES OF NEW AROMATIC SCHIFF BASE’S ESTERS

A homologous series of new Schiff base’s esters, 4-bromobenzylidene-4’-alkanoyloxyanilines were syn-thesized. The structure of the synthesized compounds was elucidated using IR and NMR spectroscopic techniques along with mass spectrometric method. Nine homologous members were prepared whereby the length of alkanoyloxy chain, Cn-1H2n-1COO is varied from n = 2, 4, 6, 8, 10, 12, 14, 16 and 18. Their thermotropic properties were analyzed using differential scanning calorimetry and polarized optical mi-croscopy techniques. Whilst short chain members (number of carbons, n = 2, 4) are non-mesogens, member with n = 6 exhibited monotropic smectic A (SmA) phase. As the n increased to 8, this com-pound becomes the only member that exhibited SmA and SmB as enantiotropic phase. Varying from n = 10 to n = 16, these members displayed enantiotropic SmA and monotropic SmB phases. Longest chain member, n-octadecanoyloxy derivatives exhibited only single mesophase (monotropic SmA). The ester linkage and polar terminal (bromo) group in the present series are essential for the smectic polymorphism in Schiff bases

Influence of molecular architecture, hydrogen bonding, chemical moieties and flexible chain for smectic polymorphism in (4)EoBD(3)AmBA:nOBAs

A novel series of calamitic, complementary hydrogen-bonded (HB), meta-extended Schiff base core containing liquid crystalline (LC) complexes with ethoxy chain at one end and varied flexible chain at other end, viz., (4)EoBD(3)AmBA:nOBAs for n = 3–12 is reported. 1H, 13C NMR and FTIR spectroscopy confirm complex formation. LC phases, phase transition temperatures (T C) and enthalpy are characterized by polarizing optical microscopy and differential scanning calorimetry(DSC). SmA, SmC, SmBcryst,SmF, SmI, SmE, SmG and SmH phases are exhibited. Clearing and melting temperatures exhibit odd–even effect. Schiff base bridged by oxygen atom promoted of smectic phase occurrence. Complementary HB-induced and -enhanced tilted smectic phase range. Large LC phase variants, i.e., tetra, penta and hexa are observed. Hexa LC phase variant for n = 6 and 12. Unlike in p-n-alkoxy benzoic acids (nOBAs), Nematic phase is quenched. Quasi two-dimensional LC SmF and SmI phases identified. Lower (for n = 3,5), intermediate (for n = 6,7) and higher (n = 9) homologues exhibit 3D SmBcryst phase. Monotropic LC phase occurrence is also noticed. Apart from other LC phase transitions, SmA–SmC transition is mostly found to be second order or very weakly first order nature. Phase diagram by DSC infers existence of multi-critical points in the vicinity of SmF and SmI phase structures with psuedo hexagonal packing. Influence of meta-extended core, complementary HB and flexibility component are discussed for LC phase abundance. Role of orientational disorder induced by flexible component discussed. In comparison with LC phase occurrence in (4)MeoBD(3)AmBA:nOBAs, shielded inductomeric effect is argued for large thermal range of tilted smectic phases. Integrated and differential heat energies are estimated in successive heating and cooling scans to investigate optimized realization of LC phases. Results are discussed in the wake of contemporary reports in other LCs.

Induction of smectic polymorphism in bent-core derivatives of the 6-oxoverdazyl by partial fluorination of alkyl chains

A series of bent-core mesogens 2[m,n] derived from the 6-oxoverdazyl radical and containing partially fluorinated alkyl tails was prepared and investigated by thermal, optical, XRD and EPR methods.

Three armed star mesogens based on 1,3,5-benzenetricarboxylic acid: Synthesis and mesophase characterization

A series of star mesogens based on 1,3,5-benzenetricarboxylic acid central core, side arms containing two phenyl rings, linked through azomethine groups and hexamethylene spacers, which connect the central core and side arms were synthesised via divergent approach. The mesogens synthesised and their intermediates were structurally well characterized, using FT-IR, 1H NMR, 13C NMR spectroscopy and elemental analysis techniques. The mesophase characterization was carried out using a hot stage-polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) techniques. Enantiotropic mesophases were observed for all the mesogens synthesised. An increase in the terminal chain length stabilized the smectic phase. Nematic phase was observed for the mesogens with short terminal chain length, whereas smectic polymorphism was observed on increasing the terminal chain length. XRD results suggested a tripod-like assembly of molecules in the mesophase.

Star mesogens — Synthesis and structural characterization using 1D and 2D solution NMR techniques and mesophase characterization

Novel star mesogens based on trimesic acid and symmetrical side arm cores with terminal alkoxy groups were synthesized via a divergent approach. The central core and side arms were connected through alkyl spacers. All the synthesized mesogens and their intermediates were characterized thoroughly using Fourier transform infrared (FT-IR), 1H nuclear magnetic resonance (NMR), 13C NMR, and mass spectrometers. One representative mesogen was subjected to the two-dimensional (2D) NMR experiments to ascertain the structure of the mesogens. The mesophase characterization was carried out using hot-stage optical polarizing microscopy (HOPM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) techniques. Many of the molecules with an ethyloxy spacer were found to be nonmesogenic, whereas all the molecules with a butyloxy spacer showed liquid crystalline phases. The increase of terminal chain length decreased the transition temperatures. The nematic phase was observed for the mesogens with short terminal chain length, whereas smectic polymorphism was observed on increasing the terminal chain length. The results of a variable temperature powder X-ray diffraction of the representative sample support the smectic layer ordering.

Isophthalic acid based mesogenic dimers: Synthesis and structural effects on mesophase properties

Two series of mesogens based on isophthalic acid and side arms containing two and three phenyl rings linked through azomethine or ester groups were synthesized via divergent approach. Another type of structurally similar mesogen was synthesized via convergent approach in which the azomethine groups were replaced by ester groups. All the synthesized mesogens and their intermediates were structurally well characterized using FT-IR, 1H NMR, 13C NMR and EI-MASS spectrometry. The mesophase characterization was carried out using hot stage-polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) techniques. The structural changes made in the mesogens i.e. increase of terminal chain length and replacement of azomethine groups with ester groups decreased the transition temperatures whereas the increase of side arm length improves the phase stability to a wide temperature range. Nematic phase was observed for the mesogens with short terminal chain length whereas the smectic polymorphism was observed on increasing the terminal chain length. The structural modification (three ring) on the side arm core stabilizes the mesophases enantiotropically.

Schiff base liquid crystals with terminal IODO group: synthesis and thermotropic properties

A new homologous series of Schiff base esters comprising a terminal iodo substituent was studied. The iodo substituent contributes to the molecular polarizability, thus, affecting intermolecular interactions, hence, resulting in smectic polymorphism. The mesomorphic properties were studied using differential scanning calorimeter (DSC) and polarizing optical microscope (POM). Whilst the lower members, n-ethanoyloxy, n-propanoyloxy, n-butanoyloxy and n-pentanoyloxy derivatives were non-mesogens, enantiotropic smectogenic A phase was observed in the medium members (n-hexyloxy and n-heptanoyloxy derivatives). Further lengthening of the alkyl chain from C8 to C18 members saw the induction of new phase which was identified as smectic B phase under microscope. The homologous members were compared with structurally related series to establish their chemical structure-mesomorphic behavior relationships. Key words: 4-{[(4-iodophenyl) imino] methyl} phenyl alkanoate, thermotropic properties, smectic A, smectic B, polarizability.

Phase Transition Studies of 6.O12O.6 and 7.O6O.7 Using Density Measurements

The symmetric liquid crystal dimer homologous series, α,ω-bis-(4-n-alkylaniline benzylidene-4′-oxy) alkanes, show a rich variety of smectic polymorphism. We report here the nature of different transitions exhibited by two of these dimers using density measurements as a function of temperature. The compound 6.O12O.6 exhibits only nematic phase; while 7.O6O.7 exhibits smectic A and smectic F phases. All the phase transitions studied viz., isotropic – nematic, isotropic – smectic A, and smectic A – smectic F are confirmed to be of first order nature. The calculated density jumps, thermal expansion coefficient data and pressure dependence of transition temperatures are analysed in the light of available literature data.

Side-Chain Liquid Crystalline Polymer Networks: Exploiting Nanoscale Smectic Polymorphism To Design Shape-Memory Polymers

Herein, we investigate the influence of nanoscale smectic polymorphism within end-on fixed side-chain liquid crystalline polymer networks (SCLCNs) on macroscopic shape-memory and actuation properties. We have synthesized a series of SCLC-type linear (TP-n) and cross-linked random terpolymers (XL-TP-n) with varying length of flexible methylene spacers (n = 5, 10, and 15) between polynorbornene main-chain and cholesteryl ester side-chains. Thermal and mechanical analyses by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) confirm a glass transition (T(g)), a clearing temperature (T(cl)), and a network structure in the XL-TP-n. Detailed structural investigation conducted using wide-angle and small-angle X-ray scattering (WAXS and SAXS) at room temperature proves self-assembled smectic A (SmA) polymorphism of the XL-TP-n which evolves from non-interdigitated bilayer (SmA(2)) for n = 5 to mixed layers of monolayer-like highly interdigitated layer (SmA(1)) and SmA(2) for n = 10 and to SmA(1) for n = 15. In addition, TP10 at temperatures above 60 °C interestingly shows transformation of SmA structure from mixed layer (SmA(1) + SmA(2)) to interdigitated structure (SmA(d)). The SmA polymorphism developed in TP-n during shape-memory cycles (SMCs) significantly impacts the ultimate strain responses. A mechanism for the unique interdigitation-based thermostrictive behavior is proposed. More importantly, this new actuation mechanism observed in these XL-TP-n can be exploited to develop intelligent thermal actuators.

Mesogenic Schiff base esters with terminal chloro group: Synthesis, thermotropic properties and X-ray diffraction studies

A new homologous series of Schiff base esters comprising a terminal chloro substituent was studied. The chloro substituent contributes to the molecular polarizability, thus, affecting intermolecular interactions, hence, resulting in smectic polymorphism. The mesomorphic properties were studied using differential scanning calorimetry, polarizing optical microscopy and temperature-dependent X-ray diffractometry. Whilst the n-pentanoyloxy and n-heptanoyloxy derivatives exhibited enantiotropic smectogenic A and B phases, monotropic smectogenic A and B phases were observed in the n-butanoyloxy and n-hexanoyloxy derivatives. Enantiotropic smectogenic A and monotropic smectogenic B phases were observed in the n-octanoyloxy to n-hexadecanoyloxy derivatives. The highest member of this series, the n-octadecanoyloxy derivative, exhibited an enantiotropic smectogen A phase. The homologous members were compared with structurally related series to establish their chemical structure-mesomorphic behavior relationships.

Synthesis, Mesomorphism, and Luminescent Properties of Calamitic 2-Phenylpyridines and Their Complexes with Platinum(II)

The liquid crystal and luminescent properties of two related series of rod-like, ortho-platinated complexes bearing a β-diketonate co-ligand are reported. The parent ligands exhibit a rich, smectic polymorphism, but when modified with a fused cyclopentene ring, nematic phases dominate. Reaction of the ligands with tetrachloroplatinate(II) leads to poorly soluble, dimeric complexes that can be cleaved using dimethylsulfoxide; the resulting monomeric complexes are then readily converted to the β-diketonate complexes. All of the complexes are mesomorphic, and the β-diketonate complexes have excited state lifetimes of 27 μs with emission quantum efficiencies exceeding 0.5, the highest yet reported for materials of this type.

Smectic polymorphism of 4-decyloxybenzylidene- 4′-alkyloxyanilines and their mixtures with polar standards of mesophases

This article describes liquid crystalline properties of the homologous series of 4-decyloxybenzylidene-4′-alkyloxyanilines. Based on the polarization microscopy (POM and TOA methods) and the calorimetric (DSC) measurements a rich polymorphism has been detected. Investigated compounds exhibit nematic, smectic A, smectic C, smectic B, smectic I and G (smectic G) mesophases with their characteristic configurations. The presence of these mesophases was confirmed by the miscibility method, using 4-heptyloxybenxylidene-4′-pentylaniline as a mesophase reference. Nine other smectic standards with different polarity have been applied in order to investigate their influence on the mesophase stability.

Structural effects in three-ring mesogenic derivatives of p-carborane and their hydrocarbon analogues

Several series of structurally related three-ring esters containing benzene, cyclohexane, bicyclo[2.2.2]octane and p-carborane rings were synthesized and their mesogenic properties investigated using thermal analysis and optical microscopy. Carborane derivatives show only nematic phases, while the richest smectic polymorphism (up to three phases) was observed in the biphenyl series D. The structure–property relationships were studied by comparison of T NI between series. The ring effectiveness in stabilization of nematic phases generally follows the order carborane<benzene∼cyclohexane<bicyclo[2.2.2]octane. The results indicate that fill fraction plays a significant role in the stabilization of the mesophase. A notable positional effect of the carborane ring on T NI was also observed.

Polymesomorphism in N-(p-n-Alkoxybenzylidene)-p-n-Alkylanilines (nO.m) Compounds

The polymesomorphism exhibited by the famous benzylidene aniline homologues, N-(p-nalkoxybenzylidene)- p-n-alkylanilines (nO.m’s) is presented from the systematic studies carried on these compounds at the centre. The salient features observed are listed. The importance of alkoxy chain length over the alkyl chain in the manifestation of smectic polymorphism in higher homologues of these materials is also discussed.

Higher Homologues of Mesomorphic Benzylidene Anilines. Manifestation of Smectic Polymorphism

The synthesis and characterisation of higher homologues of the well-known benzylidene anilines, viz. N-(p-n-tridecyloxy, tetradecyloxy, and hexadecyloxy benzylidene)-p-n-alkylanilines, 13O.m, 14O.m and 16O.m liquid crystalline compounds have exhibited mono, bi and tri phase variant smectic polymorphism. The different liquid crystalline phases are characterized by an optical polarising microscope equipped with a PC based temperature controller. The phase transitions are studied by using differential scanning calorimetry. The results are discussed in the light of the data available on other nO.m compounds.

Influence of hydrogen bonding on the generation and stabilization of liquid crystalline polyesters, poly(esteramide)s and polyacrylates

Induction and stabilization of liquid crystallinity through hydrogen bonding (HB) are now well-established. Interesting observations made on the influence of HB on LC behaviour of amido diol-based poly(esteramide)s, poly(esteramide)s containing nitro groups and azobenzene mesogen-based polyacrylates will be discussed. The use of amido diol as an important precursor for the synthesis of novel PEAs containing inbuilt di-amide linkage enabled generation of extensive hydrogen bondings between the amide-amide and amide-ester groups which stabilized the mesophase structures of the PEAs. The contributions of hydrogen bonding to the generation and stabilization of mesophase structures were plainly evident from the observation of liquid crystallinity even in PEAs prepared from fully aliphatic amido diols. Replacement of terephthaloyl units by isophthaloyl moiety totally vanquished liquid crystalline phases while biphenylene and naphthalene units did only reduce the transition temperatures as expected. The occurrence of the smectic phases in some of the polymers indicated possibly self-assembly through the formation of hetero intermolecular hydrogen bonded networks. A smectic polymorphism and in addition, a smectic-to-nematic transition, were observed in the monomers and polymers based on 1,4-phenylene[bis-(3-nitroanthranilidic acid)] containing nitro groups. A smectic polymorphism was also observed as a combined effect of hydrogen bonded carboxyl groups and laterally substituted alkyl side chains in the case of azobenzene mesogen containing side chain polyacrylates. It was further shown that the presence of the mesophase enhances the non-linear optical (NLO) response of these polymers.

Dielectric studies of two liquid crystalline benzoates with coupled and decoupled CN groups

The results of dielectric studies performed in a broad frequency range for two compounds, 4-cyanophenyl 4-n-heptylbenzoate (7BBCN) and 4-(4-cyanobutyloxy)phenyl 4- n-heptylbenzoate (7BB4CN), are compared. They have the same molecular core whereas the strongly polar CN group is attached to the benzene ring in 7BBCN or is separated from it by the butyloxy chain in 7BB4CN. 7BBCN has a nematic phase, whereas 7BB4CN exhibits a monotropic nematic and smectic A2 polymorphism. Large differences in the dielectric properties of the two substances were found. The analysis of the results led to the conclusion that the antiparallel dipole-dipole associations are considerably stronger in the substance with a decoupled CN group.

New switchable smectic phases in banana-shaped compounds

A new series of achiral compounds with banana-shaped molecules ('E n ') has been synthesized and studied by the classical techniques (optical microscopy, differential scanning calorimetry, X-ray diffraction, miscibility studies and electro-optic investigations). The short homologues (E7-E10) give switchable smectic mesophases. Nevertheless their textures clearly differ from those of the B2 mesophase. In addition, the E10 compound displays a smectic polymorphism involving a new bilayered structure.

Density Studies on Two Homologues of α,ω-Bis(4-n-Alkylanilinebenzylidine-4′-Oxy) Alkanes

α,ω-bis(4-n-alkylanilinebenzylidine-4′-oxy)alkanes exhibit a rich smectic polymorphism. We have studied the variation of density as a function of temperature across the phase transitions in two compounds of the series viz., 7.O4O.7 and 7.O5O.7. Both of these compounds exhibit a rare phase transition of Sm A - Sm F. The results confirmed first order nature of the transitions of Isotropic - Sm A and Sm A - Sm F, but with a smaller jump than expected in density as compared to monomers across the transitions. The estimated pressure dependence of transition temperature is also reported. The results are discussed in the light of available literature data.