Uma Hiremath | Centre For Nano and Soft Matter Sciences (original) (raw)
Papers by Uma Hiremath
Materials Chemistry and Physics, 2011
Optical absorption, thermo-optical and dielectric studies have been performed on three guest-host... more Optical absorption, thermo-optical and dielectric studies have been performed on three guest-host nematic mixtures featuring photo-sensitized flexoelectric polarization. The liquid crystal 4-butylcyclohexane carboxylic acid 4-pentyloxy-phenyl ester, a room temperature nematic with a negative dielectric anisotropy served as the host material. Three different azo-dye compounds, also exhibiting liquid crystallinity were employed as the low-concentration guest component in the mixtures. In each case illumination of the sample with actinic (UV) light leads to strong photoisomerization driven effects. Correlations among the shift of the isotropic-nematic phase transition temperature, population of the cis isomers, and the change in the length of the azo-dyes upon UV illumination, are discussed. The UVinduced changes in the dielectric constant values have been shown to be dependent on the nature of the photoactive dopant. The article also describes the first observation of the lowering of the bend elastic constant upon photoisomerization, a feature ascribed to the formation of the bent-shaped cis isomers.
Journal of Materials Chemistry C, 2013
We describe here X-ray, electrical switching and dielectric measurements on a binary system compr... more We describe here X-ray, electrical switching and dielectric measurements on a binary system comprising achiral bent core and chiral rod-like components [1]. While the pure bent core compound exhibits a single mesophase, namely, the B2 phase, the rod-like compound shows smectic A and
Soft matter, Jan 28, 2014
We have studied composites of a ferroelectric liquid crystal mixture with a simple organic gelati... more We have studied composites of a ferroelectric liquid crystal mixture with a simple organic gelating agent, employing structural, thermal, electrical and mechanical probes, investigating the influence of the coupling between the polarization and the tilt angle on the ferroelectric properties of smectic gels. The calorimetric data, presenting clear signatures of the gelation occurring in the smectic A (SmA) phase or the isotropic phase, depending on the concentration of the gelator, help in constructing a rich diagram in the temperature-gelator concentration phase space. The atomic force microscopy imaging brings out the interesting feature of the transfer of chirality from the ferroelectric liquid crystal (FLC) to the gel strands, as exemplified by the creation of nanorope structures which have attracted much attention in recent times. The influence of gelation on the magnitude of the tilt angle appears to be dependent on the probe employed: there is no change in the values obtained ...
Thermochimica Acta, 2006
High pressure investigations have been carried out on the photo-stimulated phase transition exhib... more High pressure investigations have been carried out on the photo-stimulated phase transition exhibited by a guest-host system, where the host is a non-photoactive liquid crystal. As guest molecules we chose successive homologues of a dimeric photoactive azobenzene liquid crystalline molecule to understand the influence of the length and parity of the alkylene spacer on such a transition. Our studies show that application of pressure as well as the parity of the spacer has a drastic influence both on the static and dynamic characters of the photo-stimulated phase transition.
Thermochimica Acta, 2012
ABSTRACT We report the first systematic work on the effect of hydrostatic pressure on the splay a... more ABSTRACT We report the first systematic work on the effect of hydrostatic pressure on the splay and bend elastic constants of a nematic (N) liquid crystal. At a relative temperature, the magnitudes of both the elastic constants increase significantly with increasing pressure, with the variation for the bend parameter being slightly higher. Pressure also influences the rate at which the elastic constants grow as the smectic A (SmA) phase is approached from the N phase. The studies further show that the temperature range of the N phase may not play an important role in the increase of the splay constant near the transition to the SmA phase.
The Journal of Physical Chemistry B, 2008
We characterize the structural transitions in an initially homeotropic bent-rod nematic liquid cr... more We characterize the structural transitions in an initially homeotropic bent-rod nematic liquid crystal excited by ac fields of frequency f well above the dielectric inversion point f i. From the measured principal dielectric constants and electrical conductivities of the compound, the Carr-Helfrich conduction regime is anticipated to extend into the sub-megahertz region. Periodic patterned states occur through secondary bifurcations from the Freedericksz distorted state. An anchoring transition between the bend Freedericksz (BF) and degenerate planar (DP) states is detected. The BF state is metastable well above the Freedericksz threshold and gives way to the DP state, which persists in the field-off condition for several hours. Numerous +1 and-1 umbilics form at the onset of BF distortion, the former being largely of the chiral type. They survive in the DP configuration as linear defects, nonsingular in the core. In the BF regime, not far from f i , periodic Williamslike domains form around the umbilics; they drift along the director easy axis right from their onset. With increasing f, the wave vector of the periodic domains switches from parallel to normal disposition with respect to the c vector. Well above f i , a broadband instability is found.
The Journal of Physical Chemistry B, 2007
We characterize the sequence of bifurcations generated by ac fields in a nematic layer held betwe... more We characterize the sequence of bifurcations generated by ac fields in a nematic layer held between unidirectionally rubbed ITO electrodes. The material, which possesses a negative dielectric anisotropy a and an inversion temperature for electrical conductivity anisotropy σ a , exhibits a monostable tilted alignment near T IN , the isotropic-nematic point. On cooling, an anchoring transition to the homeotropic configuration occurs close to the underlying smectic phase. The field experiments are performed for (i) negative σ a and homeotropic alignment, and (ii) weakly positive σ a and nearly homeotropic alignment. Under ac driving, the Freedericksz transition is followed by bifurcation into various patterned states. Among them are the striped states that seem to belong to the dielectric regime and localized hybrid instabilities. Very significantly, the patterned instabilities are not excited by dc fields, indicating their possible gradient flexoelectric origin. The Carr-Helfrich mechanism-based theories that take account of flexoelectric terms can explain the observed electroconvective effects only in part.
The Journal of Physical Chemistry B, 2008
Tetrahedron Letters, 2013
Abstract The first examples of mesogenic bis( N -salicylideneaniline)s (BSANs), wherein two lipop... more Abstract The first examples of mesogenic bis( N -salicylideneaniline)s (BSANs), wherein two lipophilic (half-disk shaped) entities are interlinked through the dihydroxydiformylbenzene core, were synthesized and characterized. In particular, three constitutional (positional) isomeric BSANs were prepared by the facile twofold condensation of 3,4,5-tris(alkoxy)anilines with 2,3-dihydroxyterephthalaldehyde, 4,6-dihydroxyisophthalaldehyde, and 2,5-dihydroxyterephthalaldehyde and their structures were established by elemental analyses, FT-IR, 1 H NMR, and 13 C NMR. Proton NMR experiments demonstrated their existence in enol–imine (OH) form solely. Polarizing optical microscopic, differential scanning calorimetric, and powder X-ray diffraction studies evidenced the occurrence of columnar mesomorphism in two sets of isomers.
Tetrahedron Letters, 2005
The molecular design, synthesis and characterization of the first examples of both classical and ... more The molecular design, synthesis and characterization of the first examples of both classical and non-conventional chiral mesoionic (mesomeric + ionic) liquid crystals derived from sydnones are described. The occurrence of chiral smectic phases in these novel compounds was evidenced by optical microscopy, calorimetry and X-ray studies.
Soft Matter, 2006
Synthetic and molecular structural characterization details The procedure for the synthesis of 2,... more Synthetic and molecular structural characterization details The procedure for the synthesis of 2,3-difluoro1-decyloxybenzene (4) A solution of 2,3-difluorobenzene (1 g, 7.6 m.mol, 1 equiv.), 1-bromodecane (1.2 g, 9.12 m.mol, 1.2 equiv.), potassium carbonate (4.1 g, 30.4 m.mol, 4 equiv.) was taken in dry DMF (20 ml) solvent. The resulting mixture was stirred for 16 h under dry nitrogen atmosphere at 80 °C. The reaction was quenched by adding ice water (300 ml) and the crude product was extracted with dichloromethane (2 × 200 ml). The combined organic extracts was washed with water (200 ml) followed by brine and dried over anhydrous Na 2 SO 4. Evaporation of solvent furnished the crude product, which was further purified by column chromatography using silica gel (100-200 mesh). The column was initially eluted with hexanes to remove the unreacted 1-bromodecane and then with 5% EtOAchexanes to get the pure product. Yield: 90%. A colourless liquid; R f = 0.55 (10% EtOAchexanes); IR (neat) ν max in cm
Physical Review E, 2008
Flexoelectricity is a unique property of liquid crystals; it is a linear coupling between electri... more Flexoelectricity is a unique property of liquid crystals; it is a linear coupling between electric polarizations and bend and/or splay distortions of the direction of average molecular orientation. Recently it was shown ͓J. Harden et al., Phys. Rev. Lett. 97, 157802 ͑2006͔͒ that the bend flexoelectric coefficient in bent-core nematic liquid crystals can be three orders of magnitude higher than the effect with calamitic ͑rod-shaped͒ molecular shape. Here we report the converse of the flexoelectric effect: An electric field applied across a bent-core liquid crystal sandwiched between thin flexible substrates produces a director distortion which is manifested as a polarity-dependent flexing of the substrates. The flex magnitude is shown to be consistent with predictions based upon both the measured value of the bend flexoelectric constant and the elastic properties of the substrates. Converse flexoelectricity makes possible a new class of microactuators with no internal moving parts, which offers applications as diverse as optical beam steering to artificial muscles.
Molecular Crystals and Liquid Crystals, 2003
Oligomeric liquid crystals that are formed by joining two or more mesogenic segments axially thro... more Oligomeric liquid crystals that are formed by joining two or more mesogenic segments axially through a flexible spacer (or multiple spacers) seem to be interesting materials from the points of view of both basic research and prac-tical applications owing to their multifunctional ...
Molecular Crystals and Liquid Crystals, 2011
Here we present new results on guest-host nematic systems featuring an optical degree of freedom ... more Here we present new results on guest-host nematic systems featuring an optical degree of freedom based on the trans-cis photoizomerization of a single azo-bond or two azo-bonds in guest mesogens, with or without a longitudinal dipole moment. Homeotropic nematic ...
Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals, 2001
Abstract We report the synthesis and physical characterization of two unsymmetrical dimeric liqui... more Abstract We report the synthesis and physical characterization of two unsymmetrical dimeric liquid crystals containing two different chiral mesogenic segments, namely, a tolan unit with (S)-2-octyloxy tail and a cholesteryl ester unit which are separated by a n-butyl (even) and ...
Liquid Crystals, 2002
The molecular design, synthesis and thermal behaviour of the rst liquid crystalline tetramers com... more The molecular design, synthesis and thermal behaviour of the rst liquid crystalline tetramers composed of four non-identical mesogenic entities connected covalently through para nic spacers are presented. These new molecular structures consisting of a tolan (half disc-shaped), ...
Liquid Crystals, 2003
The phase behaviour of two achiral bent core banana-shaped compounds, the hexyloxy (compound I) a... more The phase behaviour of two achiral bent core banana-shaped compounds, the hexyloxy (compound I) and decyloxy (compound II) members of the 1, 3-phenylene bis [N-(2-hydroxy-4-n-alkoxybenzylidene)-4′-aminobenzoate] series was investigated under hydrostatic ...
Materials Chemistry and Physics, 2011
Optical absorption, thermo-optical and dielectric studies have been performed on three guest-host... more Optical absorption, thermo-optical and dielectric studies have been performed on three guest-host nematic mixtures featuring photo-sensitized flexoelectric polarization. The liquid crystal 4-butylcyclohexane carboxylic acid 4-pentyloxy-phenyl ester, a room temperature nematic with a negative dielectric anisotropy served as the host material. Three different azo-dye compounds, also exhibiting liquid crystallinity were employed as the low-concentration guest component in the mixtures. In each case illumination of the sample with actinic (UV) light leads to strong photoisomerization driven effects. Correlations among the shift of the isotropic-nematic phase transition temperature, population of the cis isomers, and the change in the length of the azo-dyes upon UV illumination, are discussed. The UVinduced changes in the dielectric constant values have been shown to be dependent on the nature of the photoactive dopant. The article also describes the first observation of the lowering of the bend elastic constant upon photoisomerization, a feature ascribed to the formation of the bent-shaped cis isomers.
Journal of Materials Chemistry C, 2013
We describe here X-ray, electrical switching and dielectric measurements on a binary system compr... more We describe here X-ray, electrical switching and dielectric measurements on a binary system comprising achiral bent core and chiral rod-like components [1]. While the pure bent core compound exhibits a single mesophase, namely, the B2 phase, the rod-like compound shows smectic A and
Soft matter, Jan 28, 2014
We have studied composites of a ferroelectric liquid crystal mixture with a simple organic gelati... more We have studied composites of a ferroelectric liquid crystal mixture with a simple organic gelating agent, employing structural, thermal, electrical and mechanical probes, investigating the influence of the coupling between the polarization and the tilt angle on the ferroelectric properties of smectic gels. The calorimetric data, presenting clear signatures of the gelation occurring in the smectic A (SmA) phase or the isotropic phase, depending on the concentration of the gelator, help in constructing a rich diagram in the temperature-gelator concentration phase space. The atomic force microscopy imaging brings out the interesting feature of the transfer of chirality from the ferroelectric liquid crystal (FLC) to the gel strands, as exemplified by the creation of nanorope structures which have attracted much attention in recent times. The influence of gelation on the magnitude of the tilt angle appears to be dependent on the probe employed: there is no change in the values obtained ...
Thermochimica Acta, 2006
High pressure investigations have been carried out on the photo-stimulated phase transition exhib... more High pressure investigations have been carried out on the photo-stimulated phase transition exhibited by a guest-host system, where the host is a non-photoactive liquid crystal. As guest molecules we chose successive homologues of a dimeric photoactive azobenzene liquid crystalline molecule to understand the influence of the length and parity of the alkylene spacer on such a transition. Our studies show that application of pressure as well as the parity of the spacer has a drastic influence both on the static and dynamic characters of the photo-stimulated phase transition.
Thermochimica Acta, 2012
ABSTRACT We report the first systematic work on the effect of hydrostatic pressure on the splay a... more ABSTRACT We report the first systematic work on the effect of hydrostatic pressure on the splay and bend elastic constants of a nematic (N) liquid crystal. At a relative temperature, the magnitudes of both the elastic constants increase significantly with increasing pressure, with the variation for the bend parameter being slightly higher. Pressure also influences the rate at which the elastic constants grow as the smectic A (SmA) phase is approached from the N phase. The studies further show that the temperature range of the N phase may not play an important role in the increase of the splay constant near the transition to the SmA phase.
The Journal of Physical Chemistry B, 2008
We characterize the structural transitions in an initially homeotropic bent-rod nematic liquid cr... more We characterize the structural transitions in an initially homeotropic bent-rod nematic liquid crystal excited by ac fields of frequency f well above the dielectric inversion point f i. From the measured principal dielectric constants and electrical conductivities of the compound, the Carr-Helfrich conduction regime is anticipated to extend into the sub-megahertz region. Periodic patterned states occur through secondary bifurcations from the Freedericksz distorted state. An anchoring transition between the bend Freedericksz (BF) and degenerate planar (DP) states is detected. The BF state is metastable well above the Freedericksz threshold and gives way to the DP state, which persists in the field-off condition for several hours. Numerous +1 and-1 umbilics form at the onset of BF distortion, the former being largely of the chiral type. They survive in the DP configuration as linear defects, nonsingular in the core. In the BF regime, not far from f i , periodic Williamslike domains form around the umbilics; they drift along the director easy axis right from their onset. With increasing f, the wave vector of the periodic domains switches from parallel to normal disposition with respect to the c vector. Well above f i , a broadband instability is found.
The Journal of Physical Chemistry B, 2007
We characterize the sequence of bifurcations generated by ac fields in a nematic layer held betwe... more We characterize the sequence of bifurcations generated by ac fields in a nematic layer held between unidirectionally rubbed ITO electrodes. The material, which possesses a negative dielectric anisotropy a and an inversion temperature for electrical conductivity anisotropy σ a , exhibits a monostable tilted alignment near T IN , the isotropic-nematic point. On cooling, an anchoring transition to the homeotropic configuration occurs close to the underlying smectic phase. The field experiments are performed for (i) negative σ a and homeotropic alignment, and (ii) weakly positive σ a and nearly homeotropic alignment. Under ac driving, the Freedericksz transition is followed by bifurcation into various patterned states. Among them are the striped states that seem to belong to the dielectric regime and localized hybrid instabilities. Very significantly, the patterned instabilities are not excited by dc fields, indicating their possible gradient flexoelectric origin. The Carr-Helfrich mechanism-based theories that take account of flexoelectric terms can explain the observed electroconvective effects only in part.
The Journal of Physical Chemistry B, 2008
Tetrahedron Letters, 2013
Abstract The first examples of mesogenic bis( N -salicylideneaniline)s (BSANs), wherein two lipop... more Abstract The first examples of mesogenic bis( N -salicylideneaniline)s (BSANs), wherein two lipophilic (half-disk shaped) entities are interlinked through the dihydroxydiformylbenzene core, were synthesized and characterized. In particular, three constitutional (positional) isomeric BSANs were prepared by the facile twofold condensation of 3,4,5-tris(alkoxy)anilines with 2,3-dihydroxyterephthalaldehyde, 4,6-dihydroxyisophthalaldehyde, and 2,5-dihydroxyterephthalaldehyde and their structures were established by elemental analyses, FT-IR, 1 H NMR, and 13 C NMR. Proton NMR experiments demonstrated their existence in enol–imine (OH) form solely. Polarizing optical microscopic, differential scanning calorimetric, and powder X-ray diffraction studies evidenced the occurrence of columnar mesomorphism in two sets of isomers.
Tetrahedron Letters, 2005
The molecular design, synthesis and characterization of the first examples of both classical and ... more The molecular design, synthesis and characterization of the first examples of both classical and non-conventional chiral mesoionic (mesomeric + ionic) liquid crystals derived from sydnones are described. The occurrence of chiral smectic phases in these novel compounds was evidenced by optical microscopy, calorimetry and X-ray studies.
Soft Matter, 2006
Synthetic and molecular structural characterization details The procedure for the synthesis of 2,... more Synthetic and molecular structural characterization details The procedure for the synthesis of 2,3-difluoro1-decyloxybenzene (4) A solution of 2,3-difluorobenzene (1 g, 7.6 m.mol, 1 equiv.), 1-bromodecane (1.2 g, 9.12 m.mol, 1.2 equiv.), potassium carbonate (4.1 g, 30.4 m.mol, 4 equiv.) was taken in dry DMF (20 ml) solvent. The resulting mixture was stirred for 16 h under dry nitrogen atmosphere at 80 °C. The reaction was quenched by adding ice water (300 ml) and the crude product was extracted with dichloromethane (2 × 200 ml). The combined organic extracts was washed with water (200 ml) followed by brine and dried over anhydrous Na 2 SO 4. Evaporation of solvent furnished the crude product, which was further purified by column chromatography using silica gel (100-200 mesh). The column was initially eluted with hexanes to remove the unreacted 1-bromodecane and then with 5% EtOAchexanes to get the pure product. Yield: 90%. A colourless liquid; R f = 0.55 (10% EtOAchexanes); IR (neat) ν max in cm
Physical Review E, 2008
Flexoelectricity is a unique property of liquid crystals; it is a linear coupling between electri... more Flexoelectricity is a unique property of liquid crystals; it is a linear coupling between electric polarizations and bend and/or splay distortions of the direction of average molecular orientation. Recently it was shown ͓J. Harden et al., Phys. Rev. Lett. 97, 157802 ͑2006͔͒ that the bend flexoelectric coefficient in bent-core nematic liquid crystals can be three orders of magnitude higher than the effect with calamitic ͑rod-shaped͒ molecular shape. Here we report the converse of the flexoelectric effect: An electric field applied across a bent-core liquid crystal sandwiched between thin flexible substrates produces a director distortion which is manifested as a polarity-dependent flexing of the substrates. The flex magnitude is shown to be consistent with predictions based upon both the measured value of the bend flexoelectric constant and the elastic properties of the substrates. Converse flexoelectricity makes possible a new class of microactuators with no internal moving parts, which offers applications as diverse as optical beam steering to artificial muscles.
Molecular Crystals and Liquid Crystals, 2003
Oligomeric liquid crystals that are formed by joining two or more mesogenic segments axially thro... more Oligomeric liquid crystals that are formed by joining two or more mesogenic segments axially through a flexible spacer (or multiple spacers) seem to be interesting materials from the points of view of both basic research and prac-tical applications owing to their multifunctional ...
Molecular Crystals and Liquid Crystals, 2011
Here we present new results on guest-host nematic systems featuring an optical degree of freedom ... more Here we present new results on guest-host nematic systems featuring an optical degree of freedom based on the trans-cis photoizomerization of a single azo-bond or two azo-bonds in guest mesogens, with or without a longitudinal dipole moment. Homeotropic nematic ...
Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals, 2001
Abstract We report the synthesis and physical characterization of two unsymmetrical dimeric liqui... more Abstract We report the synthesis and physical characterization of two unsymmetrical dimeric liquid crystals containing two different chiral mesogenic segments, namely, a tolan unit with (S)-2-octyloxy tail and a cholesteryl ester unit which are separated by a n-butyl (even) and ...
Liquid Crystals, 2002
The molecular design, synthesis and thermal behaviour of the rst liquid crystalline tetramers com... more The molecular design, synthesis and thermal behaviour of the rst liquid crystalline tetramers composed of four non-identical mesogenic entities connected covalently through para nic spacers are presented. These new molecular structures consisting of a tolan (half disc-shaped), ...
Liquid Crystals, 2003
The phase behaviour of two achiral bent core banana-shaped compounds, the hexyloxy (compound I) a... more The phase behaviour of two achiral bent core banana-shaped compounds, the hexyloxy (compound I) and decyloxy (compound II) members of the 1, 3-phenylene bis [N-(2-hydroxy-4-n-alkoxybenzylidene)-4′-aminobenzoate] series was investigated under hydrostatic ...