- Email: [email protected]
Relative permittivity of a few H-bonded liquid crystals
Journal of Molecular Liquids 109 (2004) 39–43
Relative permittivity of a few H-bonded liquid crystals ´ ´ ´ Szalaib Monika Valisko´ a,*, Janos Liszia, Istvan a
´ H-8201 Veszprem, P.O. Box 158, Hungary Department of Physical Chemistry, University of Veszprem, b ´ H-8201 Veszprem, P.O. Box 158, Hungary Department of Physics, University of Veszprem, Received 11 December 2002; accepted 3 June 2003
Abstract This paper presents the experimentally determined relative permittivities of a few 4-n-alkoxybenzoic acids and their binary mixtures with 4,49-bipyridine. Selective recognition between a benzoic acid derivative and a nonmesogenic molecule through Hbond results in liquid crystalline properties. Due to the formed intermolecular hydrogen bonding, the liquid crystal behaviour becomes stronger. The measured relative permittivities are higher for mixtures than in the case of pure alkoxybenzoic acids. Because of the H-bonding, the dielectric anisotropy becomes negative and its absolute value is larger compared to the pure 4-nalkoxybenzoic acids, where the anisotropy is positive and lower. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Relative permittivity; Liquid crystal; Hydrogen bond
1. Introduction Hydrogen bond is one of the most important interactions in nature because of its role in molecular recognition and assembly. It is only recently that the liquid crystalline behaviour of hydrogen-bonded complexes has started to be extensively studied. The properties of the mesophase depend on the shape of the molecule, and the magnitude and direction of the molecular interactions between the molecules w1x. The importance of the dipole–dipole interactions has been established for a long time and the role of the intermolecular hydrogen bonding in the formation of mesophase is studied widely w2 x . Many publications deal with the formation of the hydrogen-bonded liquid crystals w3–8x. Complexes producing liquid crystals usually involve carboxylic acids and their derivatives as donor molecules, and pyridine, 4,49-bipyridine (4,49-BPy) or stilbazole derivatives as acceptor molecules. The donor and acceptor molecules do not always show liquid crystalline phases by themselves. The complexes having linear structures show *Corresponding author. Fax: q36-88-423-409. ´ E-mail address: [email protected] (M. Valisko).
high thermal stability and a new mesophase can appear if one of the two interacting molecules happens itself to be mesogenic. Kato et al. w3–6x have reported a novel family of liquid crystals in which different molecules are selfassembled through selective recognition between hydrogen-bonding donor and acceptor moieties. In their work they demonstrated how H-bonding can be used to build a complex with stable linear structure using a nonmesogenic rigid molecule as a core. Kato et al. w3x selected the 4,49-BPy as rigid H-bonding acceptor that is capable of recognizing H-donor molecules at each of its piridyl ends. Sideratou et al. w8x used N-(p-methoxy-o-hydroxybenzlidene)-p-amino-pyridine as acceptor molecule and determined the phase diagram of the binary mixtures of 4-n-alkoxybenzoic acid (nOBA) with this acceptor molecule. Most of the publications focus on the preparation of the hydrogen-bonded liquid crystals and the determination of the phase diagrams and the structure w3–8x. To our best knowledge, there are only a few publications dealing with the experimental determination of the dielectric properties of H-bonded liquid crystals w9,10x. One of the reasons might be the technical difficulties in measuring the relative permittivities. In most cases the phase transition temperature is higher than 100 8C, so stabilization of the temperature encounters problems.
0167-7322/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. PII: S 0 1 6 7 - 7 3 2 2 Ž 0 3 . 0 0 2 0 8 - 3
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M. Valisko´ et al. / Journal of Molecular Liquids 109 (2004) 39–43
Fig. 1. The proposed structure w3x of the studied H-bonded liquid crystals.
Dielectric behaviour of pure liquid crystalline carboxylic acids was published by Kresse w9x, while Araki et al. w10x examined the dielectric properties of a hydrogenbonded side-chain liquid crystalline polysiloxane over a wide range of frequency and temperature. The dielectric anisotropy, dispersion and relaxation have received practical interest since the realisation of technical applications of the electro-optical effects w11x. The static dielectric behaviour of liquid crystal compounds in mesophases can be divided into three groups on the basis of the temperature dependence of the anisotropy of the dielectric constant w12x. In the first group, the compounds have positive dielectric anisotropy in the nematic phase and there is no significant change at the N–S phase transition, so the dielectric anisotropy remains positive. In the next group, the liquid crystals have positive dielectric anisotropy in the nematic phase; but in smectic phase the sign of the dielectric anisotropy changes because of the strong temperature dependence of the ´H and ´≤ components. The third group is represented by negative dielectric anisotropy in the whole temperature range. Great variation in sign and magnitude of the static dielectric anisotropy of different types of mesogenes can occur. In this paper, we present the relative permittivities of some 4-n-alkoxybenzoic acids and the relative permittivities of their binary mixtures with 4,49-BPy. We found small positive dielectric anisotropy for the pure alkoxybenzoic acids, and higher and negative dielectric anisotropy in the case of mixtures with 4,49-BPy. A smectic A phase is induced for a mixture of butyloxybenzoic acid (4OBA) and 4,49-BPy. 2. Experimental The used alkoxybenzoic acids were products from Sigma-Chemical Company Inc., the 4,49-BPy was the product of Fluka Chemie AG. The purity of the studied compounds was better than 98%. The pure 4-n-alkoxybenzoic acids investigated by us are liquid crystals and defined by a polymorphic scheme; the phase transition temperatures can be found in Ref. w13x. The phase transition temperature measured by us is in good agreement with the data in Refs. w13,14x. The studied derivatives show enantiotropic liquid crystal phases over a wide range of temperature: only
the nematic phase appears for the alkoxybenzoic acid from butyloxy to hexyloxy (6OBA) and both nematic and smectic phases occur in the case of octyloxy (8OBA) and dodecyloxybenzoic acids (12OBA). Crytalline nOBA and 4,49-BPy in the required mole ratios were dissolved in pyridine. The nOBA–4,49-BPy complexes were prepared by slow evaporation from this solution w3x. According to the proposed structure of the hydrogen-bonded liquid crystals w3x, the 4,49-BPy plays the role of the core unit (Fig. 1). The liquid crystal behaviour of the pure alkoxybenzoic acids and their binary mixtures with 4,49-BPy was studied by differential scanning calorimetry (PerkinElmer DSC-2C, with a heating rate of 5 Kymin). The static relative permittivities were measured at 100 kHz with a Hewlett-Packard 4192A LF impedance analyser. The liquid crystal sample was placed within a planar, stainless steel capacitor. The distance between the electrodes was 0.2 mm. For the calibration of the measuring cell, carbon tetrachloride was used at room temperature, and cyclohexanone and naphthalene at higher temperatures. The estimated uncertainty of the dielectric constant measurement is "0.02. For the orientation of the liquid crystal samples a Weiss-type electromagnet was used. The inductance of the applied magnetic field, B, was approximately 0.5 T. The permittivities ´≤ and ´H of the ordered mesophases were measured with the mentioned 100-kHz AC electric field parallel and perpendicular to the magnetic field, respectively. As we mentioned in the introduction, temperaturestabilization encounters many difficulties. The measuring cell was inserted into an aluminum block and the heating was realized by two heating filaments. The temperature of the aluminum block was controlled electronically within "0.2 8C accuracy. The in situ temperature determination was carried out by a thermistor in the cell; the accuracy of temperature measurement was approximately "0.1 8C. Previously, the thermistor was calibrated to a THERM 3340 digital thermometer (Ahlborn, Holzkirchen, Germany). 3. Results and discussion Figs. 2–4 show the temperature dependence of the relative permittivities of the studied liquid crystals (pure
M. Valisko´ et al. / Journal of Molecular Liquids 109 (2004) 39–43
Fig. 2. Temperature dependence of the relative permittivities for 4OBA. The solid curves in the upper set show ´H and ´≤, while the dashed line represents the mean permittivity, ´¯ . The bottom set shows the dielectric anisotropy, D´.
4OBA–6OBA), while Figs. 5 and 6 show it for the mixtures of these liquid crystals with 4,49-BPy: ´H and ´≤ in the mesophases and ´ in the isotropic phase, the mean permittivity ´¯ s(´Iq2´H)y3, and the dielectric anisotropy, D´s´≤y´H. These compounds show quite low polarity: the difference between the polarizabilities of the studied nOBA liquid crystals is very low. Let us consider the isotropic phase first. It has been found that the relative permittivity data can be correlated by linear equations in the studied temperature ranges. Table 1 shows the relative permittivities of pure nOBAs and their complexes in isotropic phase at a given temperature. It can be seen that in the case of the pure alkoxybenzoic acids the relative permittivity in the isotropic phase decreases with the increase of the length of the alkoxy-chain. The same is true for the stoichiometric complexes with 4,49-BPy as seen from Table 1. The alkoxybenzoic acids in pure state are completely dimerized at room temperature because of the existence of the stable hydrogen bonding and the dimer itself may be considered as a single component w15x. Dissociation into single molecules starts after heating to relatively high temperature w8x. In isotropic phase the relative permittivity changes slightly with the temperature in the case of alkoxybenzoic acids (Figs. 2–4). On the contrary, the mixtures
41
Fig. 3. Temperature dependence of the relative permittivities for 5OBA. The presentation is as for Fig. 2.
Fig. 4. Temperature dependence of the relative permittivities for 6OBA. The presentation is as for Fig. 2.
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M. Valisko´ et al. / Journal of Molecular Liquids 109 (2004) 39–43 Table 1 Relative permittivities of the pure 4-n-alkoxybenzoic acids and the hydrogen-bonded (nOBA)2 –4,49-BPy complexes in isotropic phase at 165 8C temperature Isotropic phase
´ 12OBA 8OBA 6OBA 5OBA 4OBA
Fig. 5. Temperature dependence of the relative permittivities of 4OBA–4,49-BPy mixtures for various compositions. The solid, dashed and dotted lines represent the 2:1, 1.5:1 and 1.25:1 compositions, respectively. The upper set shows ´H and ´≤, while the bottom set shows the dielectric anisotropy, D´.
with 4,49-BPy show more significant temperature dependence in isotropic phase as seen in Figs. 5 and 6. Let us investigate the dielectric properties of the mesophase in the case of pure alkoxybenzoic acids. The
2.9 3.23 3.29 3.46 3.6
´ (12OBA)2 –4,49-BPy (8OBA)2 –4,49-BPy (6OBA)2 –4,49-BPy (5OBA)2 –4,49-BPy (4OBA)2 –4,49-BPy
3.1 – 3.5 3.8 3.95
mean permittivity shows a quite large negative jump at the phase transition from isotropic to nematic phase. The value of the dielectric anisotropy is positive and very small as seen in Figs. 2–4. The measured relative permittivities are in good agreement with data published by Kresse w9x. In the case of octyloxy and dodecyloxybenzoic acids, where the length of the paraffin chain is quite large, we could not measure ´H and ´≤ separately in the mesophase because the difference between the capacitances were too small to detect. Nevertheless, both the isotropic to nematic and the nematic to smectic phase transitions were clearly observed by the help of the dielectric measurement. This observation was confirmed by the DSC records. As it can be seen in Figs. 2–4, in the case of the ordered alkoxybenzoic acids, the relative permittivities ´H and ´≤ decrease with the increase of the length of the paraffin chains as in the case of the isotropic phase. These results are in complete agreement with the observation of Kresse w9x. Fig. 5 presents the temperature dependence of the permittivities of the binary mixture of 4OBA with 4,49BPy at different compositions. In the case of the 2:1 stoichiometric composition, the isotropic–nematic and the nematic–smectic phase transitions are clearly observed and these results are in complete agreement with the determined phase transition temperatures given by Kato et al. w3x as well as with those obtained from the DSC measurement by us. The phase transition temperatures of the H-bonded mixtures determined by the help of DSC are collected in Table 2. For (4OBA)2 –4,49-BPy liquid crystal, a new, smectic mesophase appeared that is not observed for either pure components. In the complex a mesogen is formed through intermolecular hydrogen bonding, where the Table 2 The phase transition temperature of the 4OBA–4,49-BPy mixtures at various compositions determined from DSC thermograms Phase transition temperatures
Fig. 6. Temperature dependence of the relative permittivities of the stoichiometric composition complexes for 4OBA (on the left) and 5OBA (on the rigth) with 4,49-BPy. The presentation is as for Fig. 2.
1.25:1 1.5:1 2:1
N 155 8C I SA 151 8C N 156 8C I SA 152 8C N 159 8C I
M. Valisko´ et al. / Journal of Molecular Liquids 109 (2004) 39–43
4,49-BPy functions as a core unit in the liquid crystal. The effect of the smectic phase appeared in the permittivity parallel to the magnetic field. For the mixture of 1.5:1 4OBA–4,49-BPy we also found both smectic and nematic phases, but the change in the permittivity at the nematic–smectic phase transition is not so significant. In the case of 1.25:1 composition we did not detect a new mesophase. The DSC records confirmed our observation. The phase transition temperature decreased with the mole fraction of the 4OBA. We measured the temperature dependence of the dielectric constant of the pure 4,49-BPy in the temperature range of 115–170 8C. The value of the relative permittivity is much lower than in the case of the pure nOBA or the H-bonded mixtures. The data can be correlated by the following linear equation in the studied temperature range: ´(T)sy2.93=10y3 Tq2.82, the corresponding correlation factor is 0.98. Accordingly, we can draw the conclusion that the increase in relative permittivities of the nOBA–4,49-BPy mixtures is due to formed chemical bonds. The formed intermolecular H-bonding via association makes the liquid crystalline character stronger. Fig. 6 shows the temperature dependence of the permittivities of the 2:1 stoichiometric composition complex of 5OBA–4,49-BPy. We did not observe new mesophase as it can be seen in Fig. 6, where the (4OBA)2 –4,49-BPy is also delineated for comparison. Due to the formed intermolecular hydrogen bonding, the measured permittivities are higher, the dielectric anisotropy is negative and its absolute value is larger than in the case of the pure nOBA. For the influence of the 4,49-BPy and hereby the formed H-bond, the sign of the dielectric anisotropy changes. In order to confirm our conclusions, we measured the 4:1 composition of 4OBA–4,49-BPy, and according to our expectations, the determined relative permittivity in the isotropic phase is between the values for the pure 4OBA and for the stoichiometric composition as seen in Fig. 7. Infrared study of Kato et al. w14x suggests that the hydrogen bond is of non-ionic type with a double minimum potential energy. The stability of the hydrogen-bonded complex suddenly decreases when the complex becomes an isotropic disordered phase. The stability of the hydrogen bond is not simply a function of temperature, it is greatly dependent on the state of molecular ordering. We have experimentally determined relative permittivities of H-bonded liquid crystal systems whose dielectric properties, to our best knowledge, were not investigated so far. By the help of dielectric measurement, we proved that the complex formed via association makes the liquid
43
Fig. 7. The relative permittivity as a function of the mole fraction of 4OBA for 4OBA–4,49-BPy mixtures in isotropic phase at 165 8C.
crystal behaviour stronger, which was rendered probable only by the DSC data up to present. Acknowledgments This research was supported by the Hungarian National Research Fund (OTKA-T029327). References w1x G.W. Gray, J.W. Goodby, Smectic Liquid Crystals, Leonard Hill, Glasgow, 1984. w2x R. Miethchen, J. Holz, H. Prade, A. Liptak, Tetrahedron 48 (1992) 3061. w3x T. Kato, P.G. Wilson, A. Fujishima, J.M.J. Frechet, Chem. Lett. 11 (1990) 2003. w4x T. Kato, A. Fujishima, J.M.J. Frechet, Chem. Lett. 6 (1990) 919. w5x T. Kato, H. Adachi, A. Fujishima, J.M.J. Frechet, Chem. Lett. 2 (1992) 265. w6x T. Kato, J.M.J. Frechet, J. Am. Chem. Soc. 111 (1989) 8533. w7x U. Kumar, T. Kato, J.M.J. Frechet, J. Am. Chem. Soc. 114 (1992) 6630. w8x Z. Sideratou, D. Tsiourvas, C.M. Paleos, A. Skoulios, Liquid Cryst. 22 (1997) 51. w9x H. Kresse, Fortschritte der Physik 30 (1982) 507. w10x K. Araki, T. Kato, U. Kumar, J.M.J. Frechet, Macromol. Rapid Commun. 16 (1995) 733. w11x H. Keller, R. Hatz, Handbook of Liquid Crystals, Verlag Chemie, Weinheim, Deerfield, 1980. w12x L. Bata, Advances in Liquid Crystal Research and Applications, Pergamon Press, Oxford, 1980. w13x D. Demus, H. Demus, H. Zaschke, Flussige ¨ Kristalle in ¨ Grundstoffindustrie, Leipzig, Tabellen, Deutcher Veralg fur 1974. w14x T. Kato, J.M.J. Frechet, P.G. Wilson, et al., Chem. Mater. 5 (1993) 1094. w15x G.W. Gray, B.J. Jones, J. Chem. Soc. 4 (1953) 4179.
Relative permittivity of a few H-bonded liquid crystals ´ ´ ´ Szalaib Monika Valisko´ a,*, Janos Liszia, Istvan a
´ H-8201 Veszprem, P.O. Box 158, Hungary Department of Physical Chemistry, University of Veszprem, b ´ H-8201 Veszprem, P.O. Box 158, Hungary Department of Physics, University of Veszprem, Received 11 December 2002; accepted 3 June 2003
Abstract This paper presents the experimentally determined relative permittivities of a few 4-n-alkoxybenzoic acids and their binary mixtures with 4,49-bipyridine. Selective recognition between a benzoic acid derivative and a nonmesogenic molecule through Hbond results in liquid crystalline properties. Due to the formed intermolecular hydrogen bonding, the liquid crystal behaviour becomes stronger. The measured relative permittivities are higher for mixtures than in the case of pure alkoxybenzoic acids. Because of the H-bonding, the dielectric anisotropy becomes negative and its absolute value is larger compared to the pure 4-nalkoxybenzoic acids, where the anisotropy is positive and lower. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Relative permittivity; Liquid crystal; Hydrogen bond
1. Introduction Hydrogen bond is one of the most important interactions in nature because of its role in molecular recognition and assembly. It is only recently that the liquid crystalline behaviour of hydrogen-bonded complexes has started to be extensively studied. The properties of the mesophase depend on the shape of the molecule, and the magnitude and direction of the molecular interactions between the molecules w1x. The importance of the dipole–dipole interactions has been established for a long time and the role of the intermolecular hydrogen bonding in the formation of mesophase is studied widely w2 x . Many publications deal with the formation of the hydrogen-bonded liquid crystals w3–8x. Complexes producing liquid crystals usually involve carboxylic acids and their derivatives as donor molecules, and pyridine, 4,49-bipyridine (4,49-BPy) or stilbazole derivatives as acceptor molecules. The donor and acceptor molecules do not always show liquid crystalline phases by themselves. The complexes having linear structures show *Corresponding author. Fax: q36-88-423-409. ´ E-mail address: [email protected] (M. Valisko).
high thermal stability and a new mesophase can appear if one of the two interacting molecules happens itself to be mesogenic. Kato et al. w3–6x have reported a novel family of liquid crystals in which different molecules are selfassembled through selective recognition between hydrogen-bonding donor and acceptor moieties. In their work they demonstrated how H-bonding can be used to build a complex with stable linear structure using a nonmesogenic rigid molecule as a core. Kato et al. w3x selected the 4,49-BPy as rigid H-bonding acceptor that is capable of recognizing H-donor molecules at each of its piridyl ends. Sideratou et al. w8x used N-(p-methoxy-o-hydroxybenzlidene)-p-amino-pyridine as acceptor molecule and determined the phase diagram of the binary mixtures of 4-n-alkoxybenzoic acid (nOBA) with this acceptor molecule. Most of the publications focus on the preparation of the hydrogen-bonded liquid crystals and the determination of the phase diagrams and the structure w3–8x. To our best knowledge, there are only a few publications dealing with the experimental determination of the dielectric properties of H-bonded liquid crystals w9,10x. One of the reasons might be the technical difficulties in measuring the relative permittivities. In most cases the phase transition temperature is higher than 100 8C, so stabilization of the temperature encounters problems.
0167-7322/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. PII: S 0 1 6 7 - 7 3 2 2 Ž 0 3 . 0 0 2 0 8 - 3
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M. Valisko´ et al. / Journal of Molecular Liquids 109 (2004) 39–43
Fig. 1. The proposed structure w3x of the studied H-bonded liquid crystals.
Dielectric behaviour of pure liquid crystalline carboxylic acids was published by Kresse w9x, while Araki et al. w10x examined the dielectric properties of a hydrogenbonded side-chain liquid crystalline polysiloxane over a wide range of frequency and temperature. The dielectric anisotropy, dispersion and relaxation have received practical interest since the realisation of technical applications of the electro-optical effects w11x. The static dielectric behaviour of liquid crystal compounds in mesophases can be divided into three groups on the basis of the temperature dependence of the anisotropy of the dielectric constant w12x. In the first group, the compounds have positive dielectric anisotropy in the nematic phase and there is no significant change at the N–S phase transition, so the dielectric anisotropy remains positive. In the next group, the liquid crystals have positive dielectric anisotropy in the nematic phase; but in smectic phase the sign of the dielectric anisotropy changes because of the strong temperature dependence of the ´H and ´≤ components. The third group is represented by negative dielectric anisotropy in the whole temperature range. Great variation in sign and magnitude of the static dielectric anisotropy of different types of mesogenes can occur. In this paper, we present the relative permittivities of some 4-n-alkoxybenzoic acids and the relative permittivities of their binary mixtures with 4,49-BPy. We found small positive dielectric anisotropy for the pure alkoxybenzoic acids, and higher and negative dielectric anisotropy in the case of mixtures with 4,49-BPy. A smectic A phase is induced for a mixture of butyloxybenzoic acid (4OBA) and 4,49-BPy. 2. Experimental The used alkoxybenzoic acids were products from Sigma-Chemical Company Inc., the 4,49-BPy was the product of Fluka Chemie AG. The purity of the studied compounds was better than 98%. The pure 4-n-alkoxybenzoic acids investigated by us are liquid crystals and defined by a polymorphic scheme; the phase transition temperatures can be found in Ref. w13x. The phase transition temperature measured by us is in good agreement with the data in Refs. w13,14x. The studied derivatives show enantiotropic liquid crystal phases over a wide range of temperature: only
the nematic phase appears for the alkoxybenzoic acid from butyloxy to hexyloxy (6OBA) and both nematic and smectic phases occur in the case of octyloxy (8OBA) and dodecyloxybenzoic acids (12OBA). Crytalline nOBA and 4,49-BPy in the required mole ratios were dissolved in pyridine. The nOBA–4,49-BPy complexes were prepared by slow evaporation from this solution w3x. According to the proposed structure of the hydrogen-bonded liquid crystals w3x, the 4,49-BPy plays the role of the core unit (Fig. 1). The liquid crystal behaviour of the pure alkoxybenzoic acids and their binary mixtures with 4,49-BPy was studied by differential scanning calorimetry (PerkinElmer DSC-2C, with a heating rate of 5 Kymin). The static relative permittivities were measured at 100 kHz with a Hewlett-Packard 4192A LF impedance analyser. The liquid crystal sample was placed within a planar, stainless steel capacitor. The distance between the electrodes was 0.2 mm. For the calibration of the measuring cell, carbon tetrachloride was used at room temperature, and cyclohexanone and naphthalene at higher temperatures. The estimated uncertainty of the dielectric constant measurement is "0.02. For the orientation of the liquid crystal samples a Weiss-type electromagnet was used. The inductance of the applied magnetic field, B, was approximately 0.5 T. The permittivities ´≤ and ´H of the ordered mesophases were measured with the mentioned 100-kHz AC electric field parallel and perpendicular to the magnetic field, respectively. As we mentioned in the introduction, temperaturestabilization encounters many difficulties. The measuring cell was inserted into an aluminum block and the heating was realized by two heating filaments. The temperature of the aluminum block was controlled electronically within "0.2 8C accuracy. The in situ temperature determination was carried out by a thermistor in the cell; the accuracy of temperature measurement was approximately "0.1 8C. Previously, the thermistor was calibrated to a THERM 3340 digital thermometer (Ahlborn, Holzkirchen, Germany). 3. Results and discussion Figs. 2–4 show the temperature dependence of the relative permittivities of the studied liquid crystals (pure
M. Valisko´ et al. / Journal of Molecular Liquids 109 (2004) 39–43
Fig. 2. Temperature dependence of the relative permittivities for 4OBA. The solid curves in the upper set show ´H and ´≤, while the dashed line represents the mean permittivity, ´¯ . The bottom set shows the dielectric anisotropy, D´.
4OBA–6OBA), while Figs. 5 and 6 show it for the mixtures of these liquid crystals with 4,49-BPy: ´H and ´≤ in the mesophases and ´ in the isotropic phase, the mean permittivity ´¯ s(´Iq2´H)y3, and the dielectric anisotropy, D´s´≤y´H. These compounds show quite low polarity: the difference between the polarizabilities of the studied nOBA liquid crystals is very low. Let us consider the isotropic phase first. It has been found that the relative permittivity data can be correlated by linear equations in the studied temperature ranges. Table 1 shows the relative permittivities of pure nOBAs and their complexes in isotropic phase at a given temperature. It can be seen that in the case of the pure alkoxybenzoic acids the relative permittivity in the isotropic phase decreases with the increase of the length of the alkoxy-chain. The same is true for the stoichiometric complexes with 4,49-BPy as seen from Table 1. The alkoxybenzoic acids in pure state are completely dimerized at room temperature because of the existence of the stable hydrogen bonding and the dimer itself may be considered as a single component w15x. Dissociation into single molecules starts after heating to relatively high temperature w8x. In isotropic phase the relative permittivity changes slightly with the temperature in the case of alkoxybenzoic acids (Figs. 2–4). On the contrary, the mixtures
41
Fig. 3. Temperature dependence of the relative permittivities for 5OBA. The presentation is as for Fig. 2.
Fig. 4. Temperature dependence of the relative permittivities for 6OBA. The presentation is as for Fig. 2.
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M. Valisko´ et al. / Journal of Molecular Liquids 109 (2004) 39–43 Table 1 Relative permittivities of the pure 4-n-alkoxybenzoic acids and the hydrogen-bonded (nOBA)2 –4,49-BPy complexes in isotropic phase at 165 8C temperature Isotropic phase
´ 12OBA 8OBA 6OBA 5OBA 4OBA
Fig. 5. Temperature dependence of the relative permittivities of 4OBA–4,49-BPy mixtures for various compositions. The solid, dashed and dotted lines represent the 2:1, 1.5:1 and 1.25:1 compositions, respectively. The upper set shows ´H and ´≤, while the bottom set shows the dielectric anisotropy, D´.
with 4,49-BPy show more significant temperature dependence in isotropic phase as seen in Figs. 5 and 6. Let us investigate the dielectric properties of the mesophase in the case of pure alkoxybenzoic acids. The
2.9 3.23 3.29 3.46 3.6
´ (12OBA)2 –4,49-BPy (8OBA)2 –4,49-BPy (6OBA)2 –4,49-BPy (5OBA)2 –4,49-BPy (4OBA)2 –4,49-BPy
3.1 – 3.5 3.8 3.95
mean permittivity shows a quite large negative jump at the phase transition from isotropic to nematic phase. The value of the dielectric anisotropy is positive and very small as seen in Figs. 2–4. The measured relative permittivities are in good agreement with data published by Kresse w9x. In the case of octyloxy and dodecyloxybenzoic acids, where the length of the paraffin chain is quite large, we could not measure ´H and ´≤ separately in the mesophase because the difference between the capacitances were too small to detect. Nevertheless, both the isotropic to nematic and the nematic to smectic phase transitions were clearly observed by the help of the dielectric measurement. This observation was confirmed by the DSC records. As it can be seen in Figs. 2–4, in the case of the ordered alkoxybenzoic acids, the relative permittivities ´H and ´≤ decrease with the increase of the length of the paraffin chains as in the case of the isotropic phase. These results are in complete agreement with the observation of Kresse w9x. Fig. 5 presents the temperature dependence of the permittivities of the binary mixture of 4OBA with 4,49BPy at different compositions. In the case of the 2:1 stoichiometric composition, the isotropic–nematic and the nematic–smectic phase transitions are clearly observed and these results are in complete agreement with the determined phase transition temperatures given by Kato et al. w3x as well as with those obtained from the DSC measurement by us. The phase transition temperatures of the H-bonded mixtures determined by the help of DSC are collected in Table 2. For (4OBA)2 –4,49-BPy liquid crystal, a new, smectic mesophase appeared that is not observed for either pure components. In the complex a mesogen is formed through intermolecular hydrogen bonding, where the Table 2 The phase transition temperature of the 4OBA–4,49-BPy mixtures at various compositions determined from DSC thermograms Phase transition temperatures
Fig. 6. Temperature dependence of the relative permittivities of the stoichiometric composition complexes for 4OBA (on the left) and 5OBA (on the rigth) with 4,49-BPy. The presentation is as for Fig. 2.
1.25:1 1.5:1 2:1
N 155 8C I SA 151 8C N 156 8C I SA 152 8C N 159 8C I
M. Valisko´ et al. / Journal of Molecular Liquids 109 (2004) 39–43
4,49-BPy functions as a core unit in the liquid crystal. The effect of the smectic phase appeared in the permittivity parallel to the magnetic field. For the mixture of 1.5:1 4OBA–4,49-BPy we also found both smectic and nematic phases, but the change in the permittivity at the nematic–smectic phase transition is not so significant. In the case of 1.25:1 composition we did not detect a new mesophase. The DSC records confirmed our observation. The phase transition temperature decreased with the mole fraction of the 4OBA. We measured the temperature dependence of the dielectric constant of the pure 4,49-BPy in the temperature range of 115–170 8C. The value of the relative permittivity is much lower than in the case of the pure nOBA or the H-bonded mixtures. The data can be correlated by the following linear equation in the studied temperature range: ´(T)sy2.93=10y3 Tq2.82, the corresponding correlation factor is 0.98. Accordingly, we can draw the conclusion that the increase in relative permittivities of the nOBA–4,49-BPy mixtures is due to formed chemical bonds. The formed intermolecular H-bonding via association makes the liquid crystalline character stronger. Fig. 6 shows the temperature dependence of the permittivities of the 2:1 stoichiometric composition complex of 5OBA–4,49-BPy. We did not observe new mesophase as it can be seen in Fig. 6, where the (4OBA)2 –4,49-BPy is also delineated for comparison. Due to the formed intermolecular hydrogen bonding, the measured permittivities are higher, the dielectric anisotropy is negative and its absolute value is larger than in the case of the pure nOBA. For the influence of the 4,49-BPy and hereby the formed H-bond, the sign of the dielectric anisotropy changes. In order to confirm our conclusions, we measured the 4:1 composition of 4OBA–4,49-BPy, and according to our expectations, the determined relative permittivity in the isotropic phase is between the values for the pure 4OBA and for the stoichiometric composition as seen in Fig. 7. Infrared study of Kato et al. w14x suggests that the hydrogen bond is of non-ionic type with a double minimum potential energy. The stability of the hydrogen-bonded complex suddenly decreases when the complex becomes an isotropic disordered phase. The stability of the hydrogen bond is not simply a function of temperature, it is greatly dependent on the state of molecular ordering. We have experimentally determined relative permittivities of H-bonded liquid crystal systems whose dielectric properties, to our best knowledge, were not investigated so far. By the help of dielectric measurement, we proved that the complex formed via association makes the liquid
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Fig. 7. The relative permittivity as a function of the mole fraction of 4OBA for 4OBA–4,49-BPy mixtures in isotropic phase at 165 8C.
crystal behaviour stronger, which was rendered probable only by the DSC data up to present. Acknowledgments This research was supported by the Hungarian National Research Fund (OTKA-T029327). References w1x G.W. Gray, J.W. Goodby, Smectic Liquid Crystals, Leonard Hill, Glasgow, 1984. w2x R. Miethchen, J. Holz, H. Prade, A. Liptak, Tetrahedron 48 (1992) 3061. w3x T. Kato, P.G. Wilson, A. Fujishima, J.M.J. Frechet, Chem. Lett. 11 (1990) 2003. w4x T. Kato, A. Fujishima, J.M.J. Frechet, Chem. Lett. 6 (1990) 919. w5x T. Kato, H. Adachi, A. Fujishima, J.M.J. Frechet, Chem. Lett. 2 (1992) 265. w6x T. Kato, J.M.J. Frechet, J. Am. Chem. Soc. 111 (1989) 8533. w7x U. Kumar, T. Kato, J.M.J. Frechet, J. Am. Chem. Soc. 114 (1992) 6630. w8x Z. Sideratou, D. Tsiourvas, C.M. Paleos, A. Skoulios, Liquid Cryst. 22 (1997) 51. w9x H. Kresse, Fortschritte der Physik 30 (1982) 507. w10x K. Araki, T. Kato, U. Kumar, J.M.J. Frechet, Macromol. Rapid Commun. 16 (1995) 733. w11x H. Keller, R. Hatz, Handbook of Liquid Crystals, Verlag Chemie, Weinheim, Deerfield, 1980. w12x L. Bata, Advances in Liquid Crystal Research and Applications, Pergamon Press, Oxford, 1980. w13x D. Demus, H. Demus, H. Zaschke, Flussige ¨ Kristalle in ¨ Grundstoffindustrie, Leipzig, Tabellen, Deutcher Veralg fur 1974. w14x T. Kato, J.M.J. Frechet, P.G. Wilson, et al., Chem. Mater. 5 (1993) 1094. w15x G.W. Gray, B.J. Jones, J. Chem. Soc. 4 (1953) 4179.