Dipolar Aggregation and the Static Dielectric Permittivity of Some

Article pubs.acs.org/IECR

Dipolar Aggregation and the Static Dielectric Permittivity of Some Liquid Crystalline Materials ,† ́ Jan Jadzẏ n,† Jolanta Swiergiel,* Iwona Płowaś,† Roman Dąbrowski,‡ and Urszula Sokołowska† †

Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznań, Poland Military University of Technology, Institute of Chemistry, S. Kaliskiego 2, 00-908 Warsaw, Poland

‡

ABSTRACT: In the paper, we show how some modifications in the chemical structure of mesogenic molecules can lead to an essential increase of the dielectric permittivity of liquid crystalline material due to the reduced molecular ability to the antiparallel dipolar aggregation. As an example, the static permittivities of two nematogenic homologous series, CnH2n+1PhPhCN and CnH2n+1PhCOOPhCN, are analyzed. However, the structural modifications of mesogenic molecules lead often to unfavorable changes in some important physical properties of liquid crystalline material, as a shift in the clearing temperature, for example, or can lead even to a disappearance of the mesophase. This paper presents such an event observed for the compound composed of C5H11PhPh(CH3)CN molecules, the methyl derivatives of C5H11PhPhCN molecules, which form a well-known nematic liquid crystal. structural element, an ester group, −COO−, placed between two phenyl rings, quite markedly differentiates the shape of the molecules of these two series and as a consequence differentiates their abilities to the dipolar aggregation. However, it happens quite often that the structural change indeed reduces that aggregation ability but simultaneously leads to a disappearance of the mesogeneity of the molecules. The phenomenon is illustrated in this paper by comparison of the dielectric behavior of nematogenic 4-cyano-4′-pentylbiphenyl, C5H11PhPhCN, and its simple methyl ortho-derivative, 4-cyano3-methyl-4′-pentylbiphenyl, C5H11PhPh(CH3)CN.

1. INTRODUCTION A rapid progress is observed in development of technologies used in construction of many kinds of modern displays, but up to now, the predominant technology is related to nematic liquid crystals. Liquid crystalline displays (LCDs) are still finding wide commercial uses because of their relatively low power consumption, design flexibility and flatness, view ability in a bright light, and finally, low costs. The development of LCD technology depends mainly on the design and production of novel liquid crystalline materials of suitable physical properties. The dielectric anisotropy, that is, the difference between the permittivities measured along the direction of the macroscopic molecular orientation, director n, and perpendicular to it, Δε = ε// − ε⊥, is of primary importance, because that quantity determines the electro-optical parameters of LCDs, such as the threshold voltage, the response times, and the multiplexing capability. In practice, an increase of Δε results mainly from an increase of ε//, which can be achieved by synthesis of the mesogenic molecules of high dipole moment directed to the long molecular axis.1 However, very often, an effort in that matter can be (fortunately partially) destroyed by the dipolar antiparallel molecular aggregation leading to an essential decrease of ε// in comparison to the permittivity expected for an assembly of nonaggregated dipolar molecules. The aggregation effect is especially significant for the linear molecules presenting rotational symmetry, but the effect is very sensible to the geometrical structure of the molecules. In a recent paper,2 the problem was illustrated by the difference between the dielectric properties of two liquids, acetonitrile and dimethyl sulfoxide, both composed of highly polar molecules, but due to their quite different symmetries, the degree of dipolar aggregation in these two liquids are extremely different. As a consequence, the static permittivity of dimethyl sulfoxide is higher by about 25% in comparison to that of acetonitrile. This paper presents the static dielectric properties investigated for two homologous series of the liquid crystalline compounds: CnH2n+1PhPhCN and CnH2n+1PhCOOPhCN. The © 2013 American Chemical Society

2. EXPERIMENTAL PROCEDURE The subject of our studies are two nematogenic homologous series: 4-cyano-4′-alkylbiphenyls, CnH2n+1PhPhCN, abbreviation nCB, n = 5−9, and 4-cyanophenyl-4′-alkylbenzoates, CnH2n+1PhCOOPhCN, nCPAB, n = 4−10, as well as a nonmesogenic methyl-ortho-substituted 5CB, 4-cyano-3-methyl-4′-pentylbiphenyl, C5H11PhPh(CH3)CN, 5MCB. All the compounds were synthesized and purified at the Institute of Chemistry, Military University of Technology, Warsaw. The mass fraction purity of the compounds, checked by chromatography, was better than 0.995. The temperatures of the isotropic to nematic phase transition (TNI) of the nCB3,4 and nCPAB5−7 series are depicted in Figure 1. As can be seen in the figure, an introduction of the −COO− group between the phenyl rings in cyanobiphenyl molecules leads to an essential increase of the isotropic (Iso) to nematic (Nem) phase transition temperature. The TNI recorded for nCPABs exceeds that of nCBs by about 20 K, for n = 5, and next, the difference gradually decreases up to about 10 K, for n = 9. Received: Revised: Accepted: Published: 4109

January 7, 2013 February 25, 2013 February 27, 2013 February 27, 2013 dx.doi.org/10.1021/ie400071x | Ind. Eng. Chem. Res. 2013, 52, 4109−4112

Industrial & Engineering Chemistry Research

Article

Figure 1. Phase diagrams of nematogenic series of compounds: 4cyano-4′-alkylbiphenyls, nCB, n = 5−9, and 4-cyanophenyl-4′alkylbenzoates, nCPAB, n = 4−10. TNI denotes the temperature of the phase transition from the Iso liquid to the Nem phase.

The complex permittivity spectra were recorded with the use of an HP 4194A impedance/gain phase analyzer in the frequency range from 50 kHz to 100 MHz. The measuring homemade plane capacitor was used. The homogeneous orientation of the molecules in the nematic phase was obtained by means of a dc biasing voltage (5 V) applied to electrodes of the measuring capacitor, so the component of the permittivity, parallel to the director n, ε∥*(ω), was recorded. The temperature of the measuring cell was controlled with a Scientific Instruments temperature controller, model 9700. Details on the experimental setup were presented elsewhere.8

Figure 2. Real part of the complex permittivity spectrum of 7CB (a) and 7CPAB (b) recorded in the Iso and Nem phases of the compounds. In the Nem phase, the permittivity was measured along the macroscopic orientation of the long molecular axes (the director n∥E).

3. RESULTS AND DISCUSSION Figure 2 presents, as an example, the real part of the dielectric relaxation spectra, recorded in the Iso and Nem phases of two representatives of the studied homologous series: 7CB (a) and 7CPAB (b). The real and imaginary parts of the spectra, recorded for these two compounds in the Nem phase at the temperature T = TNI − 4 K, are presented in Figure 3. The spectra of the remaining compounds, both from the nCB and nCPAB series, are similar. As can be seen in the figure, the lowfrequency plateau of the real part of the spectrum presents the static permittivity (εs) of the compound under investigation. As it was mentioned above, in the Nem phase of the compounds, the permittivity is measured along the macroscopic orientation of the long molecular axes (ε∥s ), forced by dc biasing electric field. Figure 4 depicts the temperature dependences of the static permittivities of 7CB and 7CPAB, resulting from the data presented in Figure 2a and b, respectively. An increase of the permittivity value of 7CPAB in comparison to that of 7CB is remarkable, both in the Iso and Nem phases. The static permittivity in the Iso phase of 7CPAB is practically as high as the permittivity ε∥s , measured in the oriented Nem sample of 7CB. The value of the permittivity ε∥s , measured in the Nem phase of 7CPAB, is almost twice as large as that of 7CB. The observation concerns all members of the studied nCB and nCPAB homologous series. Figure 5 presents the static permittivity measured in the Iso and Nem phases of some compounds for which the phase diagram (Figure 1) allows one to compare the dielectric data at the same (or near-by) temperatures. An essential increase of the permittivity of nCPAB compounds in comparison to nCB ones can results from two

Figure 3. Real and imaginary parts of the dielectric spectra recorded for 7CB and 7CPAB in their Nem phase, at the temperature T = TNI − 4 K.

main reasons: (i) from an increase of the dipole moment of a single CnH2n+1PhCOOPhCN molecule due to an insertion of the polar ester group and/or (ii) from a decrease of the ability of the nCPAB molecules to the antiparallel dipolar aggregation, resulting also from an existence of the ester group as a structural element of these molecules. A comprehensive paper by Saltz et al.9 allows one to estimate an increase of the dipole moment due to the presence of the −COO− group in nCPAB molecules to about 5.8 D (±0.2 D), in comparison to 4.8 D (±0.2 D) for nCB10,11 molecules. The contribution of the increased dipole moment of nCPAB molecules in an increase of the static permittivity is somewhat compensated by simulta4110

dx.doi.org/10.1021/ie400071x | Ind. Eng. Chem. Res. 2013, 52, 4109−4112

Industrial & Engineering Chemistry Research

Article

indicate, the permittivities of the isotropic liquids within the corresponding series only weakly depend on n. For the following values of the parameters of eq 1, the density of the two compounds d ≈ 1 g/cm3, ε∞ ≈ 2.7, T = 333 K, the dipole moment of a single 7CB molecule equals to μ1 = 4.8 D, and for a 7CPAB molecule, μ1 = 5.8 D, the static permittivity εs = 10.22 (for 7CB) and 15.44 (for 7CPAB), one obtains for the gk factor the following values: 0.51 for 7CB and 0.70 for 7CPAB. The conclusion resulting from that simple estimation is the following: an increase of molecular dipole moment, what formally should lead to an increase of the dipole−dipole interaction and as a consequence to increase of the degree of antiparallel dipolar aggregation, reveals here a quite opposite effect than expected. An insertion of the polar −COO− group into the cyanobiphenyl molecule indeed causes an increase of the molecular dipole moment, but at the same time, that substitution leads to distortion of the simple, closed to the linear shape of the molecule, mainly due to the lateral carbonyl CO group. That circumstance leads to a decrease of the antiparallel aggregation ability of the more polar nCPAB molecules as compared to the less polar nCB molecules. However, the value of 0.70 for the gk factor of 7CPAB shows that the decrease of the aggregation ability is here rather moderate and one still detects a quite important antiparallel dipolar association of nCPAB molecules. On the other side, the data presented in this paper prove that even a moderate increasing in the gk factor can reflects itself in a significant increasing of the static permittivity of liquid crystalline material. The effect can be technologically important. Of course, in the field of molecular engineering discussed here, the subtle balance between the structure and polarity of the molecules can be changed but in the frame where the mesogenic property of the molecules is maintained. An example presented below proves that the prediction in that matter is really not easy. Figure 6 presents an example, where a seemingly small change in the structure of the nematogenic molecule leads to

Figure 4. Comparison of the temperature dependences of the static permittivities of 7CB and 7CPAB measured in the Iso and Nem phases of the compounds.

Figure 5. Comparison of the static permittivities measured in the Iso and Nem phases of some compounds from the nCB and nCPAB homologous series for different numbers n of the carbon atoms in the hydrocarbon tails of the molecules.

neous reduction (of about 15%) of the dipoles density in liquid nCPABs: the −COO− group gives an additional 44 cm3/mol to the molar volume of nCBs (VM ≈ 280 cm3/mol of 7CB, for example). However, the most important and interesting is an analysis of the dipolar coupling in the studied compounds in the isotropic phase, which can be quite easily detected by determination of the Kirkwood correlation factor gk:12 (εs − ε∞)(2εs + ε∞) 9kT VM = μ12 g k 4πNA εs(ε∞ + 2)2

(1)

where μ1 is the dipole moment of a single molecule, k is Boltzmann’s constant, NA is Avogadro’s number, VM is the molar volume, T is the absolute temperature, and ε∞ is the permittivity measured in so high frequencies to prevent the dipolar reorientation. The factor gk was introduced by Kirkwood13 for expressing the intermolecular dipolar coupling in liquids, namely, the fractional values of the factor, gk < 1, corresponds to the antiparallel dipolar self-association leading to the reduction of the apparent dipole moment of molecule, gk > 1 corresponds to parallel association leading to an increase of the moment, and gk = 1 means a lack of the dipolar coupling in liquid under investigation. An estimation of the gk factor was performed for the Iso phase of 7CB and 7CPAB, as the representatives of the two homologous series studied. As the results from Figure 5

Figure 6. Temperature dependence of the static permittivity of mesogenic 5CB and its ortho-methyl derivative, nonmesogenic 5MCB.

the lost of its nematogeneity. The lateral substitution of the methyl group in the ortho-position in respect to the CN group in the phenyl ring of 5CB reduces the ability to dipolar aggregation of 5MCB molecules so far as one observes no longer the transition from the Iso liquid to the Nem phase. As can be seen in Figure 6, an introduction of a weakly polar (0.4 D) not too large methyl group to the structure of the 5CB molecule modifies not only the value of the static permittivity, 4111

dx.doi.org/10.1021/ie400071x | Ind. Eng. Chem. Res. 2013, 52, 4109−4112

Industrial & Engineering Chemistry Research

Article

but first of all, it changes the temperature dependence of the permittivity of isotropic liquids11,14 from a typical for nematogenic compounds (5CB) to a typical for nonnematogenic ones (5MCB).

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +48 61 86 95 162. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work was partially supported by the Polish Ministry of Sciences and Higher Education, Key Project POIG.01.03.01-14016/08 “New Photonic Materials and their Advanced Applications”.

■

REFERENCES

(1) De Gennes, P. G. Physics of Liquid Crystals; Oxford University Press: Oxford, U.K., 1974. ́ (2) Jadżyn, J.; Swiergiel, J. On intermolecular dipolar coupling in two strongly polar liquids: dimethyl sulfoxide and acetonitrile. J. Phys. Chem. B 2011, 115, 6623−6628. (3) Dąbrowski, R.; Witkiewicz, Z.; Kenig, K. New simple method for obtaining 4-n-alkyl-4′-cyanobiphenyls. Mol. Cryst. Liq. Cryst. 1980, 58, 251−257. (4) Jadżyn, J.; Dąbrowski, R.; Lech, T.; Czechowski, G. Viscosity of the homologous series of n-alkylcyanobiphenyls. J. Chem. Eng. Data 2001, 46, 110−112. (5) Czupryński, K. Destabilization of the Orthogonal Smectic Phases. Thesis, Military University of Technology, Warsaw, Poland, 1995. (6) Jadżyn, J.; Sokołowska, U.; Déjardin, J.-L. Temperature behavior of the electric field-induced entropy increment within a homologous series of nematogenic compounds. J. Phys. Chem. B 2008, 112, 9050− 9052. (7) Chrzumnicka, E.; Kobielski, A.; Szybowicz, M.; Bauman, D. Molecular orientation and odd-even effect in homologous series of 4cyanophenyl-4′-alkylbenzoate. Mol. Cryst. Liq. Cryst. 2011, 541, 118− 131. (8) Świergiel, J.; Jadżyn, J. Static dielectric permittivity of homologous series of liquid cyclic ethers, 3n-crown-n, n = 4 to 6. J. Chem. Eng. Data 2012, 57, 2271−2274. (9) Salz, E.; Hummet, J. P.; Flory, P. J.; Plavšić, M. Direction of the dipole moment in the ester group. J. Phys. Chem. 1981, 85, 3211− 3215. (10) Kędziora, P.; Jadżyn, J. Dipole-dipole association of mesogenic molecules in solution. Liq. Cryst. 1990, 8, 445−450. (11) Van Roie, B.; Leys, J.; Denolf, K.; Glorieux, C.; Pitsi, G.; Thoen, J. Weakly first-order character of the nematic-isotropic phase transition in liquid crystals. Phys. Rev. E 2005, 72, 041702. (12) Böttcher, C. J. F.; Bordewijk, P. Theory of Electric Polarization: Dielectric in Time-Dependent Fields; Elsevier: Amsterdam, The Netherlands, 1992; Vol. II. (13) Kirkwood, J. G. The dielectric polarization of polar liquids. J. Chem. Phys. 1939, 7, 911−919. (14) Jadżyn, J.; Czechowski, G.; Legrand, C.; Douali, R. Static and dynamic dielectric properties of strongly polar liquids in the vicinity of first order and weakly first order phase transitions. Phys. Rev. E 2003, 67, 041705.

4112

dx.doi.org/10.1021/ie400071x | Ind. Eng. Chem. Res. 2013, 52, 4109−4112