Magnetic Alignment Study of Rare-Earth-Containing Liquid Crystals

J. Phys. Chem. B 2007, 111, 13881-13885

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ARTICLES Magnetic Alignment Study of Rare-Earth-Containing Liquid Crystals Yury G. Galyametdinov,*,†,‡ Wolfgang Haase,§ Bart Goderis,# Dries Moors,# Kris Driesen,# Rik Van Deun,# and Koen Binnemans*,# Physical and Colloid Chemistry Department, Kazan State Technological UniVersity, Karl Marx Street 68, 420015 Kazan, Russia, Kazan Physical-Technical Institute, Russian Academy of Sciences, Sibirsky Tract 10/7, 420029 Kazan, Russia, Institute of Physical Chemistry, Darmstadt UniVersity of Technology, Petersenstrasse 20, D-64287 Darmstadt, Germany, and Department of Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200F - bus 2404, B-3001 LeuVen, Belgium ReceiVed: September 6, 2007; In Final Form: October 4, 2007

The liquid-crystalline rare-earth complexes of the type [Ln(LH)3(DOS)3]swhere Ln is Tb, Dy, Ho, Er, Tm, or Yb; LH is the Schiff base N-octadecyl-4-tetradecyloxysalicylaldimine; and DOS is dodecylsulfatesexhibit a smectic A phase. Because of the presence of rare-earth ions with a large magnetic anisotropy, the smectic A phase of these liquid crystals can be easier aligned in an external magnetic field than smectic A phases of conventional liquid crystals. The magnetic anisotropy of the [Ln(LH)3(DOS)3] complexes was determined by measurement of the temperature-dependence of the magnetic susceptibility using a Faraday balance. The highest value for the magnetic anisotropy was found for the dysprosium(III) complex. The magnetic alignment of these liquid crystals was studied by time-resolved synchrotron small-angle X-ray scattering experiments. Depending on the sign of the magnetic anisotropy, the director of the liquid-crystalline molecules was aligned parallel or perpendicular to the magnetic field lines. A positive value of the magnetic anisotropy (and parallel alignment) was found for the thulium(III) and the ytterbium(III) complexes, whereas a negative value of the magnetic anisotropy (and perpendicular alignment) was observed for the terbium(III) and dysprosium(III) complexes.

Introduction One of the most interesting properties of liquid crystals is the possibility to align these materials by external electric and magnetic fields.1 The alignment of liquid crystals in electric fields is well known and well studied, because switching of the liquid-crystal molecules by an electric field is the operational principle of liquid-crystal displays (LCDs).2,3 However, the alignment of liquid crystals by a magnetic field is much less investigated, although this phenomenon has also several scientific applications. For instance, mesophases can be aligned in a magnetic field to study the molecular symmetry and order of the mesophases by X-ray diffraction. NMR studies of organic molecules in liquid crystal solvents also take advantage of the alignment of the liquid crystal solvent by the magnetic field of the NMR spectrometer.4,5 Magnetic alignment has been used to determine the order parameter of nematics.6,7 An advantage of alignment of a liquid crystal by a magnetic field is that larger samples can be aligned than by the use of an electric field. Other advantages of magnetic alignment are that no redox processes can be induced in the metal-containing samples and that magnetic alignment also allows orientations other than those * Corresponding authors. E-mail (Y.G.G.): [email protected]. E-mail (K.B.): [email protected]. † Kazan State Technological University. ‡ Kazan Physical-Technical Institute. § Darmstadt University of Technology. # Katholieke Universiteit Leuven.

parallel or perpendicular to the liquid-crystal cell. The threshold magnetic field for mesophase alignment depends on the viscosity of the sample and on the magnetic anisotropy of the liquidcrystal molecules. Because of the lower viscosity of the nematic phase in comparison with the smectic phases, a nematic phase is much easier to align in a magnetic field than a smectic phase. Therefore, most studies on the alignment of mesophases in a magnetic field have been performed on nematogenic liquid crystals.8-13 The study of the alignment of the smectic A mesophase is attracting more and more interest.14-17 There are only a few reports on the magnetic-field-induced alignment of columnar mesophases.18-20 Smectic phases of paramagnetic liquid crystals should be easier to align than smectic phases of conventional liquid crystals. Paramagnetic liquid crystals can be obtained by incorporation of a stable radical21-23 or by incorporation of paramagnetic metal centers in the liquid-crystal molecules. Examples of paramagnetic metal-containing liquid crystals that have been used for magnetic studies are mesogenic complexes of copper(II),24,25 vanadyl,25,26 or the trivalent rare-earth ions.27-30 In fact, the possibility to create magnetic liquid crystals by incorporation of a transition metal center in a liquid crystal is one of the driving forces for metallomesogens research.27,31-35 However, paramagnetism as such is not a sufficient condition for easy alignment of the mesophase of these liquid crystals. The crucial property is the magnetic anisotropy. The magnetic anisotropy ∆χ is the difference between the magnetic suscep-

10.1021/jp0771724 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2007

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Galyametdinov et al. TABLE 1: Mesophase Behavior of the [Ln(LH)3(DOS)3] Complexes49

Figure 1. Structure of the Schiff base ligand LH in the [Ln(LH)3(DOS)3] complexes (Ln ) Tb, Dy, Ho, Er, Tm, Yb; DOS ) dodecylsulfate).

tibility parallel (χ|) and perpendicular (χ⊥) to the director n: ∆χ ) χ| - χ⊥. The director is the preferred direction of the molecular long axes of the mesogenic molecules. Therefore, paramagnetic gadolinium(III)-containing liquid crystals show no advantage over diamagnetic lanthanum(III)-containing liquid crystals for magnetic alignment studies because of the zero magnetic anisotropy of the gadolinium(III) ion. However, the complexes of the heavy lanthanide ions exhibit often a large magnetic anisotropy, and they are of special interest for magnetic alignment studies. The magnetic anisotropy of rare-earthcontaining liquid crystals with Schiff base ligands has been the topic of several detailed experimental and theoretical studies.36-41 These studies indicate that especially terbium(III), dysprosium(III), and thulium(III) complexes are the most promising candidates for smectogenic liquid crystals that can easily be aligned in an external magnetic field. In this paper, we report on the magnetic anisotropy of the Schiff base complexes [Ln(LH)3(DOS)3], where Ln is Tb, Dy, Ho, Er, Tm, or Yb, LH is the ligand N-octadecyl-4-tetradecyloxysalicylaldimine (Figure 1), and DOS is dodecylsulfate. The magnetic alignment of the terbium(III)-, dysprosium(III)-, thulium(III)-, and ytterbium(III)-containing liquid crystals was studied by time-resolved synchrotron small-angle X-ray scattering (SAXS). Special attention was paid to the orientation of the director of the liquid-crystal phases with respect to the magnetic field lines and to the degree of alignment of the mesophases. Experimental Section Magnetic Susceptibility Measurements. The temperature dependence of the magnetic susceptibility of the samples was measured by a home-built Faraday balance.42 With the aid of a microbalance (Cahn RG-2000 electrobalance) the force was determined with which the sample is attracted or expelled by the inhomogeneous magnetic field. The sample was placed in a bowl-like quartz sample holder, which was suspended on the balance with a long thin quartz thread. The sample was located between the pole shoes (Faraday-type shoes) of an electromagnet (Bruker B-E-20) which created a magnetic field with a strength of ca. 0.9 T at a pole shoe distance of 6 cm. The pole shoes have such a shape that the magnetic field gradient multiplied by the field strength is constant over a given region between the pole shoes. During the measurement, the sample was heated in a cylindrical furnace and the temperature was measured at the sample position by a Chromel-alumel thermocouple. Both the balance chamber and the furnace were evacuated to a pressure of 100 mbar. Typically, a sample size between 20 and 40 mg was used. From the measured mass change for the sample with and without applied magnetic field, the molar susceptibility was determined using eq 1:

χexp )

∆mgM - χdia dH mH dz

( )

(1)

where χexp is the experimental molar susceptibility (in cm3 mol-1), ∆m is the change in mass of the sample (in g), m is the mass of the sample (in g), M is the molar mass of the compound

lanthanide Tb Dy Ho Er Tm Yb

transition temperatures (°C)a Cr ‚ 61 ‚ SmA ‚ 80 ‚ I Cr ‚ 61 ‚ SmA ‚ 82 ‚ I Cr ‚ 62 ‚ SmA ‚ 84 ‚ I Cr ‚ 61 ‚ SmA ‚ 85 ‚ I Cr ‚ 60 ‚ SmA ‚ 86 ‚ I Cr ‚ 60 ‚ SmA ‚ 87 ‚ I

a Cr ) crystalline phase, SmA ) smectic A phase, I ) isotropic liquid.

(in g mol-1), and g is the gravity constant (981 cm s-2). H is the strength of the magnetic field (0.9 T), and dH/dz is the field gradient. χdia is the correction for the underlying diamagnetism. This correction factor was calculated applying Pascal’s scheme.43 A constant χdia value of 1917 × 10-6 cm3 mol-1 was used for all samples.44 The field gradient dH/dz was determined by using the calibration standard Hg[Co(SCN)4].45 X-ray Diffraction Measurements. Synchrotron small-angle X-ray scattering (SAXS) measurements at elevated temperatures were made on the DUBBLE SAXS-WAXS beam line (BM 26B) at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France).46,47 For all the measurements, an X-ray wavelength of 0.83 Å was used. The samples were filled as powders in quartz capillaries. The capillaries were placed in a special sample holder, which allowed application of a magnetic field. The incident X-ray beam was perpendicular to the applied magnetic field and perpendicular to the long axis of the capillary. The magnetic field was generated by two permanent magnets. The magnetic field was adjustable between 0.3 and 0.8 T by changing the gap between the permanent magnets. Measurements were performed at a magnetic field strength of 0.6 T. The temperature of the sample could be controlled between room temperature and 420 K. The scattered X-rays were detected by a 2D multiwire gas-filled SAXS detector and corrected for the detector response. In the 2D scattering patterns the logarithms of the intensities are depicted using a color scale. As a reference, the scattering pattern of silver behenate at room temperature was measured as illustrated in Figure S2 (Supporting Information).48 The (001) reflections of the smectic layers in this work occur at approximately the second-order reflection of silver behenate (at a spacing of about 30 Å). The magnetic field was oriented horizontally with respect to the scattering patterns. Results and Discussion The synthesis and the thermal behavior of the [Ln(LH)3(DOS)3] complexes were previously reported.49 All complexes exhibit a smectic A phase (Table 1). Although complexes of all the elements of the lanthanide series (except cerium and promethium) were prepared, only complexes of the heavy lanthanides (terbium, dysprosium, holmium, erbium, thulium, and ytterbium) were selected for the magnetic studies. Lanthanum(III) and lutetium(III) are diamagnetic, gadolinium(III) is magnetically isotropic, and earlier experiments have shown that the light lanthanide(III) ions give complexes with only small values for the magnetic anisotropy.37 Two different types of measurements were performed to study the magnetic alignment of the rare-earth-containing liquid crystals. The sign of the magnetic anisotropy was determined from SAXS measurements on oriented samples. The absolute value of the magnetic anisotropy was determined by temperature-dependent measurements of the magnetic susceptibility (or magnetic moment). The isotropic magnetic susceptibility (χiso)

Magnetic Alignment Study of [Ln(LH)3(DOS)3] Complexes

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TABLE 2: Magnetic Properties of the [Ln(LH)3(DOS)3] Complexesa lanthanide Tb Dy Ho Er Tm Yb

χiso

χor

χor - χiso

∆χexp

38470 48790 41820 35820 24240 7790

39650 44850 43480 35185 22550 8200

1180 3940 1160 635 1690 410

-3540 -11820 -4980 +952 +2535 +615

a All the magnetic susceptibility values χ are expressed in 10-6 cm3 mol-1. The calculation of the experimental magnetic anisotropy ∆χ exp from (χor - χiso) is described in the text. The ∆χexp value was determined at 40 °C.

was obtained from measurements at 40 °C on samples in the (magnetically isotropic) polycrystalline state and was compared with the magnetic susceptibility measured at 40 °C in the oriented state (χor), obtained by cooling the samples from the high-temperature isotropic state in the presence of the magnetic field down to 40 °C. χor is equal to (or less than, if alignment is not complete) the greater of the two components χ⊥ or χ|, and χor corresponds to the maximum of the χ-tensor. The actual value of the experimental magnetic anisotropy ∆χexp depends on the relative values of χ⊥ or χ| (i.e., on the sign of ∆χexp):

3 ∆χexp ) (χor - χiso) 2 ∆χexp ) -3(χor - χiso)

χ| > χ⊥ (∆χ > 0)

(2)

if χ| < χ⊥ (∆χ < 0)

(3)

if

Based on the SAXS measurements discussed below, a positive sign was found for the thulium(III) and the ytterbium(III) complex, whereas a negative sign was found for the terbium(III) and dysprosium(III) complex. Although not measured, the signs of the erbium(III) and holmium(III) complexes were considered to be positive and negative, respectively, since the sign is not expected to change within one isostructural series.38,50,51 The results of the magnetic susceptibility measurements are summarized in Table 2. The absolute values of the magnetic anisotropy are in decreasing order dysprosium(III) > holmium(III) > terbium(III) > thulium(III), whereas much smaller values are found for the erbium(III) and ytterbium(III) complexes. The fact that the experimental magnetic anisotropy values are smaller than the values predicted by theoretical models for this type of Schiff base complexes can be explained by the high viscosity of these liquid crystals preventing complete alignment of the director parallel or perpendicular to the magnetic field.38,50,51 In fact, the reported magnetic susceptibilities are temperature dependent, i.e., they decrease with increasing temperature according to the Curie-Weiss law. In calculating the effective magnetic moment, this bare temperature dependence is removed:

µeff )

x

3k ‚ xχT ) 2.828 xχT NA

(4)

where µeff is the effective magnetic moment (in Bohr magneton, µB), k is the Boltzmann constant, NA is Avogadro’s number, χ is the magnetic susceptibility, and T is the absolute temperature. With this effective magnetic moment one can evaluate changes to the magnetic susceptibility other than those induced by temperature, e.g., as a result of molecular alignment. The temperature dependence of the effective magnetic moment of [Dy(LH)3(DOS)3] during heating and subsequent cooling is shown in Figure 2. The temperature-independent, constant value during heating the initial, polycrystalline sample points to a pure

Figure 2. Effective magnetic moment µeff (in Bohr magneton units) of the liquid-crystalline complex [Tb(LH)3(DOS)3] as a function of the temperature.

Figure 3. SAXS patterns of [Dy(LH)3(DOS)3] in an magnetic field. At 84 °C, the molecules are still in the isotropic phase. At 80 °C, the first signs of alignment in the smectic A phase are observed, and the alignment becomes gradually better upon slow cooling. Only the firstorder reflection is visible in the X-ray diffractograms. The molecules are aligned perpendicular to the magnetic field lines. At 57 °C, the sample starts to crystallize and the alignment is partially lost.

Curie-Weiss behavior for the solid state, the following (unoriented ∼ magnetically isotropic) smectic A phase and the isotropic phase. On the other hand, when the sample is cooled down from the isotropic phase to the mesophase (in the presence of a magnetic field) a sharp increase in the magnetic moment µeff occurs in the vicinity of the clearing point. Upon further cooling, the magnetic properties varied according to the CurieWeiss law, but with a different higher µeff than in the heating run. This behavior can be considered as a magnetic-field-induced orientation in the liquid-crystalline phase of magnetically anisotropic molecules with their axis of maximum magnetic susceptibility parallel to the magnetic field. The other complexes exhibited a similar behavior. Alignment of the smectic A phase was not observed during heating of the sample because of the high viscosity of the mesophase at temperatures close to the melting point.

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Galyametdinov et al.

Figure 4. Alignment of mesophases of [Ln(LH)3(DOS)3] (Ln ) Tb, Dy, Tm, Yb) in a magnetic field, studied as a function of the temperature by SAXS measurements. The terbium(III) and dysprosium(III) samples are aligned perpendicular to the magnetic field lines, whereas the thulium(III) and ytterbium(III) samples are aligned parallel to the magnetic field lines.

To determine the sign of the magnetic anisotropy and to study the kinetics of the magnetic alignment, the samples were investigated by synchrotron SAXS measurements in an external magnetic field. The samples were filled into quartz capillaries and placed in the sample holder in the gap between two permanent magnets. The magnetic field strength was 0.6 T. The sample holder was first heated to a temperature of about 15 K above the clearing point and equilibrated for 5 min, to make sure that all smectic domains in the liquid were destroyed. The samples were then cooled down at a controlled rate of 1 K min-1. The incident X-ray beam was perpendicular to the applied magnetic field and perpendicular to the long axis of the capillary. For all samples, the first signs of alignment were observed at a temperature which corresponded quite well to the clearing points determined by polarizing optical microscopy. From the experiments one could determine that the thulium(III) and ytterbium(III) samples align parallel to the magnetic field lines (and thus have a positive value for the magnetic anisotropy ∆χ), whereas the terbium(III) and dysprosium(III) complexes align perpendicular to the magnetic field lines (and thus have a negative value for the magnetic anisotropy ∆χ). The typical diffraction patterns of aligned mesophases are shown in Figures 3 and 4. The X-ray diffractrograms show that the alignment in the mesophase is not perfect. In case of a perfect alignment, one would expect to observe fine diffraction spots for the first-order diffraction peaks of the smectic layers rather than the actually observed arcs. The alignment becomes better after a period of time (observation times extended up to 1 h), but complete alignment was never obtained. However, the compounds with a large magnetic anisotropy (the terbium and dysprosium samples) show qualitatively a better alignment than the compounds with a smaller value of the magnetic anisotropy. The difficulty to obtain well-aligned samples can be attributed to two causes: the relatively high viscosity of the mesophase and the fact that the magnetic anisotropy is provided by the rare-earth center rather than by a rigid core (as is the case for organic liquid crystals). The higher the viscosity, the more difficult alignment in the magnetic field will be. Therefore viscous smectic mesophases show a higher resistance to

alignment in the magnetic field than a less viscous nematic phase. Because the magnetic anisotropy is provided by the rareearth ion rather than by the whole molecule, the torque by the magnetic field acts essentially on the coordination unit rather than on the total molecular unit. This causes only a partial alignment of the mesogenic molecules. As a further consequence of the rare-earth-centered magnetic anisotropy, the long molecular axis does not necessarily coincide with the main axes of the magnetic susceptibility tensor. This was also evident from some of our experiments in which we observed a small angle (5-10°) between the X-ray diffraction maxima and the direction of the magnetic field lines (or to the perpendicular line to the magnetic field direction in case of compounds with negative magnetic anisotropy). Upon further cooling of the samples, the ordering of the molecules in the aligned mesophase is retained. Because of the high viscosity of the mesophase, the molecular mobility is very low and the mesophase order is frozen into the glassy state. This aligned vitreous mesophase can be considered as an optically anisotropic glass.28 However, in some cases crystallization of the sample is observed. This is illustrated in Figure 3. Upon crystallization the alignment of the molecules is partially lost. Conclusions The magnetic alignment of the mesomorphic Schiff base complexes of the trivalent lanthanides with dodecylsulfate counterions was investigated by temperature-dependent magnetic susceptibility measurements and temperature-dependent synchrotron SAXS measurements. The experiments gave the signs and values of the magnetic anisotropy of the lanthanide complexes. Although the complexes of the heavy lanthanides exhibit a huge magnetic anisotropy, the alignment of the mesophase samples is far from perfect and the alignment is slow. This can be attributed to the high viscosity of the smectic A mesophase and to the fact that the rare-earth center is only a minor component in these large molecules. The best alignment results were obtained for the dysprosium(III) complex.

Magnetic Alignment Study of [Ln(LH)3(DOS)3] Complexes Acknowledgment. B.G., R.V.D., and K.D. are postdoctoral fellows of the FWO-Flanders (Belgium). This work was performed within the framework of a bilateral project between Flanders and Russia (BIL 05/31), a bilateral project between Germany and Russia (BMBF-IB RUS 05/003), and the Russian grant (RFBR 05-03-34818). Additional financial support by the K.U.Leuven (project GOA 03/03) and by the FWO-Flanders (project G.0508.07) is gratefully acknowledged. The authors acknowledge the technical assistance by the team of the DutchBelgian Beamline at the ESRF (Grenoble, France) and especially Dr. Igor Dolbnya. Supporting Information Available: Graphical representation of the values of the magnetic anisotropy (Figure S1). Reference SAXS diffraction pattern of silver behenate at room temperature (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Blinov, L. M. Electro-optical and Magneto-Optical Properties of Liquid Crystals; Wiley: Chichester, 1983. (2) Schadt, M. Ann. ReV. Mater. Sci.1997, 27, 305-379. (3) Collings, P. J.; Patel, J. S. Handbook of Liquid Crystal Research; Oxford University Press: Oxford, 1997. (4) Ottiger, M.; Bax, A. J. Biomol. NMR 1998, 12, 361-372. (5) Celebre, G.; Deluca, G.; Longeri, M.; Catalano, D.; Veracini, C. A.; Emsley, J. W. J. Chem. Soc., Faraday Trans. 1991, 87, 2623-2627. (6) Mitra, M.; Paul, R. Mol. Cryst. Liq. Cryst. 1987, 148, 185-195. (7) Ibrahim, I. H.; Haase, W. Z. Naturforsch. A 1976, 31, 1644-1650. (8) Pieranski, P.; Brochard, F.; Guyon, E. J. J. Phys. 1973, 34, 3548. (9) Motoc, C.; Iacobescu, G. J. Magn. Magn. Mater. 2006, 306, 103107. (10) Courtieu, J.; Alderman, D. W.; Grant, D. M.; Bayles, J. P. J. Chem. Phys. 1982, 77, 723-730. (11) Noel, C.; Monnerie, L.; Achard, M. F.; Hardouin, F.; Sigaud, G.; Gasparoux, H. Polymer 1981, 22, 578-580. (12) Ciampi, E.; Emsley, J. W. Liq. Cryst. 1997, 22, 543-547. (13) Poggi, Y.; Aleonard, R. Compt. Rend. Ser. B 1973, 276, 643645. (14) Emsley, J. W.; Long, J. E.; Luckhurst, G. R.; Pedrielli, P. Phys. ReV. E 1999, 60, 1831-1839. (15) Emsley, J .W.; Luckhurst, G. R.; Pedrielli, P. Chem. Phys. Lett. 2000, 320, 255-261. (16) Bras, W.; Emsley, J. W.; Levine, Y. K.; Luckhurst, G. R.; Seddon, J. M.; Timimi, B. A. J. Chem. Phys. 2004, 121, 4397-4413. (17) Bras, W.; Levine, Y. K.; Polimeno, A. Nuclear Instr. Meth. Phys. Res. 2005, 238, 1-6. (18) Lee, J. H.; Kim, H. S.; Pate, B. D.; Choi, S. M. Physica B 2006, 385, 798-800. (19) Lee, J. H.; Choi, S. M.; Pate, B. D.; Chisholm, M. H.; Han, Y. S. J. Mater. Chem. 2006, 16, 2785-2791. (20) Pate, B. D.; Choi, S. M.; Werner-Zwanziger, U.; Baxter, D. V.; Zaleski, J. M.; Chisholm, M. H. Chem. Mater. 2002, 14, 1930-1936. (21) Zheng, M. Y.; An, Z. W. Chin. J. Chem. 2006, 24, 1754-1757. (22) Ikemoto, H.; Akutsu, H.; Yamada, J.; Nakatsuji, S. Tetrahedron Lett. 2001, 42, 6873-6875. (23) Nakatsuji, S.; Mizumoto, M.; Ikemoto, H.; Akutsu, H.; Yamada, J. Eur. J. Org. Chem. 2002, 1912-1918.

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