Electro-optic characterization of novel tolane-based nematic liquid crystals

Journal of Molecular Liquids 129 (2006) 169 – 172 www.elsevier.com/locate/molliq

Electro-optic characterization of novel tolane-based nematic liquid crystals H. Khoshsima a , A. Ghanadzadeh b,⁎, H. Tajalli a , R. Dabrowski c a

Research Institute for Applied Physics and Astronomy, University of Tabriz, Tabriz, Iran b Department of Chemistry, Faculty of Science, Guilan University, Rasht, Iran c Institute of Chemistry of the Military Technical Academy, Warsaw, Poland Received 10 January 2006; accepted 20 February 2006 Available online 21 June 2006

Abstract Four new tolane-based liquid crystals, 4′-pentyl-, 4′-hexyl-, and 4′-heptyl-3-fluoro-4-isothiocyanatotolane and 4′-hexyloxy-3′-fluoro-4isothiocyanatotolane have been investigated for the first time to determine their electro-optical behavior and third order non-linearity by the static Kerr effect method. The temperature dependence of electric Kerr constant in the isotropic phase and the pretransitional behavior, related to order parameter fluctuations, have been investigated for four new high birefringence nematic liquid crystals, laterally fluorine substituted isothiocyanatotolanes in the isotropic phase. All the compounds had a positive and large Kerr constant which increased with decreasing temperature. The Landau–De Gennes model was obeyed for these compounds. © 2006 Elsevier B.V. All rights reserved. Keywords: Tolane-based liquid crystals; Isothiocyanatotolanes; Nematic

1. Introduction Nematic liquid crystals possess large optical anisotropy owing to their large molecular anisotropy and intermolecular ordering. The strong optical nonlinearity of nematic liquid crystals arises from their large refractive index anisotropy coupled with the collective molecular reorientation [1]. Materials with nematic liquid crystalline properties are applied in electro-optical displays, optical storage devices and nonlinear optics. Therefore, the design and synthesis of new stable liquid crystals are always needed in the wide range of electro-optic applications. In order to improve optical operation or optimization of nonlinearity in nematic liquid crystals, the knowledge of the optical data and the influence of molecular structure on the nonlinear optical behavior of the liquid crystals are urgently required. The electro-optical Kerr effect, which depends both on polarity and polarizability of the molecules, has been demonstrated to be an effective technique for studying the molecular anisotropy, intermolecular ordering and pre-transitional behavior in liquid crystals [1–9]. This effect is strongly related both to ⁎ Corresponding author. E-mail address: [email protected] (A. Ghanadzadeh). 0167-7322/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2006.02.004

the chemical structure of a molecule and to intermolecular interactions [10]. Also the third order nonlinear susceptibility, which is an important material parameter, can be obtained using the Kerr effect measurements [11]. In this investigation we are concerned with new liquid crystals of the high birefringence nematic type with positive dielectric anisotropy and high chemical and thermal stability. The investigation reported here is concerned with the determination of the Kerr constant and the third order nonlinearity of four isothiocyanatotolanes in the isotropic state. All the chosen compounds have a fluorine lateral group in common. The effect of temperature on these systems has also been studied. Due to the strong permanent dipole moment and highly conjugated π-electron systems along their long molecular axis, 4′-alkyl-(4-alkoxy-) 3-fluoro-4-isothiocyanatotolanes molecules have extremely large and positive Kerr constant and dielectric anisotropy. These compounds have also relatively low viscosity, and good chemical, photo and thermal stability. One of the principal aims of the investigation was to see how the combination of tolane and NCS groups with an additional fluorine atom located in the neighborhood of the NCS or the alkoxy groups affects the electro-optical behavior of the liquid crystals. The laterally fluorine substituted isothiocyanatotolanes

170

H. Khoshsima et al. / Journal of Molecular Liquids 129 (2006) 169–172

exhibit only nematic phase. Note that the phase is monotropic for 4′-pentyl-3-fluoro-4 isothiocyanatotolane (5FBT) and 4′hexyl-3-fluoro-4 isothiocyanatotolane (6FBT). 2. Theory The electric birefringence of the medium is defined as the difference between the refractive indices for light polarized parallel (n||) and perpendicular (n⊥) to the orienting field direction, and is related to optical retardation or optical phase difference, δ, d ¼ 2klBE 2 ¼

2klðnt −n8 Þ k

ð1Þ

Where λ is the wavelength of the light, l is the optical path length, and B is called the “Kerr constant” of the substance, and is used to denote the magnitude of the Kerr effect [12]. The electrically induced birefringence, Δn, may also be conveniently defined by (Kerr law): Dn ¼ kBE 2

d 4

ð3Þ

Eliminating δ from Eqs. (3) and (4) and rearranging gives: a¼

kLBE 2 2

ð4Þ

If the Kerr law is observed, a plot of α versus the square of applied electric field (E2) also should give a straight
 line graph passing through the origin with a gradient of pLB 2 , from which the Kerr constant B may be determined. The Kerr constant can be related to the third order non-linear susceptibility tensor, χ(3), in isotropic media, which is an important material parameter, using the following formula [14]: B¼

liquid crystals. According to this model the Kerr constant in the isotropic phase is given by

ð2Þ

For the nulled intensity method of measuring Kerr effect, which involves the nulling of an optical response resulting from the application of an external electric field across the Kerr cell [13], the rotation of the plane of polarization, α, is related to the phase difference, δ, by a¼

Fig. 1. Chemical structures of the tolane-based liquid crystalline compounds.

24k ð3Þ v nk

ð5Þ

The theory of Landau–De Gennes [15], has been used to describe the pre-transitional behavior in the isotropic phase of

B¼

eo Dno Deo pffiffi 4ak eðT −T ⁎ Þ

Where a is the temperature independent coefficient, T⁎ is the second-order pre-transitional temperature, which is an extrapolated temperature just below the isotropic–nematic phase transition, Δεo is the low frequency dielectric anisotropy in the completely ordered phase. From the above equation, there is a reciprocal relationship between the Kerr constant, B, and temperature, i.e.Δn ∝ (T − T⁎)− 1.

3. Experimental 3.1. Materials The liquid crystalline materials (laterally fluorine substituted isothiocyanatotolanes) were synthesized in the Institute of Chemistry of the Military Technical Academy, Warsaw, Poland. The chemical structure of these compounds is shown in Fig. 1. The clearing points or nematic to isotropic transition temperatures for these nematogens were measured using DSC and polarizing microscopy methods and are given in Table 1.

Table 1 The clearing temperature, TC, and pre-transitional temperature, T ⁎, of the tolane compounds Compounds

TC (°C)

T ⁎ (°C)

4′-pentyl-3-fluoro-4 isothiocyanatotolane (5FBT) (Monotropic transition) 4′-hexyl-3-fluoro-4 isothiocyanatotolane (6FBT) (Monotropic transition) 4′-heptyl-3-fluoro-4 isothiocyanatotolane (7FBT) 4′-hexyloxy-3-fluoro-4 isothiocyanatotolane (6OFBT)

(40.6)

39.2

(31.7)

30.4

43.6 70.7

42.4 69.1

Fig. 2. Diagram of apparatus used to measure the electro-optical Kerr effect. P, Polarizer; QWP, quarter-wave plate; A, analyzer; PMT, photomultiplier.

H. Khoshsima et al. / Journal of Molecular Liquids 129 (2006) 169–172

14 12 10

6

0.10 0.05

4 2 42

44

46

48

50

0.5

20

0.4

15

0.3

10

0.2

0.1

5

0

0.00 40

7FBT

0.0 42

52

44

46

Fig. 3. Temperature dependence of B and B− 1 for 5FBT obtained using the AC field method. (Open circles for B and black filled circles for B− 1).

3.2. Kerr effect apparatus A diagram of the apparatus used to measure the phase difference is shown in Fig. 2. A spectra physics He–Ne laser emitting at a wavelength of 632.8 nm with a power of 5 mW was used as a probe beam. For the electro-optical investigation in the isotropic phase, a sample holder similar to that described by O'Konski and Haltner [16] was constructed. The sample holder was a quartz spectrophotometer cell with a path length of 10 mm. The stainless steel electrodes were inserted into the Kerr cell, leaving an active column of liquid 2 × 2 × 10 mm. A Teflon spacer was used to provide insulation between the electrodes and to maintain an electrode gap of 2 mm. The lower part of the Kerr cell fitted into a thermostated metal jacket, which was cut away to allow passage of the light beam. The temperatures were determined with a copper constantan thermocouple. The estimated accuracy in the measurement of temperature is ± 0.1 °C. The detection of the Kerr signal was achieved using a photomultiplier tube, type E.M.I. 9816B. The optical signal from the photomultiplier was displayed using a digital

Fig. 5. Temperature dependence of B and B− 1 for 7FBT obtained using the AC field method. (Open circles for B and black filled circles for B− 1).

storage oscilloscope (Tektronix 300 MHz, model TDS3032B) and a personal computer. The polarizer and analyzer were Glan–Thompson double refraction type prisms of commercial origin and adjusted such that they crossed each other, making an angle 45° with respect to the applied AC field (1 kHz). The quarter wave plate used in these experiments was mica cut specifically for use at 632.8 nm and mounted between glass discs. The electric field is generated by means of a high-voltage power supply that was applied, as a short duration rectangular shaped pulse from a pulse generator, to the electrodes of the Kerr cell. 3.3. Measurements of Kerr constants (B) All measurements of the Kerr constant were made using the nulled intensity method. A parallel, plane-polarized beam of monochromatic light is passed through the Kerr cell such that the plane of polarization of the light is at an angle of 145° relative to the direction of the applied electric field. In the presence of an electric field the light leaving the cell is generally elliptically polarized. After passing through an

10

0.20 0.15

15

1/B

20

0.10 0.05

6OFBT 0.5

8

0.4

6

0.3

T*= 69.1

25

Kerr constant B /10-11 mV-2

0.25

T*=30.4

Kerr Constant B/10-11 mV-2

54

0.6

30

5

52

0.30

6FBT

10

50

Temperature (°C )

Temperature (°C )

35

48

4

1/B

8

0.15

1/B

0.20

T*=39.2

Kerr Constant B /10-11 mV-2

0.25

Kerr Constant B/10-11 mV-2

25

16

1/B

0.30

5F BT

T*=42.4

18

171

0.2 0.1

2 0

0.00 30

32

34

36

38

40

42

0.0 68

70

72

74

76

78

80

82

Temperature (°C)

Temperature (°C )

Fig. 4. Temperature dependence of B and B− 1 for 6FBT obtained using the AC field method. (Open circles for B and black filled circles for B− 1).

Fig. 6. Temperature dependence of B and B− 1 for 6OFBT obtained using the AC field method. (Open circles for B and black filled circles for B− 1).

172

H. Khoshsima et al. / Journal of Molecular Liquids 129 (2006) 169–172

4.5

6F BT

temperature. For the samples studied the inverse of the Kerr constant varies linearly with temperature, i.e.γ ≈ 1, as predicted by the Maier–Saupe mean-field theory for liquid crystals [17]. However, close to the TC, a small deviation from the mean-field theory is observed [18]. The electro-optical Kerr effect is used to obtain the third order non-linear susceptibility, χ(3). Fig. 7 shows the variation of the susceptibility with temperature for 4′-alkyl-3-fluoro-4 isothiocyanatotolane liquid crystals in the isotropic phase. The strong optical nonlinearity of these materials can be due to the present tolane core coupled with the polar NCS group, which increase remarkably conjugation length, and therefore induce dipole moment and polarizability.

4.0

χ(3) / 10-18 m2V-2

3.5

7FBT

3.0 2.5

5F BT

2.0

6O FBT

1.5 1.0 0.5 0.0 32

36

40

44

48

52

72

76

80

84

88

92

Temperature (°C ) Fig. 7. Variation of susceptibility, χ(3), for 5FBT, 6FBT, 6OFBT and 7FBT.

oriented quarter-wave retarder the light can be extinguished by the analyzer. 4. Results and discussion The Kerr constant, B, of a material, at a given temperature may be obtained from the gradient of a graph of induced birefringence, Δn, plotted against the square of the applied electric field (i.e.Δn = λBE2). In all the samples examined the electrically induced birefringence was found to be directly proportional to the square of the applied electric field (E2), and is positive in all cases (Δn 〉 0). The dependence of the Kerr constant, B, and the inverse of Kerr constant, B− 1, with temperature for the nematogens (5FBT, 6FBT, 7FBT, and 6OFBT) in the isotropic phase are shown in Figs. 3–6. This data show that the birefringence depends strongly on temperature. The high Kerr constant of these materials in isotropic phase is due to the high polarizability tolane-based structure and polar head group (− NCS), which conducts to high optical anisotropy. However, the existence of lateral flourine substituent in these compounds is very important and necessary in the nematic formation. The pre-transitional effect exhibited by liquid crystal materials has been found to influence the isotropic value close to the nematic–isotropic transition temperature. In the pretransition region the residual short-range nematic ordering persists well in the isotropic phase. According to the Landau– De Gennes model, the Kerr constant in the isotropic phase is given by Δn ∝ (T − T ⁎)−γ. The second-order pre-transitional temperature T ⁎, which is slightly less than TC, can be obtained by a linear extrapolation of the Kerr constant (B− 1) vs.

5. Conclusions The tolane-based liquid crystals have high Δn and high third order non-linearity susceptibility, χ(3). This is a consequence of the long molecular structure and the coupling between high polarizable tolane and NSC groups. The presence of a − C`Clinkage group, which increases the conjugated length and molecular polarizability, leads to a marked effect on the physical properties such as birefringence and susceptibility in the tolanebased liquid crystal. The steric effect due to the presence of laterally fluorine atom in these compounds is very important and necessary in the nematic formation. In spite of a small deviation in the pretransitional behavior from the mean-field theory, close to the TC, for these compounds, there is a good agreement with the Landau–De Gennes model. References [1] R. Muenster, M. Jarasch, X. Zhuang, Y.R. Shen, Phys. Rev Lett. 78 (1996) 42. [2] Philip, T.A Prasada Rao, Phys. Rev., A 46 (1992) 2163. [3] J.P. Pouligny, J.R. Marcerou, H.J. Lalanne, Mol. Phys. 49 (1983) 583. [4] H.J. Coles, S.V. Kershaw, Mol. Cryst. Liq. Cryst. Lett. 2 (1–2) (1985) 29. [5] H.J. Coles, S.V. Kershaw, J. Chem. Soc. Faraday Trans 2 84 (1988) 987. [6] J. Philip, T.A. Prasada Rao, J. Mol. Liq. 50 (1991) 215. [7] J. Philip, T.A. Prasada Rao, J. Phys., D, Appl. Phys. 25 (1992) 1231. [8] A. Ghanadzadeh, M.S. Beevers, J. Mol. Liq. 107 (2003) 77. [9] A. Ghanadzadeh, M.S. Beevers, J. Mol. Liq. 112 (2004) 141. [10] M.S. Beevers, Mol. Cryst. Liq. Cryst. 31 (1975) 333. [11] A. Sinha, T.A. Prasada Rao, V.R.K. Murthy, Liq. Cryst. 27 (2000) 191. [12] J. Kerr, Philos. Mag. 50 (1875) 337. [13] M.S. Beevers, G. Khanarian, Aus. J. Chem. 3 (1979) 263. [14] C.C. Wang, Phys. Rev. 152 (1966) 149. [15] P.G. De Gennes, J. Prost, The Physics of Liquid Crystals, 2nd edition, Oxcford University Press, New York, 1995. [16] C.T. O'Konski, A.J. Haltner, J. Am. Chem. Soc. 78 (1956) 3604. [17] P.G. de Gennes, Mol. Cryst. Liq. Cryst. 12 (1971) 193. [18] B. Malraison, Y. Poggi, J.C. Filippini, Solid State Commun. 31 (1979) 843.