STUDY OF NEMATIC LIQUID CRYSTALS BY MEANS OF THE GUEST DYE DICHROISM

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STUDY OF NEMATIC LIQUID CRYSTALS BY MEANS OF THE GUEST DYE DICHROISM

L. Blinov, V. Kizel, V. Rumyantsev, V. Titov

To cite this version:

L. Blinov, V. Kizel, V. Rumyantsev, V. Titov. STUDY OF NEMATIC LIQUID CRYSTALS BY MEANS OF THE GUEST DYE DICHROISM. Journal de Physique Colloques, 1975, 36 (C1), pp.C1- 69-C1-76. �10.1051/jphyscol:1975111�. �jpa-00215888�

JOURNAL DE PHYSIQUE Colloque C1, supplkment au no 3, Tome 36, Mars 1975, page C1-69

Classification

Physics Abstracts 7.130

STUDY ^' OF NEMATIC LIQUID CRYSTALS BY MEANS OF THE GUEST DYE DICHROISM

L. M. BLINOV, V. A. KIZEL, V. G. RUMYANTSEV and V. V. TITOV Institute for Organic Intermediaries and Dyes, Moscow, U. S. S. R.

RBsum6. - En mesurant le dichrolsme de mol6cules allongees en solution dans une matrice nematique, on peut obtenir la valeur du param5tre d'ordre d'orientation du cristal liquide (SLC) sans connaissance prealable des proprietes de ce matkriau dans sa phase solide. Dans ce cas il ,est important que les mol6cules du solute et du solvant soient isomorphes, et il faut tenir compte de I'anisotropie du champ electrique local de I'onde lumineuse. A cet effet, les indices de refraction de plusieurs cristaux liquides ont etk mesur6s au spectrophotom6tre et le dichrolsme d'ions sphkriques a BtB determine. Les parametres d'ordre des cristaux liquides ont kt6 deduits des valeurs du rapport de dichrolsme mesurkes avec des colorants specialement choisis et les polarisabilites molBculaires principales ont eti5 calcul6es pour les mgmes substances, en utilisant nos valeurs de SLC et les indices de refraction. Les resultats prksentks ici ont Bti5 obtenus pour la PAA, MBBA, Merck (Phase IV) et quelques cristaux liquides ayant une forte anisotropie dielectrique positive.

Abstract. - By measuring the dichroism of rod-like probe molecules dissolved in a nematic matrix the value of the liquid crystal orientational order parameter (SLC) can be obtained with- out any prior knowledge of the properties of this material in a solid state. In this case it is impor- tant that the guest and host molecules be isomorphic, and the anisotropy of the local light field be taken into account. To this effect the refractive indices of some liquid crystals have been measured spectrophotometrically and the dichroism of spherical ions has been investigated. The order parameters of liquid crystals were deduced from values of the dichroic ratio measured with the specially choosen probe dye and the principal molecular polarizabilities were calculated for the same substances, using our SLC values and refractive indices. The results reported havebeen obtained for p-azoxyanisole, MBBA, Merck (Phase IV) and'some liquid crystals with a strong positive dielectric anisotropy.

1. Introduction. - Nematic liquid crystals are cha- racterized by a high degree of orientational order of the molecules, which is quantitatively described by the order parameter

where O,, is the angle between the long axis of the molecule and the uniaxial direction of the liquid crystal [I]. The parameter S,, can be derived from a statistical theory of nematic phase, and hence its expe- rimental determination is very important for the pur- pose of verifying the theory. Of the methods for mesur- ing the order parameter 121, some require complex equipment (X-ray and NMR spectroscopy), while others are based on a comparison of optical or magne- tic properties of the liquid crystals with respect to the properties of the same substances in the solid state. The growth of single crystals is a difficult task, particularly in case when the substance is nematic at room tempe- rature. To measure the dichroism (the optical absorp- tion of polarized light) in electronic or vibrational

bands of nematic crystals themselves [2], one must solve other serious problems, for example, measure- ment of high extinction coefficients and taking account of anisotropy of the local light wave field in the ultr*

violet spectral region, identifying the bands and determining the oscillator directions in the infra-red spectrum, etc.

One can avoid these difficulties by investigating the dichroism not in the bands of the liquid crystal itself (host) but in the absorption bands of impurities (guests) isomorphic to the nematic matrix. Such guests are the optical probes for a given liquid crystal. There may exist rod-like dye molecules the direction of the absorption oscillator of which coincides with the long molecular axis [3]. In the case of dilute solutions it is easy to measure the dichroism in the visible spectrum and to take into account the local field anisotropy if the refractive indices of the pure nematic are known.

In this work we first report the spectral dependences of the refraction indices n,, , nl for some liquid crystals uniformly oriented by cell walls or by electric fields (subscripts 11, I are related to the parallel and perpen-

6

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975111

C1-70 L. M. BLINOV, V. A. KIZEL, V. G . RUMYANTSEV AND V. V. TITOV dicular directions with respect to the crystal optical

axis). Then, we study the absorption dichroism of spherical ions: dissolved in the room-temperature nematic mixture. In that case the dichroism is due only to the local light field anisotropy and we can estimate the values of correction factors used in subsequent experiments. Then, the order parameter SLc is deter- mined from optical dichroism experiments with dye molecules isomorphic to the nematic matrix. Finally, from these results and from data on n,,, n, we calculate the principal molecular polarizabilities of the nematic crystals investigated.

2. Materials. - It seemed to us that first of all it was reasonable to test our experimental method on the classical nematic crystal p-azoxyanisole (PAA, I) and then to apply it to the new liquid crystals. Some of them (p-methoxy-benzylidene-p'-butylaniline, MBBA, I1 or p-butyl-p'-methoxyazoxybenzene, Merck-IV, 111) have not been studied enough ; the others have not been optically investigated at a11 (p-butoxyphenyl ester- p'-hexyloxybenzoic acid, BEHA, IV, or p-alkoxy- benzylidene-p'-cyanoanilines (V) and p'-cyanophenyl esters-p-alkyl benzoic acid (VI) with strong positive dielectric anisotropy).

(V) n = 2 , 4 , 7 (V1) ¹¹ = 4, 6 , 7

Nematic matrices (IV), (VI) are transparent in the The optical probes were chosen to have absorption visible and near-UV ranges (I 2 330 nm) and conve- bands in a wavelength region where liquid crystals are nient for a study of guest dichroism, particularly in transparent. Their molecules have similar chemical relation to the problem of local field anisotropy. The properties but differ considerably in length.

other substances absorb below 430 nm.

( 2 ,,,,,, = 370 nm) K - l

STUDY OF NEMATIC LIQUID CRYSTALS C1-71 In addition we took the molecules K-5 and K-6 respectively but absorbed light in another wavelength which were geometrically equivalent to K-2 and K-3 region.

= 430 nm) K-5

The electronic transition moments (for longwave bands) of all our guest molecules were rigorously parallel to the long molecular axes. This is clear from the electronic structure of these molecules and, in addition, may be easily verified by means of freezing the dye solutions in p-azoxyanisole : in that case the absorption of light polarized perpendicular to the crystal optical axis is completely absent (D, = 0) and the dichroic ratio of the solid solution is infinite (Ns ⁼ Dll/Dl + co). The weight concentrations of the solute molecules were 0.1-1 %. I t has been checked that the guests do not appreciably change the order parameter or the refractive indices of the liquid crystals studied.

All substances were purified by multiple recrystalli- zations ; in some cases vacuum distillation (MBBA), sublimation (substances V, VI and some dyes) or chromatography on aluminium oxide were used.

3. Technique. - The refractive indices in the nema- tic phase and the polarized absorption spectra of dyes in a region of A = 320-700 nm were measured with a Hitachi EPS-3T Spectrophotometer. The liquid crystal samples were inserted in heated sandwich cells which were constructed of strictly parallel quartz plates with tin oxide conducting electrodes spaced by teflon strips of thickness 10-200 p. Rybbing of the conducting surfaces with cotton-wool prior to cell fabrication results in the homogeneous (planar) orientation of the liquid crystal film. In the cases of substances with a negative dielectric anisotropy (I-IV), an a. c. field (of high enough frequency f x lo3-lo4 Hz to avoid any electrohydrodynamic effects) can be applied normal to the film to enhance the degree of molecular alignment and to diminish the light scattering and depoiarization.

Spectra of nll and n , were determined in the foliow- ing way. First of all, on wedge-shaped samples at 1 = 546 nm the temperature dependence of n,, and n, was studied by measuring the period of interference fringes [4]. Then a parallel-walled cell was constructed and the thickness of the empty cell measured by record- ing the oscillations of its I-dependent optical transmis- sion. These oscillations were due to interference of multiple reflections of nonpolarized light from the inner cell walls ; the thickness was calculated with an accuracy of

+

neighbouring transmission maxima 151. Then the cell was filled with a substance in the isotropic phase and -spectral oscillations were recorded again. The position of the intensity maxima is determined by the phase difference q ( A ) of two interfering beams

One can easily find the number of the maximum near I ⁼ 546 nm if the values of d and ni (546 nm) are known. Then, one must identify the remaining maxima and find the respective values of ni(l) with eq. (2a).

Unfortunately, it was very difficult to determine in the same way rill and n, separately because of scattered light in the nematic phase. It is easier to measure An(1) = nll(l) - n,(1) by recording the phase diffe- rence between ordinary and extraordinary beams [ 5 ] . In this case the cell was placed between parallel pola- roids so that its optical axis was at the angle of 450 to the light electric vector. Now the phase difference of beams with wavelength A depends on d (not on 2 d) and An :

The transmitted intensities oscillate with 1 and are recorded automatically, figure 1 (curve 1). Again, to

FIG. 1. - Wavelength dependences of : 1 - light intensity after the analyzer (thickness of BEHA film - 46 ^p, t = 78 O C , the voltage on the electrodes 100 V, frequency 2 kHz) ; 2 , 3 -optical densities of the K-5 solution in BEHA, Dl, and D L respectively ;

4 - observed dichroism N = Dll/Dl.

C1-72 L. M. BLINOV, V: A. KIZEL, V. G. RUMYANTSEV AND V. V. TITOV calculate An(il) it is necessary to find the number of the

intensity maximum near 1 = 546 nm and to identify the remaining maxima. Hence, An(&) = k&,fd. It should be noticed that we didn't utilize the simplified equation An z A2/d A1 [5], which neglects the disper- sion of An(&) and resulted in too large values of An at short wavelengths.

To find nil and n, at each wavz'ength we use the relations

An = rill - n, , - _n = .5(nll

+

where i;i is the mean refractive index in the nematic phase. It has b,een obtained by an extrapolation of the value n; into temperature 'region below, the clearing point having taken into account the temperature change of substance density. Density measurements were carried out in a glass capillary pycnometer on approximately 0.2 ml samples.

In the birefringence measurements of nil and n, the error appears to be relatively large (+ 0:Ol) but, on the other hand, this method gives the spectral dependence rill, n, in a wide range of wavelengths and provides a high degree of molecular alignment.

The optical density spectra of dyes solutions in nematic matrices were recorded with the polarizer being oriented parallel ( D l , ) or perpendicular (D,) to the rubbing direction, figure 1 (curves 2, 3). From them one can obtain the wavelength dependence of the observed dichroism N(A)

-

Genuine values of the dichroic ratio N* are related to the observed ones by equation

where g(A) is a factor taking into account the polariza- tion field anisotropy in the liquid crystal 121. The ratio N* does not have to depend on wavelength in the region of one absorption band with the same orienta- tion of transition moment. In general, the degree of order of dye molecules (Sdye) does not coincide with S,, but the eq. (I) remains valid for S,,, if Odye replaces O,,. It can be shown [2, 31, that the following relations between the orientation parameter Sdy, and the dichroic ratios are valid :

where asterisks denote that optical densities and the dichroic ratio N* ⁼ D ~ D ? have been corrected by taking local field anisotropy into account.

4. Optical properties. - The results of our measu- rements of refraction indices n i ( l ) and optical an'iso- tropy An(;l) for PAA and MBBA are shown in figure 2.

The samples were oriented by the cell boundaries and by an a. c. electric field, U =, 100 V, f = 2 kHz. For comparison the values of ' A n ( l ) taken from refe- rences [6-91 are given in the saine:figure. The agreement of the results is,good enough to justify: the optical technique described.

FIG. 2. - Refraction index Hi in the isotropic phase and the optical anisotropy An in the nematic -phase as a function of wavelength : i - ni a t 137 OC for PAA, 2 - A n a t 120 OC for PAA, 3 - A n a t 20°C for MBBA (open symbols : our data,

black symbols : data from ref. [6-81).

FIG. 3. - Refraction indices ni in the isotropic phase and the optical anisotropy An in the nematic phase as a function of wavelength : 1 - ni a t 103 OC, 2 - ^{A n} a t 80 OC for BEHA ;

3 - ni a t 75 OC, 4 - An at 20 OC for Merck-IV.

Spectra of refraction indices ni and optical aniso- tropy for Merck-IV and BEHA are shown in figure 3.

Our results for Merck-IV at 1 = 583 nm are in agree- ment with those in reference [lo].

Taking into account the density data for Merck-IV [lo] and measuring the densities of BEHA

(p100 OC = 1.004 & 0.001 g ~ m - ~ , d p / d ~ = - 1.05 x g cm-3 grad-f) we calculated values nil and n, for these two liquid crystals. In figure 4 our results. are given for 20 OC and 80 ,@C:ibr Mercbf V\ and BEHA respectively.

STUDY OF NEMATIC LIQUID CRYSTALS C1-73

FIG. 4. - Refraction indices n,, , nl as.a functionof wavelength : 1 - n i l , 2-nl for BEHA at 80°C; 3 - n i l , 4-nI for Merck-IV at 20 O C (Crosses - nl, and n, for Merck-IV from

ref. [lo]).

5. Local field anisotropy. - In order to calculate the order parameter for guest molecules in a nematic matrix using dichroism data, eq. (5), specific models taking into account the polarization field in anisotropic medium have to be considered. The factor g(l) is determined by the equation for a local field in the liquid crystal. The extension of Lorentz's formula

( F being the external field, i = 11, I) to the case of an anisotropic medium resulting in correction factors of the form

can not be proved theoretically and contradicts experi- ment [Ill. In a semiempirical approach 1121 the Lorentz formula is corrected by introducing a certain structure parameter for a given crystal lattice. This model [2, 131 also results in the correction factors g, < 1. Vuks [I I] has assumed that the local field in anisotropic crystals is independent of direction and equal to

where

n2

For liquid crystals studied the ratio nll/n, increases with dexeasing wavelength. In the Neugebauer theory [12] this increase would result in some enhan-

cement of the observed dichroic ratio (N = N*/g) with decreasing 1. On the contrary, the Vuks formula predicts its reduction. In our experiments the spectral dependences of observed dichroism are in accordance with Vuks' formula and contradicts the Neugebauer theory. For example, taking from figures 1 and 4 the values N = DII/D, and g, = nll/nl for wavelengths 1 = 360 nm and 490 nm (N = 5.75 and 6.30, ^g, = 1.22 and 1.13 respectively) we will obtain the approximately equal values of genuine dichroic ratios, N* = 7.0 and 7.1, for these J's. We observed analogous dichroism spectra for other guest-host pairs. The decrease in N with decreasing wavelength is also confirmed by measu- rements of the dichroic ratios at the absorption maxi- mum for a pair of dyes with similar molecular geometry but different absorption spectra dissolved in the same nematic matrix (for instance, K-2 and K-5 or K-3 and K-6 in BEHA). Thus, Vuks' formula is in a qualita- tive agreement with experiment although it has no theoretical justification. For this reason we considered another method of evaluating g(l) using the theory of Dunmur [I 51.

In [IS] the oriented nematic liquid crystal is repre- sented as a simple tetragonal lattice and the Lorentz factor tensor is found for that case. The correction coefficient for the dichroism may be written as :

where L,, and L,, = L,, are the diagonal components of Lorentz tensor tabulated [15] as a function of the ratio c/a (c is the length of the unit cell in the longitu- dinal 2-direction, a is the unit cell length in the x or y directions).

The eq. (6), as well as Vuks' formula, describes the observed dichroism spectra. To find the parameters c and a from the experiment one can measure the dichroism of spherically symmetric optical probes such as the complex ion [PMo,,o,,]~- in liquid crystal matrices. For this purpose we used the nematic solution of heterocompound Na,[PMo,,O,,] x H 2 0 with absorption maximum at 1 = 3 10 nm. This probe itself is globular [I61 and cannot be optically anisotropic (N* = 1). Therefore, the dichroism observed is completely due to the anisotropy of the local light field (N = l/g). Polarized absorption spectra of the globular probe in the ternary mixture (VI) at t = 20 OC is shown in figure 5. In this case the dichroic ratio is found to be less than unity (Dl, < D,) and g > 1.

Unfortunately, large errors due to the overlapping of the ion absorption band and the matrix absorption edge prevent quantitative analysis of the spectral dependence of dichroism. We could obtain the value of g = Dll/D, w 1.25 with sufficient accuracy in the region 340-360 nm only. From eq. (6), taking into account the refractive indices rill = 1.9, n, = 1.55, we found L,,]L,, w 0.53 and by means of Table I of [15] we calculated L,, = Lyy = 0.40, L,, = 0.21

C1-74 L. M. BLINOV, V. A. KIZEL, V. G. RUMYANTSEV A N D V. V. TITOV and c/a x 1.13. It may be assumed that Lorentz

tensor components Lii are approximately equal for all our liquid crystals because of the similarity of their molecular geometry.

molecular theory of nematic crystals [17] and will later be verified experimentally. For dyes isomorphic to liquid crystals, order parameter SL, may be calculated usiilg dye dichroism data and eq. (5). We think that the molecules of the two-ring dye K-2 are the most suitable to probe the degree of order of substances I-VI. The curve N in figure 6 represents the temperature depen-

RG. 5. - Wavelength dependences of optical densities D l , , D , in polarized light for the ion [PMo12040]3- in mixture VI ( t = 20 OC, cell thickness 65 p) ; refraction indices rill, nl of mixture VI at 20 OC. (Absorption edge of mixture VI is shown

with a broken line.)

tc-t ^,

FIG. 6. - Dichroism (h?) of K-2 dye in PAA and order It is interesting that with an accuracy parameter (S) PAA as a function of temperature the, correction factor g z 1.25 coincides with the ratio _{( t ,} being clearing point).

g, = n,,/n, resulting from Vuks' formula. Thus, in our case the Vuks' formula was theoretically justified : the ratio of the lengths of the liquid crystal's unit cell is such that the local field in nematics is practically independent of direction. For that reason and for the sake of simplicity we shall use the value of g = nll/n, and Vuks' formula in subsequent calculations of SLc and molecular polarizabilities.

dence of the observed dichroism at the absorption maximum (A,,,, = 490 nm) of the K-2 solution in p-azoxyanisole. SLc has been calculated (curve S in Fig. 6) with the correction factor g = nll/n, at I = 490 nm, rill and n, being temperature dependent.

The values of SLc (for instance, S,, = 0.57 a t t = 120 OC) are in accordance with the most reliable data of NMR-spectroscopy (SLc = 0.568 [18]) and diamagnetic measurements (S,, = 0.57 [2]). Thus, the assumption that the guest and the host molecules 'are isomorphic (S,,, = S,,) is valid.

6. Order parameters. - It is reasonable to assume that the order parameter of guest molecules S,,, of the 'same length as liquid crystal molecules coincides with S,,. This assumption results from general ideas of

TABLE I

Calculated values of molecular polarizabilities (*) Compound

- PAA MBB A Merck-IV BEHA V, n = 2 V , n = 4 V,n = 7

(*) Error in y is approximately

+

STUDY OF NEMATIC LIQUID CRYSTALS C1-75

FIG. 7. - Order parameter (S) as a function of reduced tem- perature : 1 - MBBA, 2 - BEHA, 3 - Merck-IV.

In the same way we found the order parameters as a function of temperature for MBBA, Merck-IV and BEHA, see figure 7. The results for the substances V have been given earlier in [4] without correction for local field anisotropy which was taken into account here in calculations of molecular polarizabilities, Table I.

Now, it is possible to analyze the dependence of the parameter Sdye on the length of guest molecules and to discuss these results from the point of view of the statistical theory of nematic phase [17]. In figure 8 the dashed line, which gives the value of SLc as a function of the dimensionless energy parameter b, was nume- rically computed from eq. (1) by means of the equation

[*"

-- _{J 0}

cosZ 8 =

in"

(7) sin 8 d9

The parameter b describes also the guest-host interaction potential that, in accordance with [19, 201, is proportional to the linear term 2 a,, - a,, - a,,, where a,, are the components of the polarizability tensor of the guest molecule. For very long molecules a,, % a,,, a,, and b must be a linear function of the molecular length, b = tl. By measuring the dichroism of dyes K - l , 2 , 3 , 4 in BEHA and dyes K-2,3,4 in PAA one can obtain the dependences of Sdy, on molecular length I, figure 8. Each experimental curve was adjusted to the theoretical one in the following manner : since for dye K-2 S,,, was equal to SLc, b was defined from figure 8. Then if one measured the length of the K-2 molecule by means of a Stewart-Briegleb model, the coefficient

e

of

t

As can be seen from figure 8, Sdye(I) is neither pro- portional to I (or 2 a,, - a,, - a,,, as in [20]) nor coincides with the theoretical curve. The experimental curves are rather smooth and, in our opinion, this tendency reflects an influence of short-range forces on dynamics of molecules which weren't taken into account in the theory. In fact, to essentially increase Sdye one must choose the guest molecule of a length which is comparable with the size of a small cluster but not with the size of the individual host molecule.

FIG. 8. - 1 -calculated dependence S d y e on the energy parameter b ; 2, 3 - experimental curves ^{S d y e} as a function of length of dye molecules in nematic matrices PAA (2) and

BEHA (3).

7. Polarizabilities. -If the values of rill, nl, density (p) and molecular weight (M) of the substance are known, the principal polarizabilities ai of a nematic medium can be calculated with Vuks' formula [l 1, 14, 41 :

where No is Avogadro's number.

Using the relations

it is easy to find the values of longitudinal (yll) and transverse (7,) components of the molecular polariza- bility

S,,-values were deduced from dye dichroism data with the correction for local field anisotropy (according to Vuks). The calculations of yll and y, were performed in

C1-76 L. M. BLINOV, V. A, KIZEL, V. G. RUMYANTSEV AND V. V. TITOV

the whole temperature range of nematic phases. The obtain valid information about the molecular configu- data are presented in Table I. ration for a number of different guests resulting from

We have shown that the study of the dichroism of the known structure of nematic matrices [21].

certain guests is a very useful method of probing the

structure of liquid crystals (measurements of order Acknowledgments. - The authors are grateful to parameters, the determination of local field anisotropy, Drs. E. I. Kovshev, E. A. Lukjanetz, J. F. Freimanis, etc.). In principle, the inverse problem can also be L. F. Kozhokina, E. I. Balabanov and V. T. Lazareva solved ; namely, by measuring the dichroism we can for synthesis and purification of compounds studied.

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