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CIRCULAR DICHROISM IN LIQUID CRYSTALS
J. Shashidhara Prasad
To cite this version:
J. Shashidhara Prasad. CIRCULAR DICHROISM IN LIQUID CRYSTALS. Journal de Physique
Colloques, 1975, 36 (C1), pp.C1-289-C1-292. �10.1051/jphyscol:1975148�. �jpa-00216227�
JOURNAL DE PHYSIQUE Colloque C 1, suppl&ment au no 3, Tome 36, Mars 1975, page C1-289
Classification
Physics Abstracts
7.130
CIRCULAR DICHROISM IN LIQUID CRYSTALS
J. SHASHIDHARA PRASAD
Department of Physics, University of Mysore, Mysore 570 006, India
Rksumk. - Des mesures du dichroi'sme circulaire de cholesteriques purs et de melanges cholesterique-cholesterique et cholesterique-nematique ont ete faits pour plusieurs epaisseurs. Les rksultats exptrimentaux sont en bon accord avec les predictions thhriques sur 1a variation d u dichro'isme circulaire en fonction de I'kpaisseur.
Abstract. - Measurements of circular dichroism of pure cholesteric substance and mixtures of cholesteric-cholesteric and cholesteric-nematic substances have been made for several thicknesses.
The experimental data are in very good agreement with the theoretical predictions of the behaviour of circular dichroism with varying thickness.
Introduction. - The optical properties such as very high rotatory power, selective reflection, circular dichroism of cholesteric liquid crystals have been a subject of many theoretical [l-181 and experimental [4, 7, 10, 12, 15, 17 & 19-23] studies. One of the unique optical properties of cholesteric liquid crystals, viz., circular dichroism is occurred by the difference between the reflection coefficients for right and left hand cir- cularly polarized waves, instead of the absorption coefficients for them as is usual in the case of ordinary materials exhibiting the optical activity. One circularly polarized component of the incident beam is trans- mitted without attenuation, whereas the other is reflected. Depending on the material, either the left or the right circular polarized component may be affected.
The sense of the circular polarization of the reflected light remains the same as that of the incident light. This is contrary to what is observed in the reflection from normal dielectrics.
The characteristics of circular dichroism are clearly related to the selective reflection observed in cholesteric liquid crystals
( I ) .Essentially the various theoretical discussions predict a flat topped maximum for reflec- tion from a specimen of infinite thickness, signifying a total reflection. But for a film of finite thickness, the primary maximum will be flanked by the secondary maxima on either side in the region of selective reflec- tion. The wavelength separation form the central peak to the secondary peaks become larger when the thickness is reduced. A much thinner specimen shows a broadened out maximum having a very small value for reflection with no secondaries. Since R plotted as a
(1)
Infact, the relation between the circular dichroism and reflection for wave propagation along the helical axis at normal incidence is shown to be D
=R/2 - R (ref. [7]).
function of wavelength exhibits primary and secon- dary maxima, D plotted as a function of wavelength also exhibits the same, except that the maximum value and half width of the central lobe are slightly smaller.
Even though there are sufficiently large number of studies on the optical properties of cholesteric liquid crystals, very little work has been done on the experi- mental side of circular dichroism. We present in this study a large number of detailed and refined measure- ments of circular dichroism in pure cholesteric substance and mixtures of cholesteric-cholesteric and cholesteric-nematic (induced cholesterics) substances.
Experimental. - The circular dichroism measure- ments were made on plane texture preparations between perfect optical flats. Lead, aluminum and silver foil spacers were used for thicknesses 25, 15, 10 and 5 p respectively. Except cholesteryl cinnamate, all the other mixtures were liquid crystalline at room temperature (24 OC). Cholesteryl cinnamate was main- tained in the liquid crystalline range (1 50 OC) by placing the optical flats containing the substance at the centre of an electric oven with a stabilized power supply. An Electronics Corporation of India Ltd. (model GS 865) monochromator with a resolution better than 3 A was used for the light source. The circular dichroism was obtained by measuring the intensity of the transmitted light for right and left circular polarization and applying the formula
Circular polarization was obtained by a Fresnel Rhomb which gave perfectly polarized light to better than 98 %
between the wavelength range 4 000-8 000 A. This pro- cedure eliminated the error arising from the reflections
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975148
C1-290 J. SHASHIDHARA PRASAD
FIG. 1 . - Circular dichroism of cholesteryl cinnamate for FIG. 3. - Circular dichroism of a mixture of cholesteryl ditferent thicknesses : - 5 p, - - - 10 p, l o
=5 210 A. benzoate, cholesteryl acetate and cholesteryl palmitate -15 p,
- - - 25 p, l o = 5 320 A.
FIG. 2. - Circular dichroism of a mixture of cholesteryJ FIG. 4. - Circular dichroism of a mixture of cholesteryl benzoate, cholesteryl acetate anc cholesteryl palmitate -5 p, nonanoate and cholesteryl chloride - S p , - - - 10 p.
- - - 10 p, Lo = 5 320 -9. l o
=6008 -4.
from glass surfaces and spectral variation of the source and the sensitivity of the detector (photomultiplier tube was used as the detector), etc.
Figure 1 gives the circular dichroism measurements for pure cholesteric cinnamate for 5 and 10 p thick- nesses. ).,, the wavelength of maximum circular dichroism is 5 210 A. Figures 2 and 3 give the circular dichroism values for a mixture of equal parts by weight of cholesteryl benzoate, cholesteryl acetate and cholesteryl palmitate. The wavelength of maximum circular dichroism is 5 320 A. Figures 4 and 5 give the
data for a mixture of 0.120 3 gm. of cholesteryl nonanoate and 0.052 3 gm. of cholesteryl chloride with I-, 6 008 .A, for 5, 10, 15 and 25 p thicknesses. Figures 6 and 7 give the circular dichroism data for a mixture of 0.662 5 gm. of p-methoxybenzylidene-pf-butylaniline (MBBA), a room temperature nematic, and 0.478 0 gm.
of cholesteryl-oleyl carbonate, a room temperature
cholesteric substance. A, in this case is 4 250 A. The
measurements below 4 000 A are not very much reliable
because of the non perfection of circular polarization
below 4 000 A.
CIRCULAR DICHROISM IN LIQUID CRYSTALS C1-291
FIG. 5. - Circular dichroism of a mixture of cholesteryl nonanoate and cholesteryl chloride - 15 p, - - - 25 p,
l o
=6 008 A.
FIG. 7.
-Circular dichroism of a mixture of MBBA and cholesteryl oleyl carbonate - 15 p, - - - 25 p, l o
=4 250 A.
Discussion. - We note from the various measure- ments, how the band width varies for different substances (width of the principal lobe 200 to 350 A).
This clearly illustrates the variation of the width with dielectric anisotropy constant discussed by Aihara and - Inaba [9]. No attempt has been made here to give a quantitative idea of the variation of the width with the
- dielectric anisotropy constant which needs the refrac-
- tive index data. In all these cases we note that irrespec- tive of the width of the principal lobe, all the
- compounds exhibit secondary maxima, when the thicknesses considered are small and become flat topped without secondary lobes as the thickness is
- increased. These results agree very well with all the theoretical results obtained by several authors.
Acknowledgments. - The author wishes to thanks Prof. D. Krishnamurti for his interest and the Head of the Department of Physics, University of Mysore, and
-400
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200 7. 200 4007-7. IN A
