Lawrence Verbit State University of New York at Binghamton, New York 13901
Liquid Crystds-Synthesis
An experiment for the integrated organic and physical laboratory
The division of matter into solids, liquids, and gases is not an absolute classification; many substances of intermediate properties are known. One example is the class of compounds known as liquid crystals. The reawakening of interest in this fascinating but long-neglected area has occasioned the introduction of the subject at the undergraduate level. Because most textbooks presently give only passing reference to liquid crystals, the following laboratory experiments are preceded by a brief introduction (1). Most organic solids have sharp melting points at which the crystal undergoes a change to an isotropic liquid. However, certain compounds pass through an intermediate stage upon heating; they melt to a liquid, often cloudy, which exhibits many properties usually associated with the crystalline state. At a higher temperature the cloudy liquid suddenly changes to a clear, isotropic liquid. The first observation of this phenomenon was published in 1885 by Friedrich Reinitzer, an Austrian botanist (2). While determining the then unknown formula of cholesterol, Reinitzer prepared cholesteryl benzoate and observed that it melted to a turbid liquid at 14G.G°C and then became clear at 180.G°C. It is interesting to note from our vantage point almost 85 years later that because of his meticulous laboratory work, Reinitzer was convinced of the correctness of his results, even though he did not understand their cause. Since 1888, approximately 2000 compounds have been found to possess the unusual properties of what are commonly referred to as liquid crystals. The intermediate liquid state has been given a number of different names in addition to liquid crystal such as "ordered liquid," "anisotropic fluid," "crystalline liquid," and "mesophase" (Greek "meso" = between). Whichever term is used, it is important to make clear that the phase in question is a true liquid, taking the shape of its container, and forming a perfect sphere when a drop is placed in an immiscible liquid of equal density. However, liquid crystal phases differ from normal liquids in that the molecules in them are oriented to a certain degree and hence, they exhibit
' Those liquid crystals whose phase transitions are formed by means of heat. Lyatropic liquid crystals, where the mesophases are produced by the forces of solvation are not considered here. However, the best known members of this latter group, aqueous soap solutions, deserve mention. Animtropic-not the same in all directions. Anisotropic substances m e usually more highly ordered than isotropic ones, with preferred directions and hence, properties whose values depend upon the direction in which they are measured. 36
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Journal o f Chemicol Education
and Properties
some properties characteristic of the crystalline state. The weak intermolecular attraction resulting in the observed orientation is favored by a rod shaped molecular geometry. It is found that most liquid crystals have a large molecular length-to-width ratio and are relatively rigid. Some representative examples are given in the table. Two broad classes of thermotropic liquid crystals1 are recognized; the smectic and nematic. I n the older literature a cholesteric phase is described. We now know that this phase is a twisted nematic liquid crystal. The smectic phase is the most highly ordered with the molecules arranged in layers, their long axes parallel to each other, and the layers aligned so that a plane may be passed between them (see (a) in the figure). The term smectic (Greek soap-like) was coined in 1922 by the French scientist Friedel (3) who studied the birefringent patterns (a typical crystal property) of mesophases under the polarizing microscope and noted the similarity.of the smectic mesophase to that observed for soap solutions. The viscosity of the smectic mesophase is generally higher than that of the other two types of mesophases and because the planes can slide over each other in the liquid, the viscosity is ani~otropic.~ Smectic liquids, as is the case with the nematic type, behave optically in the sa,me way as uniaxial crystals. That is, there are two indices of refraction so that the speed of light passing through a smectic liquid is de-
Schematic representation of molecular order in liquid cryrtolr: l a ) m e c t i s phase Idrowing represents Smectic A), each block represents a stratum; ( b ) nematic phore; and (cl M i r t e d nematic kholestericl phase.
Transition Data for Some Representative Liquid Crystals Transition temperatures, 'C
Compound
cH
,
,
O
~
C
~
~
~
~
~
~
~
Ref.
H
~
~
Alkyl groups are normal except where specified. Symbols usedare: K, crystal: S, smectic; N, nematic; C, twisted nematic (cholesteric); I, isotropic.
pendent upon the direction in which it is travelling. Such double refraction (birefringence) is characteristic of an ordered structure and gives rise to the beautiful colors exhibited by liquid crystals under the polarizing microscope. The nematic phase is less ordered than the smectic, with the main restriction being that on the average the molecules maintain a parallel arrangement of their long axes (see (b) in the figure). The term nematic, from the Greek word for thread, refers to the threadl i e lines observed with the polarizing microscope. These lines represent discontinuities in the sample hetween independently ordered regions. A unique property of nematic liquids and one of potential commercial importance, is that under certain conditions of disturbance, such as caused by the passage of ions, the liquid displays discontinuity lines of sufficient size to cause light scattering. As a thin film between transparent electrodes, the nematic liquid appears clear. The passage of an extremely small current causes the liquid to scatter light and appear opaque. Removal of the current restores the liquid to its clear state. This intruiging electrooptic property of nematic liquids, termed "dynamic scattering," is finding use in new types of display devices (4) (see Experimental). The twisted nematic liquid crystal, sometimes called the cholesteric liquid, appears in many derivatives of cholesterol. Other steroids, as well as several nonsteroid compounds, also exhibit the twisted nematic mesophase. A common feature of this type of molecule is that they are optically active. The model of this structure is that of a layered nematic liquid
twisted about an axis perpendicular to the molecular layers (see (c) of the figure). A gradual change in molecular orientation, presumably due to the chirality of the molecules, is found on going in the direction of the twist axis, giving a helical macrostructure to the mesophase. This unique molecular architecture gives rise to several interesting optical properties, perhaps the most spectacular of which is the display of various colors over a definite temperature range. Certain cholesterol compounds, although colorless themselves, are able to selectively scatter light into different colors. The pitch of the twisted nematic helix determines the color of the reflected light. Physical effects which wind or unwind the helix (such as temperature or pressure) result in observed color changes. Not all cholesterol derivatives give rise to vivid colors but mixtures may be chosen so that as the temperature is raised, the reflected colors range through the entire visible sped ~ m . ~ A few more points serve to complete the student's introduction to liquid crystals. A compound may possess only a single mesophase, say a nematic, in which case two phase transitions will occur. AH.'
crystal
AHb'
nematic liquid
73isotropic liquid
Thermodynamically, these phase transitions are exactly the same kind as occur during the melting or boiling of non-liquid crystal substances. That is, AGO 8 Plastic encapsulated cholesteric mixtures displaying these color changes are sold in toy shops under names such as "TouchMe" and "Fidgit."
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1972
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so that AHo = TASO and the transitions are of the first-order (a discontinuity in the heat capacity curve occurs together with a phase change). Some compounds possess more than one liquid crystal phase (polymorphism). For example, a compound may have a smectic phase which on further heating changes to a nematic phase. See the table for some examples. Our students carry out the synthesis of two types of liquid crystals, one a twisted nematic (cbolcsteric) and the other a nematic, during the course of one or two laboratory periods. The following session is devoted to a study of selected physical properties. Although a student has his choice of synthesis the class ratio is close to 50:50, perhaps because both reactions require similar levels of effort. The preparation of cholesteryl benzoate parallels Reinitzer's original work (2). However, the use of benzoyl chloride and pyridine as solvent cuts the reaction time to five minutes and leads to yields of 6&75%. = 0
HO
cholesterol
mesophase which a t 178-17(IDC becomes clear. A hlne-violet coloration is ohsowed upon cooling of the isotropic melt to the turbid mesophase and again upon solidificat,ionof the sample. Para-mdho~ybenz~jlidcnc para-n-hut$anilina (AfnnA). Pammethoxybenmldchyde (6.8 5 , 0.01, mole! and para-n-hut,ylaniline (7.5 g, 0.0.7 mole) nre placed in a 21,-n11 round bott,om flask fifibt,ed wlth a reflux condenser. Five millilit,ers of absolute ethanol and s few boiling rhilx are added and the solniion is refl~txedfor 1 hr. The flask is then set, np for disl.illstion and t,he ethanol-water azent,rope is removed. The yellow, opaque MBBA remaining in the flask is taken np in 40 ml of ether, dried ( a t least 1 hr) over 94% yield, mhyd. Na801, and filtered. The MBBA, 12.6 g, still eont,ains a. smxll amount of impurities (residnal ethanol, water, ethcr, starting materials) which serve to lower the phase transit,ions somewhat. However, it is sufficiently pure to exhibit the characteristic properties of the nematic mesophase. I n a capillary tuhe, the MBRA has a e~ystal-nematicpoint at, 14-1RT to give a cloudy yellow liqldd whirhundergoes anemalic-isotropic transition a t 38-39°C. If desired, it may be further purified by recrystallisation from methanol a t low temperature (6). For a pure compound one needs to vacnum distill and dry over PsOi. If this is done the transition temperature is raised to 47-4RDC. Ezamination in Polarized 1,ight. A drop of MBBA between 18 mm round microscope cover glasses is placed on the stage of a polarizing microscope. Notice the chnraeteristic threaded text,ure (dark lines) of the nematic meeophase between crossed polarieers and the diszppearanee of birefrin~ence when the polarizers are uncrossed. Experiment with the effect8 of cover slip displacement on the orientation of the sample. Raise the temnerature' snd observe the nematic-isotropic transition (loss of birefringence). If the polarizing mierascope is equipped with a suitable heating stage, the phase transitions of cholesteryl benzoate and the charartnriabio mosaic texture of the twisted nematic (cholesteric) phase may be examined. Dwamie Scattering. Using a thin Teflon or Mylar spacer' (0.00025 to 0.001 in. thickness) to confine it, place a drop of MBBA on one of two pieces of electrically conduct,ivo glass.8 Complete the sandwich by covering the liquid crystsl with the other piece of glass, offsetting them so that alligator clip electrical leads may he attached to each bus bar. Two spring clips of the type a.vailahle in stationery stores serve to hold the glass plates firmly together. Note that as a thin film the MBBA appears clear and transmits light. Now apply a dc or s c v o l t q e (30-60 V) and obselw the cell become opaque. This is the dynamic scattering mode (4). Place a small mirror behind the cell and note that the brightness of the reflected light is a direct function of the inciden t light.
-
~
crystal
149C '
0
I1
wsc-0 1780C
twisted nematic & isotropic (cholesteric)
In contrast to cholesteryl benzoate, first reported in 1888, the nematic example is adapted from a recent paper (5) which describes the first synthesis of a room temperature nematic liquid crystal, para-methoxybenzylidene para-n-butylaniline (MBBA). The student preparation differs from that given by Kelker and Scheurle (5) in its use of azeotropic rather than vacuum distillation.
lPC
40-C
crystal d nematic ti isotropic
The reaction of carbonyl compounds with amine derivatives to give imines (Schiff's bases) and water is a common one in organic chemistry. The reactions usually require acid catalysts, but in the presence of absolute ethanol MBBA forms rapidly on heating and yields around 90% are obtained. Experimental4 Cholesteryl Benzoate. Cholesterol (9.7 g, 0.025 mole) and benzoyl chloride (4.2 g, 0.03 mole)$ are dissolved in 25 ml of pyridine and refluxed for 3-5 min. The solution is allowed to cool slowly and the product is filtered by su6tion, then washed with a little cold pyridine followed by cold ethoxyethanol. Recrystallization from ethyl scetste (50-80 ml) affords approximately 8.6 g (70% yield) of cholesteryl benzoate as white granules. I n a capillary tuhe using a stirred oil bath, the ester exhibits a. crystal-twisted nematic transition a t 149-150°C to give s. cloudy
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Journal of Chemical Education
Literature Cited (1) Several books and
review articles have appeared reoently: nnows, G. H., DOAWE, J. W., AND Nmw. Y. D.. "Critical Review of Solid State Phyaios." 1, 303 (19711; BROWN, G. 11. (cdiloi). "Liquid Crystals 2," Parts I and 11. Gordon and Breach Science Publishers, Ine.. New York. N. Y.; nnowli. 0. H.. DIENBB. G. .I.. A N D LAAE., M. M. (editors). "Liquid Crystals," Gordon and Breach Science Publishers, Inc.. New York. N. Y., Gnnr. G. W.. "Moleeulhr Stmolure and t h e Properties of Liquid Crystkls," Aoademio Press. New York. N. Y., 1962; Bnowlr, G. H., A n d . Chem., 41. 26A (19691; SAOPE, A,. Angeu. Chem. Intarnot. Ed%, 7.97 (1968): Bnoww, G. H.,
Chemistry, 40, 10 (1967). (2) R m w ~ ~ z eF., n . Monatsh. Chem., 9,421 (1888). (3) FRIEDEL. O., Ann. Phyaique (Pans).18,273 (1922). (4) H e r ~ a ~ r G. ~ nH.. , Scientijtc Amw.. 222, 100 (1970).
'
Temperatures are correct,ed. Starting materials were E a s e man White Label which could be used without further purification. We find it convenient to use a stock solution of benzoyl chloride in pyridine, 0.4 g/mL The excess reagent takes into account the heneoic acid present in commercial benaayl chloride. ' A convenient variable temperature stage which will fit most L. AND H.Ar,saaT, T. It., microscopes is described by V ~ R B I T J. CHBM.EDUC.,48, 773 (1971). Warm sir fram a hair dryer may also be used without harming the microscope. 7 Mylar film is available from Brownell, Inc., 8.5 Tent,h Am., New Yark, N. Y. 10011. We have found Type 50s (0.00030 in.) to he most satisfactory. 8 A 2 X 2L/2in. piece of tin oxide coat,ed glass with a silver bus bar, avaihhle fram Trsri-Light Corp., Cincinnati, Ohio 45242, costs approximittely $1.75.
(5) Km~ran.H..AND SCHB=RL=.B.. AWCUI. Chsn. Intermt. Edit.. 8, 884 (1989). K ~ r eI. . A.. J . CREW. Eonc.. 46.697 (1969). K s ~ r ~ H.. n . AND S o a ~ o n m B.. . J . Physiqus. 30.C4.104 (1969). WIEQAND. C.. 2.Nolololff~~~h.. 4B, 249 (1949). G A ~ E R X A NL., N , * l i ~RITBCHRE, A,, Ber., 23, 1738 (1890).
(6) (7) (8) (9)
(10) (11) (12) (13) (14) (15)
Grim. G. W.. J . Chsn. Soc.. 552 (1958). YOUNG, W. R., Mol. Crysl. ondliq. Cryat., 10,237 (1970).
K x l o n ~G. . A,. AND SHAW, B. D., J . Cham. Soc., 682 (1938). V o n ~ i r r o sD., ~ , Bar.. 43,3132 (1910). V e ~ e mI ,..AND Toooer, R. L..Mol. Cryat. and Lip. Cwsi., in press. WEYOAND. C.. A N D G*BLER,R.. 2.Phvsik. Chem..B46,270 (1940).
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