- Email: [email protected]
Optical rotatory power of liquid crystal mixtures
Volume 6. number 1
CHEMICAL PHYSICS LETTERS
1 July 1930
\
OPTICAL
ROTATORY
POWER
OF
LIQUID
CRYSTAL
MIXTURES
H. STEGEMEYER and K.-J. MAINUSCH XzcanN. Stmnski-Institlct (II. Institut fi& Pkysikaliscke Ckctnie) dev Technisclzen Universitiit Berlin, 1 Be-rtia 12, Germany
Eeceived 29 April I970
Addition of Z-menthol to a nematic mesophase produces a considerable optical rotatory power, the temperature dependence of which is in excellent agreement with the optical properties of cholesteric liquid crystals. Thus, the conclusion was drawn that small opticaily active molecules which do not form a mesophase transform nematic into uholesteric mesophases.
Nearly fifty years ago Friedel [l] described the close relationship between nematic and cholesteric liquid crystals. It is well known that cholesteric mesophases will only be formed by optically active molecules. In this connection a recent paper of Buckingham and coworkers [Z] is of great value; they pointed out that a continuous change of a nematic liquid crystal to a chol-. esteric mesophase may be achieved by adding small amounts of optically active molecules which do not form a mesophase by themselves. The light-scattering power has been taken as a tool to detect phase transformation nematic cholesteric. This method, however, seems not specific enough to evidence the occurrence of a cholesteric mesophase. Thus, we tested whether addition of small optically active molecules to nematic Iiqtiid crystals would produce a large optical rotatory power characteristic of a cholcsteric liquid crystai. The nematic solvent used was j+methoxybenzylidene-p’-n-butylaniline, MBBA (nematic range 20 - 41°C) [3]*, the optically active solute was I-menthol. The rotatory power was investigated by a Zeiss Kreispolarimeter O.OI” with a thermostated special sample holder, cell-path length 20 to 60 pm. Sample orientation was achieved by surface effects [4]. For experimental details we refer to the literature [5]. At wavelengths A, = 436, 546 and 578 nm ali solutions of I-menthol in M’BBA show a rotatory power Q of remarkable magnitude. The temperature dependence of 6 given in fig. 1 is -typical * We wish to thank Dr. Xelker, Farbwerke Hcechst AG, Frankfurt/&I. for a sample of MBBA.
+roo
-50
35
Fig. 1. Optical rotatory power * versus temperriture ‘2’(OC) of a solution of Z-menthol (I.77 mote 8) in MBBA at 546 nm (dashed line). 578 nm (solid Line).
for all solutions investigated. An analogous change of sign of Q, as a function of temperature was also observed in the case of cholesteric mesophases (e.g. choiesteryl myristate in MBBA) and seems to be characteristic for this type of liquid crystal, As a reference a solution of racemic menthol in MBBA was tested but gave no rotatory power. In fig. 2 the specific amplitude of rotatory power A@ = a,, - @min (cf. fe. 1) versus mole fraction of L-merrthol is eiven. Thk effect is largest at about 2 moIe 8 of Z-&en-thol. From these f&s we conclude with respect to the de Vries theory [6] that the optically active menthol molecules transform the nematic MBBA into a cholesteric mesophase, in excelient agree5
.: Vqlumc
G. nuinlwyi i
-
:.
‘.
. .. CftE&IIC.45,PHTSICSLETTERS
-.I July 1979
shown in fig. 3. The i&ease
of .& at lower tern-perarures (cf. fig. 1) cannot be explained with Clarity until nbw. -Possibly, .there is a connection tiith‘the increase of + from negative to positive values at X : A, which recently has been,derived from theoretical considerations [lot. ’ If this interpretation is’true then there should be a wavelength region of selective light scattering [tl.9] in the solutions of Z-menthol in MBBA. Thus a problem arose. since at 1.5 temperature could any reflection colour be observed. This dilemma can be solved in the following way: Solutions bl’ !-menthol in MBBA show a large value of birefringence. The difference between the refractive indices of the ordinary and extraordinary rays is &2 0.32 161. In pure cholesteric phases this value is smaller by about a factor of-ten. On the other hand it has been shown that the spectral width AX of the total reflexion band depends on-An by the equation .ment with the results of Buckingham [Zl. The shape of tile curve 6, versus T may be explained by the following assumptions:-(i) An anomalous rotatory dispersion (Cotton effect) which had been observed in the vicinity of the total refkexion band of -pure cholesteric phases f6 - 8j also appears in the case of I-menthol MBBA solutions. (ii) According to Fergascn [91 .- the wavelength AR of miutimum scattering (total reflexion band) is red-shifted with decreasing temperature. The same holy?: for the inversionwavelength A, of the CotLn curve G versus X. By correlating + versus x and >.. versus T the ob-served curve 3 versus T may be understood as
(for perpendicular incidence). where p is the pitch-of the helix [Sj. The value of p in the solutions mentioned above is of the order of magnitude of Am. An estimate results in a value of AX-2 100 nm,[5j. Thus. the totally reflecting range is so broad that no selective reflexion colours can be seen visually. The support of this work by the Gesellschaft von Freunden der Technischen Universitat Berlin is gratefully acknowledged. REFERENCES [11 C. Friedel. Ann.Phys. (Paris) 18 (1922) 273. 121 A. D. Buckingh.xm. G. P. Censar and M-B. Dunn. Chem. Phys. Letters 3 (196% 540. [3] H. Kclker and B.Scheurle. Anger. Chem. 81 (19G9) 903. [41 W. Kzst. 2. Elektrochem. 15 (1939) 194. 151 K.-J. Mninusch. Diplomnrbcit. TU Berlin (1970). unpublished. [G] K.de Vries. ActnCryst. 4 (1951) 219. [71 H. Sackmnnn and D; Demus. Fortwhr. Chem.
Fig. 5. Wavelength and temperature dependence ui the optical rotatory power 9 of z cholesteric-type liquid crystal.
6
Forsch. 12 (1969) 349. [Sj J. L. Fergnson. Mol. Cryst. 1 (1966) 293. [9] J. L..Fergason. N. P;. Goldberg an< R. J.Nodalin. Mel. Cryst. 1 11966) 309. [lo] S. Chnndmsekhar and K.h’. Srinivasa Rao. A&a Cryst. A24 (1968) 145.
CHEMICAL PHYSICS LETTERS
1 July 1930
\
OPTICAL
ROTATORY
POWER
OF
LIQUID
CRYSTAL
MIXTURES
H. STEGEMEYER and K.-J. MAINUSCH XzcanN. Stmnski-Institlct (II. Institut fi& Pkysikaliscke Ckctnie) dev Technisclzen Universitiit Berlin, 1 Be-rtia 12, Germany
Eeceived 29 April I970
Addition of Z-menthol to a nematic mesophase produces a considerable optical rotatory power, the temperature dependence of which is in excellent agreement with the optical properties of cholesteric liquid crystals. Thus, the conclusion was drawn that small opticaily active molecules which do not form a mesophase transform nematic into uholesteric mesophases.
Nearly fifty years ago Friedel [l] described the close relationship between nematic and cholesteric liquid crystals. It is well known that cholesteric mesophases will only be formed by optically active molecules. In this connection a recent paper of Buckingham and coworkers [Z] is of great value; they pointed out that a continuous change of a nematic liquid crystal to a chol-. esteric mesophase may be achieved by adding small amounts of optically active molecules which do not form a mesophase by themselves. The light-scattering power has been taken as a tool to detect phase transformation nematic cholesteric. This method, however, seems not specific enough to evidence the occurrence of a cholesteric mesophase. Thus, we tested whether addition of small optically active molecules to nematic Iiqtiid crystals would produce a large optical rotatory power characteristic of a cholcsteric liquid crystai. The nematic solvent used was j+methoxybenzylidene-p’-n-butylaniline, MBBA (nematic range 20 - 41°C) [3]*, the optically active solute was I-menthol. The rotatory power was investigated by a Zeiss Kreispolarimeter O.OI” with a thermostated special sample holder, cell-path length 20 to 60 pm. Sample orientation was achieved by surface effects [4]. For experimental details we refer to the literature [5]. At wavelengths A, = 436, 546 and 578 nm ali solutions of I-menthol in M’BBA show a rotatory power Q of remarkable magnitude. The temperature dependence of 6 given in fig. 1 is -typical * We wish to thank Dr. Xelker, Farbwerke Hcechst AG, Frankfurt/&I. for a sample of MBBA.
+roo
-50
35
Fig. 1. Optical rotatory power * versus temperriture ‘2’(OC) of a solution of Z-menthol (I.77 mote 8) in MBBA at 546 nm (dashed line). 578 nm (solid Line).
for all solutions investigated. An analogous change of sign of Q, as a function of temperature was also observed in the case of cholesteric mesophases (e.g. choiesteryl myristate in MBBA) and seems to be characteristic for this type of liquid crystal, As a reference a solution of racemic menthol in MBBA was tested but gave no rotatory power. In fig. 2 the specific amplitude of rotatory power A@ = a,, - @min (cf. fe. 1) versus mole fraction of L-merrthol is eiven. Thk effect is largest at about 2 moIe 8 of Z-&en-thol. From these f&s we conclude with respect to the de Vries theory [6] that the optically active menthol molecules transform the nematic MBBA into a cholesteric mesophase, in excelient agree5
.: Vqlumc
G. nuinlwyi i
-
:.
‘.
. .. CftE&IIC.45,PHTSICSLETTERS
-.I July 1979
shown in fig. 3. The i&ease
of .& at lower tern-perarures (cf. fig. 1) cannot be explained with Clarity until nbw. -Possibly, .there is a connection tiith‘the increase of + from negative to positive values at X : A, which recently has been,derived from theoretical considerations [lot. ’ If this interpretation is’true then there should be a wavelength region of selective light scattering [tl.9] in the solutions of Z-menthol in MBBA. Thus a problem arose. since at 1.5 temperature could any reflection colour be observed. This dilemma can be solved in the following way: Solutions bl’ !-menthol in MBBA show a large value of birefringence. The difference between the refractive indices of the ordinary and extraordinary rays is &2 0.32 161. In pure cholesteric phases this value is smaller by about a factor of-ten. On the other hand it has been shown that the spectral width AX of the total reflexion band depends on-An by the equation .ment with the results of Buckingham [Zl. The shape of tile curve 6, versus T may be explained by the following assumptions:-(i) An anomalous rotatory dispersion (Cotton effect) which had been observed in the vicinity of the total refkexion band of -pure cholesteric phases f6 - 8j also appears in the case of I-menthol MBBA solutions. (ii) According to Fergascn [91 .- the wavelength AR of miutimum scattering (total reflexion band) is red-shifted with decreasing temperature. The same holy?: for the inversionwavelength A, of the CotLn curve G versus X. By correlating + versus x and >.. versus T the ob-served curve 3 versus T may be understood as
(for perpendicular incidence). where p is the pitch-of the helix [Sj. The value of p in the solutions mentioned above is of the order of magnitude of Am. An estimate results in a value of AX-2 100 nm,[5j. Thus. the totally reflecting range is so broad that no selective reflexion colours can be seen visually. The support of this work by the Gesellschaft von Freunden der Technischen Universitat Berlin is gratefully acknowledged. REFERENCES [11 C. Friedel. Ann.Phys. (Paris) 18 (1922) 273. 121 A. D. Buckingh.xm. G. P. Censar and M-B. Dunn. Chem. Phys. Letters 3 (196% 540. [3] H. Kclker and B.Scheurle. Anger. Chem. 81 (19G9) 903. [41 W. Kzst. 2. Elektrochem. 15 (1939) 194. 151 K.-J. Mninusch. Diplomnrbcit. TU Berlin (1970). unpublished. [G] K.de Vries. ActnCryst. 4 (1951) 219. [71 H. Sackmnnn and D; Demus. Fortwhr. Chem.
Fig. 5. Wavelength and temperature dependence ui the optical rotatory power 9 of z cholesteric-type liquid crystal.
6
Forsch. 12 (1969) 349. [Sj J. L. Fergnson. Mol. Cryst. 1 (1966) 293. [9] J. L..Fergason. N. P;. Goldberg an< R. J.Nodalin. Mel. Cryst. 1 11966) 309. [lo] S. Chnndmsekhar and K.h’. Srinivasa Rao. A&a Cryst. A24 (1968) 145.