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The effect of an electric field on the selective reflection bands of liquid crystalline blue phases
CHEMICAL PHYSICS LETTERS
Volume 125, number 4
11 April 1986
THE EFFECT OF AN ELECTRIC FIELD ON THE SELECTIVE REFLECTION BANDS OF LIQUID CRYSTALLINE BLUE PHASES * F. PORSCH and H. STEGEMEYER Institute of Physical Chemistry, Received 26 November
University Paderborn, D-4790 Paderborn, Federal Republic of Germany
1985; in final form 20 February 1986
The effect of an electric field on the Bragg reflection of the liquid-crystalline cubic blue phases BP I and II has been studied. The results can be explained by a reorientation of the cubic BP lattice in the electric field. A BP II/I phase transition cannot be induced by the field in the systems under examination.
1. Introduction Cholesteric liquid crystals with small helical pitches exhibit up to three blue phases (BPS) in a small temperature span (
2. Experimental
butyl)-biphenyl, CB 15 (BDH); nematogenic 4-cyano4’-hexoxy-biphenyl, M 18 (BDH); nematogenic mixture ZLI 1612 (Merck). The mixtures form cholesteric phases with negative dielectric anisotropy * and at least two BPS. The samples were placed between tin dioxide coated glass slides without special surface treatments, separated by 12 pm mylar spacers. The cell was driven by a squarewave generator, providing O-140 Vat 400 Hz. The temperature was controlled by a Mettler heating stage FP 5215. The reflection spectra were measured by recording the “absorbance” with a Cary 17 spectrometer with the light beam parallel to the field direction.
3. Results The mixtures under investigation exhibit two BPS: the low-temperature modification BP I and the hightemperature one BP II. As an example, we first describe the results obtained for a mixture CB 15/M 18 with 58 mol% CB 15. The behaviour of the other mixtures under investigation is quite similar and is discussed in section 3.3.
In order to obtain a strong dielectric anisotropy, mixtures of the following cyanobiphenyl derivatives were used: cholesterogenic 4-cyano-4’~(2.methyl* Dedicated to Professor Horst Sackmann on the occasion his 65th birthday.
of
0 009-2614/86/$03.500 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
The sign of the dielectric anisotropy is given with respect to the optical axis of the cholesteric phase contrarily to the designation
given by other authors [4,8].
319
3.1. Ri?silh
11 April 1986
CHEMICAL PHYSICS LETTERS
Volume 125, numbed 4
for BP i
The BP I of the mixture CB 15/M 18 (58 mol% CB 15) exhibits two SR bands in the visible: a strong band at 440 nm and a weak one at 600 nm, when the sample is prepared by heating up the cholesteric Grandjean texture (fig. la). In the following we refer to these bands as the blue and red reflection, respectively. From fig. la the effect of an ac electric field
a
I 44
0
b 11
10
4
I
I
100
500
0
I
-__
600
700
-
20
60
LO Fv
-
Qhm
Fig. 2. Wavelength of the SR bands versus applied voltage for BP I (a) and BP II (b); data from fig. 1. Blue reflection =, red reflection A (for the split blue reflection the center of the reflection band is given as AR).
6O.lV 5sov 50,ZV 45.1 V 40.1 V 31.9 v 30.5V 1OBV 0 V
i
I
I
LOO
500 -
hhm
Fig. 1. SR spectra of BP I (a) and BP II (b) of a mixture CB
1S/M 18 with 58 mol% CB 15 as a function of the applied voltage (sample thickness 12 Mm). Temperature: (a) 25 .3S°C; (b) 25.6O’C.
320
applied to the sample can be seen: (1) With increasing voltage the blue reflection band is split into two maxima. This splitting is reversible, i.e. on lowering the voltage the splitting vanishes. (2) On further increasing the voltage the intensity of the red reflection increases whereas the blue reflection decreases more and more. (3) Finally, only the red reflection survives, exhibiting a strong increase of the waveIen~ with field strength (cf. fig. Za). If the field strength is decreased the blue reflection does not reappear but the red one remains, with its wavelength decreasing down to 600 nm at zero field.
Volume 125, number 4
11 April 1986
CHEMICAL PHYSICS LETTERS
3.2. Resultsfor BP II The behaviour of BP II is rather similar to that of BP I. The sample exhibits two reflection bands at 375 and 505 nm. Again the blue reflection vanishes with increasing field strength while the red reflection becomes more intense (cf. fig. lb). The latter one is strongly redshifted with increasing voltage (cf. fig. 2b). This shift is reversible but with a relaxation time of several minutes, as was also found by Heppke et al. [6]. A shift of the blue reflection could not be measured, because this band vanishes too rapidly to be measured when a field is applied, especially at higher field strengths. The red reflection persists if the voltage is switched off, but the blue reflection does not reappear. 3.3. Resultsfor other mixtures The behaviour of the “red” reflection has been studied for several other mixtures (cf. caption of fig. 3). Intensive red reflections have been produced by application of strong electric fields to the sample, then switching the voltage off. After waiting several minutes an appropriate field was again switched on to measure the field dependence of the reflected wavelength, A(E). The zero-field reflection wavelengths of the mixtures under investigation vary from 470 to 780 mn. To compare the results of the various mixtures, the reflection wavelengths X(E) are normalized to the zero-field wavelength A(0). Furthermore, a normalized field strength E/E(BC) is introduced, where E(BC) is the field strength required for inducing the BP-tocholesteric phase transition [8]. The representation X(E)/X(O)versus E/E(BC) given in fig. 3b shows a single function for all BP II investigated with no discontinuity. A single function does not exist in the case of BP I (cf. fig. 3a). This may be due to experimental uncertainty, because both the reflection wavelength and E(BC) are strongly temperature-dependent for BP I. The function in fig. 3b may be compared with that of cholesterics [ 111. There are two striking differences: (i) the reflection wavelength of the BPSis already effected by rather weak fields, and (ii) there is a de& nite limit of the wavelength at the phase transition to the cholesteric phase.
. *a
. A
1.0
I
0
m,
.:
0 ,
,
,
,
,
I
0.5
E-ec Fig. 3. Wavelength of the SR bands versus applied voltage for BP I (a) and BP II (b) for different mixtures GxUtIPlethichw 12 Mm). n 47 mol% CB 15/M 18 (a: 36.OO”C; b: 36.50°C), A 52 mol% CB 15/M 18 (a: 30.7O“C; b: 31.20°C), l 58 mol% CB 15/M 18 (a: 25.35”C; b: 25.6o”C), l 70 mol% CB 15/M 18 (a: 12.90°C), o 57 mol% CB IS/ZLI 1612 (b: 30.3O”C).
4. Discussion Both BPSexhibit a switch in the reflection wavelength if an electric field is applied: the initial blue reflection band vanishes and a new one appears at larger wavelength (cf. fig. 1). This switch was also observed by Heppke et al. [6] in both BPs,but they only discussed that of the BP II. The SR switch of BP I, clearly to be seen in fig. 3 of their paper [6] is disregarded. Heppke et al. interpret the switch in BP II as a field321
Vohame 125, number 4
CHEMICALPHYSICS LETTERS
induced phase transition BP II/BP I. However, the switch is not caused by a phase transition because: (i) the red reflection is often observed without an electric field ad~tion~y to the blue one depending on the sample preparation [ 121, (ii) the red SR band remains when the field is switched off, and (iii) microscopic observation does not show a meld-educed phase transition BP XI/BPI [7]. Rather the switch of the reflection wavelength is caused by texture change: the orientation of the BP lattice is altered by the electric field. Because the reflection is of the Bragg-type, the new orientation has another SR wavelength. For normal incidence the wavelength is given by $kl=
zndhkl
3
(1)
where $$kl is the vacuum wavelength, n the refractive index of the sample and dhkr the spacing of the (h/cl) crystallographic planes. For cubic lattices with lattice parameter a, the spacing is given by dhkr = a/(h2 f k2 t 12)112.
(2)
The observed blue and red reflections have a zero-field wavelength ratio 1: 1.37 in both BPS.According to other authors the reflections of BP1 are attributed to the (200) and (110) crystallographic planes [2,13] while the reflections of BP II are assigned to the (L&O) and (hO0) planes (h = 1,2) [9], respectively. From the SR results available to date [8,14] it cannot be differentiated between h = 1 and 2. According to the Bragg equation (1) these resections form the wavelength ratio I : 21iz, in good agreement with the experimental results. The discontinuity in the selective reflection wavelength on increasing voltage observed in BP II by Pieranski et al. [lo] must not necessarily be caused by a phase transition but can also be explained by a lattice reorientation in the field. The observed cont~uous red-shift of the red reflection with increasing field strength indicates an increase of dhkl. Since the variation of the refractive index is expected to be negligible 181,the field shift of the “red” reflections of both BP I and BP II can be interpreted as a lattice expansion of about 15%. However, there is only a small shift of the “blue” reflections. This means that the expansion depends on the orientation of the crystal lattice within the Fteld, i.e. the expansion rate is a tensorial property. This implies a field-induced lowering of the cubic symmetry and 322
11 April 1986
hence a field-induced birefringence. Indeed, a fieldinduced optical biaxiality has been observed in BP I [9]. Generally, three lattice parameters are required in describing a deformed cubic structure. The existence of a preferred lattice orientation seems to be in contradiction to a cubic structure of the BP, which possesses dielectric isotropy. But this discrepancy may be solved by taking into account non-linear dielectric behaviour [7,9]. By this, the most stable orientation of an “isotropic” medium is that with the largest increase of the dielectric constant in the direction of the field, i.e. that with the strongest “deformation”. This matches with the experimental results: the preferred orientation exhibits a large lattice expansion. What is the preferred orientation of each BP in terms of crystallographic axes? The “red” reflection of BP I has been assigned to the (110) crystallographic plane and that of BP II to the (hO0) plane. Hence, the preferred stable lattice orientations of the two BPSare not the same: for BP I the axis 11IO] is parallel to the field direction, for BP II, however, flOO].
Acknowledgement . This work has been supported by the Deutsche Forschungsgemeinschaft , the Ministerium fur Wissenschaft und Forschung des Landes Nordrhein-Westfalen and the Fonds der Chemischen Industrie. The authors are indebted to Professor P.J. Collings and Dr. K. Hiltrop for useful discussions.
Referemxs (11 Th. Bhimel, P.J. Colhngs, H. Onusseit and H. Stegemeyer, Chem. Phys. Letters 116 (1985) 529. [ 21 S. Meiboom and M. Sammon, Phys. Rev. Letters 44 (1980) 882; D.L. Johnson, J.H. PIack and P.P. Crooker, Phys. Rev. Letters 45 (1980) 641. (31 H. Grebel, R.M. Hornreich and S. Shtrikman, Phys. Rev. A28 (1983) 1114. [4] D. Armitage and R.J. Cox, Mol. Cryst. Liquid Cry&. Letters 64 (1980) 41; G. Heppke, H.-S. Kitzerow and M. Krumrey, 10th International Liquid Crystals Conference, York, UK, July IS-21,1984, Mol. Crust. Liquid Cryst. Letters, to be published.
Volume 125, number 4
CHEMICAL PHYSICS LETTERS
[5] P.L. Finn and P.E. Cladis, Mol. Cryst. Liquid Cryst. 84 (I 982) 159. [6] G. Heppke, M. Krumrey and F. Oestreicher, Mol. Cryst. Liquid Cryst. 99 (1983) 99. [7] H. Stegemeyer and F. Porsch, Phys. Rev. A30 (1984) 3369. [S] G. Heppke, H.-S. Kitzerow and M. Krumrey, Mol. Cryst. Liquid Cryst. Letters, to be published.
11 April 1986
191 F. Porsch, H. Stegemeyer and K. Hiltrop, Z. Naturforsch. 39a (1984) 475. [lo] P. Pieranski, P.E. Cladis and R. Barbet-Massin, J. Phys. (Paris) 46 (1985) L-973. [ 1 I] F.J. Kahn, Phys. Rev. Letters 24 (1970) 209. [ 121 H. Onusseit, Thesis, University Paderborn (1983) pp. 18-22. [ 131 V.A. Kizel and V.V. Prokhorov, JETP Letters 38 (1984) 337; Soviet Phys. JETP 60 (1984) 257.
323
Volume 125, number 4
11 April 1986
THE EFFECT OF AN ELECTRIC FIELD ON THE SELECTIVE REFLECTION BANDS OF LIQUID CRYSTALLINE BLUE PHASES * F. PORSCH and H. STEGEMEYER Institute of Physical Chemistry, Received 26 November
University Paderborn, D-4790 Paderborn, Federal Republic of Germany
1985; in final form 20 February 1986
The effect of an electric field on the Bragg reflection of the liquid-crystalline cubic blue phases BP I and II has been studied. The results can be explained by a reorientation of the cubic BP lattice in the electric field. A BP II/I phase transition cannot be induced by the field in the systems under examination.
1. Introduction Cholesteric liquid crystals with small helical pitches exhibit up to three blue phases (BPS) in a small temperature span (
2. Experimental
butyl)-biphenyl, CB 15 (BDH); nematogenic 4-cyano4’-hexoxy-biphenyl, M 18 (BDH); nematogenic mixture ZLI 1612 (Merck). The mixtures form cholesteric phases with negative dielectric anisotropy * and at least two BPS. The samples were placed between tin dioxide coated glass slides without special surface treatments, separated by 12 pm mylar spacers. The cell was driven by a squarewave generator, providing O-140 Vat 400 Hz. The temperature was controlled by a Mettler heating stage FP 5215. The reflection spectra were measured by recording the “absorbance” with a Cary 17 spectrometer with the light beam parallel to the field direction.
3. Results The mixtures under investigation exhibit two BPS: the low-temperature modification BP I and the hightemperature one BP II. As an example, we first describe the results obtained for a mixture CB 15/M 18 with 58 mol% CB 15. The behaviour of the other mixtures under investigation is quite similar and is discussed in section 3.3.
In order to obtain a strong dielectric anisotropy, mixtures of the following cyanobiphenyl derivatives were used: cholesterogenic 4-cyano-4’~(2.methyl* Dedicated to Professor Horst Sackmann on the occasion his 65th birthday.
of
0 009-2614/86/$03.500 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
The sign of the dielectric anisotropy is given with respect to the optical axis of the cholesteric phase contrarily to the designation
given by other authors [4,8].
319
3.1. Ri?silh
11 April 1986
CHEMICAL PHYSICS LETTERS
Volume 125, numbed 4
for BP i
The BP I of the mixture CB 15/M 18 (58 mol% CB 15) exhibits two SR bands in the visible: a strong band at 440 nm and a weak one at 600 nm, when the sample is prepared by heating up the cholesteric Grandjean texture (fig. la). In the following we refer to these bands as the blue and red reflection, respectively. From fig. la the effect of an ac electric field
a
I 44
0
b 11
10
4
I
I
100
500
0
I
-__
600
700
-
20
60
LO Fv
-
Qhm
Fig. 2. Wavelength of the SR bands versus applied voltage for BP I (a) and BP II (b); data from fig. 1. Blue reflection =, red reflection A (for the split blue reflection the center of the reflection band is given as AR).
6O.lV 5sov 50,ZV 45.1 V 40.1 V 31.9 v 30.5V 1OBV 0 V
i
I
I
LOO
500 -
hhm
Fig. 1. SR spectra of BP I (a) and BP II (b) of a mixture CB
1S/M 18 with 58 mol% CB 15 as a function of the applied voltage (sample thickness 12 Mm). Temperature: (a) 25 .3S°C; (b) 25.6O’C.
320
applied to the sample can be seen: (1) With increasing voltage the blue reflection band is split into two maxima. This splitting is reversible, i.e. on lowering the voltage the splitting vanishes. (2) On further increasing the voltage the intensity of the red reflection increases whereas the blue reflection decreases more and more. (3) Finally, only the red reflection survives, exhibiting a strong increase of the waveIen~ with field strength (cf. fig. Za). If the field strength is decreased the blue reflection does not reappear but the red one remains, with its wavelength decreasing down to 600 nm at zero field.
Volume 125, number 4
11 April 1986
CHEMICAL PHYSICS LETTERS
3.2. Resultsfor BP II The behaviour of BP II is rather similar to that of BP I. The sample exhibits two reflection bands at 375 and 505 nm. Again the blue reflection vanishes with increasing field strength while the red reflection becomes more intense (cf. fig. lb). The latter one is strongly redshifted with increasing voltage (cf. fig. 2b). This shift is reversible but with a relaxation time of several minutes, as was also found by Heppke et al. [6]. A shift of the blue reflection could not be measured, because this band vanishes too rapidly to be measured when a field is applied, especially at higher field strengths. The red reflection persists if the voltage is switched off, but the blue reflection does not reappear. 3.3. Resultsfor other mixtures The behaviour of the “red” reflection has been studied for several other mixtures (cf. caption of fig. 3). Intensive red reflections have been produced by application of strong electric fields to the sample, then switching the voltage off. After waiting several minutes an appropriate field was again switched on to measure the field dependence of the reflected wavelength, A(E). The zero-field reflection wavelengths of the mixtures under investigation vary from 470 to 780 mn. To compare the results of the various mixtures, the reflection wavelengths X(E) are normalized to the zero-field wavelength A(0). Furthermore, a normalized field strength E/E(BC) is introduced, where E(BC) is the field strength required for inducing the BP-tocholesteric phase transition [8]. The representation X(E)/X(O)versus E/E(BC) given in fig. 3b shows a single function for all BP II investigated with no discontinuity. A single function does not exist in the case of BP I (cf. fig. 3a). This may be due to experimental uncertainty, because both the reflection wavelength and E(BC) are strongly temperature-dependent for BP I. The function in fig. 3b may be compared with that of cholesterics [ 111. There are two striking differences: (i) the reflection wavelength of the BPSis already effected by rather weak fields, and (ii) there is a de& nite limit of the wavelength at the phase transition to the cholesteric phase.
. *a
. A
1.0
I
0
m,
.:
0 ,
,
,
,
,
I
0.5
E-ec Fig. 3. Wavelength of the SR bands versus applied voltage for BP I (a) and BP II (b) for different mixtures GxUtIPlethichw 12 Mm). n 47 mol% CB 15/M 18 (a: 36.OO”C; b: 36.50°C), A 52 mol% CB 15/M 18 (a: 30.7O“C; b: 31.20°C), l 58 mol% CB 15/M 18 (a: 25.35”C; b: 25.6o”C), l 70 mol% CB 15/M 18 (a: 12.90°C), o 57 mol% CB IS/ZLI 1612 (b: 30.3O”C).
4. Discussion Both BPSexhibit a switch in the reflection wavelength if an electric field is applied: the initial blue reflection band vanishes and a new one appears at larger wavelength (cf. fig. 1). This switch was also observed by Heppke et al. [6] in both BPs,but they only discussed that of the BP II. The SR switch of BP I, clearly to be seen in fig. 3 of their paper [6] is disregarded. Heppke et al. interpret the switch in BP II as a field321
Vohame 125, number 4
CHEMICALPHYSICS LETTERS
induced phase transition BP II/BP I. However, the switch is not caused by a phase transition because: (i) the red reflection is often observed without an electric field ad~tion~y to the blue one depending on the sample preparation [ 121, (ii) the red SR band remains when the field is switched off, and (iii) microscopic observation does not show a meld-educed phase transition BP XI/BPI [7]. Rather the switch of the reflection wavelength is caused by texture change: the orientation of the BP lattice is altered by the electric field. Because the reflection is of the Bragg-type, the new orientation has another SR wavelength. For normal incidence the wavelength is given by $kl=
zndhkl
3
(1)
where $$kl is the vacuum wavelength, n the refractive index of the sample and dhkr the spacing of the (h/cl) crystallographic planes. For cubic lattices with lattice parameter a, the spacing is given by dhkr = a/(h2 f k2 t 12)112.
(2)
The observed blue and red reflections have a zero-field wavelength ratio 1: 1.37 in both BPS.According to other authors the reflections of BP1 are attributed to the (200) and (110) crystallographic planes [2,13] while the reflections of BP II are assigned to the (L&O) and (hO0) planes (h = 1,2) [9], respectively. From the SR results available to date [8,14] it cannot be differentiated between h = 1 and 2. According to the Bragg equation (1) these resections form the wavelength ratio I : 21iz, in good agreement with the experimental results. The discontinuity in the selective reflection wavelength on increasing voltage observed in BP II by Pieranski et al. [lo] must not necessarily be caused by a phase transition but can also be explained by a lattice reorientation in the field. The observed cont~uous red-shift of the red reflection with increasing field strength indicates an increase of dhkl. Since the variation of the refractive index is expected to be negligible 181,the field shift of the “red” reflections of both BP I and BP II can be interpreted as a lattice expansion of about 15%. However, there is only a small shift of the “blue” reflections. This means that the expansion depends on the orientation of the crystal lattice within the Fteld, i.e. the expansion rate is a tensorial property. This implies a field-induced lowering of the cubic symmetry and 322
11 April 1986
hence a field-induced birefringence. Indeed, a fieldinduced optical biaxiality has been observed in BP I [9]. Generally, three lattice parameters are required in describing a deformed cubic structure. The existence of a preferred lattice orientation seems to be in contradiction to a cubic structure of the BP, which possesses dielectric isotropy. But this discrepancy may be solved by taking into account non-linear dielectric behaviour [7,9]. By this, the most stable orientation of an “isotropic” medium is that with the largest increase of the dielectric constant in the direction of the field, i.e. that with the strongest “deformation”. This matches with the experimental results: the preferred orientation exhibits a large lattice expansion. What is the preferred orientation of each BP in terms of crystallographic axes? The “red” reflection of BP I has been assigned to the (110) crystallographic plane and that of BP II to the (hO0) plane. Hence, the preferred stable lattice orientations of the two BPSare not the same: for BP I the axis 11IO] is parallel to the field direction, for BP II, however, flOO].
Acknowledgement . This work has been supported by the Deutsche Forschungsgemeinschaft , the Ministerium fur Wissenschaft und Forschung des Landes Nordrhein-Westfalen and the Fonds der Chemischen Industrie. The authors are indebted to Professor P.J. Collings and Dr. K. Hiltrop for useful discussions.
Referemxs (11 Th. Bhimel, P.J. Colhngs, H. Onusseit and H. Stegemeyer, Chem. Phys. Letters 116 (1985) 529. [ 21 S. Meiboom and M. Sammon, Phys. Rev. Letters 44 (1980) 882; D.L. Johnson, J.H. PIack and P.P. Crooker, Phys. Rev. Letters 45 (1980) 641. (31 H. Grebel, R.M. Hornreich and S. Shtrikman, Phys. Rev. A28 (1983) 1114. [4] D. Armitage and R.J. Cox, Mol. Cryst. Liquid Cry&. Letters 64 (1980) 41; G. Heppke, H.-S. Kitzerow and M. Krumrey, 10th International Liquid Crystals Conference, York, UK, July IS-21,1984, Mol. Crust. Liquid Cryst. Letters, to be published.
Volume 125, number 4
CHEMICAL PHYSICS LETTERS
[5] P.L. Finn and P.E. Cladis, Mol. Cryst. Liquid Cryst. 84 (I 982) 159. [6] G. Heppke, M. Krumrey and F. Oestreicher, Mol. Cryst. Liquid Cryst. 99 (1983) 99. [7] H. Stegemeyer and F. Porsch, Phys. Rev. A30 (1984) 3369. [S] G. Heppke, H.-S. Kitzerow and M. Krumrey, Mol. Cryst. Liquid Cryst. Letters, to be published.
11 April 1986
191 F. Porsch, H. Stegemeyer and K. Hiltrop, Z. Naturforsch. 39a (1984) 475. [lo] P. Pieranski, P.E. Cladis and R. Barbet-Massin, J. Phys. (Paris) 46 (1985) L-973. [ 1 I] F.J. Kahn, Phys. Rev. Letters 24 (1970) 209. [ 121 H. Onusseit, Thesis, University Paderborn (1983) pp. 18-22. [ 131 V.A. Kizel and V.V. Prokhorov, JETP Letters 38 (1984) 337; Soviet Phys. JETP 60 (1984) 257.
323