This article was downloaded by:[Tokyo Kogyo University]
On: 6 June 2008
Access Details: [subscription number 788763157]
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Phase Transitions
A Multinational Journal
Publication details, including instructions for authors and subscription information:
http://www.informaworld.com/smpp/title~content=t713647403

Physical characterisation of 4'-butyl-4-heptylbicyclohexyl-4-carbonitrile

Surajit Dhara a; N. V. Madhusudana baSchool of Physics, University of Hyderabad, Hyderabad 500046, Indiab
Raman Research Institute, Bangalore 560080, India
Online Publication Date: 01 June 2008
To cite this Article: Dhara, Surajit and Madhusudana, N. V. (2008) 'Physicalcharacterisation of 4'-butyl-4-heptyl- bicyclohexyl-4-carbonitrile', Phase Transitions,
81:6, 561 — 569
To link to this article: DOI: 10.1080/01411590801975401
URL: http://dx.doi.org/10.1080/01411590801975401

PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf
This article maybe used for research, teaching and private study purposes. Any substantial or systematic reproduction,
re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly
forbidden.
The publisher does not give any warranty express or implied or make any representation that the contents will becomplete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be
independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings,
demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or
arising out of the use of this material.

Downloaded By: [Tokyo Kogyo University] At: 09:11 6 June 2008

Phase Transitions
Vol. 81, No. 6, June 2008, 561–569

Physical characterisation of 40 -butyl-4-heptylbicyclohexyl-4-carbonitrile
Surajit Dharaa* and N.V. MadhusudanabaSchool of Physics, University of Hyderabad, Hyderabad 500046, India;
Raman Research Institute, C.V. Raman Avenue, Bangalore 560080, Indiab(Received 7 December 2007; final version received 29 January 2008)
We report measurements of birefringence, bend elastic constant, principal
dielectric constants and conductivities as functions of temperature in the nematic
phase of 40 -butyl-4-heptyl-bicyclohexyl-4-carbonitrile (CCN-47). The compound
exhibits a small birefringence and a large negative dielectric anisotropy.
Surprisingly, it shows a negative conductivity anisotropy, which probably owes
its origin to the structural details of the molecules. The orientational orderparameters (S) are estimated from both birefringence and dielectric anisotropy,
and they agree well in the entire nematic range. The experimental results arecompared with those on CCH7, which has a similar core structure but witha longitudinal dipole moment, in place of the transverse one of CCN-47.
Keywords: liquid crystals; birefringence; dielectric anisotropy; elastic constant

1. Introduction
A large number of nematic liquid crystals with elongated molecules exhibit positive
dielectric anisotropy whereas a comparatively smaller number of compounds are known to
exhibit negative dielectric anisotropy. Liquid crystals with positive dielectric anisotropy
have been investigated extensively because of their industrial importance, and also as
compounds with strong longitudinal dipoles exhibit smectic polymorphism [1].
The study of liquid crystal materials with negative dielectric anisotropy are also
becoming important after the successful development of active matrix liquid crystal
displays using the vertically aligned mode, in-plane switching mode etc. In this paper, we
report the measurements of the temperature dependences of birefringence, dielectric andconductivity anisotropies and bend elastic constant of a nematic liquid crystal with a large
negative dielectric anisotropy. We also compare these results with the results on acompound with a similar molecular core structure, but having a longitudinal dipole
moment.

*Corresponding author. Email: sdsp@uohyd.ernet.in
ISSN 0141–1594 print/ISSN 1029–0338 online
ß 2008 Taylor & Francis
DOI: 10.1080/01411590801975401
http://www.informaworld.com

562

S. Dhara and N.V. Madhusudana

Downloaded By: [Tokyo Kogyo University] At: 09:11 6 June 2008

2. Experimental
The sample of 40 -butyl-4-heptyl-bicyclohexyl-4-carbonitrile (CCN-47) was obtained from
Merck and used without further purification. The chemical structure of the molecule isshown in Figure 1. It has the following sequence of phase transitions: Cr 28 SmA 30.6 N
59.7 I. The compound exhibits a wide temperature nematic range and a low temperaturesmecticA phase, which easily supercools to room temperature. The compound ischemically stable. Systematic dielectric investigations on the homologous series of CCN
compounds, except CCN-47, were reported by Haase et al. [2]. Detailed high electric field
experiments on CCN-47 have been reported by us recently [3–4].
Planar aligned sample cells were prepared by using two ITO (indium tin oxide)-coatedglass plates, which were treated with polyimide and cured at a temperature of 280 C for
90 min before being rubbed unidirectionally. For the homeotropic alignment the plates
were treated with ODSE (octadecyloxy silane) and cured at 150 C for 90 min. The plates of
the cell were separated by appropriate spherical glass beads and sealed with epoxy glue.
The thickness of the cell was measured by an interference technique. Typical cell thickness
used for the experiments were in the range of 6–16 mm. The electrical connections to thetwo plates were made through copper wires, which were soldered using an ultrasonicsoldering gun. The cell was filled with the sample in the isotropic phase and on slow
cooling to the nematic range, a well-aligned sample was obtained. All the experiments wereconducted while cooling the sample from the isotropic phase.
The block diagram of the experimental setup is shown in Figure 2. The cell was
mounted in a hot stage (INSTEC HS1) which itself was kept on the rotating stage of apolarising microscope (model Leitz, ORTHOPLAN). The temperature was controlled toan accuracy of 0.008 C. The dielectric properties of the sample were measured using a
lock-in amplifier (SRS-830). The sinusoidal output of the internal oscillator of the lock-in
amplifier was adjusted to an rms value of 0.2 V, and applied to the cell which was inseries with a 1 mF standard capacitor Cm as shown in Figure 2. The amplitude and phase of
the applied voltage were also checked using a reference circuit with two standard resistors(R1 ¼ 1M and R2 ¼ 100 ) in series, connected in place of the cell. The phase (’m) and therms amplitude (Vm) of the voltage measured across the series capacitor (Cm) were used to
calculate the capacitance (CS) and parallel resistance (RS) corresponding to the cell byusing the following relations [5]Y,! sin X
CS ¼ ,Y

RS ¼

Figure 1. The energy minimised configuration of a CCN-47 molecule.

ð1Þ
ð2Þ

563

Downloaded By: [Tokyo Kogyo University] At: 09:11 6 June 2008

Phase Transitions

where X ¼ cos   Q, Y ¼ sin2 þ X2/CmQ, Q ¼ Vm/Vo,  ¼ m  m, V0 and 0 are theamplitude and phase of the input signal, respectively and ! ¼ 2f, f being the frequency of
the applied signal. The typical frequency used in the experiment is 4111 Hz. We havechecked the calibration of the setup by connecting standard capacitors in parallel withstandard resistors in place of the liquid crystal cell. The measured values agree with thestandard values to within 3%. We have also checked the calibration by measuring
the capacitance and resistance using a Wayne–Kerr bridge which operates at 1.6 kHz.
The dielectric constant is given by the ratio CS/C0, where C0 is the capacitance of theempty cell.
A laser (He–Ne,  ¼ 632.8 nm) beam is passed through a polariser (POL) and made to
be incident on the sample. The transmitted beam is passed through an analyser (ANL),
which is crossed with respect to the polariser. A photodiode (PD1) is used to measure thetransmitted intensity. The stability of the laser intensity is monitored by anotherphotodiode (PD2). A multimeter (Keithley model 2000) measures the output voltages ofboth the photodiodes. The temperature variation of the transmitted intensity is measured
in the planar aligned sample. The transmitted intensity is given by

It ¼

sin2 2ð1  cos ’Þ2ð3Þ

where  is the angle made by the polariser with the optic axis and the phase difference
’ ¼2nðdÞ:


ð4Þ

PD1
MUL
ANL

HEATER

CELL

Cm
BS
R2

R1

POL

PD2

LASER

LIA

COM

Figure 2. Schematic diagram of the experimental setup: photodiodes (PD1, PD2), polariser (POL),
analyser (ANL), multimeter (MUL), voltage amplifier (AMP) lock in amplifier (LIA), resistances(R1, R2), computer (COM).

S. Dhara and N.V. Madhusudana

n ¼ ne  no, where ne and no are the extraordinary and ordinary refractive indices of theliquid crystal medium, and d is the sample thickness. The angle  is set at 45 to optimise
the measurements. The birefringence is calculated from the measured intensity.
The measurements are completely controlled by a computer, using a suitable program.

3. Results and discussion
The minimum energy configuration of the molecule was obtained using a standard
software package (Gaussian 98). The optimised structure is shown in Figure 1.
The estimated transverse dipole moment is 3.14D. The variation of n over a wide
temperature range (20 C) in the nematic phase is shown in Figure 3. n is 0.016 at
0.5 C below TNI and increases to 0.031 at 20 C below TNI. The relatively small opticalanisotropy reflects the saturated nature of the rings of the molecules. We had measured
earlier [6] the temperature dependence of birefringence of CCN-47 using a tilting
compensator and cells of thicknesses 1.5 and 6.7 mm. The birefringence and hence the orderparameter measured in the two cells were found to agree [6]. In the present study, thebirefringence has been measured from the transmitted intensity on a cell of thickness
16.8 mm and the values of n are a few percent lower than those reported earlier.
Meausurements on other samples show that the birefringence and hence the orderparameter increase as the thickness of the cell is decreased, whose origin is not yet
0understood [6]. The birefringence of trans, trans-4-n-heptyl-4 -cyanobicyclohexane(CCH-7) has been measured by Ibrahim et al. [7]. It has a molecular core structure
similar to that of CCN-47, but with a longitudinal orientation of the polar cyano groupinstead of the transverse one in the latter. We also show the variation of n of CCH-7 withtemperature in Figure 3. CCH-7 exhibits 30% larger birefringence near the NI transition
point compared to CCN-47, and the difference is increased as the temperature is loweredin the nematic phase. The longitudinal dipole moment in CCH-7 increases the longitudinalpolarisability and hence the birefringence in CCH-7 is larger as compared to that of
CCN-47 with a transverse dipole moment. Further, the temperature variation of n is
steeper in CCH-7 compared to that in CCN-47. In CCH-7 the strongly polar cyano groupis essentially along the long axis of the molecule, leading to antiparallel near neighbour
interactions [8]. This leads to the formation of antiparallel pairs, as seen by X-ray

Birefringence (∆n)

Downloaded By: [Tokyo Kogyo University] At: 09:11 6 June 2008

564

0.04

CCH-7
CCN-47

0.02

0.00
05

10
15
TNI−T

20

25

Figure 3. Variations of n as functions of temperature in CCN-47 and CCH-7. Open squares are
the experimental data on CCN-47 and the continuous line is a theoretical fit to Equation (6). Cell
thickness: 16.8 mm. Open circles correspond to data of CCH-7 obtained from ref. [7] and the dottedline is drawn as a guide to the eye.

565

scattering studies [9]. The pair formation can be expected to become efficient at lower
temperatures of the nematic phase, which effectively increases the molecular length andhence the anisotropies in polarisabilities and refractive indices. In CCN-47 the dipoles are
essentially in a transverse direction, and in the absence of the above mechanism, thetemperature variation of birefringence is weaker.
We have measured the perpendicular component of dielectric constant ("? ) using ahomogeneously aligned sample and parallel component of dielectric constant ("jj ) using ahomeotropically, aligned sample. The cell thicknesses used for the experiments are 8.5 and
7 mm, respectively, in the two cases. Both the measurements are made at an applied voltage
of 0.2 V and a frequency of 4111 Hz. The applied voltage is much lower than the thresholdvoltage for Freedericksz transition in the homeotropic cell. Electro-hydrodynamic
instability was not seen in the nematic phase in either geometry, except very close to thenematic–smecticA transition temperature. The variations of dielectric constants "jj and "?
are shown as functions of temperature in Figure 4. The dielectric constant in the isotropic
phase obtained from the two different geometries matched to within 1%. "? is larger than"jj in the nematic as well as in the smectic phases, the N-SmA transition occurring at
TNI  T ¼ 29 C. The variation of "jj across the N-SmA transition temperature is smoothwhereas a small increase is observed in "? , reflecting increased correlations of thetransverse dipoles in the layered structure. The dielectric anisotropy " ¼ ð"jj  "? Þ near
the nematic–isotropic transition point is about 3 and increases to about 6 at 30 C
below TNI. The mean dielectric constant " is calculated using the formula " ¼ ð"jj þ 2"? Þ=3.
At TNI, " ¼ "is  7:2 and as the temperature is lowered to the N-SmA transition point, "increases. The increase in " is 4% at TNI 15 C (N phase) and 10% at TNI 30 C (inthe SmA phase). The contribution of the molecular dipole moment to the average
dielectric constant is given by [10]NA  hF
F2
 þ
:
ð5Þ
" ¼ 1 þ 4M
3kB T
where NA is the Avogadro number,  the density, M the molecular weight, h, is the is thecavity field factor and F is the reaction field factor for spherical cavity and ,ε⊥10Dielectric constantDownloaded By: [Tokyo Kogyo University] At: 09:11 6 June 2008

Phase Transitions

8

6

ε

ε is

ε ||
4
0

5

10

15

20

25

30

TNI−T

Figure 4. Variations of parallel ("jj ) and perpendicular ("? ) components of dielectric constant asfunctions of temperature. Applied voltage: 0.2 V and frequency: 4111 Hz. "is is the dielectric constantin the isotropic phase. The mean value in the nematic phase is " ¼ ð"jj þ 2"? Þ=3, which is shown by
a dotted line. The vertical arrow denotes N-SmA transition temperature.

S. Dhara and N.V. Madhusudana

average polarisability. The second term in the bracket is due to the contribution of theorientational polarisation of the dipole moments. " can increase with decreasing
temperature due to the increase in the density () and the contribution from orientation
polarisation. Experimentally, it is reported that in a non-polar compound " increases by
2% at 15 C below NI transition temperature due to an increase in density alone [11].
The excess variation in the highly polar compound used by us is due to the contribution of
the orientation polarisation, which increases with decreasing temperature as seen in
Equation (5). It may be pointed out that in the case of cyanobiphenyl compounds there is aslight increase in the " when the temperature is increased from nematic to isotropic phase
due to an reduction in short range antiferroelectric order [8]. In the case of CCH
compounds, " exhibits a maximum as the temperature is increased above the NI transition
point. This trend was interpreted [12] as arising from the formation of antiparallel dimers
at lower temperatures, due to local antiferroelectric correlations between the highly polarend groups, as was discussed earlier. In CCN-47 with a transverse dipolar group, such anincrease is not observed. Similar observations have also been made by other workers onother compounds of the homologous series of CCN [2].
We have also measured the anisotropic ionic conductivities of the sample asfunctions of temperature at a frequency 4111 Hz. The variation of normalised conductivity
= is shown in Figure 5 where the mean conductivity  is calculated using the formula ¼ ðjj þ 2? Þ=3. Curiously, the perpendicular component of conductivity (? ) is larger
than the parallel component (jj ) in the nematic as well as in the smectic phases. Both theconductivities vary continuously across the NI transition. The conductivity anisotropy
 ¼ ðjj  ? Þ at TNI 3 C is 1.5  108(ohm m)1 and increases to6  108(ohm m)1 at 30 C below TNI. At TNI,   9.6  108(ohm m)1 and as thetemperature is lowered to the N-SmA transition point,  decreases to 9  108(ohm m)1. The negative anisotropy of conductivity in the entire liquid crystalline range of
CCN-47 is very unusual. In the nematic phase made of rod like molecules,  is usually
positive, as the ions can move more freely along the director than in a perpendicular
direction [1]. In the SmA phase, the ion motion is subjected to the periodic smectic
potential along the director, and the resulting hopping motion can lower jj substantially.
On the other hand, the ion motion along the liquid layers is not affected by the smectic

σ (ohm-m)−1 ×10−8

Downloaded By: [Tokyo Kogyo University] At: 09:11 6 June 2008

566

1.1

0.8

0.6

0

10

20

30

TNI−T

Figure 5. Variations of normalised conductivities ? = (open squares) and jj = (open circles) as
functions of temperature. Applied voltage: 0.2 V and frequency: 4111 Hz. The mean value of the
conductivity in the nematic phase is  ¼ ðjj þ 2? Þ=3, which is shown by a line.

567

potential, and  often changes sign near the N-SmA transition point [13], becoming
negative in the smectic phase. In the case of CCN-47, the negative anisotropy even close to
TNI probably owes its origin to the chemical structure of the compound. The unknown
ions in the sample are likely to be associated with highly polarisable species. These will
have a much larger affinity to the cyano group of the CCN-47 molecule (Figure 1) than to
the remaining weakly polarisable part. When the ions move under the action of an external
electric field, presumably, they tend to do so by hopping between the cyano groups ofneighbouring molecules. The statistically averaged separation between the cyano groups
can be expected to be considerably smaller in a direction orthogonal to the director than in
one parallel to it, in view of the rod shape of the molecules. This can in turn lead to thenegative conductivity anisotropy. As the temperature is lowered towards the smectic
phase, the smectic like short range order builds up, and the anisotropy of averageseparation, and hence that of the conductivity, also get enhanced (Figure 5).
We can estimate the orientational order parameter in the liquid crystalline phases using
the birefringence as well as dielectric anisotropy data. S ¼ n/no. The birefringence noof a completely ordered sample is determined from the temperature dependence of n,which can be approximated well for nematic liquid crystals by [14]Tn ¼ no 1 :ð6Þ
T1
where no, T1 and are adjustable parameters. A least squares fit to the experimental datawith Equation (6) is shown in Figure 3. The value of the fitted parameters are no ¼ 0.0576
and ¼ 0.23. The value of is typical for nematic liquid crystals [1]. The variation oforientational order parameter is shown as a function of temperature in Figure 6. The jumpin the order parameter at TNI is approximately 0.27. The orientational order parameter
was also estimated using the formula S  "/"o. The value of "o was estimatedcomparing the order parameter value measured from n data at TNI  T ¼ 1 C.
The temperature variations of the order parameter estimated from the two sets ofexperimental data agree well in the entire temperature range except very close to the
N-SmA transition point.
We have also measured the Freedericksz threshold voltage (Vth) at several
temperatures in a homeotropically aligned cell. The measurement is made at a frequencyof 4111 Hz and the voltage was increased in steps of 10 mV. The threshold voltage is given

Order parameter (S)Downloaded By: [Tokyo Kogyo University] At: 09:11 6 June 2008

Phase Transitions

0.6

0.4

0.2

0.0
0

5

10

15

20

25

TNI−T

Figure 6. Variation of orientational order parameter (S) with temperature estimated from thebirefringence data (solid circles) and from dielectric data (open squares).

568

S. Dhara and N.V. Madhusudana

k3 (× 10−12 N)

Downloaded By: [Tokyo Kogyo University] At: 09:11 6 June 2008

1.2

0.8
CCH-7
CCN-47
0.4

0

5

10

15

20

25

30

TNI−T

Figure 7. Variations of the bend elastic constant K3 as functions of temperature in CCN-47
and CCH-7. Points are experimental data and the continuous lines are drawn as guides to the eye.
Open circles: CCN-47 and open squares: CCH-7(from ref. [15]).

pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
by Vth ¼  K3 ="o " where " now stands for the magnitude of dielectric anisotropy,
K3 is the bend elastic constant and "o is the dielectric constant of the free space. Using Vthand " (Figure 4) we calculate the bend elastic constant (K3). The variation of K3 as afunction of temperature is shown in Figure 7. K3 is 0.2  1012 N at the temperature of
TNI  1 C and follows an S2-dependence near TNI, where S is the order parameter. K3
increases rapidly as the nematic to smectic transition point is approached, as the smectic
like short range order builds up [1]. The temperature variation of K3 of CCH-7 was
measured by Schad et al. [15], and that data is also shown in Figure 7. It is found that K3 in
CCN-47 is smaller in the entire nematic range compared to that of CCH-7. The higher
value in CCH-7 probably owes its origin to the formation of antiparallel dimers which
increase the effective length of the molecules, as described earlier.

4. Conclusions
We have measured the birefringence, dielectric and conductivity anisotropies and the bendelastic constant of the compound 40 -butyl-4-heptyl-bicyclohexyl-4-carbonitrile as functionsof temperature. The material exhibits a low birefringence and a large negative dielectricanisotropy. The estimated order parameters from the birefringence and dielectricanisotropy agree well in the entire nematic range. Unusually, the compound also exhibitsa negative conductivity anisotropy probably because of the chemical structure of thecompound. Electrohydrodynamic instabilities (EHD) are not observed in the nematic
phase, as the Carr–Helfrich mechanism does not work when both  and " arenegative [1]. However, the compound exhibits a weak EHD near the smectic transition
point under a low frequency ac field, which probably owes its origin to a flexoelectricmechanism [16]. The measured bend elastic constant (K3) increases as the temperature islowered in the nematic phase and diverges near the nematic–smectic phase transition due
to the rapid increase in smectic correlation length.

References[1] P.G. de Gennes and J. Prost, The Physics of Liquid Crystals, 2nd ed., Clarendon, Oxford, 1993.
[2] D. Ganzke, S. Wrobel, and W. Haase, Mol. Cryst. Liq. Cryst. 409 (2004), p. 323.

Downloaded By: [Tokyo Kogyo University] At: 09:11 6 June 2008

Phase Transitions

569[3] S. Dhara and N.V. Madhusudana, Europhys Lett. 67 (2004), p. 411.
[4] ——, Eur. Phys. J. E. 22 (2007), p. 139.
[5] ——, J. Appl. Phys. 90 (2001), p. 3482.
[6] ——, Eur. Phys. J. E. 13 (2004), p. 401.
[7] I.H. Ibrahim and W. Haase, Mol. Cryst. Liq. Cryst. 66 (1981), p. 189.
[8] N.V. Madhusudana and S. Chandrasekhar, Pramana Suppl. 1 (1975), p. 57.
[9] G.J. Brownsey and A.J. Leadbetter, J. Phy. Lett. 42 (1981), p. 135.
[10] W. Maier and G. Meier, Z. Naturforsch. 16a (1961), p. 262.
[11] D.A. Dunmur, A. Fukuda, and G.R. Luckhurst, Liquid Crystals: Nematics, An Inspec
Publication, p. 271.
[12] M.J. Bradshaw and E.P. Raynes, Mol. Cryst. Liq. Cryst. 72 (1981), p. 73.
[13] L.M. Blinov, M.I. Barnik, V.T. Lazareva, and A.N. Trufanov, J Phys. 40 (1979), p. c3-263.
[14] I. Haller, Prog. Solid State Chem. 10 (1975), p. 103.
[15] H. Schad and M.A. Osman, J. Chem. Phys. 75 (1981), p. 880.
[16] N.V. Madhusudana and V.A. Raghunathan, Mol. Cryst. Liq. Cryst. Lett. 5 (1988), p. 201.