Observation of exceptional 'de Vries-like' properties in a conventional aroylhydrazone based liquid crystal

Article abstract of DOI:10.1039/c6ra10028g

Smectic liquid crystals with 'de Vries-like' properties are characterized by a maximum layer contraction of ≤1% upon transition from the non-tilted SmA phase to the tilted SmC phase. A straight alkoxy chain-terminated conventional aroylhydrazone based liq

Full text of DOI:10.1039/c6ra10028g

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Observation of exceptional ‘de Vries-like’
properties in a conventional aroylhydrazone based
Cite this: RSC Adv., 2016, 6, 57799
liquid crystal†
Hemant Kumar Singh,‡a Sachin Kumar Singh,‡a Rajib Nandi,b D. S. Shankar Rao,c
Received 18th April 2016
Accepted 23rd May 2016
c
b
a
*
S. Krishna Prasad, Ranjan K. Singh and Bachcha Singh
DOI: 10.1039/c6ra10028g
www.rsc.org/advances
Smectic liquid crystals with ‘de Vries-like’ properties are characterized tilted SmC phase.4,7–15 This phenomenon stems from the diffuse
by a maximum layer contraction of #1% upon transition from the non- cone model proposed by de Vries that describes the SmA phase
tilted SmA phase to the tilted SmC phase. A straight alkoxy chain- as a lamellar structure in which mesogens have a tilted molec-
terminated conventional aroylhydrazone based liquid crystal N-(4- ular orientation and random azimuthal distribution.16 Accord-
(tetradecyloxy)-2-hydroxybenzylidene)-40((400(hexadecyloxy)benzyl) ing to this idealized model, the SmA–SmC transition is
oxy)benzohydrazide (BSac-1416) undergoes the SmA–SmC phase described as an ordering of the azimuthal distribution that
transition with a maximum layer contraction of 0.65%. The merits of results in zero layer contraction, as shown in Fig. 1. The simple
the de Vries reduction factor, i.e. R and f values for this mesogen, are reorientation of molecules at the SmA–SmC transition in de
comparable to, or even lower than, those reported for established Vries-like liquid crystals is advantageous for practical applica-
perfluorinated and polysiloxane-terminated bona fide de Vries-like tions. For instance, the low layer contraction avoids the
materials.
formation of a chevron structure and zigzag defects that usually
occur in SmA–SmC transitions of conventional ferroelectric
(SmC*) liquid crystals and that are detrimental for the optical
quality of electro-optical devices.17 High electroclinic effects
(response, tilt angle, and birefringence) can be achieved at low
voltages as compared with typical electroclinic materials on
these materials.18–20
The structure of this unusual phase has yet to be fully
elucidated, but recent theoretical studies suggest that materials
combining low orientational order and high lamellar order are
likely to exhibit this behavior.22,23 Three examples of liquid
crystal materials known to have very low layer shrinkage and
large electroclinic susceptibilities are considered as bona de
‘de Vries-like’ liquid crystals. These are the siloxane-terminated
mesogen TSiKN65 and its carbosilane-terminated analogue
Smectic A (SmA) and smectic C (SmC) liquid crystals are layered
phases possessing quasi-long-range positional order in one
dimension represented by a mass-density wave.1 The wave
vector of this density wave is along the director in the case of the
SmA phase, while it is tilted in the case of the SmC phase. Owing
to this feature, the layer spacing is signicantly lower in the
tilted SmC phase than in the orthogonal SmA phase.2 Maximum
layer-shrinkage in a typical SmA–SmC transition can be as high
as 11%.3,4
In 1972, Diele et al. reported a material in which the layer
spacing was similar in the SmA and SmC phases.5 To explain
this feature, de Vries suggested a new type of SmA phase in
which the molecules are tilted with respect to the layer normal
just as in the SmC phase but the tilt directions of different layers
are uncorrelated.6 These are now known as the de Vries smectic
phases and are characterized by a maximum layer contraction
of #1% upon transition from the non-tilted SmA phase to the
aDepartment of Chemistry (Centre of Advance Study), Institute of Science, Banaras
Hindu University, Varanasi-221005, India. E-mail: bsinghbhu@rediffmail.com
bDepartment of Physics (Centre of Advance Study), Institute of Science, Banaras Hindu
Fig. 1 Schematic representation of the molecular arrangement in (a)
SmA (b) one type of SmAdV and (c) SmC phases. The molecules are
along the layer normal in the SmA, while being tilted in the SmAdV and
SmC phases. To be noted is the fact that the tilt is azimuthally corre-
lated in the SmC but not in the SmAdV phase (dV: de Vries).21
University, Varanasi-221005, India
cCentre for Nano and So Matter Sciences, Jalahalli, Bangalore-560013, India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra10028g
‡ Authors have equal contribution.
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W599,9,24 and the 2-phenylpyrimidine mesogen 3M 8422 with
a chiral peruoroether chain10 which show maximum layer
contractions in the rage of 0.40–0.73% (Fig. 2). In our earlier
reports of X-ray, electrical, electro-optical and dielectric studies
on such bona de de Vries LCs, the effect of differential inter-
actions in different molecular end of compounds was found to
be one of the major cause of unusual behaviour and minimum
layer shrinkage in such systems.25,26 Rational design strategy
and structural investigations of de Vries-like liquid crystals was
initiated by Lemieux et al. by design and synthesis of a number
of de Vries-like liquid crystals having trisiloxane terminated
side-chain materials with 2-phenylpyrimidine core.27–29 Further,
they extended their work by incorporating chemically inert
carbosilane-terminated de Vries LCs and substituting the
2-phenylpyrimidine core with the cores known to be stronger
‘SmC-promoters’ in model compounds: 6-phenylpyridazine,
6-phenylpyridine and 2-phenyl thiadiazole.30,31 In the same
reports, drawback of hydrolytical and electrochemical lability of
siloxane end group and need of incorporation of chemically
inert nanosegregating terminal elements for fabrication of de
Vries-like LCs towards electro-optical applications are
mentioned.27,28 Thus, reports on bona de de Vries-like mate-
rials feature nanosegregating structural elements such as
siloxane end-groups or peruorinated side chains and example
of simple alkoxy chain-terminated conventional liquid crystal
showing de Vries-like properties is still rare. In this communi-
cation, we report a structurally unprecedented achiral, chemi-
cally inert, straight alkoxy chain-terminated aroylhydrazone
based liquid crystal N-(4(tetradecyloxy)-2-hydroxybenzylidene)-
Scheme 1 Reactions and reagents: (a) 1-bromohexadecane (30 mL,
0.11 mmol.), Cs2CO3 (0.1 g, 0.3 mmol), NMP, MW (150 W), 3–5 min,
87%. (b) LAH, stirring in dry THF at 0 ꢀC, 2 h, 92%. (c) SOCl2 (0.1 mL, 1.4
mmol), refluxing in dry DCM, DMF (cat.), 20 min, 82%. (d) Cs2CO3 (0.49
g, 1.5 mmol), ethyl 4-hydroxybenzoate (0.09 g, 0.5 mmol), 4-hex-
adecyloxybenzyl chloride (0.18 g, 0.5 mmol), DMF, MW, 70 ꢀC, 2–3
min, 80%. (e) NH2NH2$H2O (0.12 mL, excess), MW, 5–6 min, 82%. (f) 1-
Bromotetradecane (0.27 mL, 1.0 mmol), Cs2CO3 (1.3 g, 4.0 mmol),
NMP, MW (150 W), 3–5 min, 82%. (g) Acetic acid (cat.), EtOH, MW,
5 min, 83%.
The XRD results, microscopic optical observations and
differential scanning calorimetry (DSC) enthalpies at the phase
transitions have been used to determine mesomorphic prop-
erties and de Vries-like behaviour in the newly synthesized
compound BSac-1416. Accurate measurements of the smectic
layer spacing (d) as a function of temperature were carried out
by SAXS.
As shown in Fig. 3, the normalized d/dAC(T) proles of BSac-
1416 shows a negative thermal expansion in the SmA phase that
persists in the SmC phase and counteracts the layer contraction
caused by tilting in the SmC phase. Other bona de ‘de Vries-
like’ materials have been reported to show similar negative
thermal expansion in their d(T) proles.8,13–15,25 Maximum layer
contractions of 0.65% are observed at ca. 10 K below TAC and the
layer spacing at the SmA–SmC transition dAC is restored upon
40((400(hexadecyloxy)benzyl)oxy)benzohydrazide
(BSac-1416)
showing de Vries like properties comparable to that of per-
uorinated or siloxane end-groups containing bona de de
Vries-like liquid crystalline materials.
Compound BSac-1416 and initial precursors were obtained
by microwave assisted synthetic approach in very good yield
(Scheme 1). The detailed experimental procedures and analyt-
ical data are described in the ESI.†
further cooling to ca. 26.4 K below TAC
.
Fig. 3 Smectic layer spacing versus reduced temperature T ꢁ TAC for
compound BSac-1416. The dashed lines show the layer spacing at the
SmA–SmC transition. The solid lines represent the least-squares fit of
Fig. 2 Molecular structures of bona fide de Vries-like liquid crystals the data points in the SmA phase at T ꢁ TAC. The arrows indicate the
(TSiKN65, W599, 3M 8422, 2)8,9,18 and BSac-1416.
difference in layer spacing for calculating the R and f reduction factors.
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The compound BSac-1416 forms SmA and SmC phases. A
thin lm (18 mm thickness) of the sample observed between
crossed polarizers, exhibits the characteristic fan and homeo-
tropic textures of the SmA phase that turn into broken fan
texture, upon transition to the SmC phase. In each case, the
texture analysis revealed a pronounced change in interference
colour on cooling from the isotropic phase to the SmC phase. As
shown in Fig. 4, the fan texture shows a green colour just below
the clearing point, which gradually turns reddish-green at the
SmA–SmC transition point TAC, and then red at 6 ꢀC below TAC
.
The overall colour change corresponds to a net increase in
birefringence Dn from ca. 0.07 to 0.08 consistent with an
increase in orientational order expected for de Vries-like
materials.9
Fig. 5 Optical tilt angle qopt versus reduced temperature T ꢁ TAC for
the compound BSac-1416. The solid lines represent the fits to eqn (1)
(b ¼ 0.21).
The optical tilt angle (qopt) was measured by polarized optical
microscopy (POM) as a function of reduced temperature T ꢁ TAC
in the absence of electric eld (Fig. 5); qopt, obtained as the
difference in the angle of rotation between dark states in
domains of opposite tilt orientations30 was found to be 26.1ꢀ. Its
magnitude shows a discontinuity as T approaches TAC sug-
gesting that the SmA–SmC transition may be weakly rst-order
in nature. To test this hypothesis, the qopt prole was tted to
the power law:
Here, assuming a rigid-rod model d(T) is the tilt angle
required to give the observed contraction relative to the layer
spacing at the SmA–SmC transition.8 In eqn (3), qXray(T), the X-
ray tilt required to give the observed layer contraction relative to
the SmA layer spacing dA(T) extrapolated in the SmC phase. This
extrapolation is done at a given reduced temperature using
least-squares t of the data points in the SmA phase near the
SmA–SmC transition.10 The latter gives a reduction factor that is
normalized with respect to negative thermal expansion. In each
equation, the tilt angle derived from layer spacing measure-
ments is divided by the optical tilt angle qopt(T) measured by
POM. At a reduced temperature T ꢁ TAC ¼ ꢁ10 K, qXray observed
for BSac-1416, is 6.1ꢀ, a value consistent with the data for other
“de Vries like” materials.10 The R and f values of this compound
at T ꢁ TAC ¼ ꢁ10 K were calculated to be 0.23 and 0.44
respectively. Table 1 shows R and f values for the title compound
under investigation along with other “de Vries-like” liquid
crystals. It is seen that the gures of merit for BSac-1416 are
comparable to those for the best “de Vries-like” materials re-
ported heretofore, and thus has the potential to achieve defect
free bookshelf geometry.4
q f |T ꢁ Tc|b
(1)
where b is the order parameter exponent related to the nature of
the phase transition and Tc is the critical temperature of
a hypothetical second-order transition (i.e., the super-heating
limit of the weakly rst-order transition).1 According to the
generalized mean-eld (Landau) theory of phase transitions,
a b value of 0.5 is expected in the case of a pure second-order
phase transition, whereas a b value of 0.25 is expected in the
case of a transition at the crossover (tricritical) point from
second to rst-order transition.32,33 As shown in Fig. 5, ts of the
qopt proles to eqn (1) give b value ranging 0.21, which are
consistent with the DHAC values measured by DSC and suggest
that the SmA–SmC phase transition exhibited by the BSac-1416
mesogens reported herein is weakly rst-order in nature.8
To assess the potential of a de Vries material to achieve
defect-free bookshelf geometry in the SmC phase, Radcliffe
et al. dened gures of merit R and f according to eqn (2)
and (3):
In summary, “de Vries-like” liquid crystals behaviour
comparable to earlier best reports on such liquid crystals has
been observed in a simple straight alkoxy chain-terminated
conventional liquid crystal. Ongoing efforts are focused on
structure–function relationship studies of homologous series of
such mesogens featuring aroylhydrazone core and reduction in
melting point of mesogens via structural modications, and
will be reported in due course.
R ¼ d(T)/qopt(T) ¼ cosꢁ1[dC(T)/d(TAC)]/qopt(T)
f ¼ qXray(T)/qopt(T) ¼ cosꢁ1[dC(T)/dA(T)]/qopt(T)
(2)
(3)
Table 1 Smectic layer spacings, tilt angles, and figures of merit R and f
at T ꢁ TAC ¼ ꢁ10 K
dC
(A)
dA
(A)
d(TAC
(A)
)
qopt
qXray
˚
˚
˚
Cmpds
(deg) (deg) d (deg)
R
f
BSac-1416 43.0
2a
40.8
3M 8422b 31.7
43.9
43.1
32.3
43.3
41.2
31.8
26.1
35
25
6.1 11.4
0.23 0.44
0.23 0.54
0.18 0.44
0.20 0.20
18.8
8.0
4.5
6.8
Fig. 4 Polarized photomicrographs (20ꢂ) of compound BSac-1416 in
an unaligned glass cell with a spacing of 18 mm during mesophase
transitions (from left to right, 176.2, 174.1, 171.8 and 168.2 ꢀC). The
change in interference color from (a)–(d) corresponds to an increase
in birefringence of ca. 20%.
11.0
6.8
TSiKN65c 35.70 35.95 35.95 34
Ref. 18. b Ref. 8. Ref. 9.
a
c
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16 F. Giesselmann, P. Zugenmaier, I. Dierking, S. T. Lagerwall,
B. Stebler, M. Kaspar, V. Hamplova and M. Glogarova, Phys.
Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top.,
1999, 60, 598.
Acknowledgements
HKS (Senior Research Fellow) is grateful to the Council of
Scientic and Industrial Research (CSIR), New Delhi, India, for
nancial support. BS is grateful to DRDO, India (Project No.
ERIP/ER/1203053/M/01/1542) and DST, India (Project No. SR/S1/
IC-08/2012) for providing nancial assistance. S.K.S. is grateful
to DST, SERB for providing fellowship under N-PDF (PDF/2015/
000147) scheme.
17 A. de Vries, J. Chem. Phys., 1979, 71, 25.
18 J. C. Roberts, N. Kapernaum, F. Giesselmann and
R. P. Lemieux, J. Am. Chem. Soc., 2008, 130, 13842.
19 F. Goc, C. Blanc, V. Lorman, M. Nobili, S. Samaritani,
G. Galli and W. Kuczynski, Ferroelectrics, 2006, 343, 101.
20 K. Saunders, Phys. Rev. E: Stat., Nonlinear, So Matter Phys.,
2009, 80, 11703.
Notes and references
21 S. K. Prasad, D. S. Shankar Rao, S. Sridevi, J. Naciri and
B. R. Ratna, J. Phys.: Condens. Matter, 2011, 23, 1.
22 (a) M. V. Gorkunov, M. A. Osipov, J. P. F. Lagerwall and
F. Giesselmann, Phys. Rev. E: Stat., Nonlinear, So Matter
Phys., 2007, 76, 51706; (b) M. V. Gorkunov, F. Giesselmann,
J. P. F. Lagerwall, T. J. Sluckin and M. A. Osipov, Phys. Rev.
E: Stat., Nonlinear, So Matter Phys., 2007, 75, 60701.
23 (a) K. Saunders, D. Hernandez, S. Pearson and J. Toner, Phys.
Rev. Lett., 2007, 98, 197801; (b) K. Saunders, Phys. Rev. E:
Stat., Nonlinear, So Matter Phys., 2008, 77, 61708.
24 Y. Shen, L. Wang, R. Shao, T. Gong, C. Zhu, H. Yang,
J. E. Maclennan, D. M. Walba and N. A. Clark, Phys. Rev. E:
Stat., Nonlinear, So Matter Phys., 2013, 88, 062504.
25 S. K. Prasad, D. S. Shankar Rao, S. Sridevi, C. V. Lobo,
B. R. Ratna, J. Naciri and R. Shashidhar, Phys. Rev. Lett.,
2009, 102, 147802.
1 S. Chandrasekhar, Liquid Crystals, Cambridge University
Press, Cambridge, England, 2nd edn, 1992.
2 C. V. Lobo, S. K. Prasad and D. S. Shankar Rao, Phys. Rev. E:
Stat., Nonlinear, So Matter Phys., 2005, 72, 62701.
3 S. Kumar, Phys. Rev. A: At., Mol., Opt. Phys., 1981, 23, 3207.
4 J. P. Lagerwall and F. Giesselmann, ChemPhysChem, 2006, 7,
20.
5 S. Diele, P. Brand and H. Sackman, Mol. Cryst. Liq. Cryst.,
1972, 16, 105.
6 (a) A. De Vries, in Abstracts of the Fih International Liquid
Crystal Conference, Stockholm, ed. G. H. Brown, ILC Society,
Kent, OH, 1974, p. 150; (b) A. De Vries, Mol. Cryst. Liq.
Cryst., 1977, 41, 27; (c) A. De Vries, A. Ekachai and
N. Spielberg, Mol. Cryst. Liq. Cryst., 1979, 49, 143.
7 Y. Takanishi, Y. Ouchi, H. Takezoe, A. Fukuda, A. Mochizuki
and M. Nakatsuka, Jpn. J. Appl. Phys., 1990, 2, L984.
8 M. D. Radcliffe, M. L. Brostrom, K. A. Epstein,
A. G. Rappaport, B. N. Thomas, R. Shao and N. A. Clark,
Liq. Cryst., 1999, 26, 789.
26 R. Shashidhar, B. R. Ratna, G. G. Nair and S. K. Prasad, Phys.
Rev. Lett., 1988, 61(5), 547.
27 L. Li, C. D. Jones, J. Magolan and R. P. Lemieux, J. Mater.
Chem., 2007, 17, 2313.
9 M. S. Spector, P. A. Heiney, J. Naciri, B. T. Weslowski,
D. B. Holt and R. Shashidhar, Phys. Rev. E: Stat. Phys.,
Plasmas, Fluids, Relat. Interdiscip. Top., 2000, 61, 1579.
10 J. P. F. Lagerwall, F. Giesselmann and M. D. Radcliffe, Phys.
Rev. E: Stat., Nonlinear, So Matter Phys., 2002, 66, 31703.
11 J. Naciri, C. Carboni and A. K. George, Liq. Cryst., 2003, 30,
219.
28 J. C. Roberts, N. Kapernaum, F. Giesselman and
R. P. Lemieux, J. Am. Chem. Soc., 2008, 130, 13842.
29 J. C. Roberts, N. Kapernaum, Q. Song, D. Nonnenmacher,
K. Ayub, F. Giesselmann and R. P. Lemieux, J. Am. Chem.
Soc., 2010, 132, 364.
30 Q. Song, D. Nonnenmacher, F. Giesselmann and
R. P. Lemieux, J. Mater. Chem. C, 2013, 1, 343.
12 O. E. Panarina, Y. P. Panarin, J. K. Vij, M. S. Spector and
R. Shashidhar, Phys. Rev. E: Stat., Nonlinear, So Matter
Phys., 2003, 67, 51709.
13 M. Rossle, R. Zentel, J. P. F. Lagerwall and F. Giesselmann,
Liq. Cryst., 2004, 31, 883.
14 N. Hayashi, T. Kato, A. Fukuda, J. K. Vij, Y. P. Panarin,
J. Naciri, R. Shashidhar, S. Kawada and S. Kondoh, Phys.
Rev. E: Stat., Nonlinear, So Matter Phys., 2005, 71, 41705.
15 O. E. Panarina, Y. P. Panarin, F. Antonelli, J. K. Vij,
M. Reihmann and G. Galli, J. Mater. Chem., 2006, 16, 842.
31 C. P. J. Schubert, C. Muller, M. D. Wand, F. Giesselmann and
R. P. Lemieux, Chem. Commun., 2015, 51, 12601.
32 R. J. Birgeneau, C. W. Garland, A. R. Kortan, J. D. Litster,
M. Meichle, B. M. Ocko, C. Rosenblatt, L. J. Yu and
J. Goodby, Phys. Rev. A: At., Mol., Opt. Phys., 1983, 27, 1251.
33 L. D. Landau, Statistical Physics, Pergamon Press, Oxford, 3rd
edn, 1980.
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