Bent-rod liquid crystal dimers: Synthesis and mesomorphic properties

Article abstract of DOI:10.1039/c4tc00212a

The synthesis and characterization of three bent-rod dimers are reported. Very long flexible spacers that include methylene units and either a phenyl ring or a triazole moiety as connecting structures join the bent- and rod-promesogenic cores. Polarizing microscopy, modulated differential scanning calorimetry, X-ray diffraction at variable temperature and dielectric measurements have been performed to establish the mesophase behaviour of the dimers. The results show a complex supramolecular organization for this kind of flexible dimer in the mesophase. Interestingly, the length and the chemical nature of the flexible linking spacers condition the liquid crystalline properties of these novel compounds. Additionally, it has been found that the chemical structure of the rod-like core plays a key role in determining the kind of mesophase, either nematic or lamellar, formed by these materials.

Full text of DOI:10.1039/c4tc00212a

Journal of
Materials Chemistry C
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Bent-rod liquid crystal dimers: synthesis and
mesomorphic properties
Cite this: J. Mater. Chem. C, 2014, 2,
4027
Nerea Sebastian,a Nelida Gimeno,b Jorge Vergara,b David O. Lopez,c
´
´
´
a
d
e
b
*
*
Jose Luis Serrano, Cesar L. Folcia, M. Rosario de la Fuente and M. Blanca Ros
´
´
The synthesis and characterization of three bent-rod dimers are reported. Very long flexible spacers that
include methylene units and either a phenyl ring or a triazole moiety as connecting structures join the
bent- and rod-promesogenic cores. Polarizing microscopy, modulated differential scanning calorimetry,
X-ray diffraction at variable temperature and dielectric measurements have been performed to establish
the mesophase behaviour of the dimers. The results show a complex supramolecular organization for
this kind of flexible dimer in the mesophase. Interestingly, the length and the chemical nature of the
flexible linking spacers condition the liquid crystalline properties of these novel compounds. Additionally,
it has been found that the chemical structure of the rod-like core plays a key role in determining the
kind of mesophase, either nematic or lamellar, formed by these materials.
Received 31st January 2014
Accepted 11th March 2014
DOI: 10.1039/c4tc00212a
www.rsc.org/MaterialsC
up new possibilities in dimer research. BCLCs have been the
focus of extensive research due to the novel and extraordinary
properties shown by the mesophases of these materials.4 The
Introduction
Liquid crystalline dimers consist of two mesogenic units con-
nected by a exible spacer. These mesogenic molecules have
attracted a great deal of interest in liquid crystal research for
two main reasons. On the one hand, these materials can be
considered as model compounds for higher molecular weight
structures such as main-chain and side-chain polymers. On the
other hand, these materials have become more appealing as
they have proven to show complex and novel mesomorphism
that is markedly different from that of the corresponding
monomers.1,2 In particular, some odd rod-like cyanobiphenyl
dimers exhibit a new twist-bent nematic phase.1e, f Research into
mesogenic dimers has been mainly focused on the combination
of rod-like calamitic units of equal or different nature (polar,
non-polar, chiral, and non-chiral).1 Non-symmetric dimers that
combine rod- and disc-like mesogens or symmetric dimers that
contain two discotic units have been studied to a lesser extent.2
The discovery of a new type of liquid crystal, the so-called
bent-core liquid crystals (BCLCs), by Niori et al. in 19963 opened
ability of these materials to form polar order and phase chirality
without the molecules being chiral along with their interesting
properties (such as ferro-, antiferro- and piezoelectricity,
nonlinear optical activity, and high exoelectric behaviour)
have stimulated the research in this eld.5 Moreover, the biaxial
molecular shape of these materials makes them good candi-
dates to present thermotropic biaxial nematic phases.6 The
search for biaxiality in BCLCs has been split into two main
areas: the bent-shaped molecules themselves and their linear
covalent coupling with a rod-like unit to form a nonsymmetrical
dimer in order to enhance the shape biaxiality of the rod-like
mesogen.7 Of these approaches, the research into hybrid rod-
like and bent-core liquid crystalline dimers to give biaxial
phases has attracted the most attention. Yelamaggad et al.8
reported an Nb-SmAb sequence in a dimer that combines a bent-
core unit and a rod-like cyanobiphenyl derivative linked by a
hexamethylene spacer. In this rst example, the crucial role of
the parity of the spacer was highlighted, as the same dimer with
an odd spacer was not mesomorphous. This effect is also
appealing in dimers derived from calamitic units. The same
odd–even effect and the N-SmA phase sequence have also been
described for a different series of bent-core/cyanobiphenyl
dimers by Lee et al.9 More recently, Li et al.10 have found the
same type of sequence in substituted bent-core mesogens
linked by hexamethylene spacers to cyanobiphenyl units.
However, biaxiality could be only conrmed for the SmA phase.
Other interesting phase sequences have been achieved by the
combination of bent-core and rod-like structures within the
same molecule. By modulating the parity of the methylene
a
´
´
Departamento de Fısica Aplicada II, Facultad de Ciencia y Tecnologıa, Universidad
´
del Paıs Vasco UPV/EHU, Apdo. 644, 48080-Bilbao, Spain. E-mail: rosario.
delafuente@ehu.es
bInstituto de Ciencia de Materiales de Aragon, Universidad de Zaragoza-CSIC,
´
´
´
Departamento de Quımica Organica, Facultad de Ciencias, Pedro Cerbuna, 12,
50009 Zaragoza, Spain. E-mail: bros@unizar.es
cGrup de Propietas Fısiques dels Materials (GRPFM), Departament de Fısica
i
´
´
`
Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politecnica de Catalunya, Diagonal, 647
08028 Barcelona, Spain
d
´
Instituto de Nanociencia de Aragon, Universidad de Zaragoza, Departamento de
´
´
Quımica Organica, Facultad de Ciencias, Pedro Cerbuna 12, 50009-Zaragoza, Spain
e
´
´
Departamento de Fısica de la Materia Condensada, Facultad de Ciencia y Tecnologıa,
´
Universidad del Paıs Vasco UPV/EHU, Apdo. 644, 48080-Bilbao, Spain
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 4027–4036 | 4027

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Scheme 1 Chemical structures of the bent-core and rod-core based
dimers: AZO1, AZO2 and BIPH.
Scheme 2 Synthetic route followed to synthesize dimer AZO1.
linker, blue phases and columnar phases have been obtained by
Yelamaggad et al. for non-symmetric dimers in which bent-core
units are combined with a cholesterol derivative.11 Furthermore,
azobenzene-derived calamitic units have also been coupled with
bent-core units in order to achieve photoreactive dimers.12
Thus, besides the type and size of mesogenic units, the parity
and nature of the spacer have proven to be crucial in the
properties of the dimeric systems.13
induce BCLC phases.5c,14 The preparation of bent-core and rod-
like intermediates, alkyne 3 (ref. 14) and acids 4,14 1 and 8 has
been described previously.15
The promesogenic unit of compound 2 was prepared by
esterication of acid 1 with 6-azidohexan-1-ol. The product was
then coupled with alkyne 3 using standard ‘click chemistry’
conditions to obtain dimer AZO1 (see Scheme 2). The dimer
AZO2 was synthesized by esterication of the bent-shaped acid
4 with trimethylsilyl monoprotected hydroquinone to give
compound 5, which was deprotected and reacted with rod-
shaped acid 1 under Steglich conditions to yield the nal
product (Scheme 3). An analogous sequence of reactions was
followed for the preparation of dimer BIPH, but in this case the
rod-like acid 8 was rst esteried with trimethylsilyl monop-
rotected hydroquinone to obtain compound 9. Deprotection of
Herein we report the synthesis and broad characterization of
three new non-symmetric dimers denoted as AZO1, AZO2 and
BIPH. These compounds contain the same six-ring bent-core
structure and two different rod-like moieties derived from azo-
benzene (AZO) and cyanobiphenyl (BIPH) (see Scheme 1).
Previous reports have focused on bent-rod dimers that
connect the promesogenic units by a relatively short linear
spacer (from 3 to 10 methylene units). In contrast, with the aim
of studying the effect of the promesogenic core decoupling on
the supramolecular organization of this type of bent-rod dimer,
we selected a very long and exible connection between the
promesogenic structures. This assembly consists of two long
spacers linked either by a phenyl ring or a more exible hexa-
methylene-triazole moiety. It will be shown that the role of this
part of the dimer is crucial for the mesomorphism, with two out
of the three dimers being mesomorphous. Furthermore,
appropriate selection of the rod-like core can be used to tune the
liquid crystal organization of the dimer. A detailed thermal,
structural and dielectric study of these compounds has been
carried out and the results reveal a rich and complex behaviour.
This paper has been structured as follows. The experimental
details are described in the rst section. The liquid crystalline
properties (studied by microscopy, X-ray diffraction and
modulated differential scanning calorimetry) and dielectric
characterization are presented in the second section and,
nally, the concluding remarks are covered.
Experimental section
1. Synthesis of compounds
The synthetic routes followed to prepare the three dimers are
shown in Schemes 2–4. All of the bent-rod dimers synthesized
are based on the same six-ring bent-shaped structure derived
from 3,40-biphenyl, which has a well-documented ability to Scheme 3 Synthetic route followed to synthesize dimer AZO2.
4028 | J. Mater. Chem. C, 2014, 2, 4027–4036
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Journal of Materials Chemistry C
31.9, 34.1, 34.3, 46.3, 50.2, 57.4, 64.8, 65.2, 68.3, 68.4, 68.5,
113.1, 114.4, 114.8, 118.6, 120.4, 120.9, 122.0, 122.1, 123.0,
124.7, 125.4, 126.8, 128.3, 129.9, 131.8, 132.4, 133.1, 138.0,
142.0, 150.6, 151.3, 155.4, 162.7, 163.8, 164.3, 164.4, 164.5,
172.3, 173.2, 173.7. FTIR (KBr, n cmꢁ1): 2926, 2849, 2213, 1726,
1595, 1501, 1260. MALDI-MS m/z ¼ 1640.9 [M + Na]+. EA for
C
98H116N6O15: calc.: C 72.74, H 7.23, N 5.20; found: C 72.58, H
7.25, N 5.20%.
Compound 5. Acid 4 (0.30 g, 0.30 mmol), DPTS (0.84 g, 0.30
mmol), DCC (0.84 g, 0.40 mmol) and 4-tert-butyldimethylsily-
loxyphenol15a (0.64 g, 0.30 mmol) were stirred in dry dichloro-
methane (50 mL) for 24 h. The crude product was puried by
liquid chromatography on silica gel using dichloromethane–
ethyl acetate (10 : 0.2) as eluent. Compound 5 was obtained as a
white solid. Yield: 0.31 g (87%). M.p. (ꢀC): 70. 1H NMR (400
MHz, CDCl3): d (ppm) ¼ 0.20 (s, 6H, CH3), 0.88 (t, J ¼ 6.6 Hz, 3H,
–CH3), 0.99 (s, 9H, CH3), 1.27 (m, 34H), 1.84 (m, 6H), 2.54 (t, J ¼
7.3 Hz, 2H, H2C]O), 4.07 (t, J ¼ 6.0 Hz, 4H, –CH2O), 6.80 (d, J ¼
Scheme 4 Synthetic route followed to synthesize dimer BIPH.
this compound yielded phenol 10 and this was esteried with 8.5 Hz, 2H), 7.93 (d, J ¼ 8.5 Hz, 2H), 6.99 (d, J ¼ 8.4 Hz, 4H), 7.27
bent-core compound 4 to yield the pure dimer BIPH (Scheme 4). (m, 1H), 7.31 (d, J ¼ 8.2 Hz, 2H), 7.41 (d, J ¼ 8.0 Hz, 4H), 7.47 (m,
Details of the synthesis and full characterization data for all 1H), 7.52 (d, J ¼ 4.7 Hz, 2H), 7.70 (d, J ¼ 8.1 Hz, 2H), 8.16 (d, J ¼
compounds are given below.
8.8 Hz, 4H), 8.30 (d, J ¼ 8.8 Hz, 4H). 13C NMR (100 MHz, CDCl3)
Compound 2. Acid 1 (0.50 g, 1.22 mmol), DPTS (0.36 g, 1.22 d (ppm) ¼ ꢁ6.1, 14.1, 22.7, 24.9, 25.6, 26.0, 29.1, 29.3, 29.4, 29.5,
mmol), DCC (0.35 g, 1.7 mmol) and 6-azidohexan-1-ol (0.50 g, 29.6, 29.7, 31.9, 34.3, 68.4, 114.4, 120.5, 120.9, 122.1, 122.2,
1.22 mmol) were stirred in dry dichloromethane (30 mL) for 24 122.3, 124.7, 126.8, 126.9, 128.3, 131.8, 132.4, 138.0, 142.1,
h. The crude product was puried by liquid chromatography on 151.3, 155.4, 163.8, 164.3, 164.5. FTIR (KBr, n cmꢁ1): 2926, 2848,
silica gel using dichloromethane as eluent. Compound 2 was 1735, 1610, 1486, 1252. MALDI-MS m/z ¼ 1275.7 [M + Na]+.
obtained as an orange solid. Yield: 0.36 g (53%). M.p. (ꢀC): 73.
Compound 6. Compound 5 (0.26 g, 0.2 mmol) was dissolved
1H NMR (400 MHz, CDCl3) d (ppm) ¼ 1.30–1.38 (m, 16H), 1.60 in THF (15 mL) in a plastic vessel. Hydrouoric acid (150 mL,
(m, 6H), 1.81 (m, 2H), 2.29 (t, J ¼ 7.4 Hz, 2H), 3.26 (t, J ¼ 6.8 Hz, 49% aqueous solution) was added by syringe and stirred. The
2H), 4.06 (t, J ¼ 6.5 Hz, 4H), 7.00 (d, J ¼ 8.9 Hz, 2H), 7.76 (d, J ¼ reaction was diluted with dichloromethane and quenched with
8.4 Hz, 2H), 7.92 (d, J ¼ 8.8 Hz, 4H). 13C NMR (100 MHz, CDCl3) a saturated solution of sodium bicarbonate. The resultant
d (ppm) ¼ 25.0, 25.6, 26.0, 26.4, 28.5, 28.8, 29.1, 29.3, 29.4, 29.5, mixture was washed several times with water and the crude
34.4, 51.3, 64.1, 68.5, 113.1, 114.9, 123.1, 125.5, 133.2, 146.7, product was puried by column chromatography using
154.8, 162.8, 174.9. FTIR (KBr, n cmꢁ1): 2915, 2854, 2230, 2104, dichloromethane–ethyl acetate (10/0.2) as eluent. Yield: 0.16 g
1721, 1595, 1572, 1268. MALDI-MS m/z ¼ 555.3 [M + Na]+.
of a white solid (68%). M.p. (ꢀC): 147. 1H NMR (400 MHz,
Compound AZO1. Compound 3 (0.15 g, 0.14 mmol), CDCl3): d (ppm) ¼ 0.88 (t, J ¼ 6.9 Hz, 3H), 1.27–148 (m, 34H),
compound 2 (0.08 g, 0.15 mmol) and CuBr (0.04 g, 0.28 mmol) 1.74 (m, 2H), 1.83 (m, 4H), 2.53 (t, J ¼ 7.4 Hz, 2H), 4.06 (t, J ¼ 6.4
were placed in a dry Schlenk tube. Three vacuum/argon cycles Hz, 4H), 5.07 (s, 1H), 6.79 (d, J ¼ 8.8 Hz, 2H), 6.91 (d, J ¼ 8.8 Hz,
were performed and dry and degassed THF (3 mL) and PMDETA 2H), 6.98 (d, J ¼ 8.7 Hz, 4H), 7.23 (m, 1H), 7.31 (d, J ¼ 8.6 Hz,
(58 mL, 0.28 mmol) were added. Three freeze–thaw cycles were 2H), 7.40 (d, J ¼ 8.7 Hz, 4H), 7.46 (m, 1H), 7.52 (d, J ¼ 4.8 Hz,
carried out and the mixture was stirred at room temperature for 2H), 7.66 (d, J ¼ 8.6 Hz, 2H), 8.15 (d, J ¼ 8.6 Hz, 4H), 8.29 (d, J ¼
24 h. The mixture was ltered through neutral alumina and the 8.5 Hz, 2H), 8.32 (d, J ¼ 8.6 Hz, 2H). 13C NMR (100 MHz, CDCl3):
solvent was evaporated. The crude product was puried by d (ppm) ¼ 14.3, 22.6, 24.7, 25.8, 26.4, 29.2, 29.3, 29.4, 29.5, 29.6,
column chromatography using dichloromethane–ethyl acetate 29.7, 31.8, 34.7, 68.4, 114.4, 120.5, 121.0, 122.1, 122.2, 122.6,
(7/3) as eluent. The pure product was dissolved in the minimum 124.7, 126.8, 127.0, 128.3, 131.8, 132.4, 138.2, 142.1, 151.4,
amount of dichloromethane and precipitated from hexanes. 155.4, 163.8, 164.3, 165.5. FTIR (KBr, n cmꢁ1): 3200, 2920, 2848,
Yield: 0.12 g of an orange solid (55%). M.p. (ꢀC): 108. H NMR 1738, 1605, 1540, 1251. MALDI-MS m/z ¼ 1161.6 [M + Na]+.
1
(300 MHz, CDCl3) d (ppm) ¼ 0.88 (t, J ¼ 6.5 Hz, 3H), 1.28–1.47
Compound AZO2. Acid 1 (0.05 mg, 0.12 mmol), DPTS (0.04
(m, 48H), 1.61–1.64 (m, 6H), 1.81–1.93 (m, 10H), 2.27–2.36 (m, mg, 0.12 mmol), DCC (0.04 mg, 0.17 mmol) and compound 6
4H), 4.04 (m, 8H), 4.34 (t, J ¼ 7.2 Hz, 2H), 5.21 (s, 2H), 6.98–7.04 (0.14 g, 0.12 mmol) were stirred in dry dichloromethane (20 mL)
(m, 6H), 7.20–7.23 (m, 1H), 7.32 (d, J ¼ 8.5 Hz, 2H), 7.40 (d, J ¼ for 24 h. The crude product was puried by liquid chromatog-
8.8 Hz, 4H), 7.47 (s, 1H), 7.52 (d, J ¼ 4.8 Hz, 2H), 7.60 (s, 1H), raphy on silica gel using dichloromethane–ethyl acetate
7.68 (d, J ¼ 8.6 Hz, 2H), 7.79 (d, J ¼ 8.5 Hz, 2H), 7.94 (d, J ¼ 8.7 (10 : 0.2) as eluent. Compound AZO2 was obtained as an orange
Hz, 4H), 8.17 (d, J ¼ 8.5 Hz, 4H), 8.33 (dd, J ¼ 2.9 Hz, J ¼ 8.8 Hz, solid. Yield: 0.12 g (64%). M.p. (ꢀC): see Table 1. H NMR (400
4H). 13C NMR (75 MHz, CDCl3) d (ppm) ¼ 14.1, 17.8, 22.7, 24.8, MHz, CDCl3): d (ppm) ¼ 0.89 (t, J ¼ 6.6 Hz, 3H, –CH3), 1.27–1.49
24.9, 25.3, 26.0, 26.1, 28.3, 29.1, 29.2, 29.3, 29.4, 29.5, 29.6, 30.1, (m, 48H), 1.83 (m, 8H), 2.55 (t, J ¼ 7.3 Hz, 4H), 4.06 (t, J ¼ 6.5 Hz,
This journal is © The Royal Society of Chemistry 2014
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Table 1 Transition temperatures (ꢀC) and enthalpies (DH, kJ molꢁ1
determined by DSC and MDSC for dimers AZO1, AZO2 and BIPH
)
saturated solution of sodium bicarbonate. The resultant mixture
was washed several times with water and the crude product was
puried by column chromatography using dichloromethane–
ethyl acetate (10/0.2) as eluent. Yield: 0.54 g of a white solid
(46%). M.p. (ꢀC): 147. 1H NMR (400 MHz, CDCl3): d (ppm) ¼ 1.28
(m, 12H), 1.60 (m, 2H), 1.71 (m, 2H), 2.52 (t, J ¼ 7.5 Hz, 2H), 4.00
(t, J ¼ 6.5 Hz, 2H), 6.74 (d, J ¼ 8.9 Hz, 2H), 6.86 (d, J ¼ 8.9 Hz, 2H),
7.03 (d, J ¼ 8.8 Hz, 2H), 7.68 (d, J ¼ 8.2 Hz, 2H), 7.81 (d, J ¼ 8.7 Hz,
2H), 7.84 (d, J ¼ 8.6 Hz, 2H). 13C NMR (100 MHz, CDCl3): d (ppm)
¼ 24.2, 28.3, 228.5, 228.6, 33.3, 67.4, 108.9, 115.0, 118.8, 122.2,
126.6, 130.1, 132.6, 142.5, 144.1, 154.8, 159.2, 172.0. FTIR (KBr, n
cmꢁ1): 3259, 2929, 2840, 2220, 1739, 1604, 1502, 1246.
Compound BIPH. Acid 4 (0.40 g, 0.4 mmol), DMAP (0.04 g,
0.12 mmol), DCC (0.80 g, 0.4 mmol) and compound 10 (0.20 g,
0.40 mmol) were stirred in dry dichloromethane (50 mL) for 24
h. The crude product was puried by liquid chromatography on
silica gel using dichloromethane as eluent. Compound BIPH
was obtained as a white solid. Yield: 0.27 g (47%). M.p. (ꢀC): see
Table 1. 1H NMR (400 MHz, CDCl3): d (ppm) ¼ 0.89 (t, J ¼ 6.6 Hz,
3H), 1.27 (m, 48H), 1.81 (m, 8H), 2.53 (t, J ¼ 7.5 Hz, 2H), 2.56 (t, J
¼ 7.5 Hz, 2H), 4.00 (t, J ¼ 6.5 Hz, 2H), 4.06 (t, J ¼ 6.5 Hz, 4H),
6.99 (d, J ¼ 7.5 Hz, 6H), 7.08 (s, 4H), 7.23 (m, 1H), 7.30 (d, J ¼ 8.6
Hz, 2H), 7.37 (d, J ¼ 8.8 Hz, 2H), 7.39 (d, J ¼ 8.8 Hz, 2H), 7.46 (m,
Phase transition temperature
Compound
[ꢀC] and enthalpya [kJ molꢁ1
]
AZO1b
AZO2c
Cr 108 [103.3] I
Cr 119.7 [76.9d] M 125.5e [0.26e] N
140.2 [0.59] II 140 N 123 M 120 Cr
SmY glassy 45 SmY 128.2 [45.5f ] SmX-N
BIPHc
135g [0.5g ] II 137 N-SmX 125 SmY 49 SmY glassy
a
Cr: crystal, N: nematic mesophase, M: unidentied mesophase, SmX,
SmY: lamellar mesophases, SmY glassy: SmY vitried phase, and I:
b
isotropic liquid.
Data obtained by means of standard DSC
measurements at 10 K minꢁ1
.
Data obtained by means of MDSC
c
measurements at 1 K minꢁ1
.
Heat capacity peak attributed to the Cr-
d
to-M-to-N phase transition. Enthalpy value corresponds to the set of
both transitions and is overestimated for the single Cr-to-M phase
e
transition. Temperature and enthalpy data correspond to heating
runs from 120 ꢀC aer a slow cooling from the isotropic phase.
f
Inferred by subtracting the enthalpy of the isolated N-to-I phase
transition from the value measured for the SmY-to-(SmX + N) phase
g
transition. Obtained from the isolated N-to-I phase transition,
measured at a rate of 0.5 K minꢁ1 aer slow cooling from the
isotropic phase.
6H), 6.98 (m, 6H), 7.09 (s, 4H), 7.23 (m, 1H), 7.32 (d, J ¼ 8.6 Hz, 1H), 7.53 (m, 4H), 7.67 (m, 6H), 8.16 (d, J ¼ 8.7 Hz, 4H), 8.30 (d, J
2H), 7.38 (d, J ¼ 8.8 Hz, 4H), 7.47 (m, 1H), 7.53 (d, J ¼ 4.9 Hz, ¼ 8.1 Hz, 2H), 8.31 (d, J ¼ 8.8 Hz, 2H). 13C NMR (100 MHz,
2H), 7.67 (d, J ¼ 8.6 Hz, 2H), 7.78 (d, J ¼ 8.6 Hz, 2H), 7.94 (d, J ¼ CDCl3): d (ppm) ¼ 14.0, 22.6, 24.8, 25.9, 29.0, 29.1, 29.3, 29.5,
8.7 Hz, 4H), 8.16 (d, J ¼ 8.67 Hz, 4H), 8.29 (d, J ¼ 8.7 Hz, 2H), 29.6, 31.8, 34.2, 68.0, 68.2, 68.1, 114.3, 115.0, 120.4, 122.0, 122.3,
8.32 (d, J ¼ 8.7 Hz, 2H). 13C NMR (100 MHz, CDCl3): d (ppm) ¼ 124.6, 126.4, 127.0, 128.2, 129.9, 131.0, 131.7, 132.3, 137.9,
14.1, 22.7, 24.8, 15.9, 19.0, 29.2, 29.3, 29.4, 29.5, 29.6, 31.9, 34.3, 142.0, 145.2, 147.8, 150.1, 151.2, 155.6, 159.7, 163.7, 164.4,
68.3, 68.4, 113.1, 114.4, 114.8, 120.4, 120.9, 122.1, 122.4, 123.0, 172.1. FTIR (KBr, n cmꢁ1): 2933, 2845, 2221, 1739, 1648, 1509,
124.7, 125.4, 126.8, 128.3, 129.9, 131.8, 132.4, 133.1, 138.0, 1245. MALDI-MS m/z ¼ 1523.9 [M + Na]+. EA for C95H105NO15:
142.0, 146.6, 147.9, 148.0, 150.6, 151.3, 154.8, 155.4, 162.7, calc.: C 76.02, H 7.05, N 0.93; found: C 75.82, H 7.12, N 0.93%.
163.8, 164.3, 164.4, 172.2. FTIR (KBr, n cmꢁ1): 2938, 2842, 2224,
1736, 1650, 1504, 1242. MALDI-MS m/z ¼ 1551.7 [M + Na]+. EA
2. Experimental techniques
for C95H105N3O15: calc.: C 74.63, H 6.92, N 2.75; found: C 74.71,
H 6.90, N 2.92%.
The textures of the mesophases were studied using an Olympus
Compound 9. Rod-shaped acid 8 (1.00 g, 2.6 mmol), DPTS polarizing microscope equipped with a Linkam LTSE350 hot
(0.80 g, 2.6 mmol), DCC (0.80 g, 3.9 mmol) and 4-tert-butyldi- stage and a Linkam TMS-93 temperature controller. Photomi-
methylsilyloxyphenol (0.60 g, 0.26 mmol) were stirred in dry crographs were taken using an Olympus C-5050 camera on
dichloromethane (50 mL) for 24 h. The crude product was samples in Linkam cells (5 mm thick).
puried by liquid chromatography on silica gel using
Heat capacity data at normal pressure were obtained by
dichloromethane as eluent. Compound 9 was obtained as a means of a commercial differential scanning calorimeter DSC-
white solid. Yield: 1.47 g (95%). M.p. (ꢀC): 70. 1H NMR (400 Q2000 from TA-Instruments working in modulated mode
MHz, CDCl3): d (ppm) ¼ 0.19 (s, 6H, CH3), 0.98 (s, 9H, CH3), 1.34 (MDSC). The experimental conditions were adjusted in such a
(m, 12H), 1.74 (m, 2H), 1.81 (m, 2H), 2.52 (t, J ¼ 7.5 Hz, 2H), 4.00 way that the imaginary part of the complex heat capacity data
(t, J ¼ 6.5 Hz, 2H), 6.80 (d, J ¼ 8.9 Hz, 2H), 6.90 (d, J ¼ 8.9 Hz, vanished. Likewise, by means of a special calibration procedure
2H), 7.00 (d, J ¼ 8.8 Hz, 2H), 7.53 (d, J ¼ 8.8 Hz, 2H), 7.63 (d, J ¼ in which very precise latent heat data measured from other
8.6 Hz, 2H), 7.68 (d, J ¼ 8.6 Hz, 2H). 13C NMR (100 MHz, CDCl3): homologous-compounds through adiabatic calorimetry are
d (ppm) ¼ 24.9, 25.6, 26.0, 29.1, 29.2, 29.3, 29.5, 34.3, 68.1, considered, the MDSC-technique is also suitable for quantita-
110.0, 115.1, 119.1, 120.5, 122.2, 127.0, 128.3, 131.2, 132.5, tive measurements of latent heats of rst order transitions, even
144.7, 145.3, 153.1, 159.8, 172.5. FTIR (KBr, n cmꢁ1): 2932, 2843, if they are weak. A more detailed description of the MDSC
2225, 1737, 1605, 1507, 1249. EA for C36H47O4NSi: calc.: C 73.80, technique can be found elsewhere.16 The MDSC measurements
H 8.09, N 2.39; found: C 73.49, H 8.09, N 2.39%.
were made as a standard study of the overall thermal behaviour
Compound 10. Compound 9 (1.47 g, 2.5 mmol) was dissolved of the samples and consisted of heating runs at 1 K minꢁ1 from
in THF (15 mL) in a plastic vessel. Hydrouoric acid (1.8 mL, room temperature up to the I-phase and cooling runs at the
49% aqueous solution) was added by syringe and stirred. The same rate. In some cases, other slower rates were considered.
reaction was diluted with dichloromethane and quenched with a The parameters of modulation (temperature amplitude and
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oscillation period) were ꢂ0.5 K and 60 s and the sample masses
(chosen between 2 and 3 mg) were selected to ensure a uniform
thin layer within the aluminium pans.
X-ray studies were performed on non-oriented samples in
Debye–Scherrer geometry mode using Lindemann capillaries of
diameter 0.6 mm. The materials were introduced by capillarity
in the isotropic phase. Wide-angle measurements were carried
out using a powder goniometer equipped with a high temper-
ature attachment. A linear position-sensitive detector (PSD),
with an angular resolution better than 0.01ꢀ, was employed to
detect the diffracted intensity in the 2q interval 0.5–25ꢀ (q is the
Bragg angle). A small-angle goniometer with a different
temperature controller and a xed position PSD of 4ꢀ of angular
range was also employed. Monochromatic Cu-Ka radiation (l ¼
˚
1.5418 A) was used in both cases.
Measurements of the complex dielectric permittivity 3*( f ) ¼
30( f ) ꢁ i300( f ), in the range of 103 to 1.1 ꢃ 108 Hz, were per-
formed using an impedance analyzer (HP4294A). The cell con-
sisted of two gold-plated brass electrodes (diameter 5 mm)
separated by thick silica spacers of the order of 50 mm. A modied
HP16091A coaxial test xture was used as the sample holder. This
was held in a cryostat from Novocontrol and both temperature and
dielectric measurements were computer-controlled. Dielectric
measurements were performed on cooling runs.
Fig. 1 Microphotographs of the textures observed for compound
AZO2 (a) at 139 ꢀC in the N phase, (b) at 123 ꢀC in the transition from
the N to the M phase, (c) at 121 ꢀC in the M phase, (d) at 121 ꢀC under
the application of a triangular wave-electric field of 20 V mmꢁ1 and 50
Hz, (e) the same field conditions one minute later and (f) half a minute
after removal of the field. The width of all photographs is about 300 mm
and the rubbing direction is along the stripes.
Results and discussion
Liquid crystal characterization
The mesomorphic properties of the dimers were investigated by
polarizing optical microscopy (POM), modulated differential
scanning calorimetry (MDSC), and X-ray diffraction (XRD). The diffractogram, thus conrming the N character of the phase.
transition temperatures and corresponding enthalpy values for However, it was not possible to study the low temperature phase
the productsaresummarizedin Table1. Detailedexplanations of by XRD as the sample crystallized during the experiment.
the liquid crystalline behaviour listed in Table 1 are given below.
In order to ascertain the nature of the lower temperature
Two out of the three dimers form mesophases. Unfortu- mesophase, its optical response to electric elds was studied
nately, dimer AZO1 did not show liquid crystalline properties (see Fig. 1c–f). Thin lms of planar aligned AZO2 (Linkam cells,
and was not studied further. However, both hybrid bent-rod 5 mm thick) could be switched from an optically transmitting
dimers containing a phenyl ring as the linker formed meso- state to a homeotropic black state. Dark lines developed in a
phases. Interestingly, in spite of the ability of both rigid cores to direction perpendicular to the rubbing direction and spread
promote mesophase formation, a long and exible spacer such over the whole sample. This behaviour is consistent with a
as the one present in AZO1 dramatically prevents the appear- positive dielectric anisotropy, which will be discussed later. In a
ance of a so-phase, in contrast to AZO2, which is based on the relatively short period of time (30 seconds) aer removal of the
same bent- and rod-cores. Furthermore, the thermal behaviour eld, a quite homogenous planar texture was recovered. Such
of the two mesogenic dimers AZO2 and BIPH is quite complex switching behaviour would not be expected in a SmA meso-
and it was studied in detail by different techniques.
phase, where the transition in the texture is in general accom-
Firstly, we will consider compound AZO2. The nature of the panied by the appearance of defects. This behaviour is more
mesophases formed by this compound were assigned by both reminiscent of that found in other dimers for which a low
POM and XRD. On cooling from the isotropic liquid, a sample temperature nematic mesophase is proposed, such as the twist-
placed on 5 mm thick glass cells treated for parallel alignment bend nematic phase.1e,f Unfortunately the narrow temperature
showed a uniform birefringent texture that was characteristic of range made it difficult to characterize this mesophase unam-
the nematic mesophase (Fig. 1a). On further cooling, a transi- biguously and it will be denoted by the letter M.
tion to a striped texture was observed at around 123 ꢀC (Fig. 1b).
Heat capacity data as a function of temperature were recor-
This texture evolved to a fan-shaped texture that is characteristic ded in a heating run from the crystal (Cr) to isotropic liquid (I)
of a SmA arrangement (Fig. 1c). The nematic nature (N) of the phase for AZO2 and a plot is shown in Fig. 2 (black symbols).
high temperature mesophase was conrmed by XRD experi- The rst heat capacity effect at lower temperatures shows a two-
ments. Two diffuse halos, one in the low angle region and fold peak with a wide coexistence region of about 7 K. This type
another in the wide angle one, were detected in the XR of effect is usually found in multiple phase transitions and in
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Fig. 2 Specific heat capacity data as a function of temperature for a
sample of compound AZO2. Black and red symbols represent data
collected on heating and cooling runs at 1 ꢀC minꢁ1, respectively. Blue
symbols correspond to a heating run from 120 ꢀC after cooling from
the isotropic phase without reaching crystallization.
Fig. 3 Microphotographs of the textures of compound BIPH: (a) in the
N phase at 136 ꢀC, (b) in the N-SmX mesophase at 132 ꢀC, (c) in the
SmX at 124 ꢀC, and (d) in the SmY phase at 123 ꢀC. The width of the four
photographs is about 300 mm.
this case seems to represent the Cr-to-M-to-N phase transition.
On further heating another heat capacity peak is observed,
which according to the POM information is attributed to the N-
to-I phase transition. A subsequent cooling run from the liquid
phase at the same rate is also represented in Fig. 2 (red
symbols). The heat capacity peak corresponding to the I-to-N
phase transition is observed at almost the same temperature as
that in the heating run. At lower temperatures, two well-sepa-
rated peaks are obtained and this indicates the appearance of
another mesophase, which was identied as M according to our
POM experiments. This phase sequence is fully reversible.
Finally, on cooling the sample down to a temperature of about
We will now consider the other mesogenic dimer, i.e., BIPH.
On cooling from the isotropic phase, a characteristic texture of a
nematic mesophase was observed for this material over a narrow
temperature range. Almost simultaneously some circular and
fan-shaped domains characteristic of smectic phases started to
grow slowly as fractal nuclei, as shown in Fig. 3a. This coexis-
tence of both mesophases extended over approximately 12
degrees (Fig. 3b). XRD experiments conrmed the nematic and
lamellar character of the coexisting mesophases. The wide-angle
diffraction patterns of BIPH at two temperatures in the coexis-
tence range are shown in Fig. 4. A diffractogram characteristic of
a nematic arrangement was recorded at 137 ꢀC on cooling from
the isotropic phase (orange symbols in Fig. 4). On further cool-
ing, sharp peaks appeared that were superimposed on the
nematic pattern and these became more intense on lowering
temperature (the pattern in Fig. 4 shown in black points was
ꢀ
120 C (M mesophase) and then reheating it, the M-to-N tran-
sition could be observed clearly (blue symbols in the inset in
Fig. 2) and both the temperature and latent heat could be
calculated with a degree of accuracy.
Calculation of the latter parameter requires a special cali-
bration and the following expression for the total enthalpy
change associated with any transition (DHTOT):
ꢀ
obtained at 135 C). The structure of this new mesophase was
investigated using a low angle goniometer. The small angle
diagrams observed for three different temperatures in a cooling
run are shown in the inset in Fig. 4. Patterns in red and blue
points correspond to temperatures within the coexistence
Ð
DHTOT ¼ DH + DCpdT
(1)
The second term on the right-hand side of eqn (1) is the pre- region. These are indexed on the basis of a single periodicity of 87
˚
transitional uctuation contribution (DCp being the difference A. This value is signicantly smaller than the theoretical
˚
Cp ꢁ Cp,background due to the change in orientational order molecular length (102 A) calculated using Chemsketch. Given
intrinsic to this transition) and the latent heat is DH, which the resulting length of the combination of both spacers it is
vanishes for second order transitions. In strongly rst order difficult to determine whether the molecules are bent within the
phase transitions, the second term on the right-hand side of layers or tilted and interdigitated in a similar way to that reported
eqn (1) can be neglected due to the latent heat and the total for Janus-dendrimer analogues.14b Hence, this mesophase will be
enthalpy change is identied with the latent heat associated denoted as SmX. Finally, at about 120 ꢀC a change in the texture
with the phase transition. This is the case for the latent heat was observed and this corresponds to a transition to another
associated with the two-fold peak attributed to the Cr-to-M-to-N phase (Fig. 3d). The small angle diffraction pattern observed
phase transition. However, for the M-to-N and N-to-I transitions below this temperature is virtually identical to those observed in
the second term on the right-hand side of eqn (1) must be taken the coexistence region (diagram with green points in the inset in
into account in the calculation of the latent heats. The calcu- Fig. 4). In fact, the structure presents the same periodicity as the
lated values for compound AZO2 are listed in Table 1 together SmX phase. However, the appearance of small peaks on the wide
with the corresponding transition temperatures.
angle diffuse halo together with the dielectric behaviour, which
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Journal of Materials Chemistry C
Fig. 5 Specific-heat data as a function of temperature for a sample of
compound BIPH. Black and red symbols relate to data collected in
heating and cooling runs at 1 ꢀC minꢁ1, respectively. The inset shows
the detail of the specific heat around both the SmX-to-N and the N-
to-I phase transitions. Blue symbols correspond to a heating run on
slow cooling at 0.5 ꢀC minꢁ1 from the I phase down to 134 ꢀC.
Coexistence regions are marked by shaded areas.
Fig. 4 Wide-angle X-ray diffraction patterns for compound BIPH at
different temperatures in the phase coexistence range. The intensity is
plotted as a function of 2q, where q is the Bragg angle. Orange points in
the diagram are characteristic of a nematic structure (data at 137 ꢀC).
Black points in the diagram indicate that a lamellar phase grows within
the nematic bulk (data at 135 ꢀC). The inset represents the diffraction
patterns obtained with a small-angle goniometer. Red and blue points
correspond to the coexistence range as the temperature is lowered.
Green points correspond to the low temperature phase (data at 120
ꢀC). All the diagrams were indexed on the basis of a single periodicity.
transitions, with SmX slowly evolving towards the N mesophase.
An attempt to isolate the N–I transition was successful on slowly
cooling the sample to 134 ꢀC at 0.5 ꢀC minꢁ1. On re-heating the
ꢀ
sample a dened peak was obtained at about 135 C. Temper-
atures and latent heats associated with the observed phase
transitions are listed in Table 1.
is discussed below, strongly suggests that in this phase the
molecules have a higher degree oforder. Consequently, hereaer
Dielectric characterization
this phase will be denoted as SmY. The switching response under In order to gain further insights into the mesogenic behaviour
electric elds was also investigated for BIPH. The effect of of these two bent-core dimers, the temperature dependence of
different elds on the region of the N-SmX phase coexistence is the dielectric permittivity at 1 kHz for both materials was
consistent with reorientation of the nematic phase towards a determined in cooling runs from the isotropic phase (Fig. 6).
homeotropic dark state. In the present case, an electrooptic Dielectric results for both compounds were obtained under two
effect was not detected for the SmX and SmY phases, thus ruling different dc bias eld conditions (no eld and an applied dc
out the possibility of polar order in the mesophases.
bias voltage of 0.8 V mmꢁ1). The transitions between the
The phase sequence observed optically was conrmed by a different mesophases described in the previous section can be
careful analysis of the thermal behaviour. The heat capacity envisaged as changes in the permittivity. Due to the reor-
data for BIPH on cooling from the isotropic phase down to room ientation of the important dipole moment of the cyano-groups
temperature (red symbols) and on subsequent heating up to the of the two rod-like units, both materials show an increase in
isotropic phase (black symbols) are represented in Fig. 5. Three permittivity under eld conditions. The static permittivity of the
transitions can be identied on cooling from the isotropic nematic phase of AZO2 behaves in the same way as the nematic
phase. The rst peak at higher temperatures warrants careful phase of rod-like liquid crystals, with D3 > 0. In the case of BIPH,
analysis. The inset in Fig. 5 shows the existence of a two-fold the dielectric anisotropy is smaller than in the case of AZO2,
peak that, according to the textures, should correspond to the I- indicating that the coexistence of the nematic and lamellar
to-N and to the N-to-SmX phase transitions. The high heat mesophase hinders the reorientation of the rod-core unit under
capacity (red shaded region) is a sign of the coexistence of two dc bias conditions. Furthermore, the slow decrease in the
phases, which were identied as the N and SmX mesophases. permittivity on decreasing the temperature coincides with the
On further cooling to about 120 ꢀC a transition was detected and gradual transition from the N to the SmX phase. Finally, as can
this can be assigned to the SmX-to-SmY phase transition. be observed in Fig. 6, the dielectric response of compound BIPH
Finally, the heat capacity effect at lower temperatures (about 45 drops at the transition to the low temperature phase SmY, a
ꢀC) corresponds to a glass transition that provides evidence of nding that is consistent with a transition to a mesophase with
the disordered character, at least from a dynamic point of view, a higher degree of order, as inferred from XRD studies.
of the SmY mesophase. In the subsequent heating experiment a
The dynamic behaviour of the dimers was analysed by
similar glass transition was also observed and this was followed broadband dielectric spectroscopy measurements in the
by a single broad peak at higher temperatures, which is different mesophases as a function of temperature for both
assumed to be due to both the SmY-to-SmX and the N-to-I phase dimers. The temperature and frequency dependence of the
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Fig. 7 Three-dimensional plot of dielectric losses versus temperature
and the logarithm of the frequency for compounds AZO2 (top) and
BIPH (bottom) under 40 V dc bias field conditions.
Fig. 6 Dielectric permittivity versus temperature for compound AZO2
(top) and compound BIPH (bottom) in cooling runs.
125 ꢀC in the nematic phase are shown in Fig. 8. The dielectric
relaxation curves show different distinguishable relaxation
processes and this provides evidence that both the bent-core
imaginary part of the permittivity for AZO2 and BIPH under
eld conditions are shown in the three-dimensional plots in
Fig. 7. At rst glance, the dielectric spectra shown in Fig. 7 do
not seem to be complex. However, a closer inspection shows
several relaxation processes that complicate the interpretation
of the results. Both spectra seem to be dominated by a relaxa-
tion process whose frequency is around 1 MHz, indicating a low
thermal activation. For AZO2 the strength of this mode is
independent of temperature but for BIPH it rapidly decreases
on decreasing the temperature, i.e., with the slow conversion of
the nematic phase to the SmX phase.
In order to characterize the relaxation processes found, the
dielectric permittivity can be expressed as a sum of all the
possible contributions:
X
D3k
sdc
3ðuÞ ¼
þ 3N ꢁ i
(2)
ꢀ
ꢁ
bk
30u
ak
k
1 þ ðiuskÞ
whereD3k is thedielectricstrengthofeachrelaxationmode, sk the
relaxation time related to the frequency of maximum dielectric
loss, ak and bk are parameters that describe the shape (symmetry
and width) of the relaxation spectra (ak ¼ bk ¼ 1 corresponds to
Debye relaxation) and sdc is the dc conductivity. The summation
is extended over all relaxation modes, and each one is tted
according to the Havriliak–Negami (H–N) function.
Fig.
8 Frequency dependence of the dielectric permittivity of
compound AZO2 in the N phase (T ¼ 125 ꢀC), with no field (top) and
with a dc bias field of 40 V (bottom). Black solid lines are fittings
according to eqn (2) and coloured lines represent the deconvolution
As an example, the two components of the complex dielectric
permittivity under two different dc bias conditions for AZO2 at into elementary modes.
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Journal of Materials Chemistry C
and the rod-unit contribute to the dielectric permittivity. The the Generalitat de Catalunya of GRPFM as Emergent Research
assignment of each contribution to the corresponding molec- Group (2009-SGR-1243). The authors are also grateful to the JAE-
´
ular or intramolecular reorientation is not straightforward. Due DOC-CSIC program (N. Gimeno), the Universidad del Paıs Vasco
´
to the cumulative length of the exible spacer between the bent- for a post-doctoral grant (N. Sebastian) and to the BSCH-UZ and
core and the rod-like moieties, it seems reasonable to consider GA (project E04) (J. Vergara) fellowship program for support.
that both groups reorient independently. Under this assump- Thanks are also given to Nuclear Magnetic Resonance, Mass
tion, the frequency of the process denoted as 2 in the graphs Spectroscopy and Thermal Analysis Services from the CEQMA,
and the increase in its strength under eld conditions allow us Universidad de Zaragoza–CSIC (Spain).
to attribute this mode to the reorientation of the rod-like units,
i.e., the reorientation of the unit containing the cyano-group.
This mode shows an almost constant strength over the whole
References
temperature range, with a value of 2.7 in the nematic meso-
phase and 2.5 in the M mesophase and an activation energy of
109 kJ molꢁ1. The identication of all the different relaxation
processes requires further data and is beyond the scope of the
present work. In the case of BIPH, such analysis is even more
difficult since the effect of electric elds is weak due to the
phase coexistence.
1 (a) C. T. Imrie, Liq. Cryst., 2006, 33, 1449; (b) C. T. Imrie and
P. A. Henderson, Curr. Opin. Colloid Interface Sci., 2002, 7,
298; (c) C. T. Imrie and P. A. Henderson, Chem. Soc. Rev.,
2007, 36, 2096; (d) C. T. Imrie, P. A. Henderson and
G. Y. Yeap, Liq. Cryst., 2009, 36, 755; (e) M. Cestari, S. Diez-
Berart, D. A. Dunmur, A. Ferrarini, M. R. de la Fuente,
´
´
D. J. B. Jackson, D. O. Lopez, G. R. Luckhurst, M. A. Perez-
Jubindo, R. M. Richardson, J. Salud, B. A. Timimi and
H. Zimmerman, Phys. Rev. E: Stat., Nonlinear, So Matter
Phys., 2011, 84, 031704; (f) V. Borshch, Y.-K. Kim, J. Xiang,
Concluding remarks
´
Novel non-symmetric dimers, whose molecular design is based
on a promesogenic bent-shaped structure joined to two different
promesogenic rod-like moieties derived from cyanobiphenyl and
azobenzene units, have been synthesized. In contrast to previous
reports, and with the aim of decoupling the rigid cores, very long
spacers (two chains of 10 methylene units), linked either by a
phenyl ring or a hexamethylene-triazole moiety, have been used.
Interestingly, the use of the latter linkage prevented the appear-
ance of mesomorphism. However, in the former two cases these
unusually long spacers have proven to be suitable to promote
mesophases with a complex nature. It was also found that the
structure of the rod-like unit strongly affects the nal meso-
phasesexhibited by thematerials. Thus, for the dimercontaining
a cyanobiphenyl rod-like unit (BIPH), an unstable nematic mes-
ophase is formed and this evolved to a lamellar SmX and then to
the non-polar SmY mesophase. However, in the case of the azo-
benzene-baseddimer(AZO2) the –N]N– linking groupincreases
the length and polarizability. It confers a core structure in which
linearity is maintained, but the broadness tends to disrupt a
lamellar order thus enabling the formation of the nematic mes-
ophase, as it is commonly reported for azobenzene based liquid
crystals.17 A dielectric study of these materials revealed a complex
dynamic behaviour. Due to the cumulative length of the linking
group, the reorientational dynamics of the bent-core system and
the rod-like units are signicantly different. Furthermore, it was
shown that the strong dipole moment associated with the cyano-
group allows the reorientation of the alignment by the applica-
tion of strong electric elds.
M. Gao, A. Jakli, V. P. Panov, J. K. Vij, C. T. Imrie,
M. G. Tamba, G. H. Mehl and O. D. Lavrentovich, Nat.
Commun., 2013, 4, DOI: 10.1038/ncomms3635.
2 S. Kumar, Liq. Cryst., 2005, 32, 1089.
3 T. Niori, J. Mater. Chem., 1996, 6, 1231.
4 (a) G. Pelzl, S. Diele and W. Weissog, Adv. Mater., 1999, 11,
707; (b) D. M. Walba, Topics in Stereochemistry, Materials-
Chirality, 2003, vol. 24, p. 457; (c) C. Tschierske and
G. Dantlgraber, Pramana, 2003, 61, 455; (d) M. B. Ros,
J. L. Serrano, M. R. De La Fuente and C. L. Folcia, J. Mater.
Chem., 2005, 15, 5093; (e) W. Weissog, H. N. Shreenivasa
Murthy, S. Diele and G. Pelzl, Philos. Trans. R. Soc., A, 2006,
364, 2657; (f) R. A. Reddy and C. Tschierske, J. Mater. Chem.,
2006, 16, 907; (g) C. Pelzl, W. Weissog, Thermotropic Liquid
Crystals: Recent Advances, 2007, vol. 1, p. 1.
5 (a) J. Etxebarria and M. B. Ros, J. Mater. Chem., 2008, 18, 2919;
(b) A. Jakli, I. C. Pintre, J. L. Serrano, M. B. Ros and M. R. De La
Fuente, Adv. Mater., 2009, 21, 3784; (c) I. C. Pintre,
J. L. Serrano, M. B. Ros, J. Martınez-Perdiguero, I. Alonso,
J. Ortega, C. L. Folcia, J. Etxebarria, R. Alicante and
B. Villacampa, J. Mater. Chem., 2010, 20, 2965; (d)
J. Harden, M. Chambers, R. Verduzco, P. Luchette,
J. T. Gleeson, S. Sprunt and A. Jakli, Appl. Phys. Lett., 2010,
96, 102907; (e) A. Eremin and A. Jakli, So Matter, 2013, 9, 615.
´
´
´
´
6 (a) G. R. Luckhurst, Thin Solid Films, 2001, 393, 40; (b)
C. Tschierske, Curr. Opin. Colloid Interface Sci., 2002, 7, 69;
(c) B. Acharya, A. Primak, T. J. Dingemans, E. T. Samulski
and S. Kumar, Pramana, 2003, 61, 231; (d) M. A. Bates and
G. R. Luckhurst, Phys. Rev. E: Stat., Nonlinear, So Matter
Phys., 2005, 72, 051702; (e) B. Mettout, Phys. Rev. E: Stat.,
Nonlinear, So Matter Phys., 2005, 72, 031702.
Acknowledgements
The authors greatly appreciate nancial support from the
Spanish Government (MICINN-FEDER project MAT2009-14636-
C03 and MINECO-FEDER projects MAT2012-38538-C03-01/02/
03 and CTQ2012-35692), the Aragon-FSE (project E04) and
Basque (GI/IT-449-10) Governments, and the recognition from
7 (a) H. Takezoe and Y. Takanishi, Jpn. J. Appl. Phys., Part 1,
2006, 45, 597; (b) C. Keith, A. Lehmann, U. Baumeister,
M. Prehm and C. Tschierske, So Matter, 2010, 6, 1704; (c)
C. Tschierske and D. J. Photinos, J. Mater. Chem., 2010, 20,
4263; (d) P. Grzybowski and L. Longa, Phys. Rev. Lett., 2011,
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 4027–4036 | 4035

View Article Online
Journal of Materials Chemistry C
Paper
107, 027802; (e) M. Lehmann, Liq. Cryst., 2011, 38, 1389; (f) 13 (a) M. G. Tamba, B. Kosata, K. Pelz, S. Diele, G. Pelzl,
M. Mathews, S. Kang, S. Kumar and Q. Li, Liq. Cryst., 2011,
Z. Vakhovskaya, H. Kresse and W. Weissog, So Matter,
2006, 2, 60; (b) M. G. Tamba, U. Baumeister, G. Pelzl and
W. Weissog, Liq. Cryst., 2010, 37, 853.
¨
38, 31; (g) J. Seltmann, K. Muller, S. Klein and
M. Lehmann, Chem. Commun., 2011, 47, 6680.
8 C. V. Yelamaggad, S. Krishna Prasad, G. G. Nair, 14 (a) I. Giner, I. Gascon, J. Vergara, M. C. Lopez, M. B. Ros and
I. S. Shashikala, D. S. Shankar Rao, C. V. Lobo and
S. Chandrasekhar, Angew. Chem., Int. Ed., 2004, 43, 3429.
9 G. Lee, H. Jeong, F. Araoka, K. Ishikawa, J. G. Lee, K. T. Kang,
M. Cepic and H. Takezoe, Liq. Cryst., 2010, 37, 883.
F. M. Royo, Langmuir, 2009, 25, 12332; (b) N. Gimeno,
J. Vergara, M. Cano, J. L. Serrano, M. B. Ros, J. Ortega,
´
C. L. Folcia, S. Rodrıguez-Conde, G. Sanz-Enguita and
J. Etxebarria, Chem. Mater., 2013, 25, 286.
10 Y. Wang, H. G. Yoon, H. K. Bisoyi, S. Kumar and Q. Li, J. 15 (a) M. E. Neubert, S. S. Keast, J. M. Kim, A. G. Norton,
Mater. Chem., 2012, 22, 20363.
M. J. Whyde, M. J. Smith, R. M. Stayshich, M. E. Walsh
11 (a) C. V. Yelamaggad, I. S. Shashikala, G. Liao, D. S. Shankar
Rao, S. K. Prasad, Q. Li and A. Jakli, Chem. Mater., 2006, 18,
6100; (b) C. V. Yelamaggad, N. L. Bonde, A. S. Achalkumar,
and D. G. Abdallah Jr, Liq. Cryst., 2005, 32, 347; (b) J. Del
´
Barrio, L. Oriol, C. Sanchez, J. L. Serrano, A. Di Cicco,
P. Keller and M. H. Li, J. Am. Chem. Soc., 2010, 132, 3762.
´
D. S. Shankar Rao, S. K. Prasad and A. K. Prajapati, Chem. 16 M. B. Sied, J. Salud, D. O. Lopez, M. Barrio and J. Ll. Tamarit,
Mater., 2007, 19, 2463. Phys. Chem. Chem. Phys., 2002, 4, 2587.
12 (a) N. G. Nagaveni, V. Prasad and A. Roy, Liq. Cryst., 2013, 40, 17 Flussige Kristalle in Tabellen II, ed. D. Demus and H. Zaschke,
¨
¨
1001; (b) M.-G. Tamba, A. Bobrovsky, V. Shibaev, G. Pelzl,
U. Baumeister and W. Weissog, Liq. Cryst., 2011, 38, 1531.
VeB Deutscher Verlag Fur Grundstoffindustrie, Leipzig,
1984.
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