Chiral biphenylacetylene smectic liquid crystals

Article abstract of DOI:10.1021/jp1004727

Standard Sonogashira coupling afforded rod-like, phenylacetylene extended biphenyl mesogens substituted with different chiral alkoxy chains, starting from commercially available 4-(4a?2-bromophenyl)phenol. Depending on the position of the chiral center and the polar nature of the target molecules as determined by the electron withdrawing end-groups, different types of smectic liquid crystals are formed. The liquid crystal phases are fully characterized by polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction. In two cases, the molecular chirality is transferred to the supramolecular assembly, which is proved by circular dichroism measurements in the chiral mesophase. ? 2010 American Chemical Society.

Full text of DOI:10.1021/jp1004727

J. Phys. Chem. B 2010, 114, 4811–4815
4811
Chiral Biphenylacetylene Smectic Liquid Crystals
Laura de Vega,† Pedro D. Ortiz,†,‡ Gunther Hennrich,*,† Ana Omenat,§ Rosa M. Tejedor,§
Joaqu´ın Barbera´,§ Berta Go´mez-Lor,| and Jose´ Luis Serrano§
Departamento de Qu´ımica Orga´nica, UniVersidad Auto´noma de Madrid, Cantoblanco, 28049 Madrid, Spain,
Departamento de Qu´ımica Orga´nica, Facultad de Ciencias-Instituto de Ciencia de Materiales de Arago´n
(ICMA), UniVersidad de Zaragoza-CSIC, Pedro Cerbuna 12, 50009 Zaragoza, Spain, and Instituto de Ciencia
de Materiales de Madrid (ICMM), ICMM-CSIC, Cantoblanco, 28049 Madrid, Spain
ReceiVed: January 18, 2010; ReVised Manuscript ReceiVed: March 9, 2010
Standard Sonogashira coupling afforded rod-like, phenylacetylene extended biphenyl mesogens substituted
with different chiral alkoxy chains, starting from commercially available 4-(4′-bromophenyl)phenol. Depending
on the position of the chiral center and the polar nature of the target molecules as determined by the electron
withdrawing end-groups, different types of smectic liquid crystals are formed. The liquid crystal phases are
fully characterized by polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction.
In two cases, the molecular chirality is transferred to the supramolecular assembly, which is proved by circular
dichroism measurements in the chiral mesophase.
1. Introduction
electro-optical and high-birefringent materials.5 Sonogashira
coupling is the methodology of choice in order to obtain alkynyl-
expanded biphenyls.6 Different chiral alkyl chains are introduced
at the phenyl or biphenyl units as terminal substituents, and the
position and nature of the chiral substituent on the chirality of
the resulting LC phases is evaluated.
In smectic liquid crystal (LC) phases, the molecules present
orientational and at least short-range translational order within
a layered structure. In the smectic A phase, the long axis of the
rod-like (calamitic) mesogens is on average perpendicular to
the layer. Depending on the strength of the molecular dipole
moment, the calamitics are inclined to arrange in an antiparallel
fashion, forming bilayers as a consequence. In the smectic C
phase, the molecular axis of the mesogens is tilted with respect
to the layer normal. The orientation of the molecules responds
to an external electric field which is the basis of conventional
LC applications, primarily in display technology.1 The formation
of chiral smectic LC phases is the result of an ordered, layer
by layer propagation of the tilt angle in a way that the ensemble
of the molecular units adopts the aforementioned helical
arrangement on the macroscopic level.2 Since the early reports
on ferroelectric LCs more than three decades ago, substantial
research effort dedicated to chiral LC systems persists.3 As a
consequence of the presence of a stereocenter in the molecular
unit, the mesogens can adopt a helical conformation in the bulk.
Thus, the chirality on the molecular level is transferred on the
macroscopic scale in the LC phase, leading to interesting
applications in advanced display, sensing, and nonlinear optical
devices.4 No general rules exist which directly predict the
magnitude and sense of helical twisting in the superstructure
as the consequence of the position and absolute configuration
of the mesogenic unit. This is why one has to rely on the
extrapolation of established structure-activity relationships as
guidelines for the design of new supramolecular materials. Here,
we present the synthesis of novel chiral biphenylacetylene LCs
and study their mesomorphic and chiroptical properties. The
elongation of the biphenyl core by p-phenylethynyl units yields
molecular scaffolds which hold great promise as advanced
2. Results and Discussion
2.1. Synthesis. Commercially available 4-bromo-4′-hydroxy-
biphenyl was first alkylated with the respective bromoalkanes
to give bromobiphenyls 1a and 2a and then subjected to a
standard coupling-deprotection procedure to give the ethynyl-
biphenyls 17 and 2 as key intermediates (Scheme 1).
The target compounds 3-5 are obtained in a second coupling
step with corresponding substituted iodobenzenes in good yields
(Scheme 2). To our surprise, the supposedly straightforward
Sonogashira coupling of commercially available p-ethynylben-
zene derivatives with the bromobiphenyls 1a and 2a did not
give the desired products.
The synthesis of alkynylbiphenyl 6 with the chiral alkyl chain
situated at the biphenyl terminus is initiated by the modification
of the commercial biphenyl precursor: 4-(4′-Bromophenyl)ac-
etophenone is oxidized by tert-butyl hydroperoxide (TBHP) in
alkaline aqueous solution, catalyzed by WO3,8 to give 4′-
bromobiphenyl-4-carboxylic acid.9 After esterification of the
carboxylic acid with (S)-(-)-2-methyl-1-butanol, the ethynyl
group is introduced using a standard protocol, i.e., Pd-catalyzed
coupling of trimethylsilylacetylene (TMSA) followed by basic
cleavage of the TMS protecting group, yielding 6b.10 A second
Sonogashira coupling step employing 1-decyloxy-4-iodoben-
zene11 gives compound 6 as the final product (Scheme 3).
2.2. Experimental Section. All compounds are fully char-
acterized by 1H and 13C NMR spectra, elemental analysis, and
mass spectrometry. Full synthetic experimental procedures and
characterization of the target compounds are included in the
Supporting Information.
* To whom correspondence should be addressed. E-mail: gunther.
hennrich@uam.es.
† Universidad Auto´noma de Madrid.
‡ Present address: Departamento de Qu´ımica Inorga´nica, Universidad de
La Habana, La Habana 10400, Cuba.
§ Universidad de Zaragoza-CSIC.
Techniques. The optical textures of the mesophases were
studied with an Olympus polarizing microscope BX51 equipped
with a Linkam hot-stage and Linkam TMS 91 central processor.
| ICMM-CSIC.
10.1021/jp1004727  2010 American Chemical Society
Published on Web 03/25/2010

4812 J. Phys. Chem. B, Vol. 114, No. 14, 2010
de Vega et al.
SCHEME 1: Synthesis of Ethynylbiphenyls 1 and 2
SCHEME 2: Synthesis of Alkynylbiphenyls 3-5
SCHEME 3: Synthesis of Alkynylbiphenyl 6
TABLE 1: LC Phasesa and Transition Properties for
Compounds 3-6
The transition temperatures and enthalpies were measured by
differential scanning calorimetry with a TA Instruments Q20
instrument operated at a scanning rate of 10 °C min-1 on both
heating and cooling. The apparatus was calibrated with indium
(156.6 °C; 28.4 J g1-) as the standard. The XRD patterns were
obtained with a pinhole camera (Anton-Paar) operating with a
point-focused Ni-filtered Cu KR beam. The sample was held
in Lindemann glass capillaries (1 mm diameter) and heated,
when necessary, with a variable-temperature oven. The capillary
axis is perpendicular to the X-ray beam, and the pattern is
collected on flat photographic film perpendicular to the X-ray
beam. Spacings were obtained via Bragg’s law. UV-vis and
CD spectra of acetonitrile solution (around 10-5 M) and a thin
film of synthesized compounds were registered simultaneously
using a Jasco J-810 spectropolarimeter. Films were prepared
by casting solutions of the materials in dichloromethane onto
clean, fused silica slides and heated to the isotropic temperature
for 5 min and then sited on a metal block at 25 °C for 5 min.
Then, the films were placed in a rotating holder around the light
beam axis with temperature control. The CD spectra of the films
were registered both in a heating run starting at 5 °C above
melting temperature and measured every 5 °C up to the
isotropization temperature, and afterward in the cooling process
until the crystallization of the sample occurs. In all of the cases,
the negligible contribution of linear dichroism was confirmed
comparing the CD spectra registered rotating the sample every
60° around the light beam axis.
transition temperatures (°C)
and enthalpies (∆H/kJ mol-1
compound
)
3
4
5
6
Cr 93.9 (28.4) ordered Sm 172.5 (6.8) SmA 216.5
(2.3) I
Cr 76.3 (2.6) ordered Sm 166.5 (7.7) SmA 228.5
(7.7) I
Cr 64.2 (4.0) Cr′ 119.8 (1.5) hexatic Sm* 136.6 (2.9)
SmC* 158.4 (0.4) SmA 189.8 (4.2) I
Cr 122.5 (4.9) SmC* 147.2 (0.4) SmA 182.4 (3.7) I
a Cr, crystal; ordered Sm, ordered smectic phase; SmA, smectic
A; hexatic Sm*, chiral hexatic tilted (I or F); SmC*, chiral smectic
C; I, isotropic liquid.
named as ordered Sm in Tables 1 and 2, is followed by the
appearance of a SmA phase. The ordered character of the low-
temperature mesophase is revealed by the existence of a number
of sharp maxima at high diffraction angles in addition to a set
of equally spaced maxima at low angles. The comparison of
the experimental values obtained for the layer spacings and the
estimated molecule length using molecular modeling allows us
to deduce that the SmA phase presents a partially interdigitated
bilayer structure, which is usually found for mesogens with
highly polar groups, such as nitro or cyano.13,14 This phenom-
enon is accounted for by the antiparallel arrangement of the
mesogens. The symmetry of the orthogonal SmA phase prevents
the formation of a helical structure when introducing chirality
in the molecules forming the mesophase. However, compounds
5 and 6 exhibit tilted mesophases (hexatic I or F and C in the
case of 5 and C for 6), in which the molecules form a helical
organization in the mesophase when chiral centers are present
in the molecular structure. The exact nature (I or F) of the
hexatic mesophase cannot be determined on the basis of the
X-ray diffraction only. Table 2 gathers the layer thickness
measured by X-ray diffraction at variable temperatures for the
smectic mesophases of compounds 5 and 6. It is noteworthy
2.3. Mesomorphic Behavior. The thermal properties of the
biphenylacetylenes 3-6 were studied by differential scanning
calorimetry. Phase transition temperatures and the corresponding
enthalpy values were collected from the second heating scans.
The mesophases were identified by the observation of their
textures by polarized optical microscopy (POM),12 and by X-ray
diffraction. The data is summarized in Tables 1 and 2.
Biphenylacetylenes 3 and 4 display comparable phase be-
havior where an ordered lamellar structure, which we have

Chiral Biphenylacetylene Smectic Liquid Crystals
J. Phys. Chem. B, Vol. 114, No. 14, 2010 4813
TABLE 3: Calculated Dipole Moments for 4-6
TABLE 2: X-ray Diffraction Data for Compounds 3-6
d-layer
19
compound
µ (D)
σp
T
thickness
(Å)
tilt angle
La
4
5
6
8.07
3.01
3.21
NO2: 0.78
CO2CH3: 0.45
CO2CH3: 0.45
compound (°C)
phase
(cos-1 dC/dA) (Å)
3
100 ordered Sm
175 ordered Sm
185 SmA
34.4
34.7
36.5
37.3
32
molecules do not adopt a fully extended conformation and thus
the actual molecule length is smaller than L and much closer to
the layer spacing d measured in the high temperature range of
the SmA mesophase. Therefore, the tilt angles calculated with
the formula θ ) cos-1 dC/dA are more realistic and, obviously,
they are smaller than those calculated with the formula based
on the assumption of fully extended molecules.
The circular dichroism (CD) experiments performed on these
compounds confirm the results obtained by XRD (see below).
Comparing the SmC* and SmA layer spacings, the tilt angle of
the molecules in the SmC* mesophase can be calculated,
resulting in 27 and 29° for 5 and 6, respectively. Moreover, the
layer thickness measured for the SmA mesophase of 5 and 6
indicates that, in these cases, a monolayer structure is formed
which is expected to be due to the presence of only weakly
polar ester groups as terminal substituents of the mesogenic unit.
The transitions from the SmA to the chiral Sm* phases (SmC*
and hexactic Sm*) result in layer contractions substantially
bigger than those defined for a “de Vries-like” behavior
(e1%).1a,15
195 SmA
4
r.t.
crystal
30
175 ordered Sm
185 SmA
190 SmA
35.1
35.8
35.8
38
5
6
90
crystal
105 crystal
125 hexatic Sm*
145 SmC*
165 SmA
185 SmA
r.t.
90
135 SmC*
155 SmA
175 SmA
31.9
31.8
34.0
35.7
27°
crystal
crystal′
38
31.3
35.7
35.7
29°
a L: Estimated molecule length using Dreiding stereomodels
assuming a fully extended conformation of the hydrocarbon chains.
that the d-layer thickness of the SmA mesophase measured for
compound 5 is significantly temperature dependent. At 165 °C,
the d-layer thickness is 34.0 Å, whereas, at 185 °C, this value
increases to 35.7 Å. This indicates that the molecules are tilted
to a certain extent even in the low-temperature range of the
SmA mesophase. Such a difference in the d-layer spacing is
not observed in the case of compound 6, for which the values
measured at 155 and 175 °C are the same, although if the
experimental error of the measurement ((0.5 Å) is considered,
a certain tilt of the molecules in the SmA mesophase might
exist. The circular dichroism (CD) experiments performed on
these compounds confirm the results obtained by XRD (see
below). Comparing the SmC* and SmA layer spacings, the tilt
angle of the molecules in the SmC* mesophase can be calculated
as θ ) cos-1 dC/dA, with dC and dA being the layer thickness in
the SmC and SmA phases. The results of these calculations are
27 and 29° for 5 and 6, respectively. Moreover, the layer
thickness measured for the SmA mesophase of 5 and 6 indicates
that, in these cases, a monolayer structure is formed which is
expected to be due to the presence of only weakly polar ester
groups as terminal substituents of the mesogenic unit. Although
theoretically the layer spacing measured in the higher temper-
ature range of the smectic A mesophase of these compounds
would be expected to be equal to the molecular length, the value
obtained experimentally is smaller than the one predicted from
molecular models. This phenomenon, very common in liquid
crystals, is due to the conformational freedom of the hydrocar-
bon chains in the mesophase (presence of a number of gauche
bonds), as well as to the possibility of local fluctuations of the
direction of the molecular axes, all of which can reduce the
effective length. For compounds 5 and 6, the measured layer
spacing d is smaller by 2-2.5 Å than the molecule length L
(Table 2). It is interesting to note that the tilt angle could also
be deduced as θ ) cos-1 d/L. By using this formula, it is
assumed that, in the mesophase, the molecules are in their most
extended conformation, i.e., with their hydrocarbon chains in
the all-anti arrangement. However, as mentioned above, this is
not the case for the SmA mesophase of compounds 5 and 6. It
can be safely deduced that in the SmC* mesophase the
2.4. Electro-optical Characterization. A 5 µm thick Linkam
LC cell was filled with compound 5 in the isotropic liquid phase.
Unfortunately, it was not possible to obtain a well-oriented
sample by slow cooling of the cell and/or by applying an electric
field. Electro-optical switching was observed in the temperature
intervals corresponding to both ferroelectric mesophases, SmC*
and hexatic Sm*, by the application of an electric field
(triangular wave, 50 Vp-p, 1 Hz frequency).16 However, the
determination of the spontaneous polarization of compound 5
in these mesophases was not possible in our experimental setup.
To elucidate the differences in the mesomorphic behavior,
computational calculations (DFT//B3LYP/6-311G[d,p]) have
been performed to assess the molecular dipole moment µ of 4,
5, and 6. For simplification, the alkyl substituents in the ether
or ester groups are calculated as methyl groups. Table 3
summarizes the calculated dipole moments for the biphenyla-
etylenes 4-6 depending on the electronegativity on the terminal
phenyl units as defined by Hammet substituent constants σp.17
It is straightforward that the high dipole moment of 8.07 D for
4 is responsible for the high order in the SmA phase and the
for the formation of bilayers in the LC phase. We attribute the
distinct mesomorphism of 5 vs 6 to subtle differences in the
dipole moments of both, which in consequence affects the
alignment of the molecules in the bulk. On the molecular level,
this variation stems from the less efficient π-conjugation in the
biphenyl fragment of 5.18
2.5. Circular Dichroism. An expected result for the achiral
biphenylacetylene compound 3 is that its acetonitrile solution
is CD-silent. Under the chosen experimental conditions, aceto-
nitrile solutions of chiral compounds 4, 5, and 6 do not exhibit
CD responses either (i.e., the expected signal is below the
detection limit). Subsequently, in solution, no intrinsic CD due
to molecular chirality is detected in the UV-vis range, as the
chiral centers do not exhibit absorption bands in this spectral
range. Moreover, these chiral groups do not exert symmetry-
breaking perturbation of the electronic states of the chro-
mophores (Figures S1 and S2, Supporting Information). In order
to study the progress of the induced CD with the temperature

4814 J. Phys. Chem. B, Vol. 114, No. 14, 2010
de Vega et al.
Figure 1. CD spectra of 5 from 140 to 190 °C, registered every 5 °C. Inset: CD spectra in the 245-285 nm range from 140 to 190 °C, registered
every 5 °C.
Figure 2. CD spectra of 6 from 125 to 175 °C registered each 5 °C.
and liquid crystal order, the CD spectra of the chiral compounds
which exhibit helical mesophases, 5 and 6, were registered
within the mesogenic range every 5 °C on heating. These results
are displayed in Figures 1 and 2, respectively. CD spectra of 5
for the hexatic Sm* temperature range (125-135 °C) are
included in the Supporting Information (Figure S3).
The chiral organization of a helical liquid crystal mesophase
induces significant CD responses. In fact, chiral compounds 5
and 6 display intense CD signals in the helical mesophases
(hexatic Sm* and SmC*) and, surprisingly, also in the orthogo-
nal mesophase (SmA*). On the other hand, the achiral me-
sophase of chiral compound 4 is CD silent. The CD spectra of
compounds 5 and 6 show two Cotton effects of opposite sign.
Both of them exhibit a negative Cotton effect at shorter
wavelengths and a positive one at longer wavelengths. These
bands are attributed to the transition whose moments are
perpendicular and parallel to the molecular long axis, respec-
tively.20 Figures 3 (5) and 4 (6) show the temperature depen-
dence of the CD signal at the maximum of the positive Cotton
effect relative to the ellipticity value at the beginning of the
SmC* phase (140 °C for 5 and 125 °C for 6).
Figure 3. Temperature dependence of the CD of 5 at the maximum
of the positive Cotton effect relative to the ellipticity value at 140 °C.
CD responses and the sign of the CD signal in the SmA* phase
is always the same as that in the SmC* phase. Similar results
are obtained in the cooling scan. These results indicate the
existence of some helical structures in the SmA* phases, and
consequently, the molecular tilt is not zero.2 In fact, X-ray
diffraction data supports these results as stated above.
As a result of the liquid crystalline order, the CD signal varies
with the temperature and no discontinuity exists at the SmC*
and SmA* phase transition. In addition, the SmA* phases exhibit

Chiral Biphenylacetylene Smectic Liquid Crystals
J. Phys. Chem. B, Vol. 114, No. 14, 2010 4815
POM images for 3-6; UV-vis and CD spectra of 3, 4, 5, and
6 in acetonitrile; CD spectra of 5 in the hexatic Sm* phase;
and computational modeling. This material is available free of
charge via the Internet at http://pubs.acs.org.
References and Notes
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3. Conclusion
Straightforward synthetic protocols are presented for access-
ing various acetylenic biphenyl derivatives from commercially
available biphenyl precursors. Chiral alkyl chains can be placed
on the phenylethynyl and biphenyl terminus, likewise. Highly
polar π-systems are obtained by incorporating a terminal NO2
substituent. As a consequence, the mesogens arrange in an
antiparallel fashion, forming bilayers, and the molecular chirality
of 4 brought about by the terminal alkyl chain is canceled out
by the high supramolecular order in the bulk. In contrast, the
moderately polar ester derivatives 5 and 6 form chiral LC
phases; i.e., the molecular chirality is efficiently transferred to
the supramolecular level. In their chiral smectic (SmC*) phases,
the calamitics are considerably tilted, and especially bipheny-
lacetylene 5 displays a rich polymorphism. The differences in
their phase behavior are simply the result of the positioning of
the chiral center along the biphenylacetylene system, which
would have been impossible to predict ab initio. More than
simply giving insight into the structure-activity relationship,
which is useful for the design of LC materials, both compounds
display intriguing chiroptical properties potentially useful in
optic and photonic applications.
Acknowledgment. We thank the CCC of the UAM and the
following institution for financial support: MEC (Spain), grants
CTQ2007-65683, CTQ2006-15611-CO2-01, and MAT2008-
06522-C02.
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Supporting Information Available: General methods, full
synthetic procedures, and compound characterization for 2a-6;
JP1004727