The influence of alkoxy chain length on the ferroelectric properties of chiral fluorenol liquid crystals

Article abstract of DOI:10.1039/b804673e

A series of isomeric chiral fluorenol mesogens with different alkoxy chain length combinations have been prepared in order to test a model for the origin of polar order in the chiral SmC* phase formed by these compounds. The results show that the transposition of alkoxy side chains has no effect on the sign of spontaneous polarization, P_S, which is inconsistent with a model suggesting that polar order arises from a rotational bias of antiparallel hydrogen-bonded dimers that minimizes free volume in the tilted SmC* phase. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b804673e

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
The influence of alkoxy chain length on the ferroelectric properties of chiral
fluorenol liquid crystals†
*
Jeffrey C. Roberts, Zachary M. Hudson and Robert P. Lemieux
Received 19th March 2008, Accepted 28th April 2008
First published as an Advance Article on the web 10th June 2008
DOI: 10.1039/b804673e
A series of isomeric chiral fluorenol mesogens with different alkoxy chain length combinations have
been prepared in order to test a model for the origin of polar order in the chiral SmC* phase formed by
these compounds. The results show that the transposition of alkoxy side chains has no effect on the
sign of spontaneous polarization, PS, which is inconsistent with a model suggesting that polar order
arises from a rotational bias of antiparallel hydrogen-bonded dimers that minimizes free volume in the
tilted SmC* phase.
a preferred orientation (sign) of any dipole coupled to a stereo-
Introduction
genic center along the C2 axis of each SmC* layer. We have
applied the principles of the Boulder model to the design of
so-called ‘Type II’ dopants, in which the chiral element is located
in the rigid aromatic core (in contrast to Type I dopants, in which
the chiral element is located in one of the alkyl side chains).
Examples of such Type II dopants include the axially chiral
dinitrobiphenyl 1,6 and spirobiindandione 2.7
The ferroelectric properties of the chiral smectic C (SmC*) liquid
crystalline phase have been investigated for nearly forty years.1
The SmC* phase is comprised of a diffuse layer structure in
which the long axes of chiral calamitic (rod-shaped) molecules
are tilted at an angle q with respect to the layer normal. As
initially predicted by Meyer et al. in 1975,2 the absence of
reflection symmetry in the SmC* phase creates a net spontaneous
polarization PS along a polar axis perpendicular to the smectic
tilt plane. When aligned between conductive glass plates with
a spacing of a few microns, the SmC* phase is ferroelectric, and
coupling of PS to an external electric field allows switching
between the two possible tilt orientations.3 The birefringence of
such surface-stabilized ferroelectric liquid crystals (SSFLC)
creates the potential for designing ON/OFF light shutters with
very fast switching times, on the order of ms. The development of
novel SSFLC materials for electro-optic display applications is
currently an active field of research, which requires a thorough
understanding of the relationship between molecular structure
and ferroelectric properties.4
Although 1 and 2 function well as chiral dopants in various
achiral SmC hosts, neither of these compounds actually form
liquid crystalline phases. More recently, we have focused our
efforts on the investigation of smectogens with chiral cores such
as the fluorenol (R)-3a, which forms a broad SmC* phase with
a spontaneous polarization of ꢀ13 nC cmꢀ2 at 10 K below the
Curie point (T ꢀ TC ¼ ꢀ10 K).8,9 According to molecular
modeling, the energy-minimized conformation of (R)-3a that is
consiꢁstent with the experimentally determined optical tilt angle
Insight into the relationship between molecular chirality and
the sign and magnitude of PS is provided by the Boulder model.5
In the case of a SmC* phase induced by a chiral dopant in an
achiral SmC liquid crystal host, the Boulder model treats the
SmC phase as a supramolecular host, and the ordering of the
chiral dopant is modeled by a mean-field potential, which is
analogous to a binding site in host–guest chemistry. The zigzag
shape of the binding site, in which the rigid aromatic core is more
tilted than the alkyl side chains, is consistent with comparative
studies of smectic tilt angles derived from small angle X-ray
scattering (SAXS) and polarized optical microscopy (POM)
measurements. According to this model, the polarization PS
originates from a rotational bias of the chiral dopant about the
director n imposed by the binding site, which results in
˚
of 32 (POM) and layer spacing of 31.8 A (SAXS) is essentially
uniaxial. This suggests that the rotational bias about the director
n between +PS and ꢀPS orientations giving rise to polar order in
the SmC* phase is very small, perhaps negligible. However,
results from a series of experiments (variable temperature FT-IR
spectroscopy, dilution with achiral SmC materials, and deute-
rium exchange) suggest that the polar order of (R)-3a in the
SmC* phase is enhanced by the formation of biaxial hydrogen-
bonded dimers.9 According to this model, two chiral fluorenol
molecules associate via complimentary lateral OH–O]C
hydrogen bonds, resulting in the formation of antiparallel
dimers. Modeling of the SmC* phase formed by (R)-3a as layers
Department of Chemistry, Queen’s University, Kingston, Ontario, K7L
3N6, Canada. E-mail: lemieux@chem.queensu.ca
† Electronic supplementary information (ESI) available: Synthesis
procedures, characterization data (1H and 13C NMR, HRMS), POM
photomicrographs of liquid crystal textures, DSC data, and details of
electro-optic experiments. See DOI: 10.1039/b804673e
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of hydrogen-bonded dimers suggests that polar order arises from
a rotational bias about the director n imposed by the packing
order of the tilted smectic layers that minimizes free volume,
which would favor the ꢀPS rotational state in the case of (R)-3a
(Fig. 1a).
(R)-3a–f to those of the corresponding fluorenones 4a–f, which
cannot undergo intermolecular hydrogen bonding.
Results and discussion
Synthesis
Examination of this model reveals that the different packing
properties of the +PS and ꢀPS rotational states of (R)-3a are due
to the difference in length between the alkoxy side chains bound
to the fluorenol core (C8) and to the benzoyloxy group (C11).
Indeed, molecular modeling shows that the antiparallel dimer of
(R)-3b (the isomer of (R)-3a in which the positions of the C8 and
C11 alkoxy side chains are interchanged) would be expected to
favor the +PS rotational state instead of the ꢀPS state (Fig. 1b).
Hence, in order to test this model, we have undertaken a study of
the chiral fluorenols (R)-3a–f in which the lengths of the two
alkoxy side chains are varied while maintaining the molecular
Compounds (R)-3a–f and 4a–f were prepared according to the
route described in Scheme 1. This is a shorter, albeit less efficient
route than the directed metalation/cross coupling strategy
previously reported,8,9 which provided more rapid access to
larger amounts of the final products. The common intermediate
2,7-dihydroxy-9-fluorenone (6) was synthesized in four steps
from commercially available fluorene (5) using procedures
adapted from the work of Burke and Joullie.10 Statistical alkyl-
ation of 6 with the requisite alkyl bromides followed by reduction
with sodium borohydride gave the racemic fluorenediols 8a–f,
which were resolved by preparative chiral phase HPLC (Daicel
Chiralpak-AS). The (S) enantiomer of each fluorenediol was
selectively esterified using the appropriate benzotriazole to give
the desired fluorenols (R)-3a–f in optically pure form. The
fluorenone 4a had been prepared previously by esterification of
7a using DCC/DMAP,9 but we found that the fluorenones 4a–f
could be more readily prepared via PCC oxidation of the
corresponding fluorenols.11 All materials ((R)-3a–f and 4a–f)
were purified by chromatography on silica gel, and recrystallized
from hexanes–ethyl acetate. Synthesis procedures and charac-
terization data for all compounds are provided in the ESI.†
˚
length constant (ca. 40 A). If polar order does originate from
a rotational bias of antiparallel hydrogen-bonded dimers as per
Fig. 1, we would expect the SmC* phases formed by the
fluorenols with m < n (i.e., (R)-3a, (R)-3c, and (R)-3e) to have
a negative PS, and those formed by the fluorenols with m > n (i.e.,
(R)-3b, (R)-3d, and (R)-3f) to have a positive PS. In addition, we
sought to compare the mesogenic properties of the fluorenols
Mesophase characterization
The liquid crystalline phases formed by (R)-3a–f (Fig. 2) and 4a–f
(Fig. 3) were identified by texture analysis using POM, and phase
transition temperatures and enthalpies of transitions were
measured by differential scanning calorimetry (DSC, see ESI†
for numerical data). As shown in Fig. 2 and 3, all twelve
compounds form SmC/C* phases, as indicated by characteristic
broken fan / Schlieren textures (see ESI). None of the resolved
fluorenol materials showed periodic fringe patterns and/or
pseudo-homeotropic domains characteristic of the chiral SmC*
phase, which suggests that the helical pitch is greater than the
film thickness, probably on the order of 100 mm. The six fluorenol
materials (R)-3a–f have relatively broad SmC* temperature
ranges (47–100 K) and high clearing points (170–180 ꢁC); the
SmC* phase is predominant, and in several cases the only
mesophase observed. Increasing the chain-length mismatch
results in a slight depression of the clearing point, a narrowing of
the SmC* temperature range and, in some cases, the formation of
chiral smectic A (SmA*) and chiral nematic (N*) phases with
narrow temperature ranges. By comparison, the corresponding
Fig. 1 Molecular models (MMFF force field) of the ꢀPS and +PS
rotational states for hydrogen-bonded antiparallel dimers of (a) the
fluorenol (R)-3a (top), and (b) the fluorenol (R)-3b (bottom). The spon-
taneous polarization points from negative to positive according to the
physics convention.
3362 | J. Mater. Chem., 2008, 18, 3361–3365
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Scheme 1 Reagents and conditions: (a) acetic anhydride, AlCl3, 76%; (b) m-CPBA, TFA, CHCl3, 84%; (c) Na2Cr2O7, AcOH, EtOH, reflux, 56%; (d)
NaHCO3, H2O, MeOH, reflux, 69%; (e) CmH2m+1Br, K2CO3, DMF, 25 ꢁC, 30–35%; (f) NaBH4, Et2O, MeOH, 25 ꢁC, 90–95%; (g) preparative HPLC
resolution, Daicel Chiralpak-AS column, 5–20% IPA–hexanes, 55 mL minꢀ1; (h) 1-(4-alkoxybenzoyl)benzotriazole, 0.5 M aq NaOH, THF, 25 ꢁC, 65–
75%; (i) PCC, CH2Cl2, 25 ꢁC, 70–90%.
phase. Upon increasing the chain-length mismatch of 4a–f, the
temperature range of the N phase increases at the expense of the
SmC phase. This trend is consistent with weaker core–core
interactions in a lamellar arrangement of highly unsymmetrical
mesogens, which cannot offset as effectively the entropic cost
imposed by the translational order of the SmC phase in the
absence of hydrogen bonding. Overall, these results further
support the notion that intermolecular hydrogen bonding in (R)-
3a–f strengthens non-covalent core–core interactions, which
should enhance the nanosegregation of core and side chains, and
thus promote the formation of lamellar mesophases.
Through the course of mesophase characterization experi-
ments, the fluorenols (R)-3a–f were found to undergo thermal
degradation. According to observations by POM, prolonged
heating causes a lowering of the clearing point temperatures and
Fig. 2 Phase transition temperatures for fluorenols (R)-3a–f measured
by DSC on heating. The y-axis labels represent the alkoxy chain length
combinations.
a broadening of the phase transition points.
Similar findings were obtained by DSC, although the materials
degraded less rapidly in sealed DSC sample pans than on
microscope slides. To determine the nature of this degradation,
a neat sample of racemic 3a was heated in a conical flask open to
ꢁ
the atmosphere for two hours at 190 C, i.e., above its clearing
point. The white solid turned orange after heating, and analysis
of the residue by HPLC revealed the presence of the fluorenone
4a together with 3a in a 20 : 80 ratio. These results are consistent
with the observed color change and lowering in clearing point,
and thus indicate that the fluorenols are prone to oxidation in air
at high temperatures over extended periods of time.
Polarization measurements
The six chiral fluorenol materials were introduced into
commercial ITO glass cells with parallel-rubbed polyimide
surfaces and a spacing of 4 mm. With the exception of (R)-3f,
attempts to obtain well-aligned SSFLC films by slow cooling
from the isotropic phase (with or without a 6 V mmꢀ1 ac field)
were unsuccessful. The fluorenols (R)-3a–c, which form only
SmC* phases, gave particularly poor alignments, with contrast-
ing dark and bright texture domains (Fig. 4a). Furthermore, the
application of a 6 V mmꢀ1 ac field across these SSFLC films
typically produced two polarization reversal current peaks
instead of one (Fig. 4b). Inspection by POM indicated that both
dark and bright domains have the same sign of polarization and
Fig. 3 Phase transition temperatures for fluorenones 4a–f measured by
DSC on heating. The y-axis labels represent the alkoxy chain length
combinations.
fluorenones 4a–f have narrower SmC temperature ranges (16–
ꢁ
58 K) and lower clearing points (148–158 C). Furthermore, all
six fluorenones form nematic (N) phases in addition to the SmC
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Fig. 5 Polarized photomicrograph of a SSFLC film of (R)-3a in a 4 mm
rubbed polyimide ITO cell at T ꢀ TC ¼ ꢀ10 K after 1 h under a 6 V mmꢀ1
dc applied field (non-addressed area on left, ITO addressed area on right).
same experiment performed with a 6 V mmꢀ1 dc field applied
between PS measurements resulted in a 20% decrease in the
magnitude of PS over a one hour period. In addition, the growth
of isotropic domains was observed at the edge of the addressed
ITO area (Fig. 5), which is consistent with oxidation of the
fluorenol. Although the exact nature of this electrochemical
degradation process remains unclear, the instability of
substituted fluorenes to redox conditions has been established
previously in the field of semiconducting polyfluorenes.13
Fig. 4 Polarized photomicrographs and corresponding polarization
reversal current peaks for SSFLC films of (R)-3a in 4 mm rubbed-
polyimide ITO cell (rubbing direction horizontal) after cooling from
the isotropic phase without an applied field (a, b), and with a 3 V mmꢀ1 dc
field (c, d).
Despite the instability of fluorenols (R)-3a–f in the presence of
an applied dc field, the polarization measurements carried out
with an applied ac field proved to be reliable and reproducible.
Plots of PS and tilt angle q as a function of reduced temperature
are shown in Fig. 6 and 7, respectively, and Table 1 gives
experimental values of these measurements at T ꢀ TC ¼ ꢀ10 K.
All samples show the expected increase in PS and q with
decreasing temperature. At any given reduced temperature, PS
and q values do not change significantly upon transposition of
the alkoxy chains, and they are highest for the more symmetrical
fluorenols (R)-3a and (R)-3b, and lowest for the least symmet-
rical fluorenols (R)-3e and (R)-3f. However, if one normalizes the
polarization values with respect to tilt angle by calculating the
the same tilt angle. Dierking and co-workers previously reported
the observation of such ‘horizontal chevron’ textures in SSFLC
films, and attributed the dark and bright domains to smectic
layers tilted in different orientations with respect to the rubbing
direction.12 Hence, the dark domains are likely formed by SmC*
layers with the director n oriented parallel to the rubbing direc-
tion, which is horizontal in Fig. 4a, whereas the SmC* layers in
the bright domains are uniformly tilted in the opposite direction,
with the director n oriented away from the rubbing direction by
an angle of 2q. The two polarization reversal current peaks
would then correspond to the switching of bright domains
towards the rubbing direction (lower coercive force), followed by
switching of the dark domains away from the rubbing direction
(higher coercive force). The observation of these two nearly
resolved current peaks suggests that an unusually strong inter-
action exists between the fluorenol mesogens and the polyimide
alignment layer, which may be due to hydrogen bonding.
Uniform monodomain SSFLC films could be obtained by
cooling the chiral fluorenols from the isotropic phase at 2 ꢁC
minꢀ1 while applying a 3 V mmꢀ1 dc field; as expected, these films
gave single polarization reversal current peaks upon applying
a 6 V mmꢀ1 triangular ac field, from which accurate measure-
ments of PS were obtained (Fig. 4c, d).
Measurements of PS as a function of time (either with or
without a 6 V mmꢀ1 triangular wave ac field applied between
measurements) showed no decrease in polarization over a one
hour period at a reduced temperature of T ꢀ TC ¼ ꢀ10 K, and
subsequent inspection of the addressed ITO area by POM
revealed no apparent degradation of the liquid crystal. These
results suggest that the fluorenol samples are not oxidized to any
significant extent when confined to electro-optic cells under
standard conditions used for PS measurements. Interestingly, the
Fig. 6 Spontaneous polarization PS vs. reduced temperature T ꢀ TC for
(R)-3a (open triangles), (R)-3b (triangles), (R)-3c (open squares), (R)-3d
(squares), (R)-3e (open circles), (R)-3f (circles).
3364 | J. Mater. Chem., 2008, 18, 3361–3365
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Conclusions
A comparison of the mesogenic properties of the chiral
fluorenols (R)-3a–f with those of the corresponding fluorenones
4a–f is consistent with previous studies suggesting that inter-
molecular hydrogen bonding is stabilizing the SmC* phase
formed by the fluorenols, presumably by strengthening core–core
interactions.8,9 Furthermore, although previous results from our
laboratory strongly suggest that intermolecular hydrogen
bonding does enhance polar order in the SmC* phase formed by
the chiral fluorenol materials, the effect (or lack thereof) of
transposing alkoxy side chains on the sign of PS clearly indicates
that our model for rotational order about n based on the packing
requirement of antiparallel hydrogen-bonded dimers needs to
be revised. Indeed, the results of this study suggest that the
self-assembly of fluorenol mesogens giving rise to polar order in
the SmC* phase is likely to be more complex than initially
conceived.
Fig. 7 Tilt angle q versus reduced temperature T ꢀ TC for (R)-3a (open
triangles), (R)-3b (triangles), (R)-3c (open squares), (R)-3d (squares), (R)-
3e (open circles), (R)-3f (circles).
Acknowledgements
Table 1 Spontaneous polarization PS and tilt angle q at T ꢀ TC ¼ ꢀ10 K
for the fluorenols (R)-3a–f
We are grateful to the Natural Sciences and Engineering
Research Council of Canada for support of this work, for
a postdoctoral fellowship to J.C.R., and for an undergraduate
summer research award to Z.M.H.
(R)-Fluorenol
n/m
TC/ꢁC
PS/nC cmꢀ2
q/deg
Po/nC cmꢀ2
3a
3b
3c
3d
3e
3f
8/11
11/8
6/13
13/6
4/15
15/4
180
179
178
174
172
166
ꢀ13
ꢀ12
ꢀ9
32
32
26
27
19
17
ꢀ24
ꢀ22
ꢀ20
ꢀ22
ꢀ19
ꢀ20
ꢀ10
ꢀ6
References
ꢀ6
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reduced polarization Po ¼ PS sinqꢀ1 14
,
the data suggests that
there is relatively little difference in polar order between the six
isomers. More importantly, the spontaneous polarizations of all
six fluorenols (R)-3a–f have the same sign (negative), which is
inconsistent with the model for polar ordering described in Fig. 1
that predicts a reversal of the sign of PS upon transposing the
alkoxy side chains. This is also supported by the fact that the tilt
angle decreases with increasing length mismatch of the alkoxy
side chains, which is counter to what one would expect from the
packing of antiparallel dimers as described by Fig. 1. Instead,
these results may be explained by considering the balance that
must be achieved between out-of-layer fluctuations and core–
core interactions in minimizing the free energy of the system. As
with the fluorenones 4a–f, a greater mismatch of the alkoxy chain
lengths should reduce core–core interactions in smectic phases,
and therefore shift the balance towards out-of-layer fluctuations,
which are favored by smaller tilt angles in the SmC* phase.
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J. Mater. Chem., 2008, 18, 3361–3365 | 3365