Design of liquid crystals with de Vries-like properties: Organosiloxane mesogen with a 5-phenylpyrimidine core

Article abstract of DOI:10.1021/ja805672q

According to a new design strategy for de Vries-like liquid crystal materials, we report the synthesis and characterization of an organosiloxane mesogen with a 5-phenylpyrimidine core that forms SmA and SmC liquid crystal phases. This new material is characterized by a maximum layer contraction of 1.2% upon SmA-SmC phase transition and is comparable to the best de Vries-like materials reported heretofore. Copyright

Full text of DOI:10.1021/ja805672q

Published on Web 09/25/2008
Design of Liquid Crystals with “de Vries-Like” Properties: Organosiloxane
Mesogen with a 5-Phenylpyrimidine Core
Jeffrey C. Roberts,† Nadia Kapernaum,‡ Frank Giesselmann,‡ and Robert P. Lemieux*,†
Chemistry Department, Queen’s UniVersity, Kingston, Ontario K7L 3N6, Canada, and Institute of Physical
Chemistry, UniVersita¨t Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany
Received July 21, 2008; E-mail: lemieux@chem.queensu.ca
Ferroelectric liquid crystals (FLC) have generated a considerable
amount of research activity over the past 30 years, ranging from
fundamental studies on the molecular origins of spontaneous polar
ordering to the commercial development of high-resolution reflec-
tive liquid-crystal-on-silicon (LCOS) microdisplays with switching
times in the range of microseconds.1,2 Materials used in FLC
microdisplay applications are normally composed of an achiral
liquid crystal mixture with an isotropic-nematic-smectic A-smectic
C (INAC) phase sequence and a chiral dopant that induces a
ferroelectric polarization PS in the tilted smectic C phase. Coupling
of PS to an applied electric field enables switching of the
ferroelectric SmC* phase between opposite tilt orientations, thereby
producing an ON-OFF light shutter between crossed polarizers.3
Figure 1. Schematic representations of the SmA-SmC phase transition
One major problem in formulating FLC mixtures for display devices
according to (a) a classic rigid-rod model and (b) the diffuse cone model
proposed by de Vries.16
is the layer contraction caused by the tilting of molecules upon
transition from the orthogonal SmA phase to the SmC phase (Figure
1a), which results in a buckling of the smectic layers into a chevron
geometry and the formation of “zigzag” defects that reduce the
optical quality of the FLC film.4
To solve this problem, several groups have focused on a class
of liquid crystals characterized by layer contractions on the order
of e1% at the SmA-SmC phase transition.5-15 The structure of
the SmA phase formed by these unusual materials (so-called “de
Vries-like”) has yet to be fully elucidated,16 although recent
theoretical studies suggest that calamitic materials combining low
orientational order and high lamellar order are likely to exhibit this
behavior.17,18 This is consistent with the fact that most “de Vries-
like” materials feature nanosegregating structural elements such as
siloxane end-groups or partially fluorinated side-chains that strongly
promote lamellar order.14 Examples of such materials include 3M
8422 and TSiKN65, which show layer contractions of ca. 0.4 and
0.65%, respectively.6,7
Figure 2. Phenylpyrimidine mesogens and their phase transition temper-
atures (°C) measured by DSC on heating.
terization of this new material, which is comparable to the best
“de Vries-like” liquid crystals reported heretofore.6,7
Compound 2 was obtained by conversion of 2-chloro-5-(4-
methoxyphenyl)pyrimidine23 to 5-(4-methoxyphenyl)-2-(1-octyl-
oxy)pyrimidine by a nucleophilic aromatic substitution reaction,
followed by selective demethylation using NaSEt and alkylation
via a Mitsunobu reaction with 11-(1,1,1,3,3,5,5-heptamethyltri-
siloxanyl)undecanol (Supporting Information). The mesophases
formed by compound 2 were characterized by polarized optical
microscopy (POM) and differential scanning calorimetry (DSC).
Compound 2 forms both SmA and SmC phases, as shown by the
characteristic fan and homeotropic textures of the SmA phase that
turn into broken fan and Schlieren textures upon transition to the
SmC phase. Accurate measurements of the smectic layer spacing
(d) as a function of temperature were carried out by small-angle
X-ray scattering (SAXS). As shown in Figure 3, the layer spacing
d in the SmA phase increases with decreasing temperature, which
is likely due to an increase in orientational order, and in effective
molecular length as the alkyl side-chains become more extended
at lower temperature.6 Other “de Vries-like” materials have been
reported to show a similar negative thermal expansion in their d(T)
profiles.14 Upon SmA-SmC phase transition, the layer spacing
decreases from 41.2 Å to a minimum value of 40.7 Å, which
There is currently no rational design strategy for liquid crystals
with “de Vries-like” properties. However, we have recently shown
that combining a structural element that promotes the formation of
a SmC phase (trisiloxane-terminated side-chain) with one that
promotes the formation of a SmA phase (chloro-terminated side-
chain)19 in a mesogen with a 2-phenylpyrimidine core results in a
maximum layer contraction of 1.6%.20 This is significantly smaller
than the layer contraction of 7.1% observed with the parent
compound PhP1,21 which may reflect a frustration between the
SmA- and SmC-promoting elements in 1. To test this hypothesis,
and validate our design strategy for “de Vries-like” liquid crystals,
we designed another trisiloxane-terminated mesogen in which the
SmA-promoting element is a nonplanar 5-phenylpyrimidine core
(2).22 In this Communication, we report the synthesis and charac-
† Queen’s University.
‡ Universita¨t Stuttgart.
9
13842 J. AM. CHEM. SOC. 2008, 130, 13842–13843
10.1021/ja805672q CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Smectic Layer Spacings, Tilt Angles, and Figures of Merit
R and f at T - TAC ) -10 K
the parent compound PhP1, the R and f values for both 1 and 2
clearly support our hypothesis that combining structural elements
promoting SmA and SmC phases in a single molecule increases
de Vries character. In terms of the potential to achieve a defect-
free bookshelf geometry, the R value of compound 2 is comparable
to those of 3M 8422 and TSiKN65, which are considered to be
among the best “de Vries-like” materials.14 A detailed study of
organosiloxane mesogens featuring the 5-phenylpyrimidine core is
under way to broaden the temperature range of the SmC phase
formed by such material, and assess their electro-optical properties
in FLC display devices.
cpd
dC (Å) dA (Å) d(TAC) (Å) θopt (deg) θXray (deg) δ (deg)
R
f
PhP1a
24.8 26.0
47.7 49.4
40.8 43.1
25.9
48.5
41.2
31.8
21
24
35
25
34
17.5
15.1
18.8
11.0
6.8
16.8 0.80 0.83
10.4 0.43 0.63
8.0 0.23 0.54
4.5 0.18 0.44
6.8 0.20 0.20
1b
2
3M 8422c 31.7 32.3
TSiKN65d 35.70 35.95 35.95
a From ref 21. b From ref 20. c Estimated from data reported in ref 6.
d Estimated from data reported in ref 7 at T - TAC ) -7 K.
Acknowledgment. We are grateful to the Natural Sciences and
Engineering Research Council of Canada (NSERC) and to the
Deutsche Forschungsgemeinschaft (DFG) for support of this work.
Supporting Information Available: Synthesis of compound 2, DSC
and POM characterization, and plots of θXray and θopt vs T - TAC for
compounds 1, 2, and PhP1. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
(1) Goodby, J. W.; Blinc, R.; Clark, N. A.; Lagerwall, S. T.; Osipov, M. A.;
Pikin, S. A.; Sakurai, T.; Yoshino, K.; Zeks, B. Ferroelectric Liquid
Crystals: Principles, Properties and Applications; Gordon & Breach:
Philadelphia, 1991.
(2) Lagerwall, S. T. Ferroelectric and Antiferroelectric Liquid Crystals; Wiley-
VCH: Weinheim, 1999.
Figure 3. Smectic layer spacing d as a function of reduced temperature
T - TAC for compound 2. The dashed line corresponds to the least-squares
fit of the data points at T - TAC g 0 K.
(3) Clark, N. A.; Lagerwall, S. T. Appl. Phys. Lett. 1980, 36, 899–901.
(4) Rieker, T. P.; Clark, N. A.; Smith, G. S.; Parmar, D. S.; Sirota, E. B.;
Safinya, C. R. Phys. ReV. Lett. 1987, 59, 2658–2661.
corresponds to a maximum layer contraction of 1.2%. The fan/
broken fan texture of 2 observed by POM shows a pronounced
change in interference color upon cooling from the SmA to the
SmC phase, which is consistent with the increase in orientational
order associated with a “de Vries-like” transition (Supporting
Information).14
(5) Takanishi, Y.; Ouchi, Y.; Takezoe, H.; Fukuda, A.; Mochizuki, A.;
Nakatsuka, M. Jpn. J. Appl. Phys. 1990, 2, L984–L986.
(6) Radcliffe, M. D.; Brostrom, M. L.; Epstein, K. A.; Rappaport, A. G.;
Thomas, B. N.; Shao, R.; Clark, N. A. Liq. Cryst. 1999, 26, 789–794.
(7) Spector, M. S.; Heiney, P. A.; Naciri, J.; Weslowski, B. T.; Holt, D. B.;
Shashidhar, R. Phys. ReV. E 2000, 61, 1579–1584.
(8) Lagerwall, J. P. F.; Giesselmann, F.; Radcliffe, M. D. Phys. ReV. E 2002,
66, 031703.
To assess the potential of a material to achieve a defect-free
bookshelf geometry in the SmC phase, Radcliffe et al. defined the
figure of merit R as the ratio of the “chevron layer tilt angle” δ(T),
which is the tilt angle required to give the layer contraction relative
to d(TAC) at the SmA-SmC transition temperature, over the optical
tilt angle θopt(T) measured by POM (eq 1).6 According to this
expression, a material would approach the idealized bookshelf
geometry as R f 0. They also defined an expression for de Vries
character that takes into account negative thermal expansion (eq
2), where the “X-ray tilt angle” θXray(T) is the tilt angle required to
give the layer contraction relative to dA(T) extrapolated into the
SmC phase according to a least-squares fit of the data in the SmA
phase (dashed line in Figure 3). Measurements of θopt as a function
of temperature were carried out by POM using a mixture of 2 doped
with a chiral additive (3 mol%) in a 5 µm ITO glass cell with rubbed
Nylon alignment layers (Supporting Information).
(9) Naciri, J.; Carboni, C.; George, A. K. Liq. Cryst. 2003, 30, 219–225.
(10) Panarina, O. E.; Panarin, Y. P.; Vij, J. K.; Spector, M. S.; Shashidhar, R.
Phys. ReV. E 2003, 67, 051709.
(11) Ro¨ssle, M.; Zentel, R.; Lagerwall, J. P. F.; Giesselmann, F. Liq. Cryst.
2004, 31, 883–887.
(12) Hayashi, N.; Kato, T.; Fukuda, A.; Vij, J. K.; Panarin, Y. P.; Naciri, J.;
Shashidhar, R.; Kawada, S.; Kondoh, S. Phys. ReV. E 2005, 71, 041705.
(13) Panarina, O. E.; Panarin, Y. P.; Antonelli, F.; Vij, J. K.; Reihmann, M.;
Galli, G. J. Mater. Chem. 2006, 16, 842–849.
(14) Lagerwall, J. P. F.; Giesselmann, F. ChemPhysChem. 2006, 7, 20–45.
(15) Giesselmann, F.; Zugenmaier, P.; Dierking, I.; Lagerwall, S. T.; Stebler,
B.; Kaspar, M.; Hamplova, V.; Glogarova, M. Phys. ReV. E 1999, 60, 598–
602.
(16) The term “de Vries-like” refers to the diffuse cone model of the SmA phase
proposed by de Vries in which molecules of length L are tilted at an angle
θ but have a random azimuthal distribution described by a diffuse cone
(dA ) L cos θ). According to this model, the random azimuthal distribution
becomes biased in one direction at the SmA-SmC transition, with no
change in layer spacing (dC ) dA, as shown in Figure 1b): de Vries, A.
J. Chem. Phys. 1979, 71, 25–31.
(17) (a) Gorkunov, M. V.; Osipov, M. A.; Lagerwall, J. P. F.; Giesselmann, F.
Phys. ReV. E 2007, 76, 051706. (b) Gorkunov, M. V.; Giesselmann, F.;
Lagerwall, J. P. F.; Sluckin, T. J.; Osipov, M. A. Phys. ReV. E 2007, 75,
060701.
(18) (a) Saunders, K.; Hernandez, D.; Pearson, S.; Toner, J. Phys. ReV. Lett.
2007, 98, 197801. (b) Saunders, K. Phys. ReV. E 2008, 77, 061708.
(19) Goodby, J. W.; Saez, I. M.; Cowling, S. J.; Gortz, V.; Draper, M.; Hall,
A. W.; Sia, S.; Cosquer, G.; Lee, S. E.; Raynes, E. P. Angew. Chem., Int.
Ed. 2008, 47, 2754–2787.
R ) δ(T) ⁄ θopt(T) ) cos-1[dC(T) ⁄ d(TAC)] ⁄ θopt(T)
f ) θXray(T) ⁄ θopt(T) ) cos-1[dC(T) ⁄ dA(T)] ⁄ θopt(T)
(1)
(2)
As shown in Table 1, the optical tilt angle θopt of 2 is ca. twice
as large as the X-ray tilt angle θXray, which is consistent with “de
Vries-like” behavior. To put these results in perspective, we cal-
culated R and f values at a reduced temperature T - TAC ) -10 K
for 1, 2 and PhP1,20,21 and for 3M 8442 and TSiKN65 (Table
1).6,7 The R and f values for 2 are lower than those obtained for 1,
despite showing comparable layer contractions, which is due in
part to the greater tilt angle θopt formed by 2. When compared to
(20) Li, L.; Jones, C. D.; Magolan, J.; Lemieux, R. P. J. Mater. Chem. 2007,
17, 2313–2318.
(21) Hartley, C. S.; Kapernaum, N.; Roberts, J. C.; Giesselmann, F.; Lemieux,
R. P. J. Mater. Chem. 2006, 16, 2329.
(22) The 5-phenylpyrimidine analogue of PhP1 forms only a SmA phase (Cr
84 SmA 117 I): Hegmann, T.; Meadows, M. R.; Wand, M. D.; Lemieux,
R. P. J. Mater. Chem. 2004, 14, 185–190.
(23) Brown, D. J.; Lee, T.-C. J. Chem. Soc. C 1970, 214–219.
JA805672Q
9
J. AM. CHEM. SOC. VOL. 130, NO. 42, 2008 13843