1. Figure 3: The two mesogens were successfully synthesized. They are the
same in structure except for their perfluoro ether tails: A, [4.2.2]-perfluoro
ether tail; and B, [1.2.2.2]-perfluoro ether tail.
Figure 4: The scheme in black has been synthesized, and the scheme in blue
is future synthetic work.
Directed design and synthesis of novel asymmetric bent-
core mesogens
Alicia N. Gamble1, Ethan Tsai1,2, David M. Walba2
1Department of Chemistry, Metropolitan State University of Denver
2Department of Chemistry and Biochemistry, University of Colorado Boulder
Abstract
The field of liquid crystals (LCs) has garnered much
interest recently due to their potential applicability to
molecular electronics, such as organic photovoltaic cells
and field-effect transistors. Since shape induces certain
LC phases, many different shapes of LC molecules have
been investigated to find novel phases and properties
that are pertinent to rising technology. In this research
conducted, “hockey stick”-shaped LCs were designed by
introducing a truncated asymmetric bent-core with a
perfluorinated tail to induce de Vries phases. De Vries
materials show strong candidacy to out-perform
traditional ferroelectric display materials. Initial
exploration of these hybrid hockey stick de Vries
materials shows interesting electro-optic and x-ray
results.
Background
Isotropic
Liquid
Nematic Smectic A Smectic C
Solid
Thermotropic LCs are “in-between” phases that have
properties of liquid and solid, and like the classic phases,
are dependent upon temperature (Figure 1).
Methods Results continued…
Unique phases can be formed depending on the shape of
the molecule (i.e., bent-core, discotic, or calamitic;
Figure 2).
Figure 1: When cooling from a liquid, LCs can form three common LC
phases: Nematic, Smectic A and smectic C. As the mesogens are cooled,
they become more ordered.
A B1 C2
Figure 2: The three most common shapes of LCs: A, bent-core or banana; B1,
discotic; and C2, calamitic.
Truncated asymmetric bent-cores (or hockey stick-shape)
LCs with perfluorinated tails (Figure 3) were designed to
suppress out-of-layer fluctuation, a key property for
inducing de Vries phases. De Vries materials are strong
candidates to surpass traditional ferroelectric display
materials in performance.
A B
The overall synthesis of the [4.2.2]-perfluoro ether tail
(A) and the [1.2.2.]-perfluoro ether tail (B; Figure 4).
A and B
Results and Discussion
The [4.2.2]-tail mesogen (A) and the [1.2.2.2.]-tail
mesogen (B) exhibit interesting polar optical microscopy
(POM) and x-ray results. The POM results for both A
(Figure 5) and B (Figure 6) show a high change in
birefringence and exhibit a monotropic Smectic A phase.
However, the change in birefringence of A is much more
extreme than the change in birefringence of B, despite
the two mesogens having nearly the same atomic
length.
Figure 5: There is a drastic change in birefringence of A. The change in
birefringence is more dramatic at the higher temperatures (190-238°C), and
at the cooler temperatures (80-180°C) the birefringence shows little to no
changes, ending at a light green. The only LC phase that A exhibits is Smectic
A phase.
>238°C 238°C 236°C 234°C 232°C
230°C225°C220°C210°C200°C190°C
180°C 170°C 160°C 150°C 140°C 130°C
120°C110°C100°C90°C80°C25°C
61°C 89°C 90°C 100°C 110°C
119°C148°C164°C170°C180°C
>180°C
Figure 7: The change in birefringence of B is not as drastic of the change in
birefringence of A. The photo at 89°C is the LC changing phase from Smectic
A to solid. The only LC phase that B exhibits, like A, is Smectic A.
The x-ray data (Figure 8) shows that as the temperature
decreases, the layer spacing decreases for both A and B.
Because there is no underlying Smectic C phase, the
materials are not considered true de Vries materials.
The layer spacing changes very little; however, de Vries
materials have no change (<0.1%) in layer spacing from
Smectic A to Smectic C. Since there is little change in
layer spacing, the material shows promise for de Vries-
like homologs.
Future Research
Figure 8: The x-ray data on the left shows the layer spacing for A and the
right shows B. A transitions from isotropic liquid to Smectic A at 236°C and
from Smectic A to solid at 73°C. B transitions from isotropic liquid to
Smectic A at 215°C and Smectic A to solid at 80°C.
The overall synthesis for A and B will be completed by
making structural changes to the “nub” of the hockey
stick mesogens in making them more rigid. Their phases
will be compared to the current ones in order to explore
how a more rigid nub will affect the layer spacing and
change in birefringence. The structure will also be
modified by changing the length in the perfluoro ether
tails in order to find how the tails affect the LC phases.
More characterization of the currently synthesized
mesogens will continue.
Conclusion
Acknowledgements
The National Science Foundation for funding of this
research and the Research Experience for
Undergraduates (REU) program in the Liquid Crystals
Materials Research Center (LCMRC) at the University of
Colorado Boulder. Eva Korblova, Jacquie Richardson,
Mike Springer, and Mark Moran for guidance in the
laboratory. Shen Yongqiang for helping characterize the
materials at Brookhaven National Laboratory. The
American Chemical Society for supporting undergraduate
research. Metropolitan State University of Denver
Student Activities Department for funding the
presentation.
References
1Kent State,
http://dept.kent.edu/spie/liquidcrystals/maintypes.html
2UPC Barcelona Tech,
http://grpfm.upc.edu/research/liquid-crystals-
1?set_language=en
The synthesis of both A and B was successful, from the
POM and x-ray data. The high change in birefringence—
especially for A—is an interesting property that sets itself
apart from most LCs known today. Due to the decrease
in layer spacing with a decrease in temperature and lack
of a Smectic C phase, the materials are not considered to
be de Vries materials, but with further investigation of
modified structures, a de Vries material may be
achieved.
A B