Research Synopsis
Eric Scharrer
Liquid crystals are an industrially important class of compounds as they are used in a
variety of displays (LCD) for watches, calculators, lap-top computers, and televisions. The
utility of liquid crystalline molecules arises because they can be oriented using an electric or
magnetic field, and this reorientation drastically changes the way that light interacts with the
molecules. The ability to be oriented is due to the fact that these compounds possess
intermolecular interactions that are weaker than those found in solids and stronger than those
found in liquids. And, unlike typical substances, which melt directly from a solid to a liquid,
thermotropic liquid crystals undergo a phase transition from a solid to an intermediate phase
(mesophase) before melting to an isotropic liquid upon further heating. These phase changes
can be observed and quantified through the use of several techniques including differential
scanning calorimetry and polarizing microscopy. Many liquid crystals exhibit multiple
mesophases, such as the nematic phase (N) or the smectic phase (Sm).
The most disordered (liquid-like) liquid crystal phase is the uniaxial nematic phase.
In 2004, a new type of nematic phase, the biaxial nematic phase, was discovered in a class of
bent-core oxadiazole containing compounds (Figure 1) by researchers at the University of
North Carolina1. There was a great deal of excitement surrounding this discovery because of
the possibility that compounds which exhibit this phase might be used to generate LCDs that
switch faster and consume less power2. However, as can be seen below, the temperatures at
which this phase is present are impractically high; to be useful in device applications, the
liquid crystalline phase must be stable at or near room temperature. Therefore, in order to
explore the potential applicability of such compounds, it would be necessary to access the
biaxial nematic phase at lower temperatures.
135 o
(crystal)
(isotropic)
Figure 1. Phase behavior of liquid crystalline oxadiazole (ODBP) derivatives
I spent the 2006-07 academic year on sabbatical at UNC; my work focused on the
design and synthesis of new oxadiazole derivatives that we hoped would show lower nematic
onset temperatures. In particular, the incorporation of lateral methyl groups on the benzene
rings proved to be an effective strategy, with both the nematic onset temperatures and the
clearing temperatures significantly reduced (Figure 2)2.
N N
O
O
O
O
O
OR
RO
131 oC
R = C4H9
Cr
I
N
92 oC
R = C12H25
186 oC
99 oC
Cr2
Cr
134 oC
106 oC
Sm
I
N
Figure 2. Phase behavior of methylated oxadiazole derivatives
Upon returning from sabbatical, my students and I have continued to investigate the
effects of various structural changes on the nematic onset temperature. Much of the work
that we carry out involves the synthesis of target compounds. A typical synthetic procedure
is shown in Scheme 1 and involves preparation of the desired alkoxy substituted benzoic acid
followed by the coupling of this acid with an oxadiazole bisphenol core. After each reaction,
the products are characterized by 1H and 13C NMR spectroscopy. Purification is generally
carried out by recrystallization and/or column chromatography, and characterization of the
target compounds for liquid crystallinity is carried out using polarizing microscopy and
differential scanning calorimetry (DSC) in our department. However, we are only able to
detect the presence of a nematic phase and are unable to determine if the phase is biaxial. In
order to make this determination, samples are sent to the University of North Carolina for
sophisticated analysis by conoscopic microscopy and solid-state 2H NMR spectroscopy, and
to collaborators in Italy for analysis by x-ray diffraction.
CH3
O
HO
OH
CH3
1) EtOH
H2SO4 (cat.)
O
1) 2 M KOH
OEt
EtOH
2) 6 M HCl
RO
2) RX (1.5 equiv)
Cs2CO3 (1.5 equiv.)
DMF
N N
N N
CH3
O
O
HO
RO
O
O
OH
H3C
O
OH
O
O
CH3
OR
EDC, DMAP (cat.), CH2Cl2
RO
R = C4H9, C12H25
Scheme 1. Synthesis of oxadiazole containing target compounds
Derivatives with three lateral groups (methyl or halogen) have proven especially
promising. As seen in Figure 3, some of these derivatives supercool in the nematic phase to
room temperature. Our current efforts include the synthesis and phase analysis of related
derivatives that may show even more promising phase behavior3.
N N
H 3C
O
H 3C
O CH3
O
O
O
C4H 9O
OC4H 9
OC4 2MePh(mono3MeODBP)
Cr
99 οC
Cr 2
N
120 οC
143
οC
N
144 οC
I
I
Figure 3. Phase behavior of a trimethylated derivative
References:
1. Madsen, L. A.; Dingemans, T. J.; Nakata, M.; Samulski, E. T. Thermotropic Biaxial
Nematic Liquid Crystals Phys. Rev. Lett. 2004, 92, 14505-1-14505-4. (b) Acharya, B. R.;
Primak, A.; Kumar, S. Biaxial Nematic Phase in Bent-Core Thermotropic Mesogens Phys.
Rev. Lett. 2004, 92, 14506-14506-4.
2. Luckhurst, G. R. A Missing Phase Found at Last? Nature, 2004, 430, 413-414.
3. (a) Speetjens, F#.; Lindborg, J.#; Tauscher, T.#; LaFemina, N.#; Nguyen, J.#;
Samulski, E. T.; Vita, F.; Francescangeli, O.; Scharrer E. Low nematic onset
temperatures and room temperature cybotactic behavior in 1,3,4-oxadiazole-based bentcore mesogens possessing lateral methyl groups J. Mater. Chem. 2012, 22, 22558. (b)
Vita, F.; Tauscher, T.#; Speetjens, F.#; Samulski, E. T.; Scharrer, E.; Francescangeli, O.
Evidence of Biaxial Order in the Cybotactic Nematic Phase of Bent Core Mesogens
Chem. Mater. 2014, 26, 4671.
# denotes an undergraduate co-author.