1. Observed Phase Behavior
Optimizing Synthesis of Substituted Oxadiazole Based Liquid Crystal Compounds Blake Bordokas*, Dr. Eric Scharrer Department of Chemistry, University of Puget Sound, Tacoma, WA
Introduction
Molecules that posses a liquid crystalline phase show molecular ordering and cohesive properties similar to a solid but are able to retain fluidity at certain temperatures (figure 1).
Liquid crystals possess the ability to be reoriented upon application of an electric field; reorientation changes compounds’ optical properties, as in liquid crystal displays.
Molecules in the liquid crystal phase that can be more readily reoriented are more desirable because they will allow for faster switching, and better displays.
The uniaxial nematic phase is the most disorderly and common liquid crystal phase.
The biaxial nematic phase, proposed by Freiser possesses three dimensional ordering represented by both a major and perpendicular minor director. Switching about the minor director could lead to faster switching (figure 2).1
Project Goals
We are working to observe how specific functional groups and their positions effect liquid crystallinity.
We are also working towards developing a synthetic route which will allow for synthesis of purer target compounds and higher yields. ‘
Synthetic Efforts
By protecting the phenol group on compound 1 with an acetyl functional group, we hoped to improve the solubility of later intermediates (scheme 1).
During the conversion of 2 to 3 an unexpected byproduct formed which was determined to be the deprotected hydrazide. We suspect that the deprotection occurred via hydrazine attacking at the acetyl group.
Scheme 1. Acetylation and deprotection of intermediates.
After unsuccessful attempts at accessing intermediate 3 we decided to focus on moving forward in the synthetic route.
We found that the EDC coupling gave purer product when performed on compound 5.
This allowed us to access the same target compound, but from the other side of the molecule (scheme 2).
Scheme 2. Synthesis of bis-phenol core with 2-bromo and 3-methyl lateral substitutions.
Conversion of intermediate 7 to 8 yielded less than expected which impacted final yield.
Generally, the synthetic route in scheme 2 gave fairly pure products however 1H-NMR analysis of intermediate 9 showed some impurities and a methodology must be developed to purify it for future use.
Target compounds are accessed utilizing a 4-day EDC/DMAP coupling in dichloromethane (scheme 3).
Scheme 3. Synthesis of target compound 10 with 3-bromo and 2-methyl lateral substitutions.
*A trimethylated target compound 11 was also synthesized using the same reagents in scheme 3, however the methylated bis-phenol core and 2-methyl-4-pentoxybenzoic acid were prepared by Dr. Eric Scharrer and Gustavo Reyes, respectively, in previous research.*
Figure 1. Relative molecular ordering of a solid (A), liquid crystal (B), and liquid (C).
Figure 2. Representations of the uniaxial (Nu)
and biaxial (Nb) nematic phases.2
Previous Work
The biaxial nematic phase has been found in oxadiazole-based liquid crystals but only at temperatures that are much too high for practical application (figure 3). 2
Figure 3. Phase behavior of unsubstituted oxadiazole based liquid crystal compound.
It has been observed that lateral substituents on the structure of oxadiazole-based liquid crystal compounds can lower phase transition temperatures and alter the types of liquid crystal phases observed.
Some laterally substituted derivatives have shown very promising phase behavior.3
Figure 4. Phase behavior of oxadiazole based LC compound with lateral 3-chloro substitution.
Efforts have been made to produce asymmetric, tetrasubstituted derivatives that might have even lower transition temperatures but synthesis has proven difficult.
Some intermediates in the synthesis are difficult to purify because of their insolubility in typical organic solvents.
Figure 5. Generically substituted hydrazide and bis-phenol intermediates .
*Compounds were characterized by 1H-NMR spectroscopy and were purified by flash chromatography and recrystallization*
Future Work
Observed Phase Behavior
(polarizing microscopy)
Further optimization of the synthetic route is required. While the compound was successfully acetylated in the first step, incidental removal of this acetyl group in the second step hinders progress towards purer intermediates and higher yields.
With improved yields, our collaborators in Ancona, Italy can perform analysis by x-ray diffraction to investigate the possible presence of the biaxial nematic phase in our compounds; which could lead to the eventual utilization of oxadiazole based liquid crystals in display technology.
Both target compounds exhibited the nematic phase at some temperature; however, compound 11 remained in the nematic phase as it cooled to room temperature.
This is important because since the nematic phase is present at room temperature, this compound can potentially be utilized in liquid crystal display technology.
Further analysis by differential scanning calorimetry may give more accurate phase transition temperatures.
Acknowledgements
Special thanks to Dr. Eric Scharrer, Dr. Johanna Crane, to The University of Puget Sound Department of Chemistry, and to Gustavo Reyes ‘17 for preparing intermediates used in the synthetic procedure. Also thanks to the NASA Scholar Space Grant and the Summer Research program for project funding.
References
1) Freiser MJ. Ordered states of a nematic liquid. Phys Rev Lett. 1970;24:1041–1043.
2) Scharrer, E. RUI project description, NSF grant proposal. 2016; 1-15
3) Nguyen, Jason & Wonderly, William & Tauscher, Tatum & Harkins, Robin & Vita,
Francesco & Portale, G & Francescangeli, Oriano & Samulski, Ed & Scharrer, Eric.
(2015). The effects of lateral halogen substituents on the low-temperature cybotactic
nematic phase in oxadiazole based bent-core liquid crystals. Liquid Crystals /