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.