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1. Nematic colloids at liquid crystal–air interfaces via photopolymerization

   https://doi.org/10.1039/d0sm01311k  

   Wei, Xiaoshuang; Sbalbi, Nicholas; Bradley, Laura C. (October 2020, Soft Matter)

   null (Ed.)

   We demonstrate the preparation of colloidal crystals at nematic liquid crystal–air interfaces by simultaneous photopolymerization and assembly. Polymer colloids are produced by polymerization-induced phase separation of 2-hydroxyethyl methacrylate in the non-reactive liquid crystal (LC) 4-cyano-4′-pentylbiphenyl (5CB) using an open-cell setup. Colloids adsorbed to the nematic 5CB–air interface form non-close-packed hexagonal crystals that cover the entire interface area. We examine the mechanism of growth and assembly for the preparation of LC-templated interfacial colloidal superstructures. 

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2. Photoisomerization dynamics of a light-sensitive chiral dopant in a nematic medium

   https://doi.org/10.1080/02678292.2023.2276842  

   Jirón, Vanessa; Wang, Hao; Lavrentovich, Oleg D (November 2023, Liquid Crystals)

   Azobenzene-based chiral dopants in cholesteric liquid crystals are of interest since the properties they induce in the liquid crystal could be tuned photochemically. Here, we use a substituted binaphthyl with a halogenated azobenzene as a chiral dopant to induce a photoswitchable cholesteric phase in the nematic 4-n-pentyl-4’-cyanobiphenyl. The azobenzene group chemically attached to the chiral dopant undergoes isomerization from trans to cis upon irradiation with green light (wavelength 535 nm), and from cis to trans upon irradiation with blue light (wavelength 450 nm). The transition between the two isomers causes helicity inversion of the cholesteric, with a left-handed trans isomer and a right-handed cis isomer. We report on the kinetics of photoisomerization of both processes (trans-to-cis and cis-to-trans) in the nematic host by following the pitch evolution over time. We show that the kinetic mechanism corresponds to a two-step process: a first-order isomerization followed by a second-order autocatalytic isomerization. This mechanism differs from the typical first-order kinetics for cis-to-trans or trans-to-cis isomerization in azobenzenes. The autocatalytic process is attributed to interactions between the chiral dopant and the nematic host. 

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3. Twisted tetrathiafulvalene crystals

   https://doi.org/10.1039/D2ME00010E  

   Yang, Yongfan; Zong, Kai; Whittaker, St. John; An, Zhihua; Tan, Melissa; Zhou, Hengyu; Shtukenberg, Alexander G.; Kahr, Bart; Lee, Stephanie S. (January 2022, Molecular Systems Design & Engineering)

   Optically-active optoelectronic materials are of great interest for many applications, including chiral sensing and circularly polarized light emission. Traditionally, such applications have been enabled by synthetic strategies to design chiral organic semiconductors and conductors. Here, centrosymmetric tetrathiafulvalene (TTF) crystals are rendered chiral on the mesoscale by crystal twisting. During crystallization from the melt, helicoidal TTF fibers were observed to grow radially outwards from a nucleation centre as spherulites, twisting in concert about the growth direction. Because molecular crystals exhibit orientation-dependent refractive indices, periodic concentric bands associated with continually rotating crystal orientations were observed within the spherulites when imaged between crossed polarizers. Under certain conditions, concomitant crystal twisting and bending was observed, resulting in anomolous crystal optical behavior. X-ray diffraction measurements collected on different spherulite bands indicated no difference in the molecular packing between straight and twisted TTF crystals, as expected for microscopic twisting pitches between 20–200 μm. Mueller matrix imaging, however, revealed preferential absorption and refraction of left- or right-circularly polarized light in twisted crystals depending on the twist sense, either clockwise or counterclockwise, about the growth direction. Furthermore, hole mobilities of 2.0 ± 0.9 × 10 −6 cm 2 V −1 s −1 and 1.9 ± 0.8 × 10 −5 cm 2 V −1 s −1 were measured for straight and twisted TTF crystals deposited on organic field-effect transistor platforms, respectively, demonstrating that crystal twisting does not negatively impact charge transport in these systems. 

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4. Tunable three-dimensional architecture of nematic disclination lines

   https://doi.org/10.1073/pnas.2300833120  

   Modin, Alvin; Ash, Biswarup; Ishimoto, Kelsey; Leheny, Robert L; Serra, Francesca; Aharoni, Hillel (July 2023, Proceedings of the National Academy of Sciences)

   Disclination lines play a key role in many physical processes, from the fracture of materials to the formation of the early universe. Achieving versatile control over disclinations is key to developing novel electro-optical devices, programmable origami, directed colloidal assembly, and controlling active matter. Here, we introduce a theoretical framework to tailor three-dimensional disclination architecture in nematic liquid crystals experimentally. We produce quantitative predictions for the connectivity and shape of disclination lines found in nematics confined between two thinly spaced glass substrates with strong patterned planar anchoring. By drawing an analogy between nematic liquid crystals and magnetostatics, we find that i) disclination lines connect defects with the same topological charge on opposite surfaces and ii) disclination lines are attracted to regions of the highest twist. Using polarized light to pattern the in-plane alignment of liquid crystal molecules, we test these predictions experimentally and identify critical parameters that tune the disclination lines’ curvature. We verify our predictions with computer simulations and find nondimensional parameters enabling us to match experiments and simulations at different length scales. Our work provides a powerful method to understand and practically control defect lines in nematic liquid crystals. 

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5. Azimuthal Anchoring Strength in Photopatterned Alignment of a Nematic

   https://doi.org/10.3390/cryst11060675  

   Padmini, H. Nilanthi; Rajabi, Mojtaba; Shiyanovskii, Sergij V.; Lavrentovich, Oleg D. (June 2021, Crystals)

   null (Ed.)

   Spatially-varying director fields have become an important part of research and development in liquid crystals. Characterization of the anchoring strength associated with a spatially-varying director is difficult, since the methods developed for a uniform alignment are seldom applicable. Here we characterize the strength of azimuthal surface anchoring produced by the photoalignment technique based on plasmonic metamsaks. The measurements used photopatterned arrays of topological point defects of strength +1 and −1 in thin layers of a nematic liquid crystal. The integer-strength defects split into pairs of half-integer defects with lower elastic energy. The separation distance between the split pair is limited by the azimuthal surface anchoring, which allows one to determine the strength of the latter. The strength of the azimuthal anchoring is proportional to the UV exposure time during the photoalignment of the azobenzene layer. 

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