Organic liquid crystals in optoelectronic device applications: Field‐effect transistors, nonvolatile memory, and photovoltaics

Photophysical Properties and Metal Ion Sensing of a Pyrene-Based Liquid Crystalline Dimer

This study investigates the liquid crystalline behavior, photophysical properties, and metal ion sensing capabilities of a pyrene-based imine dimer (DPyH9). The compound exhibits monotropic nematic mesophase behavior, with a glass transition at 43 °C, as confirmed by polarized light microscopy (PLM) and differential scanning calorimetry (DSC). Its photophysical properties, including UV-vis absorption, solvatochromic fluorescence, and acidochromism, observed through spectral shifts upon HCl addition, were systematically analyzed. Notably, DPyH9 displayed selective metal ion sensing capabilities towards Sn2+ and Cu2+ with binding constants of 4.51 × 106 M-1 and 4.03 × 107 M-1 and detection limits of 1.61 × 10-5 M (Sn2+) and 4.73 × 10-5 M (Cu2+). Fluorescence titrations revealed distinct responses: Sn2+ induced an initial quenching and an enhancement at higher concentrations, while Cu2+ caused significant fluorescence quenching. These results therefore highlight DPyH9 as a potential candidate for sensing applications and optoelectronic devices.

Ferrofluid Droplet Chains in Thermotropic Nematic Liquid Crystals

Dispersing ferrofluids in liquid crystals (LCs) produces unique systems which possess magnetic functionality and novel phenomena such as droplet chaining. This work reports the formation of ferrofluid droplet chains facilitated by the topological defects within the LC director field, induced by the dispersed ferrofluid. The translational and rotational motion of these chains could be controlled via application of external magnetic fields. The process of the droplet chain formation in LCs can be stabilized by the addition of surfactants. The magnetic colloidal particles in the ferrofluid located at the interface between the ferrofluid and the LC are arranged so that a boundary layer was formed. The velocities and boundary layer thickness values of ferrofluid droplet chains in nematic 5CB (4-Cyano-4'-pentylbiphenyl) were investigated for varying average droplet sizes and number of droplets in a chain. The creation and behaviour of ferrofluid droplet chains in 5CB with the addition of the surfactant polysorbate 60 (Tween-60) and without, was comparatively investigated. The integration of liquid crystals and ferrofluids along with the incorporation of functional materials facilitates the innovative development of advanced materials for future applications.

Nonsymmetrical thermal conductivity along the director field in ferroelectric nematic liquid crystals

The synthesis of ferroelectric nematic liquid crystals (FNLCs) concludes the long wait for their existence and potential usage in multiple liquid crystal based applications. In FNLCs, electric polarization in the nematic phase significantly decreases the switching time of in-on display pixels. In this article, we report the occurrence of translation symmetry breaking for heat propagation along the director field n[over ̂] in the ferroelectric nematic phase. Due to the C_{∞V} symmetry of such a phase and close similarity to the bent-core polar liquid crystal phase, a rank-3 tensor describes its scalar order parameter and algebraic deductions. The finite element simulations show the occurrence of the nonsymmetrical thermal conductivity along n[over ̂]. The preferential heat transport in FNLCs can allow them to work as an all-thermal monophase non-nanostructured single-material thermal rectifier. We expect that this study will contribute towards the FNLCs application as functional layers and inks.

Reversible Molecular Conformation Transitions of Smectic Liquid Crystals for Light/Bias-Gated Transistor Memory

In recent years, organic photonic field-effect transistors have made remarkable progress with the rapid development of conjugated polycrystalline materials. Liquid crystals, with their smooth surface, defined layer thickness, and crystalline structures, are commonly used for these advantages. In this work, a series of smectic liquid crystalline molecules, 2,9-didecyl-dinaphtho-thienothiophene (C10-DNTT), 2,7-didecyl-benzothieno-benzothiopene (C10-BTBT), 3,9-didecyl-dinaphtho-thiophene (C10-DNT), and didecyl-sexithiophene (C10-6T), have been used in photonic transistor memory, functioning as both hole-transport channels and electron traps to investigate systematically the reasons and mechanisms behind the memory behavior of smectic liquid crystals. After thermal annealing, C10-BTBT and C10-6T/C10-DNTT are homeotropically aligned from the smectic A and smectic X phases, respectively. The 3D-ordered structure of these smectic-aligned crystals contributed to efficient photowriting and electrical erasing processes. Among them, the device performance of C10-BTBT was particularly significant, with a memory window of 21 V. The memory ratio could reach 1.5 × 106 and maintain a memory ratio of over 3 orders after 10,000 s, contributing to its smectic A structure. Through the research, we confirmed the memory and light/bias-gated behaviors of these smectic liquid crystalline molecules, attributing them to reversible molecular conformation transitions and the inherent structural inhomogeneity inside the polycrystalline channel layer.

Mesomorphism Modulation of Perfluorinated Janus Triphenylenes by Inhomogeneous Chain Substitution Patterns

Two isomeric series of compounds with "inverted" chains' substitution patterns, 7,10-dialkoxy-1,2,3,4-tetrafluoro-6,11-dimethoxytriphenylene and 6,11-dialkoxy-1,2,3,4-tetrafluoro-7,10-dimethoxytriphenylene, labelled respectively p-TPFn and m-TPFn, and two non-fluorinated homologous isomers, 3,6-dibutoxy-2,7-dimethoxytriphenylene and 2,7-dibutoxy-3,6-dimethoxytriphenylene, p-TP4 and m-TP4, respectively, were synthesized in three steps and obtained in good yields by the efficient transition-metal-free, fluoroarene nucleophilic substitution via the reaction of appropriate 2,2'-dilithium biphenylenes with either perfluorobenzene, C6 F6 , to yield p-TPFn and m-TPFn, or o-difluorobenzene, C6 H4 F2 , for p-TP4 and m-TP4, respectively. The single-crystal structures of p-TPF4, m-TPF4 and p-TP4, unequivocally confirmed that the cyclization reactions occurred at the expected positions, and that the fluorinated molecules stack up into columns with short separation, a propitious situation for the emergence of columnar mesophases. The mesomorphous properties were found to be greatly affected by both chains' length and positional isomerism: a Colhex phase is found for p-TPF4 and m-TPF4, but mesomorphism vanishes in p-TPF6, and changes for the isomeric homologs m-TPFn, with the induction for n≥6 of a lamello-columnar phase, LamColrec . As expected, both non-fluorinated compounds are deprived of mesomorphism. These compounds emit blue-violet colour in solution, independently of the chains' substitution pattern, and the absolute fluorescence quantum yields can reach up to 46 %. In thin films, fluorescence is slightly redshifted.

To Be or Not To Be Polar: The Ferroelectric and Antiferroelectric Nematic Phases

We report two new series of compounds that show the ferroelectric nematic, NF, phase in which the terminal chain length is varied. The longer the terminal chain, the weaker the dipole-dipole interactions of the molecules are along the director and thus the lower the temperature at which the axially polar NF phase is formed. For homologues of intermediate chain lengths, between the non-polar and ferroelectric nematic phases, a wide temperature range nematic phase emerges with antiferroelectric character. The size of the antiparallel ferroelectric domains critically increases upon transition to the NF phase. In dielectric studies, both collective ("ferroelectric") and non-collective fluctuations are present, and the "ferroelectric" mode softens weakly at the N-NX phase transition because the polar order in this phase is weak. The transition to the NF phase is characterized by a much stronger lowering of the mode relaxation frequency and an increase in its strength, and a typical critical behavior is observed.

Photoluminescent Thin Films of Room-Temperature Glassy Tris(keto-hydrozone) Discotic Liquid Crystals and Their Nanocomposites with Single-Walled Carbon Nanotubes for Optoelectronics

This study addresses the photoresponse of liquid-crystalline tris(keto-hydrozone) discotic (TKHD)-a star-shaped molecular structure with three branches. Object of our research interest was also TKHD filled with single-walled carbon nanotubes (SWCNTs) at a concentration of 1 wt %. At room temperature, the discotic liquid crystals in thin films (thickness 3 μm) of both TKHD and nanocomposite SWCNT/TKHD were in a glassy state. Such glassy thin films exhibited photoluminescence ranging from the deep-red to the near-infrared spectral region, being attractive for organic optoelectronics. The addition of SWCNTs to TKHD was found to stabilize the photoluminescence of TKHD, which is of significance for optoelectronic device applications. The photothermoelectrical response of highly conductive SWCNT/TKHD nanocomposite films was characterized by electrical impedance spectroscopy in the frequency range from 1 Hz to 1 MHz of the applied electric field. It was elucidated that the reversible photothermoelectrical effect in SWCNT/TKHD films occurs through SWCNTs and their network.

Lipidic lyotropic liquid crystals: Insights on biomedical applications

Liquid crystals (LCs) possess unique physicochemical properties, translatable into a wide range of applications. To date, lipidic lyotropic LCs (LLCs) have been extensively explored in drug delivery and imaging owing to the capability to encapsulate and release payloads with different characteristics. The current landscape of lipidic LLCs in biomedical applications is provided in this review. Initially, the main properties, types, methods of fabrication and applications of LCs are showcased. Then, a comprehensive discussion of the main biomedical applications of lipidic LLCs accordingly to the application (drug and biomacromolecule delivery, tissue engineering and molecular imaging) and route of administration is examined. Further discussion of the main limitations and perspectives of lipidic LLCs in biomedical applications are also provided. STATEMENT OF SIGNIFICANCE: Liquid crystals (LCs) are those systems between a solid and liquid state that possess unique morphological and physicochemical properties, translatable into a wide range of biomedical applications. A short description of the properties of LCs, their types and manufacturing procedures is given to serve as a background to the topic. Then, the latest and most innovative research in the field of biomedicine is examined, specifically the areas of drug and biomacromolecule delivery, tissue engineering and molecular imaging. Finally, prospects of LCs in biomedicine are discussed to show future trends and perspectives that might be utilized. This article is an ampliation, improvement and actualization of our previous short forum article "Bringing lipidic lyotropic liquid crystal technology into biomedicine" published in TIPS.