Ferroelectric Organic Semiconductor: [1]Benzothieno[3,2-b][1]benzothiophene-Bearing Hydrogen-Bonding -CONHC14H29 Chain

Liquid Crystalline [1]Benzothieno[3,2-b][1]benzothiophene Semiconductors with Unsymmetrical Thiomethylphenyl Substitution: Synthesis and Charge Transport

An unsymmetrical [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivative functionalized with a (4-thiomethyl)phenyl group, designed as a liquid crystalline organic semiconductor (LC-OSC), is presented. This derivative is an example of having a smectic E (SmE) liquid crystal (LC) phase in the BTBT family, forming LC semiconducting material. Phase characterization by powder X-ray diffraction (PXRD), grazing incidence wide-angle X-ray scattering (GIWAXS), and polarized optical microscopy (POM) confirms the formation of an ordered smectic E (SmE) phase at a high temperature. This two-dimensional ordered molecular packing is preserved in both bulk crystals and vapor-deposited thin films, which is promising for excellent charge transport properties. The material demonstrates high photoconductivity, reaching ϕ∑μmax = 1.6 × 10-3 cm2 V-1 s-1 (FP-TRMC), and the local hole mobility was estimated to be as high as 9.3 cm2 V-1 s-1 (FI-TRMC) at the semiconductor-insulator interface after thermal annealing at the SmE phase. Moreover, it has been found that thermal annealing in the SmE phase enhances hole mobility by preserving the ordered LC structure, whereas annealing in the smectic A or isotropic phases at higher temperatures disrupts conductive pathways due to thermal stress at the semiconductor-insulator interface. Thus, interfacial engineering at the ordered LC phase enhances conductivity, rendering thiomethyl-functionalized BTBT with improved electronic properties and thin film stability for next-generation LC semiconductors.

Ferroelectric-like Polarization Switching in Plastic Crystalline Succinonitrile

An intermediate state of the plastic crystal (PC) phase between solid and liquid has attracted much attention due to its potential applications for ionic conductors, ferroelectrics, and barocaloric materials. In general, it has been considered difficult to maintain a polarized orientation state in the PC phase due to the loss of orientational order caused by the isotropic rotation of molecules. In this study, we succeeded in realizing a unique polarized orientation state in the PC phase of sucinonitrile (SN), which exhibits hysteresis in the polarization-electric field (P-E) curve. In the PC phase, the trans and gauche conformations coexist, and the conformation and orientation of the molecules change in response to an external electric field. Interestingly, the polarization state in the PC phase relaxes upon removal of the electric field and does not exhibit the nonvolatile memory effects seen in conventional ferroelectrics. However, two different polarization mechanisms emerge due to the molecular orientational and conformational degrees of freedom. As a result, a characteristic double P-E hysteresis behavior with two different coercive fields, which is not observed in conventional ferroelectrics, is observed. This result provides an important guideline for the design of organic materials for the realization of next-generation multilevel memory.

Photodimerization of Ferroelectric N,N'-Ditetradecyl-stilbenediamide Derivative

A solid state [2 + 2] photodimerization reaction of ─C═C─ bonds in solids has been designed by controlling the molecular arrangement using supramolecular chemistry. Stilbene derivative (C14SDA) with alkyl amide chains (─CONHC14H29) forms intermolecular amide hydrogen-bonding chains and exhibits reversible successive phase transitions (S1 → S2 → S3 → L) corresponding to the dynamics of the alkyl chains. In the high-temperature solid phase S3, the alkyl chains partially melt, resulting in a one-dimensional (1D) dynamic intermolecular amide hydrogen bond and ferroelectric behavior with hysteresis in the electric field-polarization curve due to polarization reversal of the dipole moment by an external electric field. The S1 phase of C14SDA did not exhibit a photodimerization reaction, while the dynamic S2 and S3 phases exhibited a [2 + 2] photodimerization reaction to form reaction products with cyclobutane rings. In the dynamic ferroelectric S3 phase, trans-cis isomerization of stilbene was observed simultaneously with the formation of the photodimerization product. When the photodimerization reaction was attempted with an electric field applied to the S3 phase, thermal molecular fluctuations were suppressed by the electric field, increasing the distance between ─C═C─ double bonds and reducing the photoreaction yield.

Carrier Transport Switching of Ferroelectric BTBT Derivative

Alkylamide-substituted [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivative of BTBT-NHCOC14H29 (1), which has ferroelectric N-H···O= hydrogen-bonding network of alkylamide group and two-dimensional (2D) electric structure of BTBT π-cores, was prepared to design the external electric field-responsive organic semiconductors. The short-chain derivative of BTBT-NHCOC3H7 (1') revealed the coexistence of a 2D electronic band structure based on the herringbone BTBT arrangement and the one-dimensional (1D) hydrogen-bonding chain. 1 formed a smectic E (SmE) liquid crystal phase above 412 K and showed ferroelectric hysteresis in the electric field-polarization (P-E) curves at 403-433 K. The remanent polarization (Pr) and coercive electric field (Ec) of 1 at 408 K, 0.1 Hz were 24.0 μC cm-2 and 5.54 V μm-1, respectively. By thermal annealing of thin-film 1 at 443 K, the molecular assembly structure of 1 changed from a monolayer to a bilayer structure with high crystallinity, resulting in conducting layers of BTBT parallel to the substrate surface. The organic field-effect transistor (OFET) device with thermally annealed thin-film 1 showed p-type semiconducting behavior with the hole mobility of 1.0 × 10-3 cm2 V-1 s-1. Furthermore, device 1 showed switching behavior of semiconducting properties by electric field poling and thermal annealing cycle. The electric field response of ferroelectrics modulated the molecular orientation and conduction properties of organic semiconductors, resulting in external electric field control of carrier transport properties.