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

An alkylamide-substituted [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivative of BTBT-CONHC₁₄H₂₉ (1) and C₈H₁₇-BTBT-CONHC₁₄H₂₉ (2) were prepared to design the multifunctional organic materials, which can show both ferroelectric and semiconducting properties. Single-crystal X-ray structural analyses of short-chain (-CONHC₃H₇) derivatives revealed the coexistence of two-dimensional (2D) electronic band structures brought from a herringbone arrangement of the BTBT π core and the one-dimensional (1D) hydrogen-bonding chains of -CONHC₃H₇ chains. The thin films of 1 and 2 fabricated on the Si/SiO₂ substrate surface have monolayer and bilayer structures, respectively, resulting in conducting layers parallel to the substrate surface, which is suitable for a channel layer of organic field-effect transistors (OFETs). The thin film of 1 indicated a hole mobility μ_FET = 2.4 × 10^-5 cm² V^-1 s^-1 and threshold voltage V_Th = - 29 V, whereas that of 2 showed a μ_FET = 2.1 × 10^-2 cm² V^-1 s^-1 and threshold voltage V_Th = -9.7 V. Both 1 and 2 formed the smectic E (SmE) phase above 410 and 369 K, respectively, where the existence of a hole transport pathway was confirmed in the SmE phase. The ferroelectric hysteresis behavior was observed in bulk 1 and 2 in the polarization-electric field (P-E) curves at the SmE phase. 1 showed the remanent polarization P_r = 2.3 μC cm^-2 and coercive electric field E_c = 5.2 V μm^-1, whereas the P_r and E_c of 2 were 3.4 μC cm^-2 and 7.0 V μm^-1 at the conditions of 453 K and 1 Hz. Introduction of alkylamide units into the BTBT π core has the potential to develop the external stimulus-responsive organic semiconductors brought from both ferroelectricity and semiconducting properties.