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.