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

An unsymmetrical [1]benzothieno[3,2-<i>b</i>][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 (<b>SmE</b>) 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 (<b>SmE</b>) 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∑μ_max = 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 <b>SmE</b> phase. Moreover, it has been found that thermal annealing in the <b>SmE</b> 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.