Impact of alkyl chain length on the thermal, optical and semiconductor properties of the symmetric 4-alkylphenyl derivatives of [1]benzothieno[3,2-b]benzothiophene

Abstract

Luminescent organic semiconductors with high charge carrier mobility are in high demand for organic optoelectronic devices. The properties of organic semiconductors crucially depend on the length of substituents that are not directly involved in their π-conjugated system, such as alkyl chains. In this study, a series of novel symmetric 4-alkylphenyl derivatives of [1]benzothieno[3,2-b]benzothiophene (Cn-PBTBT) with C6, C8, C10 and C12 alkyl chains were synthesized and investigated through TGA, DSC, POM, SAXS/WAXS, UV-vis absorption and photoluminescence spectroscopies, and their semiconductor properties were studied in OFETs. It was found that increasing the alkyl chain length mainly influenced the thermal stability, absorption spectra in thin films, phase behavior, solubility and charge-carrier mobility. All these molecules easily formed 2D crystals, showing charge-carrier mobilities in OFETs in the range of 1–5 cm² V⁻¹ s⁻¹, with the highest values for C10-PBTBT and C12-PBTBT, while C6-PBTBT showed the highest solubility and no significant phase changes until ca. 150 °C. The alkyl chain engineering demonstrated can serve as an efficient tool for the smart molecular design of high-mobility luminescent organic semiconductors.