Impact of molecular tailoring of benzo[1,2-b:5,4-b′]dithiophene derivatives via side chain substitution on solution-processable organic field-effect transistors†

In this study, we report an investigation of the impact of molecular engineering through asymmetrical side chain substitution on the semiconducting performance of benzo[1,2-b:5,4-b′]dithiophene (BDT) derivatives in solution-processable organic field-effect transistors (OFETs). Pristine (2-(thiophen-2-yl)benzo[1,2-b:5,4-b′]dithiophene; compound 1), linear octyl-substituted (2-(5-octylthiophen-2-yl)benzo[1,2-b:5,4-b′]dithiophene; compound 2), and branched 2-ethylhexyl-substituted (2-(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:5,4-b′]dithiophene; compound 3) BDT derivatives were synthesized and characterized to evaluate the role of alkyl side chains in modulating thermal, optical, electrochemical, and charge transport properties. Despite the improved solubility and thermal stability observed for the alkylated derivatives, compound 1 without any side chains exhibited the highest field-effect mobility of up to 0.024 cm² V⁻¹ s⁻¹ and superior thin-film crystallinity. Our results demonstrate that excessive steric hindrance induced by bulky side chains can disrupt molecular packing and degrade OFET performance. This work highlights the importance of precise side chain engineering for optimizing the balance between solution-processability and charge transport in organic semiconductors.