Asymmetric phthalimide-based conjugated monomer synthesis in a flow reactor

An asymmetric design strategy for organic semiconductors enhances device efficiency through effective morphology control while broadening design possibilities. However, asymmetric monomer synthesis in conventional batch reactors is generally more complex than symmetric synthesis, requiring additional reaction steps that typically result in lower selectivity and yields. Here, we propose the flow synthesis of asymmetric monomers as an advantageous alternative. By applying our synthetic method, the steps required to synthesize a target asymmetric monomer can be reduced from five to three. Using a custom-designed flow reactor, we carried out palladium-catalyzed Stille coupling reactions to produce an asymmetric phthalimide (PhI) monomer with thiophene (Th) and bromide substituents on each side (Th-PhI-Br), starting from dibromide phthalimide (PhI-Br₂). With our highly reproducible synthesis system (the standard deviation of the yield for repeated runs was below 2 %), we screened various reaction conditions, including temperature, reaction time, flow rate, PhI-Br₂ to thiophene ratio, and reactant injection sequence. We then examined the correlations between these conditions and the reaction selectivity and yield. Our results demonstrate that the flow synthesis process is an effective method for optimizing selectivity and yield in the production of asymmetric conjugated monomers.