Synthesis of squaraine-based alternated π-conjugated copolymers: from conventional cross-coupling reactions to metal-free polycondensation

Low band-gap π-conjugated copolymers based on squaraine units alternated with thiophene or benzothiadiazole moieties were synthesized through Suzuki and Stille cross-coupling reactions, and through a metal-free polycondensation process using squaric acid. The metal-free polymerizations afforded higher molecular weights polymers, showing bright future for the synthesis of small band-gap π-conjugated polymers via less cost-effective and more environmental-friendly procedures. Additionally, the synthesized materials showed encouraging optoelectronic properties.