Thiophene-Based Conjugated Polymers with Photolabile Solubilizing Side Chains

This paper describes a series of thiophene-based conjugated polymers that become insoluble upon irradiation with ultraviolet light. Stille or Suzuki reactions of appropriately substituted 2,5-bromothiophene derivatives yielded terthiophene and polythiophene derivatives with either o-nitrobenzyl (ONB) ester or ONB ether photolabile side chains with n-octyl substituents. Light-induced cleavage of these ONB side chains with ultraviolet light at 365 nm cleaves the octyl chains responsible for solubilization of the polymers away from the conjugated main chains, rendering them insoluble. Consistent with the accepted mechanism of ONB photolysis, those structural modifications that would yield a more stable benzylic radicalmethyl substitution on the benzylic position, replacement of the ester with an ether, or bothyielded more efficient photolyses as determined by (i) quantum yields of photolysis of ONB-substituted terthiophenes, and (ii) the percentage of polymer that persists in UV-irradiated thin films upon rinsing with chloroform. These polymers behave as negative-tone photoresists, enabling both direct photopatterning of conjugated polymers, and fabrication of multilayer conjugated polymer films by irradiating with UV light after each spin-casting step. Although hole-mobility values of these polymers in thin film transistors were only ∼10–5 cm2 V–1 s–1, photolysis and rinsing did not cause significant degradation in performance.