Drastic Effects of Fluorination on Backbone Conformation of Head-to-Head Bithiophene-Based Polymer Semiconductors

This study shows that the backbone conformation of head-to-head type 3,3′-dialkyl-2,2′-bithiophene can be tuned via fluorination of the neighboring benzothiadiazole (BTz). Without fluorination, the polymer backbone is highly twisted, whereas difluorination of BTz produced a coplanar backbone. Monofluorination of BTz yielded a tunable polymer backbone conformation depending on the film annealing temperature. In organic thin-film transistors, the polymer with the head-to-head linkages showed a remarkable hole mobility of >0.5 cm² V^-1 s^-1 upon attaining a planar backbone. Thus, the head-to-head linkage does not necessarily lead to backbone nonplanarity, and achieving planar conformation of 3,3′-dialkyl-2,2′-bithiophene has profound implications in materials design for organic semiconducting devices, yielding good solubility, reduced materials synthetic steps, and improved opto-electrical properties.