Thiophene backbone-based polymers with electron-withdrawing pendant groups for application in organic thin-film transistors
Two thiophene backbone-based hole-transporting poly-mers, namely, poly[(2,2'-bithiophene-5,5'-diyl)-alt-(5,7-bis(2-butyloctyl)-4H,8H-benzo[1,2-c:4,5-c']dithiophene-4,8-dione-1,3-diyl)] (PT2-BDD) and poly[(3,3'-difluoro-[2,2'-bithiophene]-5,5'-diyl)-alt-(5,7-bis(2-butyloctyl)-4H,8H-benzo[1,2-c:4,5-c']dithiophene-4,8-dione-1,3-diyl)] (PF2-BDD), were pre-pared by using electron-withdrawing pendant groups such as ketone and fluorine moieties. They both exhibited a planar backbone with effi-cient π conjugation, which is suitable for hole transport in organic thin-film transistors (OTFTs). However, the fluori-nated one (i.e., PF2-BDD), de-spite its perfect backbone planarity and strong intra- and intermolecular interactions, could not enhance the OTFT performance; due to its sol-vent resistance, electron neg-ativity, and random orienta-tion, PF2-BDD showed 10 times lower hole mobility than the non-fluorinated polymer (i.e., PT2-BDD). Nonetheless, the two-dimensional grazing incidence X-ray diffraction and temperature-dependent absorption spectra of the syn-thesized polymers provided crucial information to under-stand the relationship be-tween their hole transport behavior and molecular struc-ture.