Regioisomerism of an alkyl-substituted bithiophene comonomer in (3E,8E)-3,8-bis(2-oxoindolin-3-ylidene)naphtho-[1,2-b:5,6-b′]difuran-2,7(3H,8H)-dione (INDF)-based D–A polymers for organic thin film transistors

Abstract

Two donor–acceptor (D–A) polymers based on (3E,8E)-3,8-bis(2-oxoindolin-3-ylidene)naphtha-[1,2-b:5,6-b′]difuran 2,7(3H,8H)-dione (INDF) and substituted regioisomeric bithiophene (BT) units with different side chain positions (head-to-head, HH, and tail-to-tail, TT) were synthesized. PINDFBT-(HH) achieved electron (μ_e) and hole (μ_h) mobilities as high as 0.33 cm² V⁻¹ s⁻¹ and 0.15 cm² V⁻¹ s⁻¹, respectively, while PINDFBT-(TT) showed an order of magnitude lower mobilities with μ_e of 0.07 cm² V⁻¹ s⁻¹ and μ_h of 0.02 cm² V⁻¹ s⁻¹. The distinctly different electrical performance of these two polymers originates from their different side chain placements on the bithiophene units and consequently different electronic structures, backbone coplanarity, chain packing, and thin film morphology. Our results showed that a bithiophene unit with an HH substitution pattern is much more favoured in terms of the charge transport property of the polymers.