Elucidating the effects of the sidechain substitution direction on the optoelectronic properties of isomeric diketopyrrolopyrrole-based conjugated polymers for near-infrared organic phototransistors

Abstract

Sidechain engineering is a powerful molecular design strategy to tune the solid-state packing and structural ordering of conjugated polymers. While the modification of the type and length of sidechains has become a key strategy in tuning the polymer properties, the effects of the sidechain direction on the optoelectronic properties of polymers have been less investigated. Herein, two new isomeric diketopyrrolopyrrole (DPP)-based conjugated polymers (namely P3BT-in and P3BT-out) with different sidechain substitution directions are designed and synthesized based on the O-alkylated DPP isomer (pyrrolo[3,4-c]pyrrol-1(2H)-one). Through comprehensive theoretical studies and experimental characterization, the effects of the sidechain-substitution direction on the optoelectronic properties of polymers and the corresponding performance of organic transistors and near-infrared organic phototransistors (NIR-OPTs) were systematically investigated. Compared to P3BT-out with sidechains spreading in the outer way, P3BT-in with sidechains in the inner way has better molecular planarity, resulting in increased crystallinity. In addition, P3BT-in has deeper LUMO energy levels and a narrower bandgap of 1.06 eV with a stronger light absorption ability, leading to increased electron mobility and n-type NIR photosensitivity. Overall, tuning the sidechain substitution direction could be a useful strategy for the rational design of functional polymers.