Synthesis and Electronic Properties of Diketopyrrolopyrrole-Based Polymers with and without Ring-Fusion

Diketopyrrolopyrroles (DPP) have been recognized as a promising acceptor unit for construction of semiconducting donor-acceptor (D-A) polymers, which are typically flanked by spacers such as thiophene rings via a carbon-carbon single bond formation. It may suffer from a decrease in the coplanarity of the molecules especially when bulky side chains are installed. In this work, the two N atoms in the DPP unit are further fused with C-3 of the two flanking thiophene rings, yielding a p-expanded, very planar fused-ring building block (DPPFu). A novel DPPFu-based D-A copolymer (PBDTT-DPPFu) was successfully synthesized, consisting of a benzo[1,2-b:4,5-b]dithiophene (BDTT) unit as a donor and a DPPFu unit as an acceptor. For comparison, the unfused DPP-based counterpart PBDTT-DPP was also synthesized. Two dodecyl alkyl chains were attached to thiophene rings of DPP moieties to ensure good solubility of the DPPFu-based polymer. The influence of the ring-fusion effect on their structure, photophysical properties, electronic properties, molecular packing, and charge transport properties is investigated. Ring-fusion enhances the intermolecular interactions of PBDTT-DPPFu polymer chains as indicated by density functional theory calculation and analysis of electrostatic potential and van der Waals potential and results in significantly improved molecular packing for both the in-plane and out-of-plane directions as suggested by X-ray measurements. Finally, we correlate the molecular packing to the device performance by fabricating field-effect transistors based on these two polymers. The charge carrier mobility of the ring-fused polymer PBDTT-DPPFu is significantly higher as compared to the PBDTT-DPP polymer without ring-fusion, although PBDTT-DPPFu exhibited a much lower number-average molecular weight of 17 kDa as compared to PBDTT-DPP with a molecular weight of 108 kDa. The results from our comparative study provide a robust way to increase the interchain interaction by ring
Funding Agencies|Swedish Research CouncilSwedish Research CouncilEuropean Commission [2015-04853, 2016-06146, 2019-04683]; Swedish Research Council FormasSwedish Research CouncilSwedish Research Council Formas; NSFCNational Natural Science Foundation of China (NSFC) [21504006, 21534003, 11272093]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [2017.0186, 2016.0059]; Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research; China Scholarship CouncilChina Scholarship Council [201908440047]; Guangzhou Municipal Science and Technology Bureau [201804010501, 201904010381, 202002030362]; Department of Education of Guangdong ProvinceNational Natural Science Foundation of Guangdong Province [2018GKTSCX041, 2017GKQNCX005]; Department of Science and Technology of Guangdong Province [2016A010103046]; Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology) [2020-skllmd-07]; Natural Science Foundation of Hunan ProvinceNatural Science Foundation of Hunan Province [2018JJ3098]; Office of Science, Office of Basic Energy Sciences, of the U.S. Department of EnergyUnited States Department of Energy (DOE) [DE-AC02-05CH11231]