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

Wenliu Zhuang,Wei Ma,Suhao Wang,Weiguo Zhu,Magnus Berggren,Ergang Wang,Simone Fabiano,Qiang Tao

doi:10.1021/acs.macromol.0c02326

Abstract

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 π-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-fusion-promoted coplanarity.

Highlights

Great efforts in molecular design have boosted the critical field-effect charge mobility of the conjugated polymers, with values exceeding 10 cm[2] V−1 s−1.3−9 One of the successful keys to achieve high mobility in semiconducting polymers is to maximize the intra- and intermolecular charge transport by means of improving: (i) polymer backbone coplanarity,[3,10] (ii) molecular weight,[11,12] (iii) orientation of polymer chains,[13−22] and (iv) intermolecular interactions between neighboring molecules.[23]. In line with such considerations, donor−acceptor (D−A) copolymers have shown great potential owing to their tunable energy levels, strong intramolecular charge transfer (ICT), and intermolecular interactions between adjacent polymer chains.[24−27] recent experimental evidences have shown that local aggregation in this class of polymers over just a few chains is a sufficient mesoscopic structure to ensure high mobility and desired swift operation in organic fieldeffect transistors (OFETs).[28]
The polymerization was realized via Pd2(dba)3-catalyzed Stille coupling of bis(trimethylstannyl)benzo[1,2-b:4,5-b′]dithiophene with dibromo-substituted monomers 3 or 4, respectively
The molecular weights of the polymers were determined by size exclusion chromatography (SEC)

Summary

■ INTRODUCTION

Solution-processable semiconducting polymers have attracted wide attention thanks to their versatile chemical synthesis and the opportunity for low-cost fabrication of large-area flexible devices such as smart cards, bendable displays, radiofrequency identification (RFID) tags, and distributed sensors.[1,2] In recent years, great efforts in molecular design have boosted the critical field-effect charge mobility of the conjugated polymers, with values exceeding 10 cm[2] V−1 s−1.3−9 One of the successful keys to achieve high mobility in semiconducting polymers is to maximize the intra- and intermolecular charge transport by means of improving: (i) polymer backbone coplanarity,[3,10] (ii) molecular weight,[11,12] (iii) orientation of polymer chains,[13−22] and (iv) intermolecular interactions between neighboring molecules.[23]. It is noted that ring-fusion of the DAD segment results in upshifted HOMO and LUMO energy levels as observed by electrochemical measurements It has been shown that high-k fluorinated dielectrics enhance hole transport in polymeric OFETs, which gives us the possibility to improve the mobility values.[28,55−57] Interestingly, hole mobility values as high as (6.0 ± 0.12) × 10−4 and (2.5 ± 0.65) × 10−3 cm[2] V−1 s−1 are detected for PBDTT-DPP and PBDTT-DPPFu, respectively (Table 2 and Figure 7)

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES