| 摘要： |
Lactam-containing conjugated molecules are important building blocks for conjugated polymers for high performance organic field-effect transistors (OFETs). The alkylation on conjugated lactam building blocks may preferably produce either O-alkylated or N-alkylated isomers, which might have different influences on the HOMO/LUMO energy levels, pi-pi stacking patterns and crystallinity of the corresponding polymers. However, the influence of O-alkylation and N-alkylation on the OFET performance of the resultant polymers has not been reported. Here, with an improved synthetic strategy, we prepared the N-alkylated isomer of dibenzonaphthyridinedione (DBND), a tetracyclic lactam building block that used to give O-alkylated product preferably, which gave us a chance to compare the influence of N-alkylated DBND (N-DBND) and O-alkylated DBND (O-DBND) on the OFET performance of the corresponding polymers. It was found that the polymer based on N-DBND exhibits a much higher hole mobility (0.55 cm(2) V-1 s(-1)), almost 100 times greater than the one based on O-DBND (0.006 cm2 s-1). The reasons for such a huge difference were thoroughly investigated theoretically and experimentally. It was found that repeating unit in the polymer based on N-DBND exhibits a much higher dipole moment (1.56 D) than that based on O-DBND (0.49 D), which results in a much stronger intermolecular binding energy (-57.2 vs -30.0 kcal mol(-1). Although both polymers exhibits very similar coplanarity and crystalline patterns, stronger intermolecular interaction of the polymer based on N-DBND leads to shorter pi-pi stacking distance (3.63 vs 3.68 angstrom), which results in a film with higher crystallinity and highly interconnected fibrillar domains, and accounts for its high charge carrier mobility, as evidenced by 2D-GIXD and AFM analysis. We come to the conclusion that the more polar amide bond in N-DBND is the major factor which governs the charge transport properties, which overwhelms the side-chain engineering effect that O-alkylation might bring in (the branching point of the side-chain of an O-DBND-based polymer is one more atom away from the polymer backbone and results in less steric hindrance). |
