Electron-Transporting Conjugated Polymers from Novel Aromatic Five-Membered Diimides: Naphtho[1,2-b:4,3-b′]-dithiophene and -Diselenophene Diimides

Despite the important advances in high-mobility electron-transporting polymers built from aromatic six- and seven-membered diimides, aromatic five-membered diimide (AFMDI)-containing polymers rarely access satisfactory n-type or ambipolar performance. Herein, a UV-photocyclization protocol is applied to create two novel AFMDI units, named as dibrominated naphtho[1,2-b:4,3-b′]dithiophene diimides (NDTI-2Br) and dibrominated naphtho[1,2-b:4,3-b′]diselenophene diimides (NDSI-2Br). Both NDTI- and NDSI-based small molecules are demonstrated to possess not only a highly π-extended conjugation backbone but also high electron deficiency and low-lying energies of lowest unoccupied molecular orbital (LUMO, as low as −3.74 eV), which is due to the incorporation of two electron-poor imide units into the fused-ring parent cores. With these attractive properties, we further disclose their applications in the construction of four novel conjugated copolymers, including NDTI and NDSI derivative acceptors coupled with a weak electron-donating vinyl unit (P1 and P2) or linked with the 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole unit (P3 and P4). It is found that the backbone structure, optical property, electronic structure (energy levels and band gap), and charge transport property of the resulting polymers are fine-tuned by regulating the copolymerization units or sulfur/selenium heteroatoms embedded in the acceptor units. All the polymers display a near-coplanar conjugation backbone, outstanding thermal stability (Td > 460 °C), and desirable reduction waves coupled with low-lying LUMO energies below −3.78 eV. Investigation of charge transport properties indicates that P1 and P2 show typical unipolar n-type characteristics with the highest electron mobility of 0.01 cm2 V–1 s–1, while P3 and P4 exhibit balanced ambipolar charge transport properties, with the maximum hole and electron mobilities of 2.0 × 10–4 and 0.005 cm2 V–1 s–1, respectively. The electron mobility (0.01 cm2 V–1 s–1) observed here can be compared to the classical n-type semiconductor PCBM (∼10–3 cm2 V–1 s–1), which is sufficient for electron transport to the electrode in the all-polymer solar cells.