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

Abstract

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 ( T d > 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 cm 2 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 cm 2 V –1 s –1, respectively. The electron mobility (0.01 cm 2 V –1 s –1 ) observed here can be compared to the classical n-type semiconductor PCBM (∼10 –3 cm 2 V –1 s –1 ), which is sufficient for electron transport to the electrode in the all-polymer solar cells.

Topics & Concepts