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

Synthesis and charge-transport properties of novel π-conjugated polymers incorporating core-extended naphtho[2,1-b:3,4-b′]dithiophene diimides

“…The key starting compounds ( 1a and 1b ) were synthesized by using the literature method. 48,49 In the presence of Pd 2 (dba) 3 and P(MeOPh) 3 , the treatment of compound 1a or 1b with thiophen-3-ylboronic acid afforded dithiophene-substituted intermediate 2a or 2b with high isolated yields of above 94%. Next, dehydrocyclization of 2a and 2b was attempted via two classic methods, namely the iodine-catalyzed photocyclization method and the MoCl 5 -mediated dehydrocyclization reaction.…”

“…46 Very recently, modification of the MDI core has become the focus of engineering organic/polymeric π-conjugated materials. 46–48 Early in 2020, the Cao group pioneered the synthesis of dichlorinated MDI motifs, which readily undergo nucleophilic substitution with ortho -disubstituted benzenes or 1,2,4,5-tetrasubstituted benzenes to yield a family of novel O/N/S co-embedded organic chromophores, featuring large π-extended conjugation and intriguing optoelectronic properties. 47 Our group recently disclosed the synthesis and structural derivation of dibrominated MDI motifs and demonstrated their applications in building solid-state luminescent chromophores 49 ( i.e.…”

“…, dibrominated naphtho[1,2- b :4,3- b ′]-dithiophene and -diselenophene diimides, abbreviated as 2Br–NDTI and 2Br–NDSI, respectively), and their D–A type electron-transporting CPSs. 48 Direct copolymerization of naphtho[1,2- b :4,3- b ′]-dithiophene or -diselenophene diimide (NDTI or NDSI) acceptor motifs with 4,7-di(thiophen-2-yl)benzo[ c ][1,2,5]thiadiazole units, however, generates a wave-like π-conjugated backbone (see the ground-state structures shown in Fig. S1†), which is unfavorable for interchain packing and film crystallization.…”

An In‐Situ Cyanidation Strategy To Access Tetracyanodiacenaphthoanthracene Diimides with High Electron Mobilities Exceeding 10 cm2 V−1 s−1

“…[a] E g opt =1240/λ onset . [b] Calibrated using ferrocene (Fc/Fc + ) as the external standard, E LUMO =−( E red +4.42) eV [15,20] . [c] E HOMO =( E LUMO – E g opt ) eV.…”

An In‐Situ Cyanidation Strategy To Access Tetracyanodiacenaphthoanthracene Diimides with High Electron Mobilities Exceeding 10 cm2 V−1 s−1

“…[b] Calibrated using ferrocene (Fc/Fc + ) as the external standard, E LUMO = À (E red + 4.42) eV. [15,20] [c] E HOMO = (E LUMO -E g opt ) eV.…”