Figure 4 (a) Chemical structure; (b) Single-crystal data of NTI-EH and NTI-Bu; (c) Device thermal stability of polymers; (d) Aging of hot PNTB-HD:N3, PNTB-2T:N3 and PM6:N3 chloroform solution (identical with condition for device fabrication) for 2 h at room temperature. Reproduced with permission.[33] Copyright, 2022 Elsevier Ltd.
3.2. Thieno-[3,4-c]-pyrrole-4,6-dione (TPD) based polymers.
TPD unit was first synthesized by Sicé in 1954 by treating thiophene-3,4-dicarbocylic acid with ammonia.[72]Tour and co-workers first reported TPD-based conjugated copolymers in 1997.[73] TPD unit has the advantages of easy synthesis, high coplanarity, and strong electron-withdrawing ability.[46] In addition, Various side groups can be introduced into the N-site of TPD to tune solubility of the resulting polymer and morphology of blend films.[39,46] To date, four synthetic routes have been reported for the preparation of TPD (Scheme 2) . Bjørnholm et al. [74] reported the synthesis of TPD from starting materials of 3, 4-dibromothiophene, which was converted to 3, 4-dicyanothiophene by Rosenmund-von Braun reaction with cuprous cyanide (Synthetic route 1). The alkaline hydrolysis of 3, 4-dicyanothiophene result in thiophenedicarboxylic acid, and then reflux in acetic anhydride to produce cyclic anhydride. The TPD was synthesized by treating 4-amino-thiophene-3-carboxylic acid with thionyl chloride. The synthesis of TPD dimer was based on the bromination of the TPD with 1 equiv of NBS, however, it makes the conditions hard to control to obtain the desired product with a good yield. In 2011, an efficient synthetic route was reported for the preparation of the 1-iodo-5-alkyl-4H-thieno[3,4-c] pyrrole-4,6(5H)-dione (TPD-I) by Leclerc et al. .[75] They synthesized3-ethyl 4-methyl 2-aminothiophene-3,4-dicarboxylate by the Gewald reaction between methyl 2-oxopropanoate and ethyl cyanoacetate. Then, the Sandmeyer-type reaction makes the amine of 3-ethyl 4-methyl 2-iodothiophene-3,4-dicarboxylate substituted by an iodide. Followed by acidic hydrolysis to obtain 3-ethyl 4-methyl 2-iodothiophene-3,4-dicarboxylate. Finally, 1-iodo-5-alkyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD-I) was obtained via pyrroledione ring formation (Synthetic route 2). In 2015, Shinichiro Fuse et al. reported the synthesis of TPD through a Pd-catalyzed carbonylative amidation reaction from commercially available 3,4-dibromothiophene in a yield of 63 % (Synthetic route 3).[76] It should be noted it is the most straightforward synthetic route to synthesize TPD by one-step reaction. In 2023, Min et al. reported a synthetic route of DT-TPD through a two-step reaction in a yield of 31 %.[77] The key ring forming reaction was conducted by heating the mixture of 4-alkylthiophene-2-carbaldehyde, S8, 1-methylpyrrolidine-2,5-dione, 2-aminobenzimidazole, NH4I, K2CO3, N-methyl-2-pyrrolidone and H2O in 140 °C for 48 hours, yielding DT-TPD (Synthetic route 4).
Scheme 2 Synthetic route of TPD.
Leclerc et al. synthesized the a TPD-based polymer, and afforded a PCE of 5.5 % when blended with PC71BM in 2010.[78] After that, Thieno[3,4-c]pyrrole-4,6-dione (TPD) unit as an electron-deficient unit has been widely used to construct conjugated polymers for application in fullerene and non-fullerene based OSCs.[39,46],[79,80] Figure 5 summarizes the structures of typical TPD-based polymers and relevant properties are listed in Table 2. Wang et al. [81]synthesized PBDT-TPD and PBDTS-TPD based on the BDT and TPD, and PCEs of 6.8 % and 7.7 % were achieved by blending with PNDI-T. Gao et al. [82] synthesized polymers PBDTT-TPD and PBDT-TPD. Compared with the alkoxy-modified polymer PBDT-TPD, the alkyl-thienyl modified polymer PBDTT-TPD possesses deeper HOMO energy level, higher extinction coefficient and better hole transport property, leading to a higher PCE of 7.15 % for PBDTT-TPD:IDIC based OSCs.