## 3 Results and Discussion  
### Molecular Design and Synthesis  
Scheme 1 shows the chemical structure of the target molecule (**M1**) and Scheme 2a depicts synthetic strategy for **M1** by showing several steps of the retrosynthesis. Since bithiophene derivatives such as 1 and 3 in Scheme 2a are commercially available, it would be convenient that based on bithiophene derivatives, the other groups were retrosynthetically analyzed first and then, since each molecule could be coupled by two types of well-established reaction depending on the type of bithiophene derivatives, as shown in Scheme 1a, we believed that the analysis was correct and feasible. Further retrosynthetic analysis was focused on the other side group containing TPD units such as 2 and 4 as described in Scheme 2b. Since the hexyl bithiophene derivative notated as 5 is also commercially available, the 2 and 4 can be retrosynthetically analyzed by two parts such as TPD derivatives and 5, as shown in Scheme 2b. From this aspect, it was found that the mono-brominated TPD (7 in Scheme 2b) and bis-brominated TPD (6 in Scheme 2b) formed the possible starting point for the desired **M1** molecule. Therefore, 1,3-dibromo-5-(2-ethylhexyl)-thieno[3,4-c]pyrrole-4,6-dione (6) was prepared with a yield of 65% by following the literature, as shown in Scheme 3a [53]. Then, one of the bromine substituents on 6 was reduced with zinc (0) so as to afford 7 with a yield of 56%, see Scheme 3a. Then, 2 was synthesized upon varying the stoichiometric ratio of 6 and 5 via a Stille cross-coupling reaction as shown in Scheme 3b. However, most of the resulting product was insoluble polymeric solid, resulting in a yield of 2 of only 7% even though an excess amount of 6 was used. Therefore, it was found that this type of reaction was not an atom economically favorable method to get 2 especially. Nonetheless, a quite reasonable amount of 4 was obtained via direct heteroallylation with a 1:1 stoichiometric ratio of 5 and 7, such as mono-brominated TPD. At this point, we lack insight to explain the side reaction in Scheme 3b via a Stille cross-coupling reaction but it is worthy to note here that TPD derivatives seem to be more suitable and stable for the direct heteroallylation condition. After 4 was obtained, several trials were attempted to obtain 2 by brominating 4 with N-bromosuccinimide (NBS), as displayed in Scheme 3c. However, Fragment 2 was not obtained from any of the trials. Notably, too many spots were detected on thin layer chromatography (TLC) substrates when the bromination was processed in the acidic condition where the TPD units were brominated. Therefore, further analysis of resulting compounds from such an acidic condition was almost impossible and we concluded that the starting materials, such as 4, could be decomposed in such acidic conditions. In relatively mild conditions, almost all starting materials remained intact for 12 h without showing any significant conversion to brominated compounds based on TLC analysis. Next, 4 and 3 were coupled to gain the **M1** with a 12% yield through direct C-H heteroaylation by modifying some of the methods for TPD-based polymers reported in the literature, as described in Scheme 4 [58, 61]. Although the reaction conditions such as reaction time and temperature were varied without changing the Pd source and phosphine ligands, the chemical yield from this type of reaction was around 12%; therefore, it was an atom economically unfavorable reaction. Since we believed there should be too many variable factors such as reagents and/or each fragment in Scheme 4 to optimize this type of reaction, it was, therefore, necessary to consider the other possible synthetic route to improve the chemical yield for **M1**. The alternative synthetic route was performed with specific reaction conditions, as shown in Scheme 5. While the previous synthetic route was designed by retrosynthesizing **M1** from the side to the core, in Scheme 5, the synthetic procedure was alternatedby expanding the core to side to reach the same target molecule.