To investigate how BDD and ester substituted thiophene groups affect polymer backbone conformation, we compared the potential energy scans of the dihedral angles corresponding to these groups by using DFT with the wB97xD method at 6-31G (d, p) basis set. This dispersioncorrected functional is a more accurate DFT method in estimating the torsional barrier and dihedral angles of conjugated molecules than the commonly used B3LYP functional which overestimates the torsional barrier and overly stabilizes the planar conjugated conformations due to its many-electron self-interaction error. In Fig. , the potential energy plots corresponding to the dihedral angle between thiophene and BDD groups (thiophene-BDD) of PM7 (red) and the dihedral angle between thiophene and ester substituted thiophene groups (thiophene-ester) of both PM7 D1 and D2 (blue) are presented and show significantly different features. We focus our discussion on comparing these dihedral angles since the potential energy scans of dihedral angles between donor and acceptor moieties were found similar for PM7 and its derivatives (Fig. ). In Fig. , 0° on the horizontal axis refers to the anti-conformation in which sulfur atoms on the adjacent thiophene groups are facing the opposite directions while at 180° sulfur atoms are syn or on the same side. Each plot indicates two local minima corresponding to the dihedral angles of anti and syn conformations. Since the energy difference between the local minima is close to 1 kT at room temperature for both plots as indicated by the dashed line in Fig. , there is no energetic preference between anti and syn conformations. Nevertheless, the minimum-energy dihedral angles of thiophene-ester groups of PM7 D1 and D2 are shifted towards the maximum dihedral angle of 90° as compared to the thiophene-BDD dihedrals of PM7 (indicated by arrows in Fig. ) suggesting that PM7 D1 and D2 exhibit larger dihedral angles than PM7 regardless of their conformation. Furthermore, by optimizing the dimers (anti-conformations) of these molecules as shown in Fig. . we demonstrate that the large dihedral angles of PM7 D1 and D2 disrupt the backbone planarity of PM7. Specifically from the side view images (Fig. ), we can clearly see that PM7 D1 and D2 dimers exhibit backbone twist angles of 31.4° and 30.6°, respectively, while PM7 has a much lower backbone twist of 18.4°. Another major difference between the potential energy plots in Fig. is the energy barrier to planarize the dihedral angles which indicates the likelihood of forming planar backbone conformation (indicated by the blue arrows in Fig. ). As shown in Fig. , the energy barriers required to planarize the dihedral angle between thiophene and ester-thiophene groups of PM7 D1 and D2 to 0° and 180° reach 9-10 kJ/mol and 13-16 kJ/mol, respectively, while the energy barriers to planarize the thiophene-BDD dihedral angle of PM7 are only around 2.5 kJ/mol and 3.5 kJ/mol (approximately 1 kT and 1.4 kT at room temperature). We attribute these high energy barriers to planarize the ester-thiophene dihedral angle of PM7 D1 and D2 to the steric hindrance originating from the alkyl side chain connected to the ester group. Indeed, when the methyl group is replaced with an ethyl group, the energy barriers to planarize this dihedral angle increase further as shown in Fig. suggesting that alkyl side chains induce strong steric hindrance, making it difficult to planarize the dihedral angle. On the other hand, for PM7, side chains are substituted further from the adjacent thiophene unit likely causing less steric hindrance as compared to side chains directly connected to the ester group in the case of PM7 D1 and D2. Moreover, we observe that the energy barrier between anti and syn conformations for the dihedral angle of PM7 D1 and D2 are lower than that for PM7 (Fig. ), suggesting that PM7 D1 and D2 are not only more torsionally distorted from planarity and less likely to planarize, but also more flexible than PM7. We note that the ester groups of PM7 D1 and D2 are rotationally flexible so the configuration where carbonyl (C=O) oxygen of ester group faces the thiophene group as shown in Fig. is also possible. However, regardless of this ester group configuration, we observe the same trend in the potential energy scans where PM7 D1 and D2 have larger minimum-energy dihedral angles, higher energy barriers to planarize, and lower energy barrier to switch from anti to syn conformation than PM7 (Fig. ).