## 3 Results and Discussion
### Optical Properties of the Polymers  
Figure 1a,b shows the normalized Ultraviolet-visible (UV-vis) absorption spectra of the PDTSDTBTDI-DMO and PDTSDTBTDI-8 polymers in (a) chloroform solutions and (b) on thin films, respectively. Table 2 summarizes the optical properties of the polymers. The absorption maxima of PDTSDTBTDI-DMO were found to be red-shifted compared to PDTSDTBTDI-8 in both solutions and thin films. This might have arisen because the former polymer's molecular weight was higher than that of the latter polymer. In the thin films, the absorption peaks of both PDTSDTBTDI-DMO and PDTSDTBTDI-8 displayed a bathochromic shift relative to their absorption in the solutions with maximal absorption at 689 and 670 nm. This could be due to the more aggregated and planar structures in the solid-state form. The band gap (E\({}_{\rm g}\)) values of the polymers were measured from the absorption onsets in the thin films. The E\({}_{\rm g}\) was found to be comparable at about 1.4 eV for both polymers. The altering alkyl chains on BTDI units from 3,7-dimethyloctyl into \(n\)-octyl chains had an insignificant effect on the resulting polymers' band gap. The E\({}_{\rm g}\) of both polymers were found to be slightly lower than PDTSDT (1.45 eV) and lower than the PDTSEHDTBT and PDTS12 DTBT counterparts (1.53 and 1.51 eV, respectively). This could be because the BTDI unit had a stronger electron-accepting property than the BT unit did. Both polymers also showed a remarkably lower E\({}_{\rm g}\) than those of their PDTSDTBTz, PDTSDTTTz-3, PDTSDTTTz-4 and PDTSTPD counterparts, which were found to be 1.85,  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Polymer**} & \multirow{2}{*}{**\% Yield**} & \multicolumn{2}{c}{**Toluene Fraction**} & \multicolumn{3}{c}{**Chloroform Fraction**} \\ \cline{3-6}  & & \(\mathbf{M_{\rm n}}\) & \(\mathbf{M_{\rm w}}\) & & & \\  & & (g mol\({}^{-1}\)) & (g mol\({}^{-1}\)) & PDI & \(\mathbf{M_{\rm n}}\) & \(\mathbf{M_{\rm w}}\) \\  & & (g mol\({}^{-1}\)) & (g mol\({}^{-1}\)) & (g mol\({}^{-1}\)) & PDI \\ \hline
**PDTSDTBTDI-DMO** & 92 & 6200 & 20,600 & 3.2 & 14,600 & 79,900 & 5.4 \\
**PDTSDTBTDI-8** & 57 & 5700 & 14,000 & 2.4 & & \\ \hline \end{tabular}
\end{table}
Table 1: The percentage of production, weight and number average molecular weights with polydispersity indexes of PDTSDTBTDI-DMO and PDTSDTBTDI-8.  
1.81, 1.76 and 1.73 eV, respectively. Compared to the E\({}_{\text{g}}\) of PDTSDTTAZ, PDTSTAZ and PDTSNTDO (1.81, 1.78 and 1.65 eV, respectively), both polymers had a significantly lower E\({}_{\text{g}}\). The significant lowering of band gaps is thought to be a result of adopting a 2D (thin film) structure since the band gap is known to be low in symmetric 2D soft matter systems [50]. As a result, this new type of polymer adopted more planar structures with a greater intrachain charge transfer alongside the polymer backbone. The absorption coefficient (\(\varepsilon\)) of PDTSDTBTDI-DMO was much higher than that of PDTSDTBTDI-8. This may be attributed to the greater molecular weight and red-shifted absorption maxima of the PDTSDTBTDI-DMO polymer.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Polymer**} & \(\varepsilon\) & \multicolumn{2}{c}{**Solution**} & \multicolumn{2}{c}{**Film**} \\ \cline{3-6}  & **(M\({}^{-1}\) cm\({}^{-1}\))** & \(\mathbf{\lambda_{\text{max}}}\) (nm)** & \(\mathbf{\lambda_{\text{max}}}\) (nm) & \(\mathbf{\lambda_{\text{onset}}}\) (nm) & **E\({}_{\text{g}}\) (eV)** \\ \hline PDTSDTBTDI-DMO & 32,300 & 627 & 689 & 873 & 1.42 \\ PDTSDTBTDI-8 & 23,400 & 613 & 670 & 863 & 1.43 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The UV-vis data and optical band gaps of the polymers.  
Figure 1: Normalized UV-vis absorption spectra of PDTSDTBTDI-DMO and PDTSDTBTDI-8 in (a) chloroform solutions; and (b) on thin films.