*3.1. DFT Calculation*

CPs consist of a large number of atoms; hence, the simulation of CPs is considered to be challenging. Thus, many studies have adopted an oligomeric model of the compound [35–37]. The absorption spectra and energy gap of the PF-co-MEH-PPV were simulated with three monomer units (n = 3) based on the HOMO-LUMO energy gap calculations. Figure 2 shows the HOMO-LUMO structure of the three-monomer model. The HOMO (Frontier orbital) densities are aligned vertically in the major part of the ring structure; however, the density diminishes in the last monomer at one end (left). The LUMO densities are highly delocalized and align horizontally along the molecular backbone. The LUMO density is higher at the end of the oligomer model where the HOMO density is diminished. The movement of the orbital densities upon excitation significantly improves the optical and electronic properties of the CP. The calculated bandgap is 2.91 eV. The HOMO lies at −6.83 and the LUMO lies at −3.92 eV. In the same manner, the structural and optoelectronic properties of CPs and CO (PCDTBT) have been examined by DFT and TD-DFT [38].

**Figure 2.** Frontier molecular orbital (HOMO-LUMO) structure of PFO-co-PPV-MEHB (tail-truncated and n = 3 model) calculated using the CAM-B3LYP/6-31G(d,p) basis set.

The UV-VIS spectrum for PFO-co-PPV-MEHB (n = 3) in vacuum was simulated using the DFT/CAM-B3LYP/6-31G(d,p) basis set, as shown in Figure 3a. The results show three singlet oscillator strengths at wavelengths of 341.66, 361.38, and 374.51 nm with high values of f = 0.633, 1.8254, and 6.8762, respectively.

Hexane has a very low dielectric contestant and is a nonpolar solvent; hence, its effect on any molecule (solute) is much less than that of most solvents. Hence, it is optimal for comparison with the results simulated in vacuum. The difference between the simulated and experimental absorption spectra (λExp − λsim) is 30 nm, as presented in Figure 3b. This difference could be attributed to the solvent dielectric constant and concentration of the CP in hexane. However, the simulated singlet oscillator strengths matched the peaks and shoulder of the hexane absorption spectra.

**Figure 3.** (**a**) UV-VIS and oscillator strength profile of PFO-co-PPV-MEHB (tail-truncated and n = 3 model). (**b**) Absorption of the low-concentration CP in hexane to identify the bandgap using the intersection principle and comparison with the stimulated spectrum.

Figure 3b shows that the bandgap calculated using the absorption and fluorescence intersection method is 2.756 eV. The purple dotted circle highlights the intersection of the absorption and fluorescence spectra, which is zoomed in on and shown in an inset of Figure 3b. This result is in good conformity with the simulated bandgap of 2.91 eV (in hexane). The disturbance could be due to the approximation of the polymer structure and repetition units (n = 3) and a change in the dielectric constant of the solvent. However, the experimental results show two distinctive bands that correlate with the simulated singlet oscillator strengths. Additionally, the experimental results contain three features, two peaks and a shoulder, which could be attributed to the three singlet oscillator strengths found using the simulation methods. The electronic circular dichroism (ECD) result is shown in Figure S3.

Figure S4 shows an estimation of the HOMO−LUMO gaps for PFO-co-PPV-MEHB using the bandgap extrapolation of oligomers (n = 1 to 5). The calculation was performed

using the same TD-DFT method for all repetitive monomer units (n = 1 to 5), but the figures show only n = 3 for a clear presentation of the HOMO LUMO structure. The linear fitting gives the equation Eg (eV) = 2.7667 + 0.3408 × (1/n). When the value of n is large, the second term tends to become negligible. Thus, the calculated bandgap value of the polymer is 2.7667 eV using the extrapolation method. This value is comparable with the experimentally measured bandgap of 2.756 eV.
