*3.2. Absorption and Fluorescence Spectra of the CP in Toluene*

Figure 4 presents the absorption spectra of the CP in toluene and benzene at different concentrations from 195 × 10<sup>−</sup>4–12.17 × 10−<sup>4</sup> mg/mL. It was found that there are three diverse features. The first is a peak at approximately 355 nm and the second is the main peak at 425 nm, with the third being a shoulder at 370 nm. The optical density decreased from a high value to a low value as the concentration decreased. The absorption spectral profile remained the same, and the full width at half maximum (FWHM) of the maximum peak at 425 nm decreased with the decreasing PFO-co-PPV-MEHB concentration. The absorption spectra in benzene are very close to those of PFO-co-PPV-MEHB in toluene. Figure 4b shows the absorption features in benzene.

**Figure 4.** (**a**,**b**) Absorption spectra of PFO-co-PPV-MEHB in (**a**) toluene and (**b**) benzene at various concentrations.

Figure 5a demonstrates the emission spectra of PFO-co-PPV-MEHB in toluene for different concentrations ranging from 2.5 mg/mL to 195 × 10−<sup>4</sup> mg/mL. At low concentrations, fluorescence peaks occur at 485 nm and 510 nm, with a tail at 550 nm. Up to a concentration of 0.078 mg/mL, the fluorescence increases; after that, when the concentration (0.156 mg/mL) is further increased, the fluorescence intensity redshifts and the spectral profile shifts toward red. At higher concentrations, the primary peak of the fluorescence spectrum becomes a shoulder and the 515.5 nm peak becomes dominant with reduced intensity. This behavior is common to fluorescent organic molecules. At low concentrations, the fluorescence output is low because the number of molecules is low in a unit area of solution. As the concentration increases, the intensity also increases up to an optimal concentration. Beyond this concentration, the fluorescence output intensity starts decreasing due to the proximity of molecules, which suppress the certain singlet vibration and increase the reabsorption. Figure 5b shows a spectral shift of the singlet peak at around 515.5 at higher concentrations, and the shift is due to reabsorption.

**Figure 5.** (**a**) Emission spectra of PFO-co-PPV-MEHB for various concentrations in toluene. (**b**) Redshift of the peak fluorescence spectra of PFO-co-PPV-MEHB in toluene. (**c**) Fluorescence spectra in benzene from high to low concentrations. (**d**) Deconvolution of the fluorescence spectrum of PFO-co-PPV-MEHB at 0.0195 mg/mL.

The solvent effect was studied in benzene and the trends of the fluorescence intensity and spectral profile were very similar, with a 4 nm redshift in the peak wavelengths at approximately 483 and 508 nm, as displayed in Figure 5c. This shift is due to the change in the solvent dielectric constant.

Figure 5d shows the deconvolution fitting of the PFO-co-PPV-MEHB fluorescence spectral profile using Gaussian functions. The peak positions are 483.3, 508, and 535.4 nm, and the linewidths are approximately 19, 38, and 60 nm, correspondingly. These peaks can be ascribed to the fluorescence counterparts of the singlet oscillators in the simulated UV-VIS spectra. The Stokes shift was calculated in toluene and benzene and it was 98 nm and 97 nm, respectively. The large Stokes shift could be useful to reduce light scattering and self-absorption in optical materials [39].
