**5. Conclusions**

This report investigated the theoretical, optical, and mirrorless laser properties of CP PFO-co-PPV-MEHB. The calculation showed a HOMO-LUMO structure and a very high oscillator strength of 6.434 (a.u.) for the singlet-singlet transition at 374.43 nm. The experimental UV-Vis spectra obtained for the nonpolar solvent hexane were analogous to the simulated HOMO–LUMO structure. The simulation showed three oscillator strengths that matched the two peaks and shoulder manifested in experimental UV-VIS spectra, indicating that highly efficient fluorescence properties arise from singlet vibrational transitions. Under an optimal solution concentration and pump energy, CP spectral narrowing was achieved at 508 nm with an FWHM of 8.75 nm. The photochemical stability under

pulsed laser excitation was excellent, without deuteration for up to 20 k pulses. TRS studies showed a rapid intensity increase and spectral narrowing within 2 ns of fluorescence.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/polym13091430/s1. Figure S1: Optimized structure of the copolymer PFO-co-PPV-MEHB Using DFT calculations. Figure S2: (a) Charge distribution of PFO-co-PPV-MEHB (tail-truncated and n = 3 model), (b) Polarizability of PFO-co-PPV-MEHB is 229.15 Å3 (tail-truncated and n = 3 model) and (c) H bond donor acceptor of PFO-co-PPV-MEHB (tail-truncated and n = 3 model). Figure S3: Electronic circular dichroism (ECD) of PFO-co-PPV-MEHB (tail-truncated and n = 3 model) calculated using the CAM-B3LYP/6-31G(d,p) basis set. Figure S4 Estimation of HOMO−LUMO gaps for PFO-co-PPV-MEHB using bandgap extrapolation of oligomers (n = 1 to 5).

**Author Contributions:** Conceptualization, M.J.A., M.S.A. and S.P.; software, M.J.A. and S.P.; methodology, M.J.A. and S.P.; validation, S.P., M.J.A. and M.S.A.; investigation, R.H.A. and R.A.A.; formal analysis, S.P., R.H.A., M.J.A. and R.A.A., resources, M.S.A.; data curation, M.J.A. and S.P.; writing original draft preparation, M.J.A. and S.P.; writing—review and editing, M.S.A.; visualization, S.P. and M.J.A.; supervision, M.S.A.; project administration M.S.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This Project was funded by the National Plan for Science, Technology, and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award Number (3-17-04-001-0002).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All data is offered by corresponding author for reasonable request.

**Conflicts of Interest:** The authors declare no conflict of interest.
