**2. Materials and Methods**

The molecular structure of CP PFO-co-PPV-MEHB could be explained as a group of bridged structures. The molecule (A) 1-methyl-4-[(1E)-prop-1-en-1-yl] benzene shown in Seg. A is connected to both sides of the familiar CP poly[(9,9-dioctylfluorenyl-2,7 diyl)] (PFO) shown in Seg. B. Molecular segment A is connected to another polymer (1,4-diphenylene-vinylene-2-methoxy-5-{2-ethylhexyloxy}-benzene) resembling the MEH-PPV structure, as shown in Seg. C. The cumulative structure becomes [(A)-(B)-(A)-(C)] n. This approach is taken to modify the HOMO-LUMO gap. The calculated dipole moment is 1.4334525 Debye for n = 3, which is for the structure optimized using the DFT/Coulombattenuating method (Becke-3 Parameter-Lee-Yang-Parr) CAM-B3LYP/6-31G(d,p) basis set and the augmented structure is shown in Supplementary Figure S1. We used a workstation that consisted of an Intel i7 processor, 64 GB RAM, and 1 TB SSD hard disk for all our simulations.

The CP has 65,000 ± 35,000 g mol−<sup>1</sup> as its molecular weight (Mw). American Dye Source, Inc. (Baie-D'Urfé, Montreal, QC, Canada) provided the CP material. (ADS125GE) The Coumarin 510 has an Mw of 318.4 g/mol and was precured from Exciton Luxottica. A

Perkin Elmer Lambda 950 spectrophotometer (Llantrisant, UK) was utilized to record the absorption spectra. The fluorescence spectra at an excitation wavelength of 355 nm from a xenon flash lamp were investigated utilizing a spectrofluorometer (LS 55, Llantrisant, UK). To achieve transverse pumping, the tripled frequency (355 nm) of a 5 ns Nd:YAG laser was focused by a quartz cylindrical lens (5 cm focal length). The cylindrical lens focused the pulse onto a horizontal strip of 1 cm × 2 mm (length × width). One fiber was linked to an Ocean Optics spectrograph (Maybachstrasse, Ostfildern, Germany) to record the spectra. Another fiber was linked to a Princeton Instruments PI-MAX 4 ultrafastgated emICCD, which had a gate delay of less than 500 ps. The camera had a high-speed electrically controlled shutter that allowed events to be recorded with a sub-nanosecond time resolutions (please see [32]).

The monomer of PF-co-MEH-PPV is very large and requires significant computational resources. The full-scale simulation of the complete polymer structure with many monomers could be challenging. However, an oligomer structure with three connected monomers is accepted in practice for simulating polymer properties; hence, we adopted the n = 3 oligomer approach. To further reduce the computational burden, we cut short the tail of PFO and MEH-PPV segments, as shown in Figure 1b, since its contribution to optical properties is insignificant. However, practically, the long tail offers a higher solubility in many common solvents [33]. The TD-DFT calculations were performed via the following steps. The molecular structure was created using the Gaussian View 6 software. The structure was optimized with Gaussian 16 using the DFT/CAM-B3LYP/6-31G(d,p) basis set [34]. Next, the UV-VIS spectra, HOMO–LUMO structure, and dipole moment were calculated using the CAM-B3LYP/6-31G(d,p) basis set. Other calculations, such as acceptor-donor sites, polarizability, and charge distribution, were calculated using molecular dynamics (HFF) in the MarvinSketch software, as shown in Figure S2a–c. The charge distribution showed that the central ring structure was negative, more negative charges were found at oxygen sites and the edge of the ring was mostly positive. The CP polarizability was calculated for n = 3 was 229.15 *A*3, which shows that CP has a high polarizability for external electric fields, such as optical pumping. The H bond acceptor and donor site were calculated and it was found that each monomer contained 2 acceptor and 4 acceptor sites at oxygen; hence, the CPs have more solubility in a medium polar solvent.

**Figure 1.** (**a**) Molecular structure of CP poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-diphenylenevinylene-2-methoxy-5-{2-ethylhexyloxy}-benzene)] (i.e., PFO-co-PPV-MEHB). (**b**) Tail-truncated molecular structure utilized for the DFT calculation of PFO-co-PPV-MEHB.
