## 3 Results and Discussion
### Computational Results  
The energies of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), as well as the HOMO-LUMO gaps of monomers and A-A' dimeric units are collected in Table S2. Figure 2 illustrates the energy levels and shapes of thefrontier MOs of the DPP-DPP, DPP-DAP, and DPP-BTBPF units issued from the coupling of their constitutive monomers. In the case of the symmetrical DPP-DPP structure, the HOMO and LUMO were spread out evenly throughout each monomer. For A-A' systems with A = DPP and A' = DAP or A' = BTBPF, the HOMOs were primarily localized on the DPP unit, while the LUMOs were mainly spread on the A' moieties. DPP-DPP had the largest electronic gap of 3.72 eV. When DPP was linked to DAP, the gap reduced to 3.36 eV as a result of the large stabilization of the LUMO. The electronic gap of DPP-BTBPF was close to that of DPP-DAP (3.34 eV) due to the symmetrical downshift (by \(\sim\)0.1 eV) of the HOMO and LUMO energies with respect to that of DPP-DAP.  
The energies (\(\Delta\)E) and oscillator strengths associated with the S\({}_{0}\)\(\rightarrow\) S\({}_{1}\) electronic transition, as well as the quantities characterizing the spatial extent of the photo-induced charge transfer [58, 59] (namely, the amount of charge transferred \(\Delta q\), the charge transfer distance \(\Delta r\), and the dipole moment variation \(\Delta\mu\)) are gathered in Table 1. Consistently with the values of HOMO-LUMO gaps, the transition energies of the A-A' systems evolve in the order: DPP-DPP > DPP-DAP > DPP-BTBPF. As indicated by its largest oscillator strength, DPP-BTBPF also exhibited the highest light-harvesting efficiency, owing to the larger spatial extent of the photo-excitation compared with the two other systems. While the dipole moment variation was equal to zero in DPP-DPP owing to its symmetrical structure, DPP-DAP and DPP-BTBPF exhibited large \(\Delta\mu\) values, which indicate a significant charge transfer from the DPP to the A' moiety in these two systems.  
The HOMO and LUMO energies, as well as the electronic gaps calculated for increasingly large (A-A)\({}_{n}\) oligomers are summarized in Table S3, while the optical properties are  
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**System** & \(\Delta E\) & \(\lambda\) & \(f\) & \(\Delta q\) & \(\Delta r\) & \(\Delta\mu\) \\ \hline DPP & 2.69 & 461 & 0.532 & 0.310 & 0.001 & 0.002 \\ DAP & 2.41 & 516 & 0.506 & 0.736 & 0.003 & 0.006 \\ BTBPF & 2.37 & 524 & 0.856 & 0.436 & 0.003 & 0.006 \\ DPP-DPP & 2.28 & 544 & 1.817 & 0.358 & 0.007 & 0.012 \\ DPP-DAP & 2.12 & 589 & 1.677 & 0.410 & 2.844 & 5.604 \\ DPP-BTPBF & 2.03 & 610 & 1.860 & 0.481 & 3.320 & 7.673 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Characterization of the S\({}_{0}\)\(\rightarrow\)S\({}_{1}\) electron transition in DPP, DAP, and BTBPF monomers, as well as in DPP-DPP, DPP-DAP, and DPP-BTBPF A-A’ units: transition energies (\(\Delta E\), eV), wavelengths (\(\lambda\), nm), oscillator strengths (\(f\), dimensionless), amount of charge transferred (\(\,\mathrm{|\,e\,|}\)), charge transfer distances (Å), and change in dipole moments (D).  
Figure 2: Energy levels (with values in eV) and shapes of frontier MOs of A-A’ dimers with A = DPP and A’ = DPP, DAP and BTPBF.  
gathered in Tables S4 and S5. To no surprise, as the conjugated oligomers increased in size, the electronic gap lowered as a result of the continuous destabilization of the HOMO and stabilization of the LUMO. Consistently, the optical gap decreased when increasing the conjugated chain length, while the oscillator strength increased. The evolution of the transition energies \(\Delta E\) as a function of 1/N (with N the number of double bonds along the conjugated path) is shown in Figure 3. The transition energies extrapolated using Equation (1) are 1.82 eV for A\({}^{\prime}\) = DPP, and 1.62 eV for DAP and BTPBF.