The calculated absorption properties of the polymers and the terpolymers are gathered in Table . All the 5 molecules display strong absorption peaks in the visible spectrum. For the terpolymers, λcalc lies between 662 and 818 nm for TP(BACD-T-TII) and TP(BACD-T-DPP) respectively. The first main absorption peak always arises from a S0 →S1 transition and is described by HOMO-LUMO orbitals, except for TP(BACD-T-IDTTB6) with a HOMO-LUMO+1 transition. All these transitions possess a π-π* character and are delocalized through the polymer chain (see Figures ). All HOMO orbitals are centred on the aldehyde comonomer except for P(BACD-T) where the electronic density is delocalized throughout the entire molecule (Figure ). Even if all the polymers are conjugated systems, P(BACD-T) is the most planar one and it can explain the more pronounced delocalization of the electronic density over both the BACD unit and the co-monomers. For LUMO orbitals, the electronic density is mainly centred on the BACD units for the two polymers and the three terpolymers. The main absorption peak always corresponds to a HOMO/LUMO+3 transition with LUMO+3 orbitals being localized either on the entire polymer P(BACD-T), on the comonomer P(BACD-TII) or on some BACD and co-monomer units in the terpolymers). A charge transfer (CT) character can be attributed to each transition, as illustrated through the calculation of the amount of transferred charge (Table ). Due to the symmetry of the polymers, values for the charge transfer distance, dCT, and charge transfer dipole moment, ||𝜇 ⃗CT|| are very small. For terpolymers, the loss of the symmetry leads to an increase of all the CT parameters, with a transfer from either TII, IDTTB6 or DPP towards BACD unit or even BACD-T unit (for TP(BACD-T-TII)). By introducing either electron donor or electron acceptor co-monomers within the BACD-based polymer chain, the absorption behaviour of the polymer can be effectively modified. P(BACD-TII) displays the highest global charge transfer character among the polymers and has the highest f(λcalc)/f(λmax) ratio with a value equal to 0.95. It highlights its ability to absorb effectively across a large area of the visible spectrum. Terpolymers show some attenuation of the absorption process at higher wavelength. Indeed, the f(λcalc)/f(λmax) ratio is equal to 0.69, 0.24, and 0.20 for