First, we focus on protonation of PCPDT-BT with a single proton at a time with proton positions as indicated in Fig. (marked by red numbers). For simplicity, we refer to the protonated chain with the proton at position 1 and 2 as "Proton-1" and "Proton-2", respectively. Proton-1 features a proton on the left side of the donor unit D5, facing the middle of the polymer. The resulting structure remains nearly planar as in a neutral PCPDT-BT without protonation. As shown in Fig. , it forms a spinsymmetry-broken density of states (DOS) in its ground state in gas-phase. We note that the absolute energy values of the singleparticle molecular orbitals (MOs) significantly depend on the specific method used, however, for the MOs and DOS discussed here we are mostly interested in trends observed and in their relative ordering. The lowest single unoccupied molecular orbital (SUMO) in the β -channel is depicted in Fig. . It features as a bonding π-delocalized character, is situated on the left of the proton position at donor unit D5, and is delocalized from donor unit D3 to donor unit D5. This is accompanied by a high amount of positive charge on the donor units D3, D4, and D5 (a total of 0.88e), as shown in Fig. (blue bars). Moreover, the calculated vibrational spectrum of Proton-1 features a band of IVMs at wavenumbers of about 1100 cm -1 in good agreement with ex- perimental reports on IVMs of PCPDT-BT doped with F 4 TNCQ. These results suggest that the proton on position 1 induces a polaron in an electronic state that is reflected by the β -SUMO. We note that the proton-induced polaron is spatially less delocalized than found for an individual singly charged PCPDT-BT and, consequently, the IVMs are shifted to higher wavenumbers compared to the IVMs of the singly charged PCPDT-BT (cf. Fig. ). We attribute the less delocalized nature of the proton induced polaron to a strong coupling between the proton and the polaron formed. For comparison we also show results where the proton is attached in a different position, in position 2 in Fig. on the right side of donor unit D5 in a qualitatively different position with respect to the adjacent acceptor unit and facing the polymer edge (denoted as structure Proton-2). The electronic structure for this proton position in gas-phase turns out to be spin-symmetric as shown in Fig. with a lowest unoccupied molecular orbital (LUMO) localized on units D5 to A6 near the oligomer edge as illustrated in Fig. . This orbital shows a dominant antibonding character near the proton. Moreover, large amounts of positive charge are found in the spatial range of this LUMO orbital (Fig. ) and, in contrast to Proton-1, the acceptor units are also more charged. Together with the absence of an IVM (Fig. ), we conclude that the protonation in the structure Proton-2 merely induces a slightly modified electronic behavior of a neutral polymer and does not induce a polaron.