Addressing the Elusive Polaronic Nature of Multiple Redox States in a π-Conjugated Ladder-Type Polymer

Poly(benzimidazole–benzophenanthroline) (BBL) is a ladder-type conjugated polymer showing remarkable charge transport properties. Upon doping it displays various conductive regimes, leading to two insulator-to-conductor transitions. Such transitions are never fully characterized, limiting understanding of its charged states. Open issues are: i) the electron/hole polaron relaxations, ii) the structure–function relationships of multiple redox states and their connection with the conductive regimes, and iii) the role of protonation. Such knowledge-gaps are tackled via a comprehensive computational investigation of multiple redox species. Polarons show polyradicaloid character, as revealed by combining broken-symmetry density functional theory, fragment orbital density, and multireference analysis. Electron/hole polaron relaxations occur on the polymer chain, the former localizing on the benzophenanthroline moieties, the latter on the benzimidazole units. Modeling of multiple charged species, up to one electron per repeat unit (1 eru), reveals a complex scenario of quasidegenerate states each featuring different spin multiplicity. Four redox states are responsible for the BBL insulator-to-conductor transitions. The two high conductive states refer to the electron polaron (0.25 eru) and the redox species with 0.75 eru. The insulating regimes refer to the bipolaron (0.50 eru) and the redox state with 1 eru. Protonation is modeled, revealing polaron-like features in the spectroscopic properties.