Conjugated Polymers with an Oxindole-Terminated Quinoidal Unit: Energy Level Modulation Toward Air-Stable n-Doped Conductors

Developing n-doped conjugated polymers (CPs) with good air stability remains a significant obstacle to the advancement of organic thermoelectrics. In this study, three CPs (designated PmQ2F, PsQ4F, and PsQ6F) containing an oxindole-terminated quinoidal unit are synthesized. To obtain an isomeric purity quinoidal unit and tune the LUMO (lowest unoccupied molecular orbital) energies of the CPs, fluoride (F) atoms on the monomers are strategically installed. Compared to a polymer with isomeric quinoidal units (PmQ2F), polymers with an isomerically pure quinoidal unit (PsQ4F and PsQ6F) exhibited higher electron mobilities owing to their more-ordered molecular packing structures. After n-doping, PsQ4F and PsQ6F show higher electrical conductivities than PmQ2F. Moreover, the former demonstrated excellent air stability for over 2 months, which can be attributed to the synergistic effects of the quinoidal structure and the reduced LUMO energy. The work offers a novel molecular design strategy that can be expected to advance the development of n-type CPs for air-stable organic thermoelectrics.