Intrachain Electron Transport in a Naphthalene Diimide–Bithiophene Copolymer: A Mixed-Valence Approach

The mechanism of electron transport in the polymer P(NDI2OD-T2) (poly(N,N′-bis-2-octyldodecylnaphthalene-1,4,5,8-bis-dicarboximide-2,6-diyl-alt-2,2′-bithiophene-5,5′-diyl), N2200) is investigated. We use spectroelectrochemical measurements on P(NDI2OD-T2), spectroscopic studies of a chemically reduced model compound, 2,2′-(2,2′-bithiophene-5,5′-diyl)-bis(N,N′-di-n-hexylnaphthalene-1,8:4,5-bis(dicarboximide)) (NDI-T2-NDI), and electronic structure calculations to evaluate the microscopic charge-transport parameters. Experimental and computational data suggest that NDI-T2-NDI^•– is a class-II mixed-valence compound, strongly supporting the small-polaron hopping model as a charge-transport mechanism for electrons along polymer chains in P(NDI2OD-T2). The electronic coupling between the NDI redox units is at least 21 meV, while the reorganization energy is between 0.45 and 0.56 eV. Using a hopping model, we estimated the mobility and activation energy for electron transport along P(NDI2OD-T2) polymer chains to be 0.15 cm² V^–1 s^–1 and 60 meV, respectively. Our study elucidates a long-standing issue of explaining the coexistence in P(NDI2OD-T2) of localized redox sites with relatively large electron mobilities more usually achieved only in highly conjugated polymers.