Radical Anion Yield, Stability, and Electrical Conductivity of Naphthalene Diimide Copolymers n-Doped with Tertiary Amines

Doped organic semiconductors are required for applications such as organic solar cells, organic light-emitting diodes, and thermoelectric generators. To further establish structure-property relationships and improve the efficiency of these devices, electron-acceptor conjugated polymers and suitable doping schemes are required. A key criterion is a sufficiently low lowest unoccupied molecular orbital (LUMO), which enables air stability of excess electrons. In this work, a series of naphthalene diimide (NDI) copolymers with varying highest occupied molecular orbital (HOMO) and LUMO energy levels are made and used to investigate photochemically and thermally induced electron transfer from a small molecule NDI carrying dimethylaminopropyl (DMAP) side chains. Density functional theory calculations and UV-vis and electron spin resonance (ESR) spectroscopies indicate that the LUMO energy level of the NDI copolymer governs thermal electron transfer from the HOMO of the DMAP side chain and dictates air stability of the corresponding radical anions. Conversely, photoinduced electron transfer from DMAP to the NDI copolymer is governed by the position of the HOMO energy levels. Although the dicyano-substituted NDI copolymers with very low LUMO levels display the highest radical anion yield and excellent air stability, their conductivity is limited by electron mobility, which in turn is strongly influenced by backbone torsion and localized radical anions. These results establish fundamental structure-function relationships and shine light on the use of simple, cost-effective, covalently bound tertiary amines as potential n-dopants for electron-acceptor copolymers.