Electronic Structure Trumps Planarity: Unexpected Narrow Exciton Delocalization in PNDIT2 Revealed by Time‐Resolved Electron Paramagnetic Resonance (EPR) Spectroscopy

Exciton delocalization in organic semiconductors, due to its direct relation to device efficiency, is of outstanding importance. Time-resolved electron paramagnetic resonance spectroscopy of light-induced triplet excitons gives access to the delocalization length in a unique way, connecting it to both, electronic structure and overall conformational flexibility. Systematically investigating building blocks of increasing length and comparing the results with the polymer deepens the understanding of the structure-function relationship in organic semiconductors. Applying this approach to the n-type polymer poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (PNDIT2) known for its remarkable charge-carrier mobility reveals the triplet exciton in this polymer to be strongly confined to no more than two repeat units. Furthermore, neither planarity nor extent of backbone conjugation seem to be good indicators for the exciton delocalization length.