Identifying the "true" repeat unit of a copolymer using time-resolved electron paramagnetic resonance spectroscopy: a case study involving PNDIT2, NDI-T2 and T-NDI-T

Semiconducting polymers promise to revolutionise the way electronic devices can be built and deployed for a vast array of applications ranging from light-energy conversion to sensors to thermoelectric generators. Conjugated push-pull copolymers consisting of alternating donor and acceptor moieties are at the heart of these applications, due to the large tunability of their electronic structure. Hence, knowing the repeat unit and thus the chromophore of these materials is essential for a detailed understanding of the structure--function relationship of conjugated polymers used in organic electronics applications. Therefore, spectroscopic tools providing the necessary molecular resolution that allows to discriminate between different building blocks and to decide which one actually resembles the electronic structure of the polymer are of utmost importance. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy is both, perfectly suited for this task and clearly superior to optical spectroscopy, particularly when supported by quantum-chemical calculations. This is due to its molecular resolution and unique capability of using light-induced triplet states to probe the electronic structure as well as the impact of the local environment. Here, we demonstrate the power of this approach for the polymer PNDIT2 (poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)}) revealing NDI-T2 unambiguously as the "true" repeat unit of the polymer, representing the chromophore. The alternative building block T-NDI-T has a markedly different electronic structure. These results are of high importance for the rational design of conjugated polymers for organic electronics applications.