Probing the molecular weight dependent intramolecular interactions in single molecules of PCDTBT

Abstract

Push–pull conjugated polymers have shown high performance in organic photovoltaic devices and yet, there have been fewer studies of the fundamental structure–property relationship in this class of materials compared to prototypical conjugated homopolymers MEH-PPV and P3HT. Here we report a single molecule study of a push–pull polymer poly[N-(1-octylnonyl)-2,7-carbazole]-alt-5,5-[4′,7′-di(thien-2-yl)-2′,1′,3′-benzothiadiazole] (PCDTBT) to understand the origin of the substantial band gap narrowing and the sensitivity of peak position to chain length even for long polymer chains. In bulk solution, the peak of the emission spectrum was observed to shift from a higher energy emission state to a lower energy state above a critical molecular weight. This same trend was observed at the single molecule level where there were two clear emission species. At low MW the single molecules showed emission predominately from the higher energy state. Above the critical molecular weight, the single molecules changed and showed emission from the low energy species. At the transition MW, a mixture was observed with molecules emitting from one or the other of the two species. The transition from the high energy to low energy state at a particular molecular weight is likely the result of substantial intermolecular self-interactions between chain segments that leads to excitation delocalization. This is made possible at even higher molecular weight and lengths where there are more energy transfer pathways for the already long polymer chains.