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.

中文翻译：

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探索PCDTBT单分子中与分子量有关的分子内相互作用

推挽共轭聚合物在有机光伏器件中已显示出高性能，但与原型共轭均聚物MEH-PPV和P3HT相比，此类材料的基本结构与性质关系的研究较少。此处我们报告一个推挽聚合物聚单个分子研究[ Ñ - （1-辛基壬）-2,7-咔唑] -中高音-5,5- [4'，7'-二（噻吩-2-基）-2'，1'，3'-苯并噻二唑]（PCDTBT）了解基本的带隙变窄的起源和峰的敏感性即使对于长的聚合物链，也可以保持链长。在本体溶液中，观察到发射光谱的峰从高于临界分子量的较高能量的发射状态转变为较低能量的状态。在存在两个清晰的发射物种的单分子水平上也观察到了相同的趋势。在低MW下，单分子主要显示出来自较高能态的发射。高于临界分子量，单分子发生变化并显示出来自低能物质的发射。在转变MW处，观察到混合物与从两种物质中的一种或另一种发出的分子。在特定分子量下从高能状态到低能状态的转变可能是链段之间大量分子间自相互作用的结果，这导致激发离域。这甚至可以在更高的分子量和更长的长度上实现，其中对于已经很长的聚合物链来说，存在更多的能量转移途径。