Department of Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan.
J Am Chem Soc. 2011 May 4;133(17):6852-60. doi: 10.1021/ja201591a. Epub 2011 Apr 8.
Four isomeric naphthodithiophenes (NDTs) with linear and angular shapes were introduced into the polythiophene semiconductor backbones, and their field-effect transistor performances were characterized. The polymers bearing naphtho[1,2-b:5,6-b']dithiophene (NDT3), an angular-shaped NDT, exhibited the highest mobilities of ∼0.8 cm(2) V(-1) s(-1) among the four NDT-based polymers, which is among the highest reported so far for semiconducting polymers. Interestingly, the trend of the mobility in the NDT-based polymers was contrary to our expectations; the polymers with angular NDTs showed higher mobilities than those with linear NDTs despite the fact that naphtho[2,3-b:6,7-b']dithiophene (NDT1), a linear-shaped NDT, has shown the highest mobility in small-molecule systems. X-ray diffraction studies revealed that angular-NDT-based polymers gave the highly ordered structures with a very close π-stacking distance of 3.6 Å, whereas linear-NDT-based polymers had a very weak or no π-stacking order, which is quite consistent with the trend of the mobility. The nature of such ordering structures can be well understood by considering their molecular shapes. In fact, a linear NDT (NDT1) provides angular backbones and an angular NDT (NDT3) provides a pseudostraight backbone, the latter of which can pack into the highly ordered structure and thus facilitate the charge carrier transport. In addition to the ordering structure, the electronic structures seem to correlate with the carrier transport property. MO calculations, supported by the measurement of ionization potentials, suggested that, while the HOMOs are relatively localized within the NDT cores in the linear-NDT-based polymers, those are apparently delocalized along the backbone in the angular-NDT-based polymers. The latter should promote the efficient HOMO overlaps between the polymer backbones that are the main paths of the charge carrier transport, which also agrees with the trend of the mobility. With these results, we conclude that angular NDTs, in particular NDT3, are promising cores for high-performance semiconducting polymers. We thus propose that both the molecular shapes and the electronic structures are important factors to be considered when designing high performance semiconducting polymers.
四种具有线型和角型结构的并四苯并二噻吩（NDTs）被引入到聚噻吩半导体主链中，并对它们的场效应晶体管性能进行了表征。含有萘并[1,2-b:5,6-b']二噻吩（NDT3）的聚合物具有角型结构，其迁移率高达 0.8 cm(2) V(-1) s(-1)，是目前报道的半导体聚合物中最高的之一。有趣的是，NDT 基聚合物的迁移率趋势与我们的预期相反；尽管萘并[2,3-b:6,7-b']二噻吩（NDT1）具有线性结构，在小分子体系中表现出最高的迁移率，但具有角型结构的 NDT 基聚合物的迁移率却高于具有线型结构的聚合物。X 射线衍射研究表明，角型-NDT 基聚合物具有高度有序的结构，其π-堆积距离非常接近 3.6 Å，而线型-NDT 基聚合物的π-堆积顺序非常弱或不存在，这与迁移率的趋势非常一致。通过考虑它们的分子形状，可以很好地理解这种有序结构的性质。事实上，一个线性的 NDT（NDT1）提供了角型的主链，而一个角型的 NDT（NDT3）提供了一个准直的主链，后者可以堆积成高度有序的结构，从而促进电荷载流子的输运。除了有序结构之外，电子结构似乎与载流子输运性质有关。MO 计算得到的结果，得到了电离势测量的支持，表明虽然在基于线型-NDT 的聚合物中 HOMO 相对局限在 NDT 核内，但在基于角型-NDT 的聚合物中 HOMO 明显沿主链离域。后者应该促进聚合物主链之间有效的 HOMO 重叠，这是电荷载流子输运的主要途径，这也与迁移率的趋势一致。有了这些结果，我们得出结论，角型 NDTs，特别是 NDT3，是高性能半导体聚合物的有前途的核心。因此，我们提出分子形状和电子结构是设计高性能半导体聚合物时需要考虑的重要因素。
