Department of Chemistry, University College London , 20 Gordon Street., London WC1H 0AJ, U.K.
Cavendish Laboratory, University of Cambridge , JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
J Am Chem Soc. 2017 Aug 16;139(32):11073-11080. doi: 10.1021/jacs.7b03327. Epub 2017 Jun 23.
The presence of energetically low-lying triplet states is a hallmark of organic semiconductors. Even though they present a wealth of interesting photophysical properties, these optically dark states significantly limit optoelectronic device performance. Recent advances in emissive charge-transfer molecules have pioneered routes to reduce the energy gap between triplets and "bright" singlets, allowing thermal population exchange between them and eliminating a significant loss channel in devices. In conjugated polymers, this gap has proved resistant to modification. Here, we introduce a general approach to reduce the singlet-triplet energy gap in fully conjugated polymers, using a donor-orthogonal acceptor motif to spatially separate electron and hole wave functions. This new generation of conjugated polymers allows for a greatly reduced exchange energy, enhancing triplet formation and enabling thermally activated delayed fluorescence. We find that the mechanisms of both processes are driven by excited-state mixing between π-π*and charge-transfer states, affording new insight into reverse intersystem crossing.
三重态的低能态的存在是有机半导体的一个标志。尽管它们具有丰富的有趣的光物理性质，但这些光学暗态显著限制了光电设备的性能。最近在发射电荷转移分子方面的进展开创了减少三重态和“明亮”单重态之间能隙的途径，允许它们之间的热种群交换，并消除设备中的一个重要损耗通道。在共轭聚合物中，这种间隙被证明难以修饰。在这里，我们引入了一种通用的方法，使用给体-正交受体模体来空间分离电子和空穴波函数，从而在完全共轭聚合物中降低单重态-三重态能量间隙。这新一代的共轭聚合物允许大大降低交换能，增强三重态的形成，并实现热激活延迟荧光。我们发现，这两个过程的机制都是由π-π*和电荷转移态之间的激发态混合驱动的，这为反向系间窜跃提供了新的见解。
