Li Wenbo, Li Zhaoning, Si Changfeng, Wong Michael Y, Jinnai Kazuya, Gupta Abhishek Kumar, Kabe Ryota, Adachi Chihaya, Huang Wei, Zysman-Colman Eli, Samuel Ifor D W
Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, UK.
Organic Semiconductor Centre, EaStCHEM, School of Chemistry, University of St Andrews, St Andrews, Fife, KY16 9ST, UK.
Adv Mater. 2020 Nov;32(45):e2003911. doi: 10.1002/adma.202003911. Epub 2020 Oct 7.
Organic long-persistent luminescence (OLPL) is one of the most promising methods for long-lived-emission applications. However, present room-temperature OLPL emitters are mainly based on a bimolecular exciplex system which usually needs an expensive small molecule such as 2,8-bis(diphenyl-phosphoryl)dibenzo[b,d]thiophene (PPT) as the acceptor. In this study, a new thermally activated delayed fluorescence (TADF) compound, 3-(4-(9H-carbazol-9-yl)phenyl)acenaphtho[1,2-b]pyrazine-8,9-dicarbonitrile (CzPhAP), is designed, which also shows OLPL in many well-known hosts such as PPT, 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), and poly(methyl methacrylate) (PMMA), without any exciplex formation, and its OLPL duration reaches more than 1 h at room temperature. Combining the low cost of PMMA manufacture and flexible designs of TADF molecules, pure organic, large-scale, color tunable, and low-cost room-temperature OLPL applications become possible. Moreover, it is found that the onset of the 77 K afterglow spectra from a TADF-emitter-doped film is not necessarily reliable for determining the lowest triplet state energy level. This is because in some TADF-emitter-doped films, optical excitation can generate charges (electron and holes) that can later recombine to form singlet excitons during the phosphorescence spectrum measurement. The spectrum taken in the phosphorescence time window at low temperature may consequently consist of both singlet and triplet emission.
有机长余辉发光（OLPL）是最具前景的长寿命发光应用方法之一。然而，目前的室温OLPL发光体主要基于双分子激基复合物体系，该体系通常需要昂贵的小分子，如2,8-双（二苯基磷酰基）二苯并[b,d]噻吩（PPT）作为受体。在本研究中，设计了一种新型热激活延迟荧光（TADF）化合物3-（4-（9H-咔唑-9-基）苯基）苊并[1,2-b]吡嗪-8,9-二腈（CzPhAP），它在许多知名主体中也表现出OLPL，如PPT、2,2',2″-（1,3,5-苯三嗪基）-三（1-苯基-1-H-苯并咪唑）（TPBi）和聚甲基丙烯酸甲酯（PMMA），且不形成任何激基复合物，其OLPL持续时间在室温下达到1小时以上。结合PMMA制造的低成本和TADF分子的灵活设计，纯有机、大规模、颜色可调且低成本的室温OLPL应用成为可能。此外，发现对于掺杂TADF发光体的薄膜，77K余辉光谱的起始点不一定能可靠地确定最低三重态能级。这是因为在一些掺杂TADF发光体的薄膜中，光激发会产生电荷（电子和空穴），这些电荷随后在磷光光谱测量期间可以重新组合形成单重态激子。因此，在低温磷光时间窗口拍摄的光谱可能同时包含单重态和三重态发射。
