Conjugation-Induced Thermally Activated Delayed Fluorescence: Photophysics of a Carbazole-Benzophenone Monomer-to-Tetramer Molecular Series,The Journal of Physical Chemistry A

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Conjugation-Induced Thermally Activated Delayed Fluorescence: Photophysics of a Carbazole-Benzophenone Monomer-to-Tetramer Molecular Series
The Journal of Physical Chemistry A ( IF 2.7 ) Pub Date : 2021-02-08 , DOI: 10.1021/acs.jpca.0c08977
Qiang Wei _{1,

2} , Paulius Imbrasas ₃ , Enrique Caldera-Cruz ₄ , Liang Cao _{1,

2} , Nanan Fei _{1,

2} , Heidi Thomas ₃ , Reinhard Scholz _{3,

4} , Simone Lenk ₃ , Brigitte Voit _{4,

5} , Sebastian Reineke ₃ , Ziyi Ge _{1,

2}

Affiliation

Materials exhibiting thermally activated delayed fluorescence (TADF) have been extensively explored in the last decade. These emitters have great potential of being used in organic light-emitting diodes because they allow for high quantum efficiencies by utilizing triplet states via reverse intersystem crossing. In small molecules, this is done by spatially separating the highest occupied molecular orbital from the lowest unoccupied molecular orbital, forming an intramolecular charge-transfer (iCT) state and leading to a small energy difference between lowest excited singlet and triplet states (ΔE_ST). However, in polymer emitters, this is harder to achieve, and typical strategies usually include adding known TADF units as sidechains onto a polymer backbone. In a previous work, we proposed an alternative way to achieve a TADF polymer by repeating a non-TADF unit, polymerizing it via electron-donating carbazole moieties. The extended conjugation on the backbone reduced the ΔE_ST and allowed for an efficient TADF polymer. In this work, we present a more in-depth study of the shift from a non-TADF monomer to TADF oligomers. The monomer shows non-TADF emission, and we find the delayed emission to be of triplet–triplet annihilation origin. An iCT state is formed already in the dimer, leading to a much more efficient TADF emission. This is confirmed by an almost two-fold increase of photoluminescence quantum yield, a decrease in the delayed luminescence lifetime, and the respective spectral lineshapes of the molecules.

中文翻译：

共轭诱导的热活化延迟荧光：咔唑-苯甲酮单体对四聚物分子系列的光物理

在过去的十年中，对具有热激活延迟荧光（TADF）的材料进行了广泛的研究。这些发射器具有巨大的潜力，可用于有机发光二极管，因为它们通过反向系统间交叉利用三重态实现了高量子效率。在小分子中，这是通过将最高占据的分子轨道与最低的未占据分子轨道在空间上分开，形成分子内电荷转移（iCT）状态并导致最低激发单重态和三重态之间的能量差小而实现的（ΔE _ST）。但是，在聚合物发射体中，这很难实现，典型的策略通常包括将已知的TADF单元作为侧链添加到聚合物主链上。在先前的工作中，我们提出了一种替代方法，可通过重复一个非TADF单元，通过给电子咔唑基团使其聚合来获得TADF聚合物。主链上的扩展共轭降低了ΔE _ST并允许使用有效的TADF聚合物。在这项工作中，我们对从非TADF单体向TADF低聚物的转变进行了更深入的研究。单体显示出非TADF发射，我们发现延迟发射是三重态-三重态an灭的起源。iCT状态已经在二聚体中形成，导致TADF发射效率更高。这可以通过光致发光量子产率的几乎两倍的增加，延迟的发光寿命的减少以及分子各自的光谱线形来证实。

更新日期：2021-02-18

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