Design and mechanism of thermally activated delayed fluorescent (TADF) and efficient room temperature phosphorescent (RTP) molecules
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Abstract
In recent years pure organic photoluminscent materials have been shown to possess great potential for applications in the fields of sensors, optoelectronic devices, bio-imaging, data encryption, etc.. Especially thermally activated delayed fluorescent (TADF) and room temperature phosphorescent (RTP) materials received extensive attention from researchers around the world. Currently, organic electroluminescent devices based on TADF materials have achieved some success in small-size displays, but blue materials still suffer from device transport imbalance and low device lifetime. For RTP materials, the triplet state excitons are very susceptible to the influence of the external environment. Efficient and stable room temperature phosphorescent materials that are purely organic have been a challenge. The main studies of this thesis is divided into 3 parts which are summarized as follows:
Firstly, TADF and RTP materials need a fast intersystem crossing (ISC) process and a small singlet triplet splitting (ΔEST). Model emitters were designed and synthesised with simultaneous fluorescence, delayed fluorescence, and room temperature phosphorescence. It was proven experimentally and theoretically that multiple excited states are involved in the luminescence process. By adjusting the molecular structure, emitters with RTP quantum yield of more than 30% were obtained. Secondly, we designed and studied molecules for achieving phosphorescence emission in neat films. By changing the intramolecular steric hindrance, the phosphorescence radiation lifetime can be improved, also enhancing the quantum yield of phosphorescence emission in neat films. Our experimental data show that lifetimes up to 40 ms and phosphorescence quantum yields of 6.3% are achievable in neat films. All molecules have a lifetime of over 30 ms under ambient conditions. Thirdly, the introduction of fluorine atoms at the donor lowers the HOMO and LUMO energies but also affects the packing pattern of the molecules. Molecules without fluorine atoms pack more tightly compared to molecules with fluorine atoms. The molecules with tighter molecular packing achieved nearly trap-free electron transport in blue emitting molecules. We have gained a deep understanding of how to design trap-free blue luminescent materials.