China University of Science and Technology has made progress in the study of the regulatory mechanism of thermally induced delayed fluorescence

Recently, Professor Zhou Meng's research group at the University of Science and Technology of China collaborated with Professor Fu Hongbing's team at the Capital Normal University to reveal the mechanism by which aggregation effects regulate the luminescent properties of thermally delayed fluorescent materials. The research findings, titled "Aggregation Enhanced Thermally Activated Delayed Fluoroscopy through Spin Orbit Coupling Regulation," were published in the German Journal of Applied Chemistry and selected as a hot topic article.

Integrating aggregation induced emission (AIE) effects into thermally delayed fluorescence (TADF) luminescent materials can provide enormous potential for the development of efficient organic light-emitting diodes (OLEDs). Although some progress has been made in the synthesis and fabrication of such materials and devices, there is still a lack of understanding of the corresponding theoretical mechanisms. In this work, the research team aims to regulate TADF by controlling the dynamic process of excited states through aggregation effects.

Research has found that aggregation not only enhances both immediate and delayed fluorescence, but also exerts binding effects on the conformational changes of excited states of molecules. This confinement not only enhances spin orbit coupling (SOC), but also reduces the energy difference (DEST) between singlet and triplet states. This work reveals the understanding of the basic mechanism of aggregation effect regulating TADF, providing guidance for the design of efficient photoluminescence materials.

The research team first analyzed the aggregation effect of the target material DCzBF2 on the regulation of TADF under N2 and O2 atmospheres. Research has found that both in N2 and O2 atmospheres, DCzBF2 exhibits a significant aggregation enhancing luminescence effect. Meanwhile, it was found that the relative ratio of immediate fluorescence and delayed fluorescence of DCzBF2 remained unchanged with the enhancement of aggregation effect in N2 atmosphere.

Using ultrafast spectroscopy research, it was found that the excited state conformational changes of molecules after aggregation were significantly suppressed. However, the ultrafast spectrum did not capture the TADF process in the liquid phase, but it did capture the corresponding process in the membrane phase. Quantitative calculations reveal that this is due to the suppression of the conformational rotation of molecules in the membrane phase, which enhances the SOC between singlet and triplet states involved in inter system crossing (ISC) processes and reduces the corresponding DEST, resulting in a strong triplet signal. Finally, the author studied the influence of different aggregation levels on the excited state relaxation process. The study found that an enhanced aggregation effect would slow down the excited state relaxation process, and there was also an excited state conformational change process at low aggregation levels, while at high aggregation levels, the excited state conformational change was completely suppressed.

This study demonstrates the feasibility of integrating the AIE effect in TADF materials and reveals the corresponding working mechanism. Research has found that with the enhancement of aggregation effect, immediate fluorescence and delayed fluorescence gradually increase, but aggregation effect does not change the ratio between singlet radiation rate and ISC rate. In addition, ultrafast spectroscopy and theoretical calculations in solutions and thin films further reveal that enhancing SOC and reducing DEST are the essential reasons for aggregation enhanced TADF.

Zhang Weite, Associate Researcher at the University of Science and Technology of China, is the first author of the paper; Professor Zhou Meng from the University of Science and Technology of China, Associate Researcher Kong Jie, and Professor Fu Hongbing from the Capital Normal University are the corresponding authors of this paper. This work has been supported by the Chinese Academy of Sciences and the National Natural Science Foundation of China.

Source: University of Science and Technology of China