Recently, Xu Zhaochao, a researcher of the Molecular Probe and Fluorescence Imaging Research Group of the Chinese Academy of Sciences Dalian Institute of Chemical Physics, and Liu Xiaogang, a professor of the University of Science and Technology Design of Singapore, have found a method to accurately predict the TICT state of fluorescent dyes.
Twisted intramolecular charge transfer (TICT) is a photophysical process that quenches fluorescence and significantly reduces the photostability of dyes. During this process, the donor or acceptor fragments of the molecule gradually twist to the vertical configuration, causing complete charge separation. Inhibiting the occurrence of TICT can significantly improve fluorescence intensity and photostability, meeting the cutting-edge needs of single molecule detection and ultra-high spatiotemporal dynamic resolution in biology. However, predicting the existence of TICT in different fluorescence systems has always been a challenge.
The research team has deeply understood and explored the molecular luminescence mechanism through a combination of "experiments/theories", and based on a deep understanding of the TICT mechanism in the early stage (J.Am. Chem. Soc., 20161386960-6963; Angew. Chem. Int. Ed.,2019,58,7073-7077), Recently, predictions have been made for the existence of TICT in different fluorescence systems.
The research team summarized 13 different types of S1 potential energy surfaces for fluorescent dyes based on the structural characteristics of TICT. It was found that on the S1 potential energy surface, when the rotational potential barrier (ERB) is high and the driving energy (EDE) is high, molecules tend to maintain a bright state (LE or ICT state), that is, do not form TICT; When ERB is positive and EDE is negative, molecules will partially form TICT; When ERB is zero and EDE is negative, the molecule will form a large number of TICT states, greatly quenching fluorescence. Therefore, the formation of TICT can be determined based on ERB and EDE.
The research team designed PRODAN series dyes and aggregation induced luminescence (AIE) dyes to verify the discrimination law of TICT. By calculating the S1 potential energy surface of PRODAN, it was found that N-TICT occurs in water, while 0-TICT does not occur in water. Comparing the S1 potential energy surfaces of different molecules, it was found that the molecule P4 with the introduction of acridine had the highest ERB (0.38 eV) and the lowest EDE (-0.14 eV), suggesting that it is not easy to form TICT. Experiments have shown that the quantum yield of P4 (0.38) in PBS buffer is approximately twice that of P2 (introducing dimethylamine). The research team confirmed through experiments such as viscosity and ultrafast spectroscopy that the TICT state exists in both P2 and P4, but the quenching fluorescence amplitude is different. Compared to dimethylamine, the introduction of acridine effectively inhibits the formation of TICT. Through a large amount of experimental evidence, the research team has verified the reliability of the computational model and put an end to the 20-year debate over the mechanism of PRODAN - in aqueous solution, PRODAN does indeed form a TICT state, quenching fluorescence (the influence of other quenching mechanisms cannot be ruled out).
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