Recently, Nature Communications, an international leading journal published the latest findings by Prof. Li Zhen’s research group from the School of Chemistry and Molecular Sciences, Wuhan University. This research not only delves deeper in the internal mechanism of purely organic room temperature phosphorescence (in short of RTP) luminogens, but also realizes the practical application of such materials.

The thesis is titled The influence of the molecular packing on the room temperature phosphorescence of purely organic luminogens, with Wuhan University being its first signature institution and Yang Jie, a doctoral candidate its first author. Research groups led by Pu Kanyi, associate professor of Nanyang Technological University (NTU), Li Jianrong, professor   of Beijing University of Technology, and Peng Qian, associate researcher of Institute of Chemistry Chinese Academy of Sciences, also participated in this research. They respectively conducted collaborative researches on bioimaging, crystal analysis and theory calculations.

Phosphorescence is a slow luminescence phenomenon of photoluminescence (PL), which can persist after the phosphorescence excitation stops. At present, noble metals contained in most RTP systems lead to a series of problems, including high cost and potential toxicity, etc. However, the pure organic RTP molecules were seldom reported, especially those phosphorescence compounds with ultra-long lifetimes. It is due to the inadequate understanding towards the mechanism of pure organic RTP. Therefore, a reasonable theoretical guidance has not yet been in place for instructing the design of such kind of molecules.

In this research, a series of phenothiazine derivatives were designed and synthesized by Li Zhen’s team. Their RTP properties were adjusted by changing the kinds of substituent groups, achieving significant growth in phosphorescence lifetimes from 88 ms to 410 ms. Through careful crystal analysis and theory calculations, they found that increasing the electron withdrawing ability of substituents enhances the intermolecular π-π stacking interaction of crystals, which could stabilize the excited triplet state accompanying with the ultra long RTP effect. More interestingly, when trifluoromethyl group serves as the substituent group, the unusual photo-induced RTP was observed for the first time, which has never been reported before. Finally, compounds CS-F, with its characteristic of ultralong phosphorescence lifetime, could be utilized for the applications in vivo phosphorescent imaging. Thus, Li’s study paves the way for the vast development of persistent RTP materials, in both the inherent mechanism and the practical applications.

This paper put forward for the first time that Molecular Uniting Set Identified Characteristic (MUSIC) is a common phenomenon, which can vividly indicate the intermolecular condensed state interaction, just like melodies merging into a symphony.

(Rewritten by Ge Chao, edited by Zheng Lingling, Shen Yuxi, Liu Jiachen and Li Xiaoshu )