Organic π-conjugated molecules have attracted increasing attention for their potential applications in the fields of organic field-effect transistors (OFET), organic lasers, stimuli-responses and so on. Apart from the electronic nature of molecular structure, the molecular packing also shows large impact on the optoelectronic property, as functional π-conjugated molecules mainly exist in solid state for practical applications, which could be termed as Molecular Uniting Set Identified Characteristic (MUSIC): the molecules act as the movements, while they pack together and communicate each other, can a beautiful symphony be played.
Recently, organic luminogens with persistent room-temperature phosphorescence (RTP) have attracted great attention for their wide applications and hard accessibility resulting from the unclear mechanism. Li et al. integrated 9-phenyl-9H-Carbazole and Sulfonyldibenzene groups together with O and N atoms to promote n–π* transitions, and synthesized a series of phenothiazine 5,5-dioxide derivatives (CS-CH3O, CS-CH3, CS-H, CS-Br, CS-Cl, and CS-F), with the aim to carefully investigate the structure-property relationship. Excitedly, accompanying with the adjustment of the substituent groups on the 10-phenyl ring from a methoxyl or methyl group to the hydrogen atom, then to the bromine, chlorine or fluorine atoms, the RTP lifetimes in crystals of the corresponding luminogens increase from 88 ms (CS-CH3O) and 96 ms (CS-CH3) to 188 ms (CS-H), then to 268 ms (CS-Br), 256 ms (CS-Cl) and 410 ms (CS-F).
By carefully analyzing their crystal structures, it is found that the packing mode with strong π-π interactions, resulting from the introduced electronic withdrawing substituents, could be considered as the main origin for the formation of the stabilized T1* state, accompanying with the persistent RTP effect, as further confirmed by controlled experiments and theoretical calculations. Then, another substituent, the trifluoromethyl group, with even stronger withdrawing ability was introduced to yield CS-CF3. Although the longer RTP lifetime could not be observed, CS-CF3 crystal was found to possess another novel character of reversible “photo-induced room temperature phosphorescence”, which also should be originated from the reversible changes of molecular packing. Furthermore, CS-F with its long RTP lifetime could be utilized for real-time excitation-free phosphorescent imaging in living mice. Thus, this study paves the way for the development of persistent RTP materials, in both the practical applications and the inherent mechanism.
This work has been published on the Nature Communications with the title “The influence of the molecular packing on the room temperature phosphorescence of purely organic luminogens”. The research team was led by Prof. Zhen Li, in College of Chemistry & Molecular Sciences of Wuhan University, China. Jie Yang is its first author and Prof. Kanyi Pu (Nanyang Technological Universit), Prof. Jianrong Li (Beijing University of Technology), Prof. Qian Peng (Institute of Chemistry, Chinese Academy of Sciences) also made significant contribution.
Paper Link: https://www.nature.com/articles/s41467-018-03236-6.
Homepage of Prof. Zhen Li’s Group: http://ligroup.whu.edu.cn/.
