image: One the left, the molecular structure of raloxifene, and the design of stimuli-responsive and RTP-emissive raloxifene analogues based on the raloxifene framework. On the right, the molecular structures of raloxifene analogues, and their crystalline-state optical images taken in the air under daylight at 365 nm-UV light irradiation and after ceasing the UV light for 0.02 s, 0.04 s, and 0.06 s, respectively. Photo credit: Zhichao Pan.
Credit: Photo credit: Zhichao Pan.
This study is dominated by Dr. Ju Mei and Prof. Da-Hui Qu from the School of Chemistry and Molecular Engineering at East China University of Science and Technology. The synthesis, characterization, theoretical calculations, and application exploration of the raloxifene analogues were mainly conducted by Zhichao Pan, a master postgraduate of Dr. Ju Mei.
Smart materials which can rapidly respond to external stimuli possess immense potential for applications in anti-counterfeiting and encryption, data storage, sensors, bioimaging, and so on. However, most stimulus-responsive systems are designed based on controlled fluorescence emission (emission color and intensity). Due to the time-resolved characteristics of phosphorescence emission, materials possessing stimuli-responsive room-temperature phosphorescence (RTP) can also exhibit the change in emission lifetime and hence a response on the temporal dimension. Therefore, stimuli-responsive RTP materials are believed to have greater value in practical applications. Nevertheless, there are still difficulties in developing stimuli-responsive RTP materials especially the ones based on single-component pure organic compounds, because it is complicated to control stimulus-responsiveness and triplet-state emission synchronously.
Mei and Qu worked together in pursuit of novel efficient stimuli-responsive phosphors based on single-component organics. They turned their sight to raloxifene, which is both a phenylthiophene compound and a new concepted non-hormone drug against bone resorption. It also belongs to the second generation of selective estrogen receptor modulator and has hypolipidemic effect as well. Nevertheless, its photophysical properties have seldom been reported. With detailed examination on the structure and photophysical properties of the raloxifene, they carried out elaborate molecular design and successfully realized stimuli-responsive RTP in the molecular crystals of the resultant raloxifene analogues.
The crystals of these developed raloxifene analogues show distinct dual-emission properties with both blue fluorescence and orange phosphorescence. Interestingly, with the substituent on the benzoyl group varying from ‒CH3 to ‒CN, the quantum yield of the orange RTP increases from RALO-CH3 all the way to RALO-CN. Combining the crystallography analysis and theoretical calculations, it is demonstrated that the tight antiparallel molecular packing in the crystal is the crucial point to their RTP behaviors. When the substituents are electron-withdrawing, it is more favorable for the resultant compounds to form close packing, thus achieving higher RTP quantum yields and longer phosphorescence lifetime.
It is worth noting that they have successfully obtained another crystal form of RALO-OAc, namely RALO-OAc*. The RALO-OAc* crystal shows quite different shape and packing mode from the RALO-OAc crystal. RALO-OAc* crystal predominantly exhibits a fluorescent luminescence, with the RTP lifetime and quantum yields lower than those of RALO-OAc. These results further confirm the important influence of packing modes on the room-temperature phosphorescence. Moreover, taking advantage of the polymorphic transition between RALO-OAc and RALO-OAc*, a single-component multilevel stimuli-responsive platform with tunable emission color is constructed, which can respond to mechanical force, solvent vapor, and heat. Utilizing the multi-stimuli-responsiveness of the RALO-OAc crystals, the authors further explore their application potential in the advanced information encryption.
Such a work will help to understand the intrinsic RTP mechanism of organic small-molecular crystals, and to develop smart single-component organic RTP materials as well as to explore efficient RTP emitters based on known drugs. Moreover, in the light of the therapeutic effect of raloxifene, it also lays a certain foundation for researches exploring the use of raloxifene analogues as in-vivo afterglow imaging contrast agents and chemotherapeutic drugs in the future.
See the article:
Tailoring raloxifene into single-component molecular crystals possessing multilevel stimuli-responsive room-temperature phosphorescence
https://doi.org/10.1016/j.scib.2024.02.029
Journal
Science Bulletin