Facile Synthesis, Macroscopic Separation, E/Z Isomerization, and Distinct AIE properties of Pure Stereoisomers of an Oxetane-Substituted Tetraphenylethene Luminogen

Control of stereochemistry plays a key role in medicinal chemistry, material and life science. As a prominent AIE luminogen, tetraphenylethene (TPE) derivatives have E/Z isomers which are challenging to separate even by HPLC. Herein, we designed oxetane-substituted TPE (TPE-2OXE) and separated pure isomers by simple column chromatography with high yields, as confirmed by mass spectrometry, IR and NMR spectroscopy. The isomerization of the two isomers can occur by photo- and thermo-activation. Importantly, (Z)-TPE-2OXE isomer solid shows bathochromic emission with a quantum yield 5 times higher than that of (E)-TPE-2OXE. The differences in emission wavelength and quantum yield are derived from distinct emission mechanisms of locally excited (LE) state emission of (E)-TPE-2OXE and charge transfer (CT) state emission of (Z)-TPE-2OXE. The two isomers are also good piezochromic luminescent materials, which have not only an obvious emission color shift but also significantly enhanced luminescence brightness by external force. In addition, (E)-TPE-2OXE solids show self-healing ability, which can crystallize spontaneously from ground amorphous state. The higher brightness of (E)-TPE-2OXE can be retained in solution, so fluorescent AIE nanodots are prepared from the two isomers. Cell-labeling experiments also show that (Z)-TPE-2OXE AIE dots have higher labeling brightness as compared to the (E)-TPE-2OXE isomer. The synthesis and distinct properties of E/Z isomers are beneficial to further development of new TPE derivatives for various applications.