Optical Trapping of Aggregation-Induced Emission Molecules for Crystallization at the Air/Solution Surface

This study investigates the optical trapping-induced crystallization (OTIC) of aggregation-induced emission (AIE) molecules 4,4'-((1E,1'E)-anthracene-9,10-diylbis(ethene-2,1-diyl))bis(1-methylpyridin-1-ium) iodide (AEMPI) at the air-solution interface through luminescence. We synthesized the AIE molecule AEMPI, which exhibits an emission wavelength of 640 nm in powder form at room temperature. At the beginning of focusing a 1064 nm continuous-wave (CW) laser at the interface of an unsaturated AEMPI D₂O solution, its luminescence is hardly detectable. As the laser irradiation time increases, we observe nonemissive aggregates formed by liquid-like clusters that subsequently transform into crystals and exhibit bright red luminescence. This is due to the ordered molecular structure formed after crystallization effectively restricting the motion of the anthracene core and the adjacent vinyl groups, thereby activating the AIE properties. The crystal plane area and luminescence intensity steadily increased under constant laser power with a good positive correlation between them. We leveraged the unique advantages of AIE materials and the interface to develop a method for studying OTIC based on <i>in situ</i> luminescence signals, effectively addressing the challenge of nucleation detection. We believe that with further research, <i>in situ</i> optical signal detection will become an important means for studying the OTIC.