Fluorescence microspectroscopic study on laser trapping-controlled aggregation and polymorphism

• Main Authors: Lin, Jhao-Rong, 林昭容
• Other Authors: 杉山輝樹
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/xcbjgm

碩士 === 國立交通大學 === 應用化學系碩博士班 === 106 === We here present laser trapping-controlled aggregation and crystallization of a tertiary ammonium-appended tetraphenylethene (TA-TPE) showing aggregation-induced emission enhancement (AIEE). After focusing a 1064 nm-laser beam at an air/solution interface of TA-TPE solution, several sub μm-sized aggregates showing no fluorescence emission were gathered at the laser focus. After the stable trapping of the aggregate at 700 mW for about 5 min, it suddenly grew large to 2-3 μm, accompanying with strong yellow fluorescence emission. We imply that laser trapping overcomes the inherent attractive and repulsive interactions among TA-TPE molecules in the aggregate, and the resultant molecular arrangement are suitable for fluorescence emission. Next, when the laser power was decreased from 700 to 5 mW immediately after confirming the emission from the aggregate, the emission was no longer being observed with the small change in size. The emission intensity was recovered by increasing the laser power to 700 mW again, which was observable in a repetitive manner. Thus, we controlled molecular arrangement in aggregate using laser trapping, and succeeded in the switching-on/off of the fluorescence emission arbitrarily by tuning laser power. We also found that continuous laser trapping into the yellow fluorescent aggregate led to crystallization. There were observed two kinds of crystals with green and yellow-green emission, that is, polymorphism was realized by laser trapping. It is noteworthy that both polymorphs can be induced under unsaturation. Indeed, the generated crystals were dissolved into solution by stopping laser trapping. Moreover, both crystals were generated at the edge of the aggregate, not at the laser focus. These two phenomena are quite general in experiments on laser trapping-induced crystallization, as we have so far observed them for various compounds. Therefore, we consider that molecular arrangement in aggregate generated around laser focus is gradually disordered against the distance from laser focus, which make the molecules aligned spontaneously and finally triggers crystal formation. The findings in this work give us a novel insight into the dynamic and mechanism of laser trapping-controlled crystallization and polymorphism.