Ground-state Proton Transfer Dynamics of HPTS in AOT Reverse Micelles as Studied by Nanosecond Time-Resolved Infrared Spectroscopy

• Main Author: 王辰文
• Other Authors: 重藤 真介
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/94548666759372290853

碩士 === 國立交通大學 === 應用化學系分子科學碩博士班 === 100 === 1-hydroxy-3,6,8-pyrenetrisulphonic acid (commonly referred to as HPTS) is one of the most extensively studied fluorescent molecules. HPTS occurs as two forms in aqueous solution. One is the acidic form, whose absorption maximum is located at ~400 nm, and the other is the basic form, which has the absorption maximum at ~450 nm. The pKa of HPTS in the electronic ground state is about 7.2, whereas that in the first excited state drastically decreases to 0.5. Being motivated by this phototriggered change in acidity, a number of studies have been done to understand the proton transfer dynamics of HPTS in the excited state. However, most of those studies used electronic absorption and/or emission spectroscopies. Vibrational spectroscopic studies, which provide more direct information on the structure of the two forms of HPTS, are still scant. Here, we apply nanosecond time-resolved infrared (TRIR) spectroscopy to investigate the proton transfer dynamics of HPTS in the ground state. To be able to perform TRIR measurements in aqueous solution, we utilize the nanoscale water pool of reverse micelles formed by bis(2-ethylhexyl) sulfosuccinate (known as AOT), in which HPTS is dissolved. The use of reverse micelles allows us not only to effectively decrease the immense IR absorption of water but also to control the microscopic environment of HPTS from confined water to bulk-like water by varying the water-to-surfactant ratio, W0 = [H2O]/[AOT]. Transient IR spectra of HPTS in AOT reverse micelles excited at 355 nm are recorded in the sub-?酲 to ?酨 time regime. The average number of HPTS molecules in a single reverse micelle is adjusted to 0.8 so that no more than one HPTS molecule is included on average. It is shown that the observed dynamics is attributed to the reprotonation process of the basic form of HPTS in the ground state. We measure the decay profiles of the transient species with varying the W0 value from 7 to 25, which are fit to an exponential function. The exponential time constant,?n??z so determined changes drastically depending on W0: that is, ??n= 0.2 ?酨 at W0 = 7, whereas ? = 2.4 ?酨 at W0 = 25. In other words, the reprotonation rate of HPTS increases as the water pool size decreases. This result is in qualitative agreement with a diffusion model of the back proton-transfer of HPTS in the ground state. We also find that the decay profile depends on the average number of HPTS molecules present in the water pool of AOT reverse micelles.