利用奈秒時間解析光譜研究HPTS在AOT反微胞的質子轉移基態動力學研究

Abstract

1-hydroxy-3,6,8-pyrenetrisulphonic acid (常見名為HPTS) 為其中一種被廣泛研究的螢光分子。在水溶液中，HPTS存在兩種形式，一種為酸式，其吸收光譜最大峰值出現在400nm，另外一種形式則為鹼式，吸收光譜最大峰值出現在450nm。在電子基態下， HPTS的pKa值約為7.2，然而在第一激發態下急遽下降到了0.5。受到光誘發而改變酸性這一特性引起了科學家的興趣，有一系列的研究已完成而了解HPTS位於激發態的質子轉移動力學。然而，那些大部分的研究利用的是電子吸收/放射光譜。能用來提供HPTS兩種形式結構直接訊息的震動光譜術至今仍舊匱乏。 這裡我們利用了奈秒時間解析紅外光譜(TRIR)來觀察HPTS位於基態的質子轉移動力學。為了能在水溶液中進行TRIR的測量，我們利用了具有奈秒尺度的 ”水池”，其位於丁二酸二辛酯磺酸鈉(通稱AOT)形成的反微胞中，HPTS亦溶於 ”水池” 內部。使用反微胞讓我們不只可以有效降低水大量的IR吸收而且能藉由改變水-介面活性劑的比例， W0 = [H2O]/[AOT]，來控制HPTS位於從侷限水到類散水的微觀環境，我們利用次微秒至微秒的時間區段來記錄受到355nm激發，位於AOT反微胞的HPTS之瞬態IR光譜。HPTS位於單一反微胞的平均分子數調整至0.8，這樣可以使平均不超過一個HPTS分子包含在單一反微胞中。由結果顯示位於基態下觀察到的動力學反應歸因於鹼式HPTS的再質子化途徑。我們量測了在從7改變到25的瞬態物質衰變圖譜，並利用自然對數方程式來擬合。自然對數的時間常數□□會依據W0的變化而有急遽的改變，也就是在W0 = 7時□□= 0.2 s，然而W0 = 25時□□ = 2.4 s。換句話說, HPTS的再質子化結合速率會因為 ”水池” 的大小減少而增加。這樣的結果定性而言和HPTS於基態下反質子轉移的擴散模型一致。我們也發現存在於AOT反微胞內部 ”水池” 之HPTS分子的平均數目和衰變圖譜有關。

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-□s to □s 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,□□□ so determined changes drastically depending on W0: that is, □□= 0.2 □s at W0 = 7, whereas □ = 2.4 □s 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.