偶氮苯在各種不同凝態環境下的光致順反異構化動力學研究

Abstract

在這篇論文裡，我們介紹利用飛秒螢光上轉移方法研究偶氮苯在激發態的的光致順反異構化動力行為。研究主題包括： (i)偶氮苯在不同有機溶劑中的S1(n,p*)激發態螢光動力學行為； (ii)偶氮苯在低和高的黏性溶劑裡的螢光非等向性動力學；(iii)經由修改化學結構和 (iv)透過偶氮苯被嵌入到環糊精的環境限制，觀察旋轉禁制衍生物(或錯合物)的光致順反異構化動力行為。比較相關的實驗結果和理論，我們做出以下的結論︰偶氮苯在低黏滯性環境中，S1(n,p*)和S2(p,p*)激發態的光致順反異構化是由旋轉機制所主導。其激發態的緩解可以被描述成雙指數衰減的過程，兩個時間常數約150~350 fs 和2~4ps，其值會隨著偵測螢光波長位置增加而增加。我們也發現當旋轉的路徑在高黏滯性環境中，或化學結構的修飾下限制住時，其他路徑(例如雙反轉機制)將會開啟而有效地達成S1→S0內轉換的能量緩解過程。

In this thesis, we study the photoisomerization dynamics of azobenzene in theexcited state using femtosecond fluorescence up-conversion method. The main topics include, (i) the dynamic behavior of S1(n,p*) excited state of azobenzene in different organic solvents; (ii) the fluorescence anisotropy of azobenzene measured in the low and high viscosity solvents; and the relaxation of excited azobenzene derivatives with the rotation pathway restrained (iii) by modified chemical structure and (iv) by confined environment which azobenzene is embedded into cyclodextrins. By comparing with previous ultrafast experiment results and theoretical calculations, we made the following conclusions: the hotoisomerization for both of S1(n,p*) or S2(p,p*) excitation of azobenzene can take place via rotation mechanism in low viscosity solvent. The fluorescence temporal profiles can be described as bi-exponential decay in subpico- and pico-second time scale. We also found the concerted inversion mechanism can occur when rotational pathway is blocked by high viscosity solvent, or by restricted chemical structure, or by environment confined.