推拉性分子DPBMN與DPAMN在不同極性溶劑中的螢光緩解動力學研究

Abstract

本論文主要分為兩大主題。第一主題為探討2-(4(diphenylamino)benzylidene)malononitrile (DPBMN)分子在不同極性溶劑中的電荷轉移現象。我們利用飛秒螢光上轉換技術(Femtosecond fluorescence up-conversion)以及時間相關單光子技術系統（Time-Correlated Single Photon Counting，TCSPC）測量DPBMN在不同極性溶劑中的光譜動力學。實驗結果顯示DPBMN分子在非極性溶劑(正己烷)中的區域性激發態(local excited state，LE state)有兩個非輻射緩解的過程發生，首先為發生在LE能階的振動緩解( ~ 4 ps)，之後為系統內轉換的過程( ~ 220 ps)。但是在極性溶劑中(THF)，我們測量到不同於非極性溶劑中的三個非輻射緩解過程，首先為由LE能階到電荷轉移能階(charge transfer state，CT state)的電荷轉移過程( ~ 120-300 fs)，之後為發生在CT 能階的振動緩解( ~1-3 ps)，最後為CT能階內轉換到S0的過程( ~ 1.2 ns)。然而當我們提高溶劑極性時(例如DMSO)，由於其對CT能階的穩定作用造成LE能階與CT能階之間的能量障礙減小並使得CT能階的能量更接近基態，因此我們觀測到電荷轉移過程與CT能階的內轉換過程被加速( ~ 10-20 ps) 。 第二主題在探討DPBMN的衍生物2-((10-(diphenylamino)anthracen-9-yl) -methylene)malononitrile(DPAMN)分子的光物理動態學。我們利用TCSPC來測量DPAMN在不同極性溶劑中的時間解析光譜，其所觀測到的螢光衰減極為類似，此結果顯示DPAMN中並無CT能階的生成。我們從不同波長激發的steady-state螢光光譜可以發現，當高能量激發時螢光訊號都比低能量激發時高，且從不同激發波長(400 nm與490 nm)雷射激發的時間解析螢光光譜，我們也可以發現當激發較低能量時所測量到的光譜大部分是超快的螢光衰減。因此我們推論DPAMN的螢光主要是由S2能階所產生，且S1為一個不放光的能階，是一個罕見的違反Kasha’s rule 的例子。

This thesis contains two subjects. The first subject is to study the phenomenon of charge transfer of 2-(4(diphenylamino)benzylidene)malononitrile(DPBMN) in the solvents of various polarities. The time-resolved fluorescence spectra of DPBMN in polar and non-polar solvents were measured using the techniques of Femtosecond fluorescence up-conversion and Time-Correlated Single Photon Counting（TCSPC）. The results of DPBMN in n-hexane solution show that there are two non-radiative processes occurring in the local excited state. The decay time constants of ~4 ps and ~ 220 ps are due to vibration relaxation and intersystem crossing processes respectively. However, DPBMN in the polar solvent (THF) the relaxation dynamics were different with those in n-hexane. We observed triphasic fluorescence decay, the femtosecond component is attribured to the charge transfer rate from the LE state to the CT state , the picosecond component is attributed to the vibration relaxation in the CT state and the nanosecond component is attributed to the internal conversion from the CT state to S0¬ state. In DMSO, we observed the faster charge transfer rate and the faster internal conversion in the CT state because both the energy barrier between LE and CT state and the energy between CT and S0 state were decreased by the high polar solvent. For the second subject, we studied the fluorescence relaxation dynamics of the derivatives of DPBMN, 2-((10-(diphenylamino)anthracen-9-yl)-methylene)- Malononitrile (DPAMN) in different solutions. We measured the time-resolved fluorescence spectra with TCSPC. The results show that there is no CT state in DPAMN because similar time-resolved spectras of DPAMN in the polar and non-polar solvents were observed. We found the S2 fluorescence in the steady-state fluorescence specta in different excited wavelengths. We suggest that DPAMN is an example of anti-Kasha’s rule.