氣相超快光游離誘發雙官能基陽離子內之電荷轉移動態學研究

Gas-Phase Ultrafast Photoionization Induced Charge-Transfer Dynamic in Bifunctional Cations

宋桓宇(Huan-Yu Sung)

指導教授 ： 鄭博元

國立清華大學/理學院/化學系所/碩士(2019年)

摘要

本論文中，我們利用飛秒泵浦-探測光游離-光裂解(Femtosecond Pump-Probe Photoionization-Photofragmentation)光譜術結合質譜偵測技術研究一系列三碳橋官能基系統: N,N-dimethyl-3-phenylpropan-1-amine (DMPPA)、N-Methyl-N-(3-phenylpropyl)amine (MPPA)及3-phenylpropan-1-amine (PPA)陽離子的電荷轉移反應動態學並和實驗室前人所做之兩碳鏈橋系列分子: 2-phenylethyl-N,N-dimethylamine (PENNA)、N-methylphenethylamine (MPEA)及2-phenylethylamine (PEA)結果做比較。本實驗利用1+1 REMPI技術使在S0 state分子先吸收一個泵浦雷射光子(λpump = 266.1 nm)到達S1 state再吸收第二個泵浦雷射光子游離化上述分子，由於我們是利用苯端當作發色團，因此初始產生之陽離子應該在苯端產生局部游離化，亦即正電荷較集中於苯端，相當於陽離子之第一激發態(D1)，接著導入探測雷射(λprobe = 798.3 nm)將陽離子打到更高能的激發態後碎裂，藉由調控不同的泵浦-探測脈衝延遲時間擷取離子訊號，得到離子損耗瞬時光譜，並以連續反應動力學模型進行適解，進而獲取上述分子系統離子態之動態學資訊。DMPPA+、MPPA+和PPA+的損耗瞬時光譜分別得到兩至三個時間常數，三者均有次皮秒之常數τ1，我們利用無電荷轉移行為的分子PPAL (phenylpropyl alcohol)離子之瞬時訊號適解結果比較，認為DMPPA+、MPPA+及PPA+內的電子轉移過程約分別為0.2、0.22、0.16 ps，另外我們推測τ2及τ3應為電荷轉移之後構型緩解過程，但構型動態學非本次實驗目的，故不多做討論。最後，將兩組陽離子之電荷轉移時間常數作比較，我們發現碳鏈橋長度增加，並未對電荷轉移速率產生明顯系統性的變化，在雙甲基胺系列中三碳橋(DMPPA+)與兩碳橋(PENNA+)電荷轉移速率變化不大；而在單甲基胺系列中三碳橋(MPPA+)比兩碳橋(MPEA+)電荷轉移速率有些微變快現象，至於無甲基胺系列，三碳橋(PPA+)比兩碳橋(PEA+)之電荷轉移速率則是變慢。綜合以上，吾人了解到在雙官能基分子中，電荷轉移可藉由鍵或是空間中的傳遞，兩機制在不同系統皆有大小不一之貢獻。

關鍵字

電荷轉移

並列摘要

In this thesis, we study the ultrafast charge-transfer (CT) dynamics in a series of three-carbon-bridged bichromophore molecular systems: N,N-dimethyl-3-phenylpropan-1-amine (DMPPA), N-Methyl-N-(3-phenylpropyl)amine (MPPA) and 3-phenylpropan-1-amine (PPA), after photoionization using the femtosecond pump-probe photoionization-photofragmentation (fs-PIPF) spectroscopy. We also compare our results with those of previous studies on similar two-carbon-bridged molecules: 2-phenylethyl-N,N-dimethylamine (PENNA), N-methylphenethylamine (MPEA) and 2-phenylethylamine (PEA). We utilize the femtosecond 1+1 resonance-enhanced multiphoton ionization via their S1 state, and the subsequent dynamics occurring in the cations is probed by delayed pulses that result in ion fragmentation. By monitoring ion singal while scanning the pump-probe delay time, we can acquire ion depletion transients. We found that there are two or three time constants in ion depletion transients of DMPPA+, MPPA+ and PPA+, and all have a sub-picosecond time constant τ1. In comparison with the fitting results of a non-CT system, phenylpropyl alcohol (PPAL), we identify the CT process in DMPPA+, MPPA+ and PPA+ to be about 0.2, 0.22, 0.16 ps, respectively. Besides, we assigned τ2 and τ3 to the conformational relaxation process after CT. However, conformational dynamics is not the purpose of our experiment, therefore, we do not focus on the details of such processes. Finally, we observe that there is no clear systematic dependence of the CT rate on the length of the carbon bridge. In dimethyl amine series, the CT rates are almost the same；however, in monomethyl amine series, the CT rate increases in MPPA+ but in nonmethyl amine series, the CT rate decreases in PPA+. In conclusion, we speculate that the charge transfer may proceed through bonds and/or space in these molecular systems, making the bridge-length effect not pronounced.

並列關鍵字

charge transfer

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