以飛秒瞬態吸收光譜研究苯/四氰基乙烯錯合物之電荷轉移態與局部激發態之電荷轉移動力學

Femtosecond Transient Absorption Spectroscopic Studies of Charge Transfer Dynamic of Charge-Transfer and Locally Excited States of Benzene/Tetracyanoethylene Complexes

廖翊凱(Yi-Kai Liao)

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

本論文利用實驗室自行架設的飛秒瞬態吸收光譜儀(transient absorption, TA)及一套全自動數據擷取電腦程式，研究電子給體-受體(electron doner-acceptor, EDA)錯合物的分子間電子轉移動力學行為。具體研究對象為苯/四氰基乙烯(benzene/tetracyanoethylene, Bz/TCNE)錯合物溶於不同比例苯/四氯化碳混合溶劑。本論文分為兩個主題，第一個是觀察直接激發Bz/TCNE錯合物的電荷轉移(charge transfer, CT)態後的電子轉移動力學；第二個是研究激發Bz/TCNE錯合物中的TCNE至局部激發(locally excited, LE)態後所進行電子轉移動力學。在第一個主題中，我們觀察到四氰基乙烯自由基陰離子(TCNE-)與苯二聚體自由基陽離子((Bz)2+)的光譜特徵，及其時間變化之行為，並與過去本實驗室利用光學克爾光閘超快時間解析螢光光譜(time-resolved fluorescence,TRFL)研究Bz/TCNE錯合物的CT態動力學比較。於TA光譜中，我們觀察到激發錯合物CT態後，激發態錯合物會先進行較快的溶劑及振動緩解，接著為較慢的電荷重合(charge recombination, CR)反應。溶劑與振動緩解時間常數與使用TRFL技術得到的相近(分別約為0.2與8 ps)，(Bz)2+的生成速率隨苯濃度降低而有顯著的下降，與TRFL技術得到結果相近，CR過程則隨著苯濃度由xBz=0.05上升至1，其時間常數由240 ps縮短為100 ps，此趨勢雖與TRFL偵測結果類似，但TA得到的時間常數明顯較TRFL結果為長，在論文中提出許多原因來解釋這個現象。在第二個主題中，我們藉由第一個主題得到的光譜協助指認Bz/TCNE錯合物的離子激發態光譜，並提出一個近似的動力學模型來描述激發Bz/TCNE錯合物LE態後，經由電子轉移生成Bz/TCNE離子激發態錯合物以及其後續衰減動力學。我們觀察到Bz/TCNE錯合物電子轉移速率時間常數約為0.1至0.3 ps(隨苯濃度下降而變長)，為非常快的反應。在高苯濃度溶液中因TCNE激發態周圍有足夠的苯分子，使得(Bz)2+的生成速率與電子轉移速率相近。

關鍵字

電荷轉移動力學 ； 飛秒瞬態吸收光譜 ； 苯/四氰基乙烯錯合物

並列摘要

We study the charge-transfer (CT) dynamics of the electron donor-acceptor (EDA) complexes with broadband femtosecond transient absorption (TA) spectroscopy. The EDA complex studied here are binary (1:1) and ternary (2:1) benzene/tetracyanoethylene (Bz/TCNE) dissolved in a series of Bz/CCl4 cosolvents. The thesis is mainly divided into two topics. In the first topic, we directly excite the CT state of Bz/TCNE complexes to observe their relaxation dynamics and compare the results with those obtained previously using ultrafast time-resolved fluorescence spectroscopy (TRFL) implemented by optical Kerr gating in our laboratory. We observe spectral characteristics of tetracyanoethylene radical anion (TCNE-) and benzene dimer radical cation ((Bz)2+). By analyzing their time-dependent behavior, we find that after exciting the CT state of Bz/TCNE complex, the excited state complex undergoes rapid solvation and vibrational relaxation, followed by a slower charge recombination (CR) reaction. The time constants of the solvation and vibrational relaxation are similar to those obtained with TRFL technique (about 0.2 and 8 ps, respectively). (Bz)2+ formation rate decreases significantly with decreasing benzene concentration, which is similar to these observed in previous TRFL experiment. With TA spectroscopy, we find a positive correlation between Bz concentration and CR rate: when Bz concentration increases from xBz=0.05 to 1, the CR time constant decreases from 240 ps to 100 ps. Althougt similar correlations have been also observed in previous TRFL experiment, the time constant obtained by TA is substantially longer than those obtained by TRFL. We propose a few possibilities to explain this phenomenon. In the second topic, we study the electron transfer kinetics after exciting electron acceptor (TCNE) to the locally excited (LE) state of the Bz/TCNE complexes. We identify the ionic CT state spectra of Bz/TCNE complexes by comparing with those obtained from the first part. An approximate kinetic model is proposed to describe the electron transfer that generates Bz/TCNE ionic CT state complexes after excitation of the Bz/TCNE complex LE state, and its decay kinetics. We observe that the electron transfer time constants of the Bz/TCNE complex are about 0.1 to 0.3 ps, which are very fast reactions. In the high Bz concentration solutions, there are enough Bz molecules surrounding excited TCNE, so that the formation rate of (Bz)2+ is close to the electron transfer rate.

並列關鍵字

charge transfer dynamic ； femtosecond transient absorption spectrum ； benzene/tetracyanoethylene complexes

參考文獻

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