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  • 學位論文

以超快時間解析光學克爾光閘螢光光譜研究3-氰基-4-甲基-7-羥基香豆素錯合物激發態質子轉移動態學

Ultrafast time-resolved fluorescence studies of Excited-State Proton Transfer Dynamics in 3-cyano-4-methyl-7-hydroxycoumarin complexes

指導教授 : 鄭博元

摘要


本論文之目的為以飛秒雷射光譜研究激發態分子間質子轉移過程的超快動力學。我們以具有質子予體的3-氰基-4-甲基-7-羥基香豆素(3-cyano-4-methyl-7-hydroxycoumarin,簡稱3CN4MU)作為研究分子,使其與不同質子親和力的質子受體1-甲基咪唑(1-methylimidazole,簡稱1MI)及三乙胺(Triethylamine,簡稱TEA)在不同極性的溶劑甲苯(toluene,簡稱TL)及乙酸乙酯(ethyl acetate,簡稱EA)中形成不同構型之錯合物,利用靜態吸收光譜及靜態螢光光譜得到其光譜特徵,再以實驗室自行架設之飛秒雷射光譜系統取得時間解析螢光光譜。綜合各項結果以指認錯合物之不同構型並討論其激發態質子轉移(Excited-state intermolecular proton transfer,簡稱ESPT)。當以強質子親和力TEA作為質子受體時,我們發現部分錯合物在基態時已發生質子轉移,因而無法有效地觀測其ESPT現象。當以弱質子親和力1MI作為質子受體時,靜態光譜顯示其在基態時形成非質子轉移氫鍵錯合物。我們發現在TL非極性溶劑中,溶劑分子無法藉由solvation幫助錯合物質子轉移,因此在激發態時質子轉移並不顯著。當在EA極性溶劑中,EA可藉由solvation幫助錯合物質子轉移,使其整體自由能下降而穩定分子,故我們指認其在激發態時發生質子轉移。在時間解析螢光光譜中,我們以三個時間常數0.8 ps、30±2 ps、1700±100 ps擬合全螢光強度P(t)衰減曲線,我們將0.8 ps指認為質子從3CN4MU轉移至1MI上之第一步,為一solvation-controlled proton transfer過程,將30±2 ps指認為EA溶劑重新調整距離及位向之後,ion pair進一步調整自身結構分離以降低整體自由能的過程。最後,1700±100 ps則為質子轉移錯合物的激發態生命期。

關鍵字

動力學 錯合物 超快 飛秒

並列摘要


We employed a broadband ultrafast time-resolved fluorescence (TRFL) spectrometer implemented by optical Kerr gating (OKG) to study the excited-state proton transfer (ESPT) dynamics in 3-cyano-4-methyl-7-hydroxycoumarin (3CN4MU) complexes. We chose 3CN4MU as the proton donor and two bases, triethylamine (TEA) and 1-methylimidazole (1MI), of different proton affinities (PA) as the proton acceptors. We used two solvents, ethyl acetate (EA) and toluene (TL), of different polarities to study the solvent effect in ESPT. It was conclude from by steady-state spectra that the ground-state 3CN4MU transfers a proton from its phenolic group to TEA, which possesses stronger PA. Therefore, it is difficult to study ESPT in the 3CN4MU-TEA complex. On the other hand, 3CN4MU forms ground-state hydrogen-bonded complexes with 1MI (weaker PA) which suggesting that no proton-transfer reaction occurs in the ground state. Excited states are reached by 383 nm femtosecond laser pulse excitation. The observed TRFL spectra reveal that ESPT does not occur in 3CN4MU-1MI complex in TL, due to the weak solvation effect in nonpolar solvent. On the contrary, solvation-controlled proton transfer in excited state is observed in 3CN4MI-1MI complex in polar solvent EA. We used a total fluorescence intensity function P(t) to measure the excited-state population and transition moment evolution with time during the ESPT process. The P(t) function of 3CN4MU-1MI in EA can be described by three time constants. The fast initial decay component (0.8 ps) can be assigned to first step of proton transfer, which is controlled by solvation effect. The second decay component (30 ps) is assigned to the ion-pair structural relaxation to reduce the overall free energy. In the end, the 1700 ps component is assigned to the lifetime of the excited-sate proton-transferred ion pair.

並列關鍵字

Dynamics complex ultrafast femtosecond

參考文獻


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