激發態質子轉移/電子轉移偶合反應之光譜及飛秒動力學研究

中文摘要 Part 1: Para-N,N-ditolylamino-salicylaldehyde (Ia) 的激發態過程伴隨的質子及電子轉移的細節已經利用放光光譜以及飛秒動力學方法研究出來. 在環己烷中, 隨著超快速率(~2×1012s-1)的質子轉移反應發生後, 未質子轉移前的異構物(Normal, N)以及質子轉移後的異構物(Tautomer, T)的激發態快速形成平衡, 導致產生兩個波段的螢光, 分別在450和540nm, 而且這兩個波段有相同的緩解機制(360 ps-1). Normal 的放光表現出隨著溶劑極性出現極端的變化, 而且已經我們證明它是由一個發生激發態分子內的ditolylamine到carbonyl oxygen電子轉移的物質所產生的放光. 在極性溶劑中, 我們發現ESIPT和溶劑緩解的速度相當, 經過溶劑緩解之後的電子轉移激發態較為熱力學穩定, 所以會發生T*-N*這樣的逆向質子轉移反應. 額外的證據也藉由甲基化衍生物(Ia)及其他相關的衍生物的實驗證明. 這些結果闡明了詳細的質子/電子轉移偶合反應的機制以及在時間極短之內所牽涉到的溶劑緩解機制. Part 2: 4’-N,N-diethylamino-3-hydroxyflavone (IIa) and 4’-N,N-diethylamino-3-methoxyflavone (IIb) 的激發態過程伴隨的質子及電子轉移的細節已經利用飛秒螢光合頻方法研究出來. 在極性溶劑中, 如CH2Cl2 和 CH3CN, IIb 經歷了一個激發態分子內電子轉移反應(ESICT), 而且一開始的螢光衰減符合溶劑緩解應該表現的機制, 所以形成了一個連續的時間相關且位移的放光. 就IIa 來說, 除了一個與之前報導相符的, 慢的溶劑極性相關的激發態分子內質子轉移(ESIPT)速率, 一個飛秒級的緩解機制解析中清楚的顯示出質子轉移後的tautomer放光有一個幾百飛秒的生成訊號. Temporal spectral evolution 從時間零點到幾百飛秒中間顯示出有兩個清楚的放光波段組成, 一個是質子轉移後tautomer的放光, 另一個是如同IIb所觀察到的隨時間位移的電子轉移後狀態的放光, 這個結果結合ab initio所計算的normal 和 tautomer在激發態及基態的偶極矩, 致使我們明白不同狀態的偶極矩不同所衍生出的重要性, 進而瞭解這些相關狀態的相對能量, 最後闡明整個Ia的質子/電子轉移偶合的反應. 我們的結論認為基態的normal和激發態的tautomer具有相似的偶極矩, 然而因為ESICT的緣故, 激發態的normal(N*)具有相對N 及T*相當大變化的偶極矩. ESIPT因此在N*狀態時是熱力學上允許發生的, 而發生的速率是和溶劑緩解機制的速率是相當的. 一旦反應到達溶劑效應穩定後, N*和T*會形成平衡的狀態, 比如說在CH3CN中. 因為N*和T*的平衡之後的偶極矩差別很大, 所以正向或逆向的ESIPT都會面臨溶劑效應產生的能障. 後者的觀點關於IIa 的ESIPT平衡反應的現象與基於Steady-State, picosecond 及femtosecond動力學研究的結果的文獻報導相符合. 本文也討論了偶極矩操控的ESIPT/ESICT偶合反應機制的普遍適用性. Part 3: 7-N,N-diethylamino-3-hydroxyflavone(III) 的激發態表現已經藉由Steady-state, 包含溫度效應, 以及飛秒螢光合頻技術以研究此偶極矩操控的質子/電子轉移偶合反應. 當III在比如說CH2Cl2 和 CH3CN極性溶液中, 除了一個相對較慢且溶劑相關的ESIPT速率(幾十個picosecond-1)之外, 在飛秒級的時間內的動力學清楚的顯示溶劑緩解和質子轉移互相競爭的現象. Spectral temporal evolution 提供了更堅強的證據, 那就是表現出兩個清楚的放光波段, 即在短於幾個picosecond之內同時顯示出ESICT 和ESIPT的放光. 這個結果結合ab initio所計算的normal 和tautomer在激發態和基態的偶極矩, 使我們結論說它們是相似的. ESIPT在N*狀態因此是熱力學上允許發生的, 而其發生的速率是和溶劑緩解機制相當的. 相異於眾人皆知的ESICT/ESIPT偶合性質的分子4’-N,N-diethylamino-3-hydroxyflavone, N*和T*是平衡的狀態, III 的N*-T*的質子轉移反應在不同溶劑中是高度放熱的且不可逆, 更進一部的溫度效應實驗推算出在CH3CN中有溶劑影響的3.6 kcal/mol能障. 另外在III 加入4’-N,N-dimethyl的取代基變成分子IV, 7-N,N-diethylamino-4’-N,N-dimethyl-3-hydroxyflavone, 偶極矩的改變因此被抵銷了, 因此ESIT的性質類似於3HF只和溶劑極性一絲相關.這個機制的結果普遍化了在此一類型的反應, 即ESICT和ESIPT偶合反應, 偶極矩操控的特性及其所扮演的角色.

關鍵字

飛秒動力學；質子轉移；電子轉移

並列摘要

Part 1: Detailed insights into the excitation behaviors for charge versus proton transfer in para-N,N-ditolylamino-salicylaldehyde (Ia) have been gained via luminescence spectroscopy and femtosecond dynamics. In cyclohexane, following an ultrafast rate (~2.0x1012 s-1) of excited-state intramolecular proton transfer (ESIPT), fast equilibrium takes place between normal (N*) and tautomer excited states (T*), resulting in dual fluorescence maximized at 450 and 540 nm, respectively, with a common population decay rate of 360 ps-1. The normal emission exhibits drastic solvent-polarity dependence and has been concluded to originate from a charge-transfer species incorporating excited-state intramolecular charge transfer from ditolylamine to carbonyl oxygen. In dipolar solvents, competitive rates between ESIPT and solvent relaxation were observed, and the solvated charge-transfer state is thermodynamically more favorable, so that the T*-N* reverse proton transfer takes places. Supplementary supports were provided by the corresponding experiments for the methoxy derivative of Ia as well as other relevant analogues. The results shed light on detailed proton/charge transfer coupled dynamics as well as the associated solvent-relaxation dynamics at an early time domain. Part 2: The excitation behaviors for 4’-N,N-diethylamino-3-hydroxyflavone (Ia) and 4’-N,N-diethylamino-3-methoxyflavone (Ib) have been investigated via femtosecond fluorescence upconversion approaches to gain detailed insights into the mechanism of proton/charge transfer coupling reaction. In polar solvents such as CH2Cl2 and CH3CN, Ib undergoes an excited state intramolecular charge transfer (ESICT) reaction, and its early fluorescence decay correlates well with the solvent relaxation dynamics, resulting in a continuously time-dependent Stokes shifted emission. For the case of Ia, in addition to a slow, solvent-polarity dependent rate (a few tens of picoseconds-1) of excited state intramolecular proton transfer (ESIPT) reported previously, early femtosecond relaxation dynamics clearly reveal that the proton-transfer tautomer emission consists of a rise component of a few hundred femtoseconds. The temporal spectral evolution at the time domain of zero to a few hundred femtoseconds further resolves two distinct emission bands consisting of a proton transfer tautomer emission and a time-dependent Stokes shifted charge transfer emission similar to that observed in Ib. The results, in combination with ab initio calculations on the dipolar vectors for normal and tautomer species in both ground and excited states, lead us to unveil the importance of the relationship of the dipolar vectors among various states, and hence the corresponding solvation energetics, in order to shed light on the overall ESICT/ESIPT coupled reaction in Ia. We conclude a similar dipolar character between ground-state normal (N) and excited proton-transfer tautomer (T*) species, whereas due to ESICT, the normal excited state (N*) possesses a large dipolar change with respect to N and T*. ESIPT is thus energetically favorable at the Franck-Condon excited N*, and its rate is competitive with respect to the solvation relaxation process. After reaching the solvent equilibration, there exists an equilibrium between N* and T* states in e.g. CH3CN. Due to the greatly different equilibrium polarization between N* and T*, both forward and reversed ESIPT dynamics are associated with a solvent induced barrier. The latter viewpoint of the equilibrium type of ESIPT in Ia is in agreement with the previous reports based on steady state,10 picosecond,11, and femtosecond , dynamic approaches. The generalization of mechanism of the dipolar tuning ESIPT/ESICT reaction is also discussed. Part 3: Comprehensive excitation behaviors of 7-N,N-diethylamino-3-hydroxyflavone (I) have been investigated via steady state, temperature dependent emission, and fluorescence upconversion to probe the excited state intramolecular charge/proton transfer (ESICT/ESIPT) coupled reaction. For I in polar solvents such as CH2Cl2 and CH3CN, in addition to a relatively slow, solvent-polarity dependent rate (a few tens of picoseconds-1) of ESIPT, femtosecond dynamics clearly reveals the competitive rates between solvation and proton-transfer dynamics. Firm supports are rendered by the spectral temporal evolution, which resolves two distinct bands, i.e., ESICT and ESIPT emission at < few ps. The results, in combination with ab initio calculations on the dipolar vectors for various corresponding states, lead us to conclude that excited-state normal (N*) and excited proton-transfer tautomer (T*) possesses very different dipole orientation, whereas the dipole orientation of the normal ground state (N) is in between that of N and T*. ESIPT is thus energetically favorable at the Franck-Condon excited N*, and its rate is competitive with respect to solvent relaxation dynamics induced by ESICT. Unlike the well-known ESICT/ESIPT system, 4’-N,N-diethylamino-3-hydroxyflavone, in which equilibrium exists between N* and T*, N*-T* ESIPT for I is a highly exergonic, irreversible process in all solvents studied. Further temperature dependent studies deduce a solvent-polarity perturbed energy barrier of 3.6 kcal/mol in CH3CN. To extend the dipolar tuning ESICT/ESIPT mechanism, 7-N,N-diethylamino-4’-N,N-dimethyl-3-hydroxyflavone (II) was synthesized via addition of the 4’-N,N-dimethyl substituent to I. As a result, the changes of dipolar vector are largely cancelled out, and ESIPT behavior is similar to that of the parent 3-hydroxyflavne in that the proton transfer dynamics is only slightly dependent of solvent polarity. The proposed dipolar tuning mechanism thus generalizes the ESICT/ESIPT coupled reaction dynamics in polar, aprotic solvents.

並列關鍵字

femtosecond ； proton transfer ； electron transfer

參考文獻

Chapter 1:

1. For example, see: Electron and Proton Transfer in Chemistry and Biology; Muller, A.; Ratajack, H.; Junge, W.; Diemann, E., Eds.; Studies in Physical and Theoretical Chemistry, Vol. 78; Elsevier: Amsterdam, 1992.

8. Arnaut, L. G.; Formosinho, S. J. J. Photochem. Photobiol. A 1993, 75, 1.

11. Huppert, D.; Cohen, B. J. Phys. Chem. A 2000, 104, 2663.

14. Weller, A. Prog. React. Kinet. 1961, 1, 188.

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激發態質子轉移/電子轉移偶合反應之光譜及飛秒動力學研究

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