透過您的圖書館登入
IP:44.197.238.222
  • 學位論文

飛秒時間解析光譜學在激發態電子與質子轉移的研究

Femtosecond Time-Resolved Spectroscopy in Studying Excited-State Electron and Proton Transfer Reactions

指導教授 : 周必泰
若您是本文的作者,可授權文章由華藝線上圖書館中協助推廣。

摘要


第一部分主要探討內容是利用溫度效應與單光子同步計數器(TCSPC)來研究電子轉移現象,藉由調控電子提供者(donor)與接受者(acceptor)之間的距離來觀測兩者之間的交互作用,當電子提供者與接受者之間的距離較近時,可以發現到兩者的偶合是非常大的,其偶合常數(coupling constant)遠超過室溫下的波茲曼分部熱能(kbT ~ 200 cm-1 at 298 K),當提供者與接受者距離較遠時,其偶合常數會隨著距離的增加而劇烈的減少,例如兩者之間距離大約13Å的時候其偶合常數只剩下2cm-1左右,在這麼弱的彼此作用之下其電子轉移現象屬於光致電子轉移(photoinduced electron transfer),其動力學上與物理上的意義可以使用Marcus theory來解釋並且合理化所有觀察到的現象. 第二部分則是研究激發態質子轉移的反應,其中包括了溶液態的分子間與分子內的質子轉移現象,再研究雙質子轉移的反應我們設計了一個6HIQ與7-azaindole (7AI)分子有非常類似的質子提供與接受團,因此彼此可以互相形成非對稱性的二聚體結構,但6HIQ的可見紫外光吸收比7AI還要再更紅位移100nm左後,因此我們可以選擇性的激發6HIQ分子產生非對稱性的雙質子轉移,來光察到雙質子轉移過程中所存在的中間體,此項發現可以有力的證明在非對稱的雙質子轉移系統,其轉移反應是兩個質子以兩個步驟(two-step)進行的;最後在固態雙質子反應我們可以比較6HIQ與7AI衍生物的光譜現象有很大的不同,可歸納出在質子轉移的過程中,分子的結構扮演了很大的角色. 然而在分子間與分子內質子轉移現象探討了有關質子偶合電子轉移反應時,溶劑的極性對整個反應所造成的影響,質子偶合電子轉移反應可分為兩類,第一種是電子轉移先發生然後伴隨著質子轉移現象,這類的反應在質子轉移之前的結構會受到溶劑的極性影響較大,而導致質子轉移之前的正常體(normal form)螢光光譜受到溶劑的極性的影響,在此則使用了7AI的衍生物藉由接上不同位置的拉電子基(-CN)來調控質子轉移之前偶極矩的變化,來觀察這類分子在不同極向環境的質子轉移光譜現象;第二類則是質子轉移之後誘導電子轉移現象,這種反應在質子轉移之後的結構會受到溶劑很大的影響而使得質子轉移同分異構物(tautomer)光譜有很大的溶劑效應,這種現象目前只有少入數現象,例如 diCN-HBO分子,在飛秒時間解析螢光光譜可以觀察到質子轉移之後的電子轉移反應. 最後則是使用飛秒時間解析的紫外可見光螢光與瞬時態吸收光譜,來探討綠色螢光蛋白的鄰位衍生物o-HBDI,在飛秒的系統中發現到o-HBDI分子的螢光半生期與溶劑的黏滯性有很大的關聯,表示o-HBDI在激發態緩解到基態過程中的順反異構化過程中會造成分子體積很大的改變,因此這種反應可歸類為單鍵轉動造成的現象,另外在可見瞬態吸收光譜則發現到了順反異構化之後的產物可以在基態存活超過5μs,經過了很長一段時間這種異構化產物才會逐漸變回原本的起始物,最後這種產物可以用奈秒微秒的長時間解析吸收光譜來證明.

並列摘要


Contemporary progress on organic dyes undergoing excited-state electron and/or proton transfers is reviewed via three aspects: (1) the fundamental view on the mechanistic of electron transfer reaction, i.e., adiabatic versus nonadiabatic processes, and their applications in e.g. ion recognition; (2) excited-state proton transfer (ESPT) in view of hydrogen bonding configuration; (3) the role of solvation and solvation dynamics in the proton transfer coupled charge transfer reactions and (4) excited-state intramolecular proton transfer (ESIPT) reaction of ortho-green fluorescent protein core chromophore. For (1), of particular emphasis are bipolar molecules, for which the electron donor (D) and acceptor (A) are linked by non-π-conjugated covalent bonds. As for altering the distance between D and A via rigid spacers, the electron transfer process can be fine-tuned from adiabatic to nonadiabatic. Relevant fundamentals and applications are briefly reviewed and discussed. For (2), prototypes of intramolecular ESIPT are classified into five-, six-, and seven-membered ring hydrogen-bonding systems. While five- and six-membered hydrogen-bonding systems are somewhat common, seven-membered one is rare. In this section, in addition to those of 5- and 6-membered classes, paradigms of 7-membered hydrogen-bonding systems are presented by using analogue of green fluorescent protein core chromophore. On the other hand, the ESPT dyanmcis via intermolecular hydrogen bonding has also been reviewed. Particually, a four 6HIQ is strategically designed and synthesized; it possesses a central moiety of 7-azaindole (7AI) and undergoes excited-state double proton transfer (ESDPT). Despite a barrierless type of ESDPT in the 6HIQ dimer, femtosecond dynamics and a kinetic isotope effect provide indications for a stepwise ESDPT process in the 6HIQ/7AI heterodimer. In (3), various mechanisms of protic solvent assisting ESPT are reviewed, among which plausible mechanism is deduced and discussed. Particular attention is paid to reaction dynamics in alcohol and aqueous solutions, with an aim toward biological applications. In this section, the interplay between excited-state charge and proton transfer reactions in protic solvents is investigated in a series of 7AI derivatives can undergo methanol catalyzed ESDPT, resulting in dual (normal and proton transfer) emission. Instead, the normal emission undergoes prominent solvatochromism. Detailed relaxation dynamics and temperature dependent studies are carried out. The result is remarkably different from 7AI, which is also unique among most excitedstate electron/proton transfer coupled systems studied to date. Last but not least, the differences in how solvent diffusive reorganization and solvent relaxation affect the ESPT dynamics are discussed; the conclusions should provide more insight into the micro- and macro-solvation processes. Subsequently, the fluorescence spectroscopy and femtosecond relaxation dynamics of diCN-HBO anddiCN-HBT are studied to probe the ESIPT coupled electron transfer (ESIET) reaction. Unlike most of the ESIPT/ESIET systems previously designed, in which ESIET takes place prior to ESIPT, both diCN-HBO and diCN-HBT undergo ESIPT, concomitantly accompanied with the charge transfer process, such that the ESIPT reaction dynamics are directly coupled with solvent polarization effects. The long-range solvent polarization interactions result in a solvent-induced barrier that affects the overall proton transfer reaction rate. A prototypical system has been demonstrate in which the photon-induced nuclear motion (proton transfer) is directly coupled with solvent polarization and the corresponding mechanism is reminiscent of that applied in an electron transfer process. In (4), the ESIPT reaction of ortho-green fluorescent protein (GFP) chromophore, 4-(2-hydroxybenzyli-dene)-1,2-dimethyl-1H-imidazol-5(4H)-one (o-HBDI), has been studied via ultrafast time-resolved spectroscopy. o-HBDI possesses a seven-membered-ring intramolecular hydrogen bond, from which the system response limited (< 150 fs) ESIPT takes place, resulting in a ~602 nm proton-transfer tautomer emission (τf ~ 7.8 ps) in CH3CN. An aim of this study is to probe the structural evolution of o-HBDI during and after ESIPT reaction. The results of UV/Vis transient absorption spectra reveal new absorption bands, providing new insight into the structural evolution of the biomimetic systems during the ESIPT reaction. The appearance of long-lived absorption band at ~580 nm in CH3CN firmly supports the existence of ground state tautomer isomer which cannot be detected in the fluorescence upconversion. Further nanosecond transient absorption spectra resolve this transient species with a lifetime of as long as ~24μs. It is thus concluded that upon electronic excitation, ESIPT takes place in o-HBDI, forming a hydrogen bonded keto-isomer, which then undergoes cis-trans isomerization either along the one-bond-flip of exocyclic C-C double bond or via the concerted twist around the bridging C=C-C bonds, i.e. a hula type of twist, giving a long-lived trans-keto isomer at ground state followed by a 24 μs deprotonation rate. The results of viscosity dependent dynamics are in favor of the former mechanism, i.e. one-bond-flip with large amplitude motion.

參考文獻


19. Duan, H. S.; Chou, P. T. Hsu, C. C.; Hung, J. Y.; Chi, Y. Inorg. Chem. 2009, 48, 6501.
30. Catalán, J.; de1 Valle, J. C.; Claramuntb, R. M. et al J. Lumin. 1996, 68, 165-170.
14. Guldi, D. M.; Aminur Rahman, G. M.; Sgobba, V. et al Chem. Soc. Rev. 2006, 35, 471-487.
62. Carbazole: (a) Hu, N.-X.; Xie, S.; Popovic, Z. D.; Ong, B.; Hor, A.-M. Synth. Met. 2000, 111, 421. (b) Thomas, K. R. J.; Lin, J. T.; Tao, Y.-T.; Ko, C.-W. Adv. Mater. 2000, 12, 1949. (c) Zhang, Q.; Hu, Y. F.; Cheng, Y. X.; Su, G. P.; Ma, D. G.; Wang, L. X.; Jing, X. B.; Wang, F. S. Synth. Met. 2003, 137, 1111. (d) Bugatti, V.; Concilio, S.; Iannelli, P.; Piotto, S. P.; Bellone, S.e; Ferrara, M.; Neitzert, H. C.; Rubino, A.; Della Sala, D.; Vacca, P. Synth. Met. 2006, 156, 13. Oxadiazole: (e) Freeman, A. W.; Koene, S. C.; Malenfant, P. R. L.; Thompson, M. E.; Frechet, J. M. J. J. Am. Chem. Soc. 2000, 122, 12385. (f) Yang, X. H.; Jaiser, F.; Klinger, S.; Neher, D. Appl. Phys. Lett. 2006, 88, 021107/1. (g) Ichikawa, M.; Kawaguchi, T.; Kobayashi, K.; Miki, T.o; Furukawa, K.; Koyama, T.; Taniguchi, Y. J. Mater. Chem. 2006, 16, 221.
96. 6HIQ was synthesized according to the procedure in this reference: C. Shi, Q. Zhang, K. K. Wang, J. Org. Chem. 1999, 64, 925.

延伸閱讀