我們利用飛秒激發-探測光游離光裂解結合飛行時間質譜儀測量離子損耗瞬時訊號研究MNMA (methyl[(1,2,3,4-tetrahydronaphthalen-2-yl)methyl]amine)與MPBA (methyl(4-phenylbutyl)amine) 陽離子之電荷轉移及構型緩解動力學行為。另外，我們也測量預期不發生電荷轉移的PPAL (3-phenylpropyl alcohol)陽離子以支持對MNMA與MPBA陽離子電荷轉移指認的正確性。我們首先利用波長266 nm之激發脈衝雷射以1+1共振增益多光子游離方式將分子由中性S0 state經苯環端局部游離至陽離子D1/D2 state，隨後再緩解至陽離子D0 state。在緩解過程中，我們使用波長範圍480~1500 nm之探測脈衝雷射將陽離子激發至更高能態，使陽離子裂解產生離子碎片而造成母陽離子的損耗，並測量激發與探測脈衝隨延遲時間變化之離子損耗瞬時訊號，及兩陽離子於特定延遲時間下的離子損耗率，以獲取時間解析光裂解光譜。我們接著以連續動力學模型擬合MNMA與MPBA陽離子在各探測波長的離子損耗瞬時訊號，可得到三個不同的時間常數(time constant, τ)。兩陽離子之時間常數τ1約在100~200 fs，τ2及τ3分別約10~26 ps及370~600 ps。而我們擬合出的PPAL陽離子時間常數τ1約1~2 ps，此擬合結果的數量級有明顯不同，且在PPAL的離子損耗瞬時訊號中無法測量到瞬間生成與極快速衰退的尖峰訊號，可支持我們所研究的MNMA與MPBA陽離子的緩解動力學過程包含電荷轉移行為。藉由陽離子時間解析光裂解光譜配合理論計算結果，我們指認時間常數τ1為陽離子D1/D2 state電荷轉移緩解至D0 state的過程，τ2與τ3指認為陽離子在庫侖作用力影響下緩解至較穩定構型分佈的過程，而時間常數τ2及τ3相差至少一個數量級的事實暗示了構型緩解可能經過兩個有顯著差異的能障。

We utilized femtosecond pump-probe photoionization-photofragmentation in combination with time-of-flight mass spectrometry to measure the ion depletion transient signals to study the charge transfer (CT) and conformational relaxation dynamics of MNMA (methyl[(1,2,3,4-tetrahydronaphthalen-2-yl)methyl]amine) and MPBA (methyl(4-phenylbutyl)amine) cation. We also measured PPAL (3-phenylpropyl alcohol) cation, which is expected not to undergo CT, to support the validity of our assignment to CT signal in MNMA and MPBA cations. Firstly, we used a 266 nm excitation laser pulse to ionize the molecules from the neutral S0 state to the cationic D1/D2 state via 1+1 resonance-enhanced multiphoton ionization (REMPI). Subsequently, the D1/D2 state decay to the cationic D0 state. During the relaxation process, we used probe pulse lasers with wavelengths ranging from 480 to 1500 nm to excite the cations to higher energy states, causing ion fragmentation and resulting in the depletion of parent ions. We measured the ion depletion transient signals as a function of the pump-probe delay time as well as the cation depletion yield at specific delay times to obtain time-resolved photofragmentation spectra. Furthermore, we fitted the ion depletion transient signals of MNMA and MPBA cations at various probe wavelengths by using a consecutive kinetic model, yielding three different time constants (τ). For the two cations, the time constant τ1 is around 100-200 fs, τ2 is about 10-26 ps, and τ3 is approximately 370-600 ps. τ1 for PPAL cations was found to be around 1-2 ps, which is an order longer than those of MNMA and MPBA. Moreover, no fast rise and decay "peak signals" could be observed in the ion depletion transients of PPAL. This supports that the relaxation dynamics associated with τ1 of MNMA and MPBA cations is related to charge transfer. By combining the time-resolved photofragmentation spectra with theoretical calculations, we identified the time constant τ1 as the charge transfer relaxation process from the cationic D1/D2 state to the D0 state, and τ2 and τ3 as the relaxation processes to more stable conformation distributions driven by Coulombic interactions. The fact that τ2 and τ3 differ by at least one order of magnitude suggests that the conformational relaxation may involve two significantly different energy barriers.

參考文獻
1. Liu, Q.; Y.R. Guo; Z.H. Chen; Z.G. Zhang; X.M. Fang, Constructing a novel ternary Fe(III)/graphene/g-C3N4 composite photocatalyst with enhanced visible-light driven photocatalytic activity via interfacial charge transfer effect. Appl. Catal. B:Environ, 2016, 183, 231-241.
2. Kawai, K.; T. Majima, Hole Transfer Kinetics of DNA. Acc. Chem. Res., 2013, 46 (11), 2616-2625.
3. Takada, T.; K. Kawai; M. Fujitsuka; T. Majima, Direct observation of hole transfer through double-helical DNA over 100 A. Proc. Natl. Acad. Sci. U.S.A., 2004, 101 (39), 14002-14006.
4. Astakhova, T.Y.; V.N. Likhachev; G.A. Vinogradov, Long-range charge transfer in biopolymers. Russ. Chem. Rev., 2012, 81 (11), 994-1010.
5. Barton, J.K.; E.D. Olmon; P.A. Sontz, Metal complexes for DNA-mediated charge transport. Coord. Chem. Rev., 2011, 255 (7-8), 619-634.
6. Chen, X.; X. Zhang; X. Xiao; Z.J. Wang; J.Z. Zhao, Recent Developments on Understanding Charge Transfer in Molecular Electron Donor-Acceptor Systems. Angew. Chem. Int. Ed., 2023, 62 (16).
7. Lewis, F.D.; R.L. Letsinger; M.R. Wasielewski, Dynamics of photoinduced charge transfer and hole transport in synthetic DNA hairpins. Acc. Chem. Res., 2001, 34 (2), 159-170.
8. Isied, S.S.; M.Y. Ogawa; J.F. Wishart, Peptide-Mediated Intramolecular Electron Transfer Long Range Distance Dependence. Chemical Reviews, 1992, 92 (3), 381-394.
9. Gilbert, M.; B. Albinsson, Photoinduced charge and energy transfer in molecular wires. Chem. Soc. Rev., 2015, 44 (4), 845-862.
10. Adams, D.M.; L. Brus; C.E.D. Chidsey; S. Creager; C. Creutz; C.R. Kagan; P.V. Kamat; M. Lieberman; S. Lindsay; R.A. Marcus; R.M. Metzger; M.E. Michel-Beyerle; J.R. Miller; M.D. Newton; D.R. Rolison; O. Sankey; K.S. Schanze; J. Yardley; X.Y. Zhu, Charge transfer on the nanoscale: Current status. J. Phys. Chem. B, 2003, 107 (28), 6668-6697.
11. Jortner, J.; M. Bixon; B. Wegewijs; J.W. Verhoeven; R.P.H. Rettschnick, Long-Range, Photoinduced Charge Separation in Solvent-Free Donor Bridge Acceptor Molecoles. Chem. Phys. Lett., 1993, 205 (4-5), 451-455.
12. Bixon, M.; J. Jortner; J. Cortes; H. Heitele; M.E. Michelbeyerle, Energy-Gap Law for Nonradiative and Radiative Charge-Transfer in Isolated and in Solvated Supermolecules. J. Phys. Chem., 1994, 98 (30), 7289-7299.
13. Weinkauf, R.; P. Schanen; D. Yang; S. Sonkara; E.W. Schlag, Elementary processes in peptides-electron-mobility and dissociations in peptide cations in the gas-phase. J. Phys. Chem., 1995, 99 (28), 11255-11265.
14. 鄭博元. Conference in Memory of the Nobel Laureate Ahmed Zewail 演講之投影片. 2018.
15. Weinkauf, R.; L. Lehr; A. Metsala, Local ionization in 2-phenylethyl-N,N-dimethylamine: Charge transfer and dissociation directly after ionization. J. Phys. Chem. A, 2003, 107 (16), 2787-2799.
16. Lehr, L.; T. Horneff; R. Weinkauf; E.W. Schlag, Femtosecond dynamics after ionization: 2-Phenylethyl-N,N-dimethylamine as a model system for nonresonant downhill charge transfer in peptides. J. Phys. Chem. A, 2005, 109 (36), 8074-8080.
17. Cheng, W.; N. Kuthirummal; J.L. Gosselin; T.I. Solling; R. Weinkauf; P.M. Weber, Control of local ionization and charge transfer in the bifunctional molecule 2-phenylethyl-N,N-dimethylamine using Rydberg fingerprint spectroscopy. J. Phys. Chem. A, 2005, 109 (9), 1920-1925.
18. Sun, S.; E.R. Bernstein, Spectroscopy of neurotransmitters and their clusters .1. Evidence for five molecular conformers of phenethylamine in a supersonic jet expansion. J. Am. Chem. Soc., 1996, 118 (21), 5086-5095.
19. Li, S.; E.R. Bernstein; J.I. Seeman, Stable conformations of benzylamine and N,N-dimethylbenzylamine. J. Phys. Chem., 1992, 96 (22), 8808-8813.
20. Law, K.S.; E.R. Bernstein, Molecular jet study of van der Waals complexes of flexible molecules: n‐Propyl benzene solvated by small alkanesa). J. Chem. Phys., 1985, 82 (7), 2856-2866.
21. Im, H.S.B., E. R., Determination of the Minimum Energy Conformations of Benzyl Alcohol and 2-Phenethyl Alcohol. Colorado State Univ Fort Collins Dept of Chemistry., 1989.
22. Sun, S.T.; B. Mignolet; L. Fan; W. Li; R.D. Levine; F. Remacle, Nuclear Motion Driven Ultrafast Photodissociative Charge Transfer of the PENNA Cation: An Experimental and Computational Study. J. Phys. Chem. A, 2017, 121 (7), 1442-1447.
23. 顏暐儒. 光游離誘發雙官能基陽離子超快電荷轉移動態學之距離相依性研究, 碩士論文, 國立清華大學, 新竹市. 2019.
24. 紀泓瑋. 超快光游離誘發雙官能基陽離子之電荷轉移動態學研究, 碩士論文, 國立清華大學, 新竹市. 2020.
25. 楊博竣. 超快光游離誘發2-苯基乙基-N,N-二甲基胺陽離子內之電荷轉移動態學研究, 碩士論文, 國立清華大學, 新竹市. 2017.
26. 宋桓宇. 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, 碩士論文, 國立清華大學, 新竹市. 2018.
27. 呂柏昀. 以飛秒光游離-光裂解光譜法研究雙官能基陽離子電荷轉移動態學, 碩士論文, 國立清華大學, 新竹市. 2021.
28. 蘇培鈞. 以超快時間解析光分解光譜研究具剛性環烷基橋雙官能基陽離子之電荷轉移與構型緩解動態學, 碩士論文, 國立清華大學, 新竹市. 2023.
29. Walter, K.; R. Weinkauf; U. Boesl; E.W. Schlag, Spectroscopy of the Benzene Cation-Resonance-Enhanced Multiphoton Dissociation Spectra of the B(E2G)-X(E1G) Transition. Chem. Phys. Lett., 1989, 155 (1), 8-14.
30. Dryza, V.; N. Chalyavi; J.A. Sanelli; E.J. Bieske, Electronic absorptions of the benzylium cation. J. Chem. Phys., 2012, 137 (20), 6.
31. Smalley, R.E.; L. Wharton; D.H. Levy, Molecular Optical Spectroscopy with Supersonic Beams and Jets. Acc. Chem. Res., 1977, 10 (4), 139-145.
32. NIST網站. (https://webbook.nist.gov/chemistry/).
33. Frisch, M.T., G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G., Gaussian 09, revision D. 01. 2009, Gaussian, Inc., Wallingford CT: .
34. Li, A.; H.S. Muddana; M.K. Gilson, Quantum Mechanical Calculation of Noncovalent Interactions: A Large-Scale Evaluation of PMx, DFT, and SAPT Approaches. J. Chem. Theory Comput., 2014, 10 (4), 1563-1575.
35. Chai, J.D.; M. Head-Gordon, Long-range corrected hybrid density functionals with damped atom-atom dispersion corrections. Phys. Chem. Chem. Phys., 2008, 10 (44), 6615-6620.
36. Burns, L.A.; A. Vázquez-Mayagoitia; B.G. Sumpter; C.D. Sherrill, Density-functional approaches to noncovalent interactions: A comparison of dispersion corrections (DFT-D), exchange-hole dipole moment (XDM) theory, and specialized functionals. J. Chem. Phys., 2011, 134 (8), 25.
37. Yanai, T.; D.P. Tew; N.C. Handy, A new hybrid exchange-correlation functional using the Coulomb-attenuating method (CAM-B3LYP). Chem. Phys. Lett., 2004, 393 (1-3), 51-57.
38. Amirav, A.; U. Even; J. Jortner, Cooling of Large and Heavy Molecules in Seeded Supersonic Beams. Chemical Physics, 1980, 51 (1-2), 31-42.