我們結合飛秒激發-探測光譜技術與飛行時間質譜技術，觀察MNMA (methyl[(1,2,3,4-tetrahydronaphthalen-2-yl)methyl]amine)分子在光游離-光裂解機制下所產生的離子損耗瞬時訊號，以研究其在陽離子態時的緩解動力學。我們使用266 nm的激發脈衝雷射，以1+1共振增益多光子游離(1+1 REMPI)的方式將MNMA分子經由苯環端局部S1 state而達到苯環端的局部游離，形成正電荷主要位於苯環端的初始陽離子D1/D2 state，隨後陽離子D1/D2 state應可經由內轉換緩解至正電荷位於胺基端的D0 state，而此過程相當於電荷轉移(charge transfer)。在MNMA陽離子緩解的過程中，我們使用另一道波長在550～1500 nm範圍內的探測脈衝雷射將母陽離子激發至更高能態，導致其發生裂解從而獲得母陽離子的離子損耗瞬時訊號，並以此瞬時訊號研究MNMA陽離子的緩解動力學過程。我們量測在各探測波長下的MNMA陽離子損耗率，獲得MNMA超快時間解析陽離子光分解光譜，在此光譜中有三個時間行為截然不同的主要譜帶，分別為625 nm、750 nm及1250 nm附近的譜帶，且三譜帶呈現相互消長的現象。我們以連續動力學模型擬合不同探測波長下的離子損耗瞬時訊號，獲得三個不同時間尺度的時間常數(time constant, τ)，其中τ1為0.08～0.2 ps，τ2為15～26 ps，τ3為370～475 ps。我們認為τ1應為陽離子D1/D2 state緩解至D0 state的電荷轉移過程，τ2及τ3應為陽離子D0 state的構型緩解平衡過程。我們藉由理論計算估算MNMA陽離子在不同構形下的垂直激發能量，輔助解釋實驗及擬合的結果，由計算結果我們推測首先出現於625 nm附近，時間行為在亞皮秒尺度的譜帶，是由陽離子D1/D2 state的吸收所造成；接著出現於1250 nm附近的譜帶是由緩解至陽離子D0 state後形成的初始構型的吸收所造成；最後出現於750 nm附近的譜帶是由陽離子D0 state緩解至較穩定構型的吸收所造成。此三個物種之間的消長情形反映了MNMA陽離子由D1/D2 state內轉換至D0 state的電荷轉移過程以及後續在D0 state的構型緩解過程，而時間常數τ2及τ3的大小則受到構型轉換的能障所影響。

We investigate the ultrafast charge transfer (CT) dynamics of methyl[(1,2,3,4-tetrahydronaphthalen-2-yl)methyl]amine (MNMA) cation by using femtosecond pump-probe spectroscopy and time-of-flight mass spectrometry to observe the ion depletion transient produced with the photoionization-photofragmentation (PI-PF) mechanism. Through 1+1 resonance-enhanced multiphoton ionization (1+1 REMPI), the phenyl site of MNMA is selectively ionized via the local neutral S1 state by 266 nm pump pulse, leading to the formation of initial cationic D1/D2 states in which the positive charge is mainly localized at the phenyl site. Subsequently, the cationic D1/D2 state relax to the cationic D0 state in which the positive charge is mainly localized at the amine site. This internal conversion relaxation corresponds to a CT process. During the relaxation of MNMA cations, we use probe laser pulses at wavelengths within 550 to 1500 nm to excite the cations to higher excited states that can undergo fragmentation, resulting the depletion of MNMA cation signals. We measure MNMA cation depletion yield at various probe wavelengths and obtain an ultrafast time-resolved cation photofragmentation spectrum in which we find three major absorption bands near 625 nm, 750 nm, and 1250 nm. Using a consecutive kinetic model to fit the transients, we obtain three time constants (τ): τ1 in the range of 0.08 to 0.2 ps, τ2 in the range of 15 to 26 ps, and τ3 in the range of 370 to 475 ps. We assign τ1 to the internal conversion from the D1/D2 states to the D0 state, which correspond to the CT process. We assign τ2 and τ3 to the equilibrium processes among conformers in the D0 state. Additionally, theoretical calculations are used to estimate the vertical excitation energies of different conformers of MNMA cation. The results suggest that the band near 625 nm with a sub-picosecond temporal behavior is the absorption of the D1/D2 states, while the subsequent band near 1250 nm is the absorption of the initial conformer formed after the relaxation of D1/D2 states to the D0 state. Finally, the band near 750 nm is the absorption of the more stable conformer formed via the conformational relaxation in the D0 state. The sequential decay and rise of these three species reflect the CT process and the conformational relaxation process in the D0 state and the time constants τ2 and τ3 depend on the energy barriers between the conformers.

1. Barber, J.; Andersson, B. Revealing the blueprint of photosynthesis. Nature 1994, 370 (6484), 31-34.
2. Scholes, G. D.; Fleming, G. R.; Olaya-Castro, A.; Van Grondelle, R. Lessons from nature about solar light harvesting. Nat. Chem. 2011, 3 (10), 763-774.
3. Lyu, S.; Farré, Y.; Ducasse, L.; Pellegrin, Y.; Toupance, T.; Olivier, C.; Odobel, F. Push–pull ruthenium diacetylide complexes: new dyes for p-type dye-sensitized solar cells.
RSC Adv. 2016, 6 (24), 19928-19936.
4. Shin, H.; Kim, B. M.; Jang, T.; Kim, K. M.; Roh, D. H.; Nam, J. S.; Kim, J. S.; Kim, U. Y.; Lee, B.; Pang, Y. Surface State‐Mediated Charge Transfer of Cs2SnI6 and Its Application in Dye‐Sensitized Solar Cells. Adv. Energy Mater. 2019, 9 (3), 1803243.
5. Maldon, B.; Thamwattana, N. A fractional diffusion model for dye-sensitized solar cells. Molecules 2020, 25 (13), 2966.
6. Schlag, E. W.; Sheu, S. Y.; Yang, D. Y.; Selzle, H. L.; Lin, S. H. Distal charge transport in peptides. Angew. Chem. Int. Ed. 2007, 46 (18), 3196-3210.
7. Shah, A.; Adhikari, B.; Martic, S.; Munir, A.; Shahzad, S.; Ahmad, K.; Kraatz, H.-B. Electron transfer in peptides. Chem. Soc. Rev. 2015, 44 (4), 1015-1027.
8. Yu, J.; Horsley, J. R.; Moore, K. E.; Shapter, J. G.; Abell, A. D. The effect of a macrocyclic constraint on electron transfer in helical peptides: A step towards tunable molecular wires. Chem. Commun. 2014, 50 (14), 1652-1654.
9. Kawai, K.; Majima, T. Hole transfer kinetics of DNA. Acc. Chem. Res. 2013, 46 (11), 2616-2625.
10. Takada, T.; Kawai, K.; Fujitsuka, M.; Majima, T. Direct observation of hole transfer through double-helical DNA over 100 Å. Proc. Natl. Acad. Sci. U.S.A. 2004, 101 (39), 14002-14006.
11. Isied, S. S.; Ogawa, M. Y.; Wishart, J. F. Peptide-mediated intramolecular electron transfer: long-range distance dependence. Chem. Rev. 1992, 92 (3), 381-394.
12. Lewis, F. D.; Letsinger, R. L.; Wasielewski, M. R. Dynamics of photoinduced charge transfer and hole transport in synthetic DNA hairpins. Acc. Chem. Res. 2001, 34 (2), 159-170.
13. Adams, D. M.; Brus, L.; Chidsey, C. E.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A. Charge transfer on the nanoscale: current status. J. Phys. Chem. B 2003, 107 (28), 6668-6697.
14. Gilbert, M.; Albinsson, B. Photoinduced charge and energy transfer in molecular wires. Chem. Soc. Rev. 2015, 44 (4), 845-862.
15. Jortner, J.; Bixon, M.; Wegewijs, B.; Verhoeven, J. W.; Rettschnick, R. P. Long-range, photoinduced charge separation in solvent-free donor—bridge—acceptor molecules. Chem. Phys. Lett. 1993, 205 (4-5), 451-455.
16. Bixon, M.; Jortner, J.; Cortes, J.; Heitele, H.; Michel-Beyerle, M. Energy gap law for nonradiative and radiative charge transfer in isolated and in solvated supermolecules. J. Phys. Chem. 1994, 98 (30), 7289-7299.
17. Weinkauf, R.; Schanen, P.; Yang, D.; Soukara, S.; Schlag, E. Elementary processes in peptides: electron mobility and dissociation in peptide cations in the gas phase. J. Phys. Chem. 1995, 99 (28), 11255-11265.
18. 鄭博元. Conference in Memory of the Nobel Laureate Ahmed Zewail 演講之投影片. 2018.
19. Weinkauf, R.; Lehr, L.; Metsala, A. Local ionization in 2-phenylethyl-N, Ndimethylamine: charge transfer and dissociation directly after ionization. J. Phys. Chem. A 2003, 107 (16), 2787-2799.
20. Lehr, L.; Horneff, T.; Weinkauf, R.; Schlag, E. 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.
21. Cheng, W.; Kuthirummal, N.; Gosselin, J. L.; Sølling, T. I.; Weinkauf, R.; Weber, P. M. 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.
22. Li, S.; Bernstien, E.; Seeman, J. Stable conformations of benzylamine and N, Ndimethylbenzylamine. J. Phys. Chem. 1992, 96 (22), 8808-8813.
23. Sun, S.; Bernstein, E. 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.
24. Im, H.-S.; Bernstein, E. Determination of the Minimum Energy Conformations of Benzyl Alcohol and 2-Phenethyl Alcohol. Colorado State Univ Fort Collins Dept of Chemistry. 1989.
25. Law, K.; Bernstein, E. 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.
26. Sun, S.; Mignolet, B.; Fan, L.; Li, W.; Levine, R. D.; Remacle, F. 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.
27. 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. 2018.
28. 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. 2021.
29. 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. 2020.
30. 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. 2019.
31. 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. 2022.
32. Walter, K.; Weinkauf, R.; Boesl, U.; Schlag, E. Spectroscopy of the benzene cation: Resonance-enhanced multiphoton dissociation spectra of the B̃ (E2g)← X̃ (E1g) transition. Chem. Phys. Lett. 1989, 155 (1), 8-14.
33. Dryza, V.; Chalyavi, N.; Sanelli, J. A.; Bieske, E. J. Electronic absorptions of the benzylium cation. J. Chem. Phys. 2012, 137 (20).
34. Smalley, R. E.; Wharton, L.; Levy, D. H. Molecular optical spectroscopy with supersonic beams and jets. Acc. Chem. Res. 1977, 10 (4), 139-145.
35. Frisch, M.; Trucks, 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. Gaussian, Inc., Wallingford CT: 2009.
36. Burns, L. A.; Mayagoitia, Á. V.; Sumpter, B. G.; Sherrill, C. D. 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).
37. Chai, J.-D.; Head-Gordon, M. Long-range corrected hybrid density functionals with damped atom–atom dispersion corrections. Phys. Chem. Chem. Phys. 2008, 10 (44), 6615-6620.
38. Li, A.; Muddana, H. S.; Gilson, M. K. 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.
39. Yanai, T.; Tew, D. P.; Handy, N. C. A new hybrid exchange–correlation functional using the Coulomb-attenuating method (CAM-B3LYP). Chem. Phys. Lett. 2004, 393 (1-3),
51-57.
40. Amirav, A.; Even, U.; Jortner, J. Cooling of large and heavy molecules in seeded supersonic beams. Chem. Phys. 1980, 51 (1-2), 31-42.