本論文以靜態光譜、超快時間解析螢光光譜(TRFL)及超快瞬態吸收光譜(TA)技術研究不同激發波長下4-硝基聯苯(4-nitrobiphenyl, 4-NBP)及2-硝基芴(2-nitrofluorene, 2-NF)在cyclohexane (CHX)及acetonitrile (ACN)溶劑中的激發態緩解動力學，並藉由理論計算的結果來輔助解釋4-NBP及2-NF在激發態的緩解機制。在以短波長(310 nm/320 nm)及360 nm飛秒脈衝雷射激發4-NBP及2-NF在CHX及ACN溶劑的TA光譜中，我們皆觀察到初始單重激發態之 ESA訊號具有極短的initial decay (τ1 < 100 fs)。特別重要的是在4-NBP於CHX溶劑中，因為其S2(ππ*)及S1(nπ*)態之間的能量差較大，使我們能夠在4-NBP於CHX溶劑中使用310 nm或360 nm脈衝雷射分別激發到S2(ππ*)或S1(nπ*)，進而發現其分別展現出波長範圍較窄的吸收峰或broad band的ESA譜帶。我們發現當使用310 nm或320 nm激發4-NBP/2-NF至S2(ππ*)態時，在TA光譜中缺乏可被觀測到的S1(nπ*)特徵譜帶，這暗示S2(ππ*)態的主要衰減途徑是直接ISC到三重態，而S2(ππ*)態IC至S1(nπ*)態過程相對很慢，無法與S2(ππ*)態ISC至三重態形成有效競爭。而從receiver triplet states進行IC至T1態的過程，其時間常數τ2a約1~2 ps。T1態進行VR的過程，其時間常數τ3約10 ps。最後，從T1態回到基態的過程，其時間常數τ4隨溶劑極性降低有變短的趨勢(τ4從ACN中> 50 ns到CHX中~ 2 ns)。在以310 nm/320 nm及350 nm/360 nm飛秒脈衝雷射激發的4-NBP及2-NF在CHX及ACN溶劑的TRFL光譜中，我們都觀察到極短的螢光衰減訊號且其皆呈現biexponential decay (τ1< 100 fs與τ2f約0.4~2 ps)。τ1主要為單重激發態同時進行超快ISC及VR的過程，而τ2f (0.4~2 ps)為S2(ππ*)態或是S1(nπ*)態在VR過程中在較低振動能階進行較慢之ISC至三重態的過程，與TA光譜中所觀測的τ2a並不相同，且僅佔很小的比例。

In this study, the excited-state relaxation dynamics of 4-nitrobiphenyl (4-NBP) and 2-nitrofluorene (2-NF) were investigated by steady-state spectroscopy, ultrafast time-resolved fluorescence (TRFL), and ultrafast transient absorption (TA) spectroscopy at various excitation wavelengths in cyclohexane (CHX) and acetonitrile (ACN) solvents. With the aid of theoretical calculations, we proposed an excited-state relaxation mechanism of 4-NBP and 2-NF. The TA spectra revealed a rapid initial decay of the singlet excited state (τ1 < 100 fs) for both molecules when excited at 310 nm/320 nm and 360 nm. Notably, for 4-NBP in CHX, there was a significant energy gap between the S2(ππ*) and S1(nπ*) states, enabling selective excitation of the two states and leading to the observation of and characteristic narrow/broad ESA bands. This study also found that the primary decay pathway for the S2(ππ*) state was direct intersystem crossing (ISC) to triplet states, with the internal conversion (IC) to S1(nπ*) being comparatively slow and inefficient. Additionally, the subsequent relaxation in the triplet states was characterized by time constants τ2a (1-2 ps) for IC to the T1 state and τ3 (~ 10 ps) for vibrational relaxation (VR). Finally, the process of returning from the T1 state to the ground state showed a trend of decreased time constants with reduced solvent polarity, with τ4 ranging from over 50 ns in acetonitrile to approximately 2 ns in cyclohexane. In TRFL spectra, biexponential decay patterns were observed with τ1 < 100 fs and τ2f ~ 0.4-2 ps. τ1 primarily represented the ultrafast simultaneous processes of ISC and VR within the singlet excited state. τ2f (0.4–2 ps) represented a slower ISC to the triplet state during VR at lower vibrational levels of the S2(ππ*) or S1(nπ*) states. The τ2f process was different from the τ2a observed in TA spectra and constituted only a small proportion of the observed fluorescence.

1. Pitts Jr, J.N. Formation and fate of gaseous and particulate mutagens and carcinogens in real and simulated atmospheres. Environ. Health Perspect. 1983, 47115-140.
2. Baek, S.; R. Field; M. Goldstone; P. Kirk; J. LesterR. Perry. A review of atmospheric polycyclic aromatic hydrocarbons: sources, fate and behavior. WAT. AIR AND SOIL POLL. 1991, 60279-300.
3. Shen, H.; Y. Huang; R. Wang; D. Zhu; W. Li; G. Shen; B. Wang; Y. Zhang; Y. ChenY. Lu. Global atmospheric emissions of polycyclic aromatic hydrocarbons from 1960 to 2008 and future predictions. Environ. Sci. Technol. 2013, 47(12), 6415-6424.
4. Albinet, A.; E. Leoz-Garziandia; H. BudzinskiE. ViIlenave. Polycyclic aromatic hydrocarbons (PAHs), nitrated PAHs and oxygenated PAHs in ambient air of the Marseilles area (South of France): concentrations and sources. Sci. Total Environ. 2007, 384(1-3), 280-292.
5. Jariyasopit, N.; M. McIntosh; K. Zimmermann; J. Arey; R. Atkinson; P.H.-Y. Cheong; R.G. Carter; T.-W. Yu; R.H. DashwoodS.L. Massey Simonich. Novel nitro-PAH formation from heterogeneous reactions of PAHs with NO2, NO3/N2O5, and OH radicals: prediction, laboratory studies, and mutagenicity. Environ. Sci. Technol. 2014, 48(1), 412-419.
6. Atkinson, R.J. Arey. Atmospheric chemistry of gas-phase polycyclic aromatic hydrocarbons: formation of atmospheric mutagens. Environ. Health Perspect. 1994, 102(suppl 4), 117-126.
7. Arey, J.; B. Zielinska; R. Atkinson; A.M. Winer; T. RamdahlJ.N. Pitts Jr. The formation of nitro-PAH from the gas-phase reactions of fluoranthene and pyrene with the OH radical in the presence of NOx. Atmos. Environ. 1986, 20(12), 2339-2345.
8. Zhang, Q.; R. Gao; F. Xu; Q. Zhou; G. Jiang; T. Wang; J. Chen; J. Hu; W. JiangW. Wang. Role of water molecule in the gas-phase formation process of nitrated polycyclic aromatic hydrocarbons in the atmosphere: a computational study. Environ. Sci. Technol. 2014, 48(9), 5051-5057.
9. Durant, J.L.; W.F. Busby Jr; A.L. Lafleur; B.W. PenmanC.L. Crespi. Human cell mutagenicity of oxygenated, nitrated and unsubstituted polycyclic aromatic hydrocarbons associated with urban aerosols. Mutat. Res. - Genet. 1996, 371(3-4), 123-157.
10. Yang, X.-Y.; K. Igarashi; N. Tang; J.-M. Lin; W. Wang; T. Kameda; A. ToribaK. Hayakawa. Indirect-and direct-acting mutagenicity of diesel, coal and wood burning-derived particulates and contribution of polycyclic aromatic hydrocarbons and nitropolycyclic aromatic hydrocarbons. Mutat. Res. - Genet. 2010, 695(1-2), 29-34.
11. Fu, P.P. Metabolism of nitro-polycyclic aromatic hydrocarbons. Drug Metab. Rev. 1990, 22(2-3), 209-268.
12. Landvik, N.E.; M. Gorria; V.M. Arlt; N. Asare; A. Solhaug; D. Lagadic-GossmannJ.A. Holme. Effects of nitrated-polycyclic aromatic hydrocarbons and diesel exhaust particle extracts on cell signalling related to apoptosis: possible implications for their mutagenic and carcinogenic effects. Toxicology. 2007, 231(2-3), 159-174.
13. Øvrevik, J.; V. Arlt; E. Øya; E. Nagy; S. Mollerup; D. Phillips; M. LågJ. Holme. Differential effects of nitro-PAHs and amino-PAHs on cytokine and chemokine responses in human bronchial epithelial BEAS-2B cells. Toxicol. Appl. Pharmacol. 2010, 242(3), 270-280.
14. Plaza-Medina, E.F.; W. Rodríguez-CórdobaJ. Peon. Role of upper triplet states on the photophysics of nitrated polyaromatic compounds: S1 lifetimes of singly nitrated pyrenes. J. Phys. Chem. A. 2011, 115(35), 9782-9789.
15. Kovalenko, S.; R. Schanz; V. Farztdinov; H. HennigN. Ernsting. Femtosecond relaxation of photoexcited para-nitroaniline: solvation, charge transfer, internal conversion and cooling. Chem. Phys. Lett. 2000, 323(3-4), 312-322.
16. Crespo-Hernández, C.E.; G. BurdzinskiR. Arce. Environmental photochemistry of nitro-PAHs: direct observation of ultrafast intersystem crossing in 1-nitropyrene. J. Phys. Chem. A. 2008, 112(28), 6313-6319.
17. Zugazagoitia, J.S.; C.X. Almora-DíazJ. Peon. Ultrafast intersystem crossing in 1-nitronaphthalene. An experimental and computational study. J. Phys. Chem. A. 2008, 112(3), 358-365.
18. Mohammed, O.F.E. Vauthey. Excited-state dynamics of nitroperylene in solution: solvent and excitation wavelength dependence. J. Phys. Chem. A. 2008, 112(17), 3823-3830.
19. Thomsen, C.L.; J. ThøgersenS.R. Keiding. Ultrafast charge-transfer dynamics: studies of p-nitroaniline in water and dioxane. J. Phys. Chem. A. 1998, 102(7), 1062-1067.
20. Ohtani, H.; T. Kobayashi; K. SuzukiS. Nagakura. Picosecond spectroscopy studies of the intersystem crossing of aromatic carbonyl and nitro compounds in solution. Bull. Chem. Soc. Jpn. 1980, 53(1), 43-47.
21. Vogt, R.A.; C. ReichardtC.E. Crespo-Hernández. Excited-state dynamics in nitro-naphthalene derivatives: intersystem crossing to the triplet manifold in hundreds of femtoseconds. J. Phys. Chem. A. 2013, 117(30), 6580-6588.
22. Morales-Cueto, R.; M. Esquivelzeta-Rabell; J. Saucedo-ZugazagoitiaJ. Peon. Singlet excited-state dynamics of nitropolycyclic aromatic hydrocarbons: Direct measurements by femtosecond fluorescence up-conversion. J. Phys. Chem. A. 2007, 111(4), 552-557.
23. McCusker, J.K. Femtosecond absorption spectroscopy of transition metal charge-transfer complexes. Acc. Chem. Res. 2003, 36(12), 876-887.
24. López-Arteaga, R.; A.B. Stephansen; C.A. Guarin; T.I. SøllingJ. Peon. The influence of push–pull states on the ultrafast intersystem crossing in nitroaromatics. J. Phys. Chem. B. 2013, 117(34), 9947-9955.
25. Fan, Z.; R.M. Kamens; J. Hu; J. ZhangS. McDow. Photostability of nitro-polycyclic aromatic hydrocarbons on combustion soot particles in sunlight. Environ. Sci. Technol. 1996, 30(4), 1358-1364.
26. Braslavsky, S.E. Glossary of terms used in photochemistry, (IUPAC Recommendations 2006). Pure Appl. Chem. 2007, 79(3), 293-465.
27. Vogt, R.A.; C. ReichardtC.E. Crespo-Hernández. Excited-State Dynamics in Nitro-Naphthalene Derivatives: Intersystem Crossing to the Triplet Manifold in Hundreds of Femtoseconds. J. Phys. Chem. A. 2013, 117(30), 6580-6588.
28. Plaza-Medina, E.F.; W. Rodríguez-Córdoba; R. Morales-CuetoJ. Peon. Primary photochemistry of nitrated aromatic compounds: excited-state dynamics and NO· dissociation from 9-nitroanthracene. J Phys Chem A. 2011, 115(5), 577-85.
29. Rodríguez-Córdoba, W.; L. Gutiérrez-Arzaluz; F. Cortés-GuzmánJ. Peon. Excited state dynamics and photochemistry of nitroaromatic compounds. ChemComm. 2021, 57(92), 12218-12235.
30. Crespo-Hernández, C.E.; G. BurdzinskiR. Arce. Environmental Photochemistry of Nitro-PAHs: Direct Observation of Ultrafast Intersystem Crossing in 1-Nitropyrene. J. Phys. Chem. A. 2008, 112(28), 6313-6319.
31. Larsen, M.A.; J. Thøgersen; A.B. Stephansen; J. Peon; T.I. SøllingS.R. Keiding. Transient IR spectroscopic observation of singlet and triplet states of 2-nitrofluorene: revisiting the photophysics of nitroaromatics. J. Phys. Chem. A. 2016, 120(1), 28-35.
32. 陳品勳. 以超快時間解析雷射光譜研究4-硝基聯苯於溶液中之激發態緩解動力學. 國立清華大學. 2023.
33. 許友誠. 以飛秒雷射光譜研究4-硝基聯苯於液相溶液中之超快激發態動態學. 國立清華大學. 2023.
34. Boyd, R.W. Nonlinear Optics. Academic Press. 1992.
35. Kalpouzos, C.; W.T. Lotshaw; D. McMorrowG.A. Kenney-Wallace. Femtosecond laser-induced Kerr responses in liquid carbon disulfide. J. Phys. Chem. A. 1987, 91(8), 2028-2030.
36. Appavoo, K.M.Y. Sfeir. Enhanced broadband ultrafast detection of ultraviolet emission using optical Kerr gating. Rev. Sci. Instrum. 2014, 85(5), 055114.
37. Arzhantsev, S.M. Maroncelli. Design and Characterization of a Femtosecond Fluorescence Spectrometer Based on Optical Kerr Gating. Appl. Spectrosc. 2005, 59(2), 206-220.
38. 邱志忠. 以超快時間解析光學克爾光閘螢光光譜研究分子間與分子內電荷轉移. 國立清華大學. 2013.
39. Lorenc, M.; M. Ziolek; R. Naskrecki; J. Karolczak; J. KubickiA. Maciejewski. Artifacts in femtosecond transient absorption spectroscopy. Applied Physics B: Lasers and Optics. 2002, 74(1), 19-27.
40. Görner, H.D. Schulte-Frohlinde. Tripletspectra of 4-Nitrostilbenes in Polar and Non-Polar Solvents at Different Temperatures and the Configuration of the Triplet State. 1978, 82(10), 1102-1107.
41. Snellenburg, J.J.; S. Laptenok; R. Seger; K.M. MullenI.H.M. van Stokkum. Glotaran: A Java-Based Graphical User Interface for the R Package TIMP. J. Stat. Softw. 2012, 49(3), 1 - 22.
42. Morales-Cueto, R.; M. Esquivelzeta-Rabell; J. Saucedo-ZugazagoitiaJ. Peon. Singlet Excited-State Dynamics of Nitropolycyclic Aromatic Hydrocarbons:  Direct Measurements by Femtosecond Fluorescence Up-Conversion. J. Phys. Chem. A. 2007, 111(4), 552-557.
43. Plaza-Medina, E.F.; W. Rodríguez-Córdoba; R. Morales-CuetoJ. Peon. Primary Photochemistry of Nitrated Aromatic Compounds: Excited-State Dynamics and NO· Dissociation from 9-Nitroanthracene. J. Phys. Chem. A. 2011, 115(5), 577-585.
44. Plaza-Medina, E.F.; W. Rodríguez-CórdobaJ. Peon. Role of Upper Triplet States on the Photophysics of Nitrated Polyaromatic Compounds: S1 Lifetimes of Singly Nitrated Pyrenes. J. Phys. Chem. A. 2011, 115(35), 9782-9789.