本論文利用實驗室自行架設的超快時間解析螢光(time-resolved fluorescence, TRFL)光譜儀、飛秒瞬態吸收(transient absorption, TA)光譜儀以及一套自動化數據擷取電腦程式，研究反-4-硝基均二苯乙烯(trans-4-nitrostilbene, t-NSB)在不同溶劑中的激發態動力學行為。本論文分為兩個主要實驗，其中皆以383 nm之飛秒雷射脈衝激發t-NSB至其S1態，並分別利用超快時間解析螢光(TRFL)光譜和飛秒瞬態吸收(TA)光譜觀察其動力學行為。在超快TRFL光譜中，我們觀察到在極性溶劑中呈現明顯的solvation和振動緩解效應。t-NSB S1態之螢光生命期與溶劑極性相關，在acetonitrile中長達60 ps，而在cyclohexane中僅有0.26 ps。在飛秒TA光譜中，我們觀察到S1態excited state absorption (ESA)光譜以及三重態ESA光譜的演變，其中S1態ESA訊號之動力學可對應上述TRFL光譜所測得之螢光生命期。由於t-NSB S1態生命期與t-stilbene於溶液中的S1態生命期約為同數量級，故我們認為t-NSB在極性溶劑中的S1態生命期可能同時受制於ISC速率和跨越S1→1P*能障之速率，而在非極性溶劑中的S1態生命期則由ISC主導。此外，在TA光譜中也觀察到>2 ns的三重態生命期。在量子化學理論計算中，我們發現t-NSB在高低極性溶劑中的S1態皆為ππ*態，而與其能量較接近的三重態中只有T4態為nπ*態。計算結果也顯示在非極性溶劑中，沿S1緩解途徑S1(ππ*)及T4(nπ*)位能面交叉且又因其間的ISC過程滿足El-Sayed’s rule，因而導致超快速(~250 fs) ISC過程；但在高極性溶劑中S1態因solvation導致能量降低而無法直接ISC到T4態，但可能和經由vibronic coupling而混有T4(nπ*)態特徵的T3(*)或T2(*)態發生較緩慢的ISC。此外，t-NSB S1態經由快速ISC轉進三重態系統後，可能再經由連續快速的內轉換到達T1態，而T1態可無障礙地旋轉中心C=C雙鍵成3P*態，最後再慢慢地緩解至基態的P構型並分道轉回成反式t-NSB或順式c-NSB。

We study the excited-state dynamics of trans-4-nitrostilbene (t-NSB) in liquid solutions with broadband ultrafast time-resolved fluorescence (TRFL) spectroscopy and femtosecond (fs) transient absorption (TA) spectroscopy. This thesis mainly includes two types of experiments. In both experiments, we directly excited t-NSB to its S1 state with 383 nm fs laser pulses to observe its relaxation dynamics. In ultrafast TRFL experiments, solvation and vibrational relaxation behaviors were clearly observed in polar solvents. The fluorescence lifetime of the S1 state of t-NSB was found to be related to solvent polarity; it is up to 60 ps in acetonitrile and becomes as short as 0.26 ps in cyclohexane. In fs-TA experiments, we observed the evolutions of the excited-state absorption (ESA) spectra of S1 state and triplet states. By analyzing their time-dependent behaviors, we found that the kinetics of the S1-state ESA signal are consistent with the fluorescence lifetimes measured in TRFL experiments. Since the lifetimes of the S1 state of t-NSB in polar solvents are in the same order of magnitude as those of t-stilbene in solutions, we believe that the S1-state lifetimes of t-NSB in polar solvents are determined by both ISC and the S1→1P* barrier crossing rates. In non-polar solvents, the S1 state lifetime is dominated by ISC. In addition, the triplet lifetime (>2 ns) was also observed in the TA spectra. We also carried out quantum chemistry computations to aid in interpretations of experimental data. We found that the S1 state of t-NSB is a ππ* state in both polar and non-polar solvents, and among all nearby triplet states only the T4 state is a nπ*state. The calculated results also show that in non-polar solvents the S1(ππ*) and T4(nπ*) potential energy surfaces intersect along the S1 relaxation pathway, and the ISC between them satisfies the El-Sayed's rule, resulting in an ultrafast (~250 fs) ISC. On the other hand, due to solvation effect in polar solvents the S1 state is energetically unable to undergo ISC to the T4 state. Nonetheless, it may undergo slower ISC to T3(ππ*) or T2(ππ*) states having some mixed n* characteristics via vibronic coupling with the T4(nπ*) state. Once in the triplet system t-NSB can reach the T1 state through rapid sequential internal conversion and rotate the central C=C double bond into the 3P* state, where it slowly relaxes to the 1P state and finally branches back into t-NSB or c-NSB.

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