本論文透過實驗室自行架設的超快時間解析螢光(time-resolved fluorescence, TRFL)光譜儀與超快瞬態吸收(transient absorption, TA)光譜儀研究4-硝基聯苯(4-nitobiphenyl, 4-NBP)與2-硝基芴(2-nitrofluorene, 2-NF)在cyclohexane (CHX)及acetonitrile (ACN)中的激發態動力學行為，最後以理論計算的結果輔助解析分子的緩解機制。靜態吸收光譜及理論計算的結果顯示，4-NBP及2-NF在短波長(310 nm和320 nm)激發下主要被激發至1(ππ*)態，而在長波長(360 nm)激發下，依溶劑極性不同可被激發至1(nπ*)態或1(nπ*)/1(ππ*)態。而不同激發波長下的瞬態吸收光譜揭示了1(nπ*)態和1(ππ*)態的ESA訊號雖有著相同時間尺度的生命期(τ_(1 )< 100 fs)，但1(nπ*)態的ESA吸收峰範圍較1(ππ*)態寬廣許多且位於不同光區。此外，瞬態吸收光譜中還有一需以三個時間常數進行擬合的長壽命triplet state ESA訊號，τ_(2a )(~1.4-2 ps)為triplet state的internal conversion (IC)，τ_3(~10 ps)為triplet state的vibration relaxation (VR)，τ_4(數十ns) 反映能量最低的triplet state (T1)回到ground state的過程。在時間解析螢光光譜中，放光訊號隨激發波長變長而紅移，並可以biexponential decay模型擬合出τ_1小於100 fs，τ_2f約為0.4至2 ps。綜合實驗與理論計算之結果，兩分子不論被激發到S1或S2態後，皆傾向經由超快ISC緩解至triplet state，少部分會伴隨VR緩解至能量較低之vibrational state時發生稍慢之ISC。本研究首次清楚觀察到1(nπ*)態在瞬態吸收光譜中的特徵吸收及其動力學行為，為瞭解這類分子的激發態動力學提供了新的視角。

We investigate the excited-state relaxation dynamics of 4-nitrobiphenyl (4-NBP) and 2-nitroflourene in acetonitrile (ACN) and cyclohexane (CHX) solutions using a home-built ultrafast time-resolved fluorescence spectrometer (TRFL) and ultrafast transient absorption (TA) spectrometer. The experimental findings are complemented by theoretical calculations to elucidate the excited-state relaxation mechanisms of these molecules. Steady-state absorption spectra and theoretical calculations indicate that 4-NBP and 2-NF are predominantly excited to the 1(ππ*) state at shorter wavelengths (310 nm and 320 nm), while at longer wavelengths (360 nm), they can be excited to either the 1(nπ*) state or a mixture of 1(nπ*)/1(ππ*) states, depending on the solvent polarity. Transient absorption spectra at different excitation wavelengths reveal that both 1(nπ*) and 1(ππ*) state exhibit excited-state absorption (ESA) signals with similar lifetimes (τ_1< 100 fs), but the ESA band of the 1(nπ*) state is significantly broader and occurs in different spectral regions compared to the 1(ππ*) state. Additionally, the spectra show a long-lived triplet state ESA signal, which requires fitting with three time constants: τ_(2a )(~1.4-2 ps) for triplet state internal conversion (IC), τ_(3 )(~10 ps) for triplet state vibrational relaxation (VR), and τ_(4 )(tens of nanoseconds) for the return of the lowest energy triplet state (T1) to the ground state. Time-resolved fluorescence spectra show a redshift in the emission signal with increasing excitation wavelength, and a biexponential decay model fits the data with τ_1 less than 100 fs and τ_(2f )of approximately 0.4 to 2 ps. Combining experimental and theoretical results, it is evident that both molecules preferentially relax to the triplet state via ultrafast intersystem crossing (ISC) after excitation to either the S1 or S2 state, with a minor portion undergoing ISC accompanied by VR to lower vibrational states. This study provides the first clear observation of the characteristic absorption and dynamics of the 1(nπ*) state in transient absorption spectra, offering new insights into the excited state dynamics of these molecules.

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