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)光譜儀及超快瞬態吸收(transient absorption, TA)光譜儀研究4-硝基聯苯(4-nitrobiphenyl, 4-NBP)在不同溶劑中的激發態動力學行為，並藉由理論計算的結果來輔助解析4-NBP可能的激發態緩解機制。在兩個實驗當中我們皆以310 nm之飛秒雷射脈衝激發4-NBP至S2(ππ*)態，實驗與計算結果顯示出4-NBP不論在高低極性溶劑中的激發態緩解機制大致相同，我們推測可能的緩解路徑為主要由S2(ππ*)態internal conversion (IC)至S1(nπ*)態，其IC時間常數< 100 fs。其中少部分S2(ππ*)態與符合El-Sayed’s rule之三重態直接進行intersystem crossing(ISC)。隨後S1(nπ*)態再經ISC緩解至T(ππ*)態，此ISC過程不僅滿足El-Sayed’s rule且S1(nπ*)及T2(ππ*)態之間計算得出之SOC約為40 cm-1，進而導致快速ISC過程，其時間常數約為1～2 ps。此外，4-NBP於triplet manifold中對應於ISC接受態緩解至較穩定的T1的過程，則根據溶劑環境而有所不同，在ACN與CHX溶劑中的緩解過程只需要一個時間常數來描述，而在MeOH與TRI溶劑中，則可能多了與溶劑相關的specific interaction，因此需要兩個時間常數來描述。最後，由T1態緩慢回到基態過程，即T1態生命期，隨著溶劑極性減少而有變短的趨勢，從ACN中>50 ps至CHX中的2 ps。與實驗室前人所研究的反-4-硝基均二苯乙烯(trans-4-nitrostilbene, t-NSB)相比，t-NSB其ISC速率會隨著溶劑極性不同而有大幅變化，於非極性的CHX溶劑中，其ISC速率僅有0.26 ps，然而在高極性的ACN溶劑中則> 60 ps。而4-NBP之ISC速率幾乎不受溶劑極性影響，我們認為這是因為t-NSB進行ISC過程的起始態為受溶劑極性影響較大的S1(ππ*)態，而4-NBP進行ISC過程的起始態為較不受溶劑極性影響之S1(nπ*)態的緣故。

We study the excited-state dynamics of 4-nitrobiphenyl (4-NBP) in various solutions with home-built ultrafast time-resolved fluorescence (TRFL) spectrometer and ultrafast transient absorption(TA) spectrometer. With the aid of theoretical calculations, we propose an excited-state relaxation mechanism of 4-NBP. In both experiments, 4-NBP was excited to the S2(ππ*) state with femtosecond laser pulses at 310 nm. Both experimental and computational results indicate that the excited-state relaxation behaviors of 4-NBP are similar in various solvents of differing polarities. We speculate that the major deactivation pathway is the internal conversion (IC) from the S2(ππ*) state to the S1(nπ*) state with an IC time constant of < 100 fs. A minor portion of the S2(ππ*) population directly undergoes intersystem crossing (ISC) to triplet states that are allowed by the El-Sayed’s rule. Subsequently, the S1(nπ*) state undergoes ISC to the T(ππ*) state. This ISC process satisfies El-Sayed’s rule and has a calculated spin-orbit coupling (SOC) value of around 40 cm-1 between the S1(nπ*) and T2(ππ*) states, which leads to a rapid ISC process with a time constant of approximately 1 ps. The triplet manifold relaxation process from the ISC receiver state to the lowest-energy T1 state varies in different solvent environment. In ACN and CHX solvents, the relaxation process can be described with a single time constant, while in MeOH and TRI solvents, due to the additional solvent-specific interactions, it requires two time constants to describe the relaxation process. Finally, the T1 state slowly returns to the ground state, and the lifetime of T1 state tends to shorten as the polarity of the solvent decreases, ranging from >50 ps in ACN to 2 ps in CHX. Comparing with trans-4-nitrostilbene (t-NSB) studied previous in our laboratory, the ISC rate of t-NSB varies significantly with solvent polarity. In CHX, its ISC time is only 0.26 ps, while it increases to > 60 ps in ACN. In contrast, the ISC rate of 4-NBP is hardly affected by solvent polarity. This difference is attributed to the fact that the initial state for the ISC process in t-NSB is the S1(ππ*) state, which is more influenced by solvent polarity, while for 4-NBP, the initial state for primary ISC process is S1(nπ*) state, which is less affected by solvent polarity.

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