二苯乙烯分子是同時具有「螢光」(fluorescence) 與「光學異構化」(photoisomerization) 行為的小分子，一直是研究光化學行為的重要模型分子。本篇研究利用電子密度泛函理論 (DFT) 及時間相關的電子密度泛函理論 (TD-DFT) 方法，對反式-二苯乙烯 (trans-stilbene) 分子及不同位置取代的反式-二苯乙烯胺 (trans-aminostilbene) 分子進行基態 (ground state，S0) 結構與第一激發單重態 (first excited singlet state，S1)光化學行為的研究。 第一激發單重態中的分子結構特徵與基態不同，乙烯基鍵結方面，S0 狀態下呈現較明顯的單-雙鍵交錯的特性，然而，S1 狀態下則呈現較明顯的「部份雙鍵」特性；氨基方面，S0 狀態下擁有明顯的「角錐化現象」(pyramidalization)，然而，S1 狀態下此「角錐化程度」明顯下降甚至消失，導致分子在S1 狀態下 π-電子「非定域化」(delocalization) 程度增加。 基本上，計算的UV-Vis 光譜和實驗一致。從計算得到的 UV-Vis 光譜、螢光光譜以及「雙鍵扭轉能障」比對實驗結果，推測實驗觀察到的鄰位取代反式-二苯乙烯胺分子單一放光行為，主要是由其中一種旋轉異構體 (rotamer) 所貢獻；然而，間位取代反式-二苯乙烯胺分子的單一放光行為，是由兩種旋轉異構物同時貢獻。計算得到的雙鍵扭轉能障基本上和實驗相符。計算結果可解釋並推論實驗結果，進而輔助分子設計。 Chemical reactions can be stimulated by the light absorbed by molecules. One of the simple examples is the cis-trans photoisomerization of stilbene. We have studied the photochemistry of trans-stilbene and its derivatives, p-trans-aminostilbene, m-trans-aminostilbene and o-trans-aminostilbene, using first principle density functional theory and time-dependent density functional theory. Molecular geometries of ground state (So) and first excited singlet state (S1), UV-vis spectra, fluorescence spectra as well as the potential energy surfaces of photoisomerization were calculated. The molecular geometries of S1 state are differential from these of S0 state, in particular, the ethylene and amine groups. The ethylene groups own the characteristics of alternated single-double bonds at So state; however, at S1 state, these bonds have obvious characteristics of partial double bonds. The pyramidalization of amine group at So state decreases or even completely vanishes at S1 state. In general, the π-electrons on trans-stilbene and trans-aminostilbene at S1 state are more delocalization than at So state. The calculated UV-vis spectra are good agreement with experimental observations; moreover, the calculated UV-vis spectra are similar for m-trans-aminostilbene and o-trans-aminostilbene rotamers. Similar results are observed for the fluorescence spectra of m-trans-aminostilbene rotamers; furthermore, they also own similar activation energies (Ea) of cis-trans isomerization. More interestingly, the difference of emission wavelengths of o-trans-aminostilbene rotamers is up to 57 nm with one of them close to the experimental result. In addition, the activation energies of cis-trans isomerization for o-trans-aminostilbene rotamers are discrete, one with lower Ea can have fast isomerization and have weak fluorescencing and vice versa. Taking these results together, the single fluorescence of m-trans-aminostilbene from experiments is contributed from its two rotatmers simultaneously; in contrast, only one of m-trans-aminostilbene rotamers with stronger fluorescencing dominates the experimental observations.
