本篇利用時間相關的密度泛涵理論(TD-DFT)對 2-(2’-hydroxyphenyl)benzimidazole (HBI) 以及其胺基取代衍生物， HBI-NH2 和 HBI-NEt2 ，在激發態時的位能曲面進行探討。HBI分子受到光的激發之後，在激發態時會進行分子內質子轉移的行為 (ESIPT)，而形成釋放螢光的異構化結構，以S1-Tsyn簡稱之。然而當HBI分子在苯酚的對位 (para) 上接上胺基取代基後，其螢光的表現和螢光量產率與HBI分子相比卻有著明顯的不同。因此我們希望能藉由激發態位能曲線圖的計算，瞭解HBI系列分子在激發態的分子行為。 根據計算結果，在激發態HBI分子進行ESIPT時，是為不具有活化能 (activation energy) 的反應。而與HBI分子不同的是，其胺基取代衍生物，HBI-NH2 和HBI-NEt2，則具有反應活化能。此活化能的存在降低釋放螢光物種的生成，使得HBI-NEt2 的螢光量子產率較HBI分子低。另外，HBI 、HBI-NH2 、HBI-NEt2三個分子在激發態時，亦可能存在穩定而非平面的分子結構，且在C1的位置上具有角錐化 (pyramidalization)。因此，我們以扭轉N6-C1-C2-C3的二面角與在C1位置上的角錐化程度作為反應座標，進行二維位能曲線圖的計算，並分析其可能的最低能量路徑，探討此三個分子於激發態的行為。反應活化能的存在，可說明實驗上所觀測到與溫度相關之螢光量子產率的變化。當溫度升高，S1-Tsyn的比率隨之降低，S1-TCT(非輻射鬆弛)的比例則相對提高，螢光量子產率也隨之下降。但三個分子間，較為特別的是HBI分子在乙醇溶劑下，其S1-TCT能量較S1-tau高，因此當溫度升高時，逆反應的速度也隨之提高，而使得HBI分子在乙醇溶劑下，其螢光量子產率和溫度無關。 The potential energy surfaces (PES) of the first singlet excited state (S1) of 2-(2’-hydroxyphenyl)benzimidazole (HBI) and its amino substituted derivatives (HBI-NH2 and HBI-NEt2) are investigated using time-dependent density functional theory. Upon excitation, these molecules undergo the excited-state intra-molecular proton transfer process (ESIPT) to produce their excited tautomeric forms, the major species of fluorescence. Interestingly, when HBI is substituted by a diethyl amino group at para-position of phenyl group, the fluorescence quantum yield is significant reduced. We investigate the possible quenching mechanisms by searching the excited state PES connecting with the fluorescing tautomeric form. We found that the ESIPT is energy barrier-less for HBI; however, there is a low energy barrier (e.g. Ea = 1.98 kcal/mole for HBI-NEt2 in ethanol) exists retarding the fast ESIPT of HBI-NH2 and HBI-NEt2. This barrier first reduces the fluorescence quantum yield of HBI-NEt2 in relative to that of HBI. More interesting, these three molecules studied here also can adopt two stable charge-transfer forms in their S1 state. The S1-CT conformer has a non-planar structure; moreover, it is pyramidalized at benzimidazole. The formation of S1-CT state can quench the fluorescence of excited tautomeric form. The energy barrier for S1-tautomer to S1-CT reaction is similar for these three molecules (ca. 2.5 kcal/mol). Moreover, the S1-CT forms for HBI-NH2 and HBI-NEt2 have lower energy than their corresponding tautomers. Therefore, increasing the temperature will speed up the formation of S1-CT conformer and further quench the fluorescence, consistent with experimental observations. Interesting, the S1-CT form of HBI is less stable than its tautomer. Increasing the temperature though speeds up the formation of CT conformer; however, it also enhances the reverse reaction resulting in a T-independent fluorescence quantum yield.
