利用健級-鍵長關係式估算多環芳香烴之重組能

Abstract

隨有機光電材料的應用越來越多，如何尋找更好的有機半導體材料成為現今相當重要的議題。預測有機分子的重組能(reorganization energy)可以成為尋找材料的重要參考依據。因為根據能隙理論(energy gap law)，若將一分子的重組能降低，它的非輻射衰變(non-radiative decay)也會隨之降低。我們可以利用密度泛函理論(density functional theory, DFT)或 器學習模型計算各種分子的重組能，但這些方法卻無法提供我們關於重組能量值的背後原因的洞察。因此我們發展一種基於休克爾(Hückel)理論並使用一套鍵級-鍵長關係的方法，並應用在計算各種多環芳香烴(polyaromatic hydrocarbon, PAH)的激發態重組能。根據我們的結果，這個基於休克爾理論的方法成功預測分子激發後的鍵長變化，並可定性地預測重組能。透過這個模型，我們可以解釋為何大分子會有較小的重組能，以及發生激發的軌域對預測重組能的重要性。例外，我們也探討了La 以及Lb 激發態的性質，並展示我們可以從HOMO 和HOMO-1 預測偶數圓環多環芳香烴的激發態重組能。

As the importance of organic photoelectric materials goes higher and higher, how to find a better candidate molecule for such application becomes a crucial problem for researchers. Reorganization energy plays an important role in finding such materials for it is related to non-radiative decay probability according to energy gap law, and thus increase the quantum yield. DFT calculations and machine learning models can calculate reorganization energy of various molecules, but unable to give us deeper insights on causes behind reorganization energy values. We investigated a Hückel based theory with a bond order-bond length relationship on excited state reorganization energy of various polyaromatic hydrocarbons (PAHs). Our results shows that the Hückel based theory successfully predicted the bond length change after excitation, and generated semi-quantitative results on reorganization energies. With this model, we can explain the trend of smaller reorganization energy on larger molecules and the importance of transition orbitals on predicting reorganization energies. We also discussed the properties of La and Lb state and showed that we can predict the reorganization energy of PAHs consist of even-membered rings only from Hückel HOMO and HOMO-1.