以理論計算探討具高效率反向系統間跨越性質的有機化合物設計原理與其有機發光二極體應用

Abstract

近年來，有機發光二極體之領域致力於發展可有效利用75 %三重態激子的有機螢光材料。目前最受注目的提高激子使用效率方法為設計具有電荷轉移激發態之有機分子，使其三重態激子更有效地透過反向系統間跨越至單重態，進而增加螢光放光。本篇論文將利用計算化學針對新一代有機發光二極體的發光分子進行探討，期望提供此類型發光分子的新設計觀點。本篇論文內容包含以下兩個部份： (i) 根據El-Sayed’s規則，其激發態的躍遷型態改變能有助於系統間跨越的發生，而目前大部分的熱啟動延遲螢光分子，皆在電荷轉移激發態之間進行反向系統間跨越。因此，在第一部份我們將根據自旋-軌域耦合積分值大小，探討具有電荷轉移和nπ*性質混合的激發態對於系統間跨越之影響。此外，我們也利用硫原子取代螢光素之羰基上的氧原子，觀察其重原子效應之影響。 (ii) 由於含時密度泛函理論(TD-DFT)在計算電荷轉移激發態時，若使用缺少非局部密度交換能描述的密度泛函時，常會出現低估激發態能量的問題。因此，在第二部分，我們將利用三種不同的密度泛函，對存在類似熱啟動延遲螢光性質的雜化局部-電荷轉移分子系統做詳細的第一原理調查。此外，我們也使用線性響應溶劑模型與態特定溶劑模型，比較此兩種不同方式描述溶劑影響激發態能量的差異性。Recently, many efforts are devoted to converting the 75 % triplet excitons into singlet excitons in pure organic molecules for the applications in organic light-emitting diodes (OLEDs). Currently the most often applied approach involves designing a charge transfer excited state in organic molecules to facilitate efficient reverse intersystem crossing (RISC) from triplet to singlet state, hence enhancing fluorescence emission and the exciton utilization efficiency. In this thesis, we investigate these new-generation emitters by computational methods, and we expect to propose new perspectives in terms of molecular design for OLED applications. This thesis involves the following two parts: (i) According to the El-Sayed’s rule, the intersystem crossing (ISC) may be enhanced if the singlet and the triplet excited states inolve different transition types. However, in most systems that show thermally activated delayed fluorescence (TADF) phenomenon, it is currently believed that the RISC process occurs between the singlet and the triplet charge-transfer (CT) states (same transition type). Therefore, in the first part, we will investigate the influence of nπ*- or ππ*- mixing with the CT excited state to ISC process by computing the spin-orbit coupling (SOC) integral between various low-lying excited states. Moreover, we explore heavy atom effect by substituting the oxygen atom the carbonyl group to the sulfur atom. (ii) It is a well-known issue that the application of time-dependent density functional theory (TD-DFT) on charge transfer excitations often leads to a severe underestimation if the non-local exchange energy is not properly included. In the second part of this thesis, we perform a careful first-principles investigation on a hybrid local charge transfer molecular system that is known to exhibit the TADF phenomenon, using three different functionals. In addition, we evaluate the performance of two different solvation models, the linear response polarized continuum model (LR-PCM) and the state-specific PCM (SS-PCM), in predicting the charge transfer excitation energies in the solution phase.