以理論計算探索重組能之光物理效應

Abstract

近年來，隨著光物理之研究領域蓬勃發展，相關機制也日益被重視，例如：生物顯影技術之遠紅外放光量子效率之提升與有機發光二極體效率提升之微觀光物理機制。本論文利用計算化學探索重組能對以上兩大光物理現象之影響，內容分別如下： I. 為了提升遠紅外放光分子的放光量子效率，以Franck Condon Principle與Marcus Theory為根基，將基態與激發態兩能階之間的電子躍遷與遲豫至優化結構的能量差定為重組能，並以青色素分子（Cyanines）與電子予體－受體分子（Donor－acceptor compounds），兩類目前研究上最具遠紅外放光潛能之分子，以DFT以及TD－DFT計算結果比對分子差異，並探討分子結構對重組能的影響。 II. 以Fermi’s Golden Rule與Marcus Theory為基礎，將電子轉移速率推廣至第一單重激發態與第一參重激發態的轉換速率，並以絕熱狀態、低耦合為前提，在構象座標中，y軸表示以基態優化結構能量為零點的相對能量，x軸表示不同結構，兩能態得以兩拋物線表達。兩拋物線的相交點為由第一參重激發態轉換至第一單重激發態所需經過的能量障礙。透過DFT以及TD－DFT計算各能態能量並運用二次函數運算，計算不同分子之能障，討論熱延遲螢光放光現象計算上預測之判斷依據。

With booming researches in photophysics, the underlying mechanisms behind many interesting phenomena are attracting more attention, e.g. the quantum efficiency of near-IR emitting compounds used in bioimaging, and detailed mechanisms of thermally activated delayed fluorescence (TADF). In this thesis, we use computational methods to explore these two fundamental photophysical topics. I. We explore the relation between the molecular structure and the reorganization energy. In principle, the quantum efficiency formula can be enhanced by reducing the nonradiation rate constant, which depends strongly on the magnitude of the reorganization energy. Based on Franck Condon Principle and Marcus theory, the reorganization energy is defined as the the first singlet excited state (S1) state in the ground state (S0) structure and in the optimized S1 structure. Using DFT and TD-DFT methods, we compute the reorganization energies of cyanine-related compounds and other donor-acceptor compounds. Our detailed analysis indicates that the reorganization energy may be effectively reduced by modifying the molecular structure. II. We compute the intersystem crossing energy barrier between the first singlet excited state (S1) and the first triplet excited state (T1) for various systems, and discuss its effect on TADF. On the condition that S1 and T1 states are adiabatic and weakly coupled, these states may be simplified as parabolas on the conformation coordinate diagram based on Fermi’s Golden Rule and Marcus Theory. Using DFT and TD-DFT, we calculate the energy barrier by solving the quadratic function of the enegy parabola, and we attempt to propose rules for qualitative and quantitative predictions on the experimentally observed TADF phenomenon.