二聚體中單重態激發分裂電子耦合之理論計算研究

Theoretical Study on Singlet Fission Electronic Couplings in Dimer Systems

孫敬(Jing Sun)

指導教授 ： 鄭原忠

國立臺灣大學/理學院/化學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201701715

摘要

單重態分裂是一個描述分子在吸收到一個光子之後進而產生多個三重態激子的過程。這種過程由於被認為可以拿來應用在太陽能電池上而被大家所重視。雖然過去已有許多研究在討論單重態分裂，但是單重態分裂的反應機制仍然有所爭議，例如控制單重態分裂效率的條件和描述單重態分裂的激發態波函數等等。從電子結構著手，我們利用非絕熱基底近似展示了激發態的性質。並且利用有限電子活躍空間的電子組態交互作用闡述了多組態在多并苯二聚體激發態的重要性。藉由過去工作提到的三能態模型，有效哈密頓量展示了基組的選取和電子活躍空間的大小對於電子耦合常數影響很小，但對於電子激發態的能量卻影響很大。在嘗試引入完全電子活躍空間的二階微擾理論之後，結果呈現之強電子耦合及近簡併的性質也解釋了單重態分裂的反應機制，也表示在靜態和動態相關能量之間取得平衡在共軛體系也是相當重要的。最後，我們展示了二聚體中重要的有效電子耦合常數受到結構的影響。在二聚體中的有效電子耦合常數會和單體間的軌域重疊積分有著密切的關係，因此我們可以從單體間的重疊積分來得知分子間的相對方向會對單重態分裂的效率有什麼樣的影響。此方法也從分子結構方向的觀點提供了一個方向來設計高效單重態分裂的染料敏化太陽能電池。

關鍵字

單重態分裂 ； 電子組態交互作用 ； 耦合常數 ； 結構效應

並列摘要

Because of the promising potential in the high-efficiency solar cell,singlet exciton fission (SF), the molecular analog of multiexciton generation in which the absorption of one photon results in the generation of two triplet excitons, has attracted intensive research attention recently. Although the striking research are advancing, the mechanism of SF is still controversial, and some open questions remain, e.g., the key parameters manipulating the occurrence of SF and the excited state wavefunctions involved in it. From an electronic structure point of view, we construct an approximate diabatic basis to unambiguously interpret the character of the excited states, by applying the restricted active space equation of motion single and double approach to show the importance of considering multi-configurational effects in polyacene dimers. Using a three-state model, the effective hamiltonian shows that strong superexchange effective couplings depend on little on the choose of the basis set and the size of the active space while the diabatic state energies depend on a lot. Second order perturbation theory is introduced, the results show strong couplings and near degeneracy condition that explain the ultrafast SF mechanism, which shows striking the balance between static and dynamic correlation is also important in conjugated systems. Finally, we show that dominant effective couplings in a dimer system strongly correlated with the monomeric orbital overlap between two molecules. Hence, in a dimer system, the overlap between two monomers elucidate the dependence of the relative orientation of monomers on the efficiency of SF providing a criterion to the design principle of high SF efficiency dye-sensitized cells.

並列關鍵字

singlet fission ； configuration interaction ； coupling ； structural effect

參考文獻

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