多並苯二聚體系統中單重態激發子分裂之理論計算研究

Abstract

單重態激發子分裂，一個經由吸收單一光子產生兩個獨立三重態激 發子的光化學現象，由於具有應用於高效能太陽能發電材料的潛力而 備受囑目。然而，儘管大量的研究資源投入於此領域之中，科學家們 對於單重態激發子分裂的反應機制依然沒有達成共識，並且尚存在著 許多未解決的疑問。例如：驅使單重態激發子分裂發生的因素為何， 以及當中牽涉到的激發態波函數真正的性質等等。 從電子結構的觀點著手，我們利用非絕熱基底近似成功地展示了激發 態中電子躍遷圖像，以及利用有限電子活躍空間的電子組態交互作用 （考慮單電子及雙電子激發）計算闡述了多重組態效應在多並苯的電子 結構中扮演著相當重要的角色，尤其是來自雙重單重態電子組態的影 響。 藉由建立弗朗凱爾激發態-電荷轉移態-雙重單重態的三能態模型，我 們成功地解釋實驗上觀測到在飛秒尺度下發生的單重態激發子分裂， 並說明了其是藉由強烈的超交換耦合常數以及近乎重能態的條件而發 生的。我們發現到在影響單重態激發子分裂的因素之中，電荷轉移能 態的能階位置大幅地影響著超交換耦合常數的數值，因此被認為是最 重要的因素。因此，除了過去考慮第一單重態與兩倍三重態之能量差 以外，我們提出電子轉移態能階的降低應該也要納入判斷單重態激發 子分裂發生與否的考量之中。 另外，藉由從分子軌域對電子組態交互作用的分析，我們可以從單分 子的分子軌蜮來瞭解二重體中兩個分子的相對位置會對單重態激發子 分裂帶來什麼樣的影響。此方法不但能省去電子組態交互作用昂貴的 計算資源而提供對電子結構定性的分析，也能進而從分子設計的觀點 去提出發生高效率單重態激發子分裂的分子設計方向。

Because of the promising potential in the high efficiency solar cell, singlet exciton fission, the molecular analogue of multiexciton generation resulting in two independent triplet excitons by absorbing a single photon, has attracted increasing attention recently. Although the striking research works 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 configuration interaction single and double approach to show the importance of considering multi-configuration effects in polyacene dimers, especially the key role of double-excitation configurations. Using a three-state model, strong superexchange effective coupling and the near degeneracy condition that explain the ultrafast SF mechanism are obtained. We demonstrate that a crucial factor is the energetic position of the charge transferred diabatic state, which remarkably controls the amplitude of the effective coupling. Therefore, in addition to the near degeneracy of the lowest lying singlet exciton and the double-triplet state, we conclude that the lowering of the charge transferred diabatic state energy should also be considered as a key factor for the design of hign efficiency SF materials. Finally we show that dominant interactions controlling SF efficiency in a dimer can be decomposed into Conlomb interaction between monomer molecular orbitals. Trough molecular orbital based analysis of configuration interactions involved in the SF process, the driving force of SF can be determined from the relative orientation of monomers, providing an effective method to survey potential molecules and further guideline to the design principle of high SF dye-sensitizing materials.