電子轉移和單重態裂變之耦合值

電子耦合值對於預測電子轉移速率扮演著重要的角色。利用雙態模型下的 Generalized Mulliken-Hush (GMH) and Fragment Charge Difference (FCD) 方法，激發態的電子耦合值可以被計算出來。雖然這兩個方法己經成功地應用在絕大多數的分子系統中，但對於某些系統仍然有些問題，因為使用Configuration Interaction Singles方法，有時候電子轉移(CT)態會參雜著些區域性激發態(LE)。因此我們發展多態FCD方法，這是一個不用手動挑選並分類CT或LE態，而且讓程式自動產生出各種非絕熱態。這方法也適用於GMH。所以我們利用這方法測試了幾個分子系統，也得到了比雙態FCD或GMH 更局域化的電荷分布及電子耦合值。單重態裂變是從一個激發單重態裂變成兩個三重態的過程，從一個光子的吸收，最後有兩個機會產生光致電流,這有助於提升太陽能電池的效率。而三重態煙滅則是前者的反向過程，它是讓兩個三重激發態分子，有機會發出一個高頻率的光，這可應用於上轉換材料的發展。對單重態裂變之反應而言，初態是 (S1S0) 而末態是 (T1T1})，而三重態煙滅的初態是 (T1T1) 而末態是 (S1S0)。基本上，前者和後者的電子耦合值，可利用同一個理論計算方法得到。在這個論文中，我們使用 Fragment Spin Difference (FSD)　和 direct coupling with natural transition orbitals (DC-NTO) 方法來得電子耦合值。利用4個軌域模型，我們發現對稱性與電子轉移、三重態能量轉移等都不相同。因此在高度對稱的分子間堆疊中，容易產生耦合值為零的情形。另外，我們發現電子耦合值隨>著距離呈現指數大於2$angstrom^{-1}$的衰退，這是屬於近程的相互作用。除此之外，從五苯 (Pentacene)　的計算中，我們發現當初態或末態中參雜了CT態時，耦合值會變大，其理論的反應速率估計值是接近實驗值(100~fs)。因此CT態在單重態裂變中是扮演非常重要的角色。

關鍵字

電子轉移；單重態裂變；耦合值

並列摘要

The electronic coupling is an important factor for the electron transfer (ET) rate prediction. Based on the two-state model, the Generalized Mulliken-Hush (GMH) and Fragment Charge Difference (FCD) schemes have been useful approaches to calculate ET coupling from an excited state calculation. However, the ET problems are not always two-state in nature, i.e. the charge transfer (CT) state obtained is sometimes mixed with nearby local excited (LE) states in Configuration Interaction Singlets (CIS) method. Therefore, we develop a general multi-state approach for FCD without the need of manual assignment for the states and automate the process in generating the diabatic states. This scheme can be generalized for GMH as well. We test the new multi-state schemes for the performance in systems that have been studied using more than two states with FCD or GMH. We found that the multi-state approach yields much better charge-localized states and electronic couplings in these systems. Singlet fission is a process where a singlet exciton is split into a pair of triplet excitons. Since a high-energy photon is split into two lower-energy triplet excitons, it has the potential to increase the efficiency of solar cell. The reverse process, triplet-triplet annihilation (TTA) converts a high-energy exciton from two low-energy triplet excitations. It is also an important process that is capable of up-converting the photon energy. In the singlet fission process, the initial state is (S1S0) and final state is (T1T1). For TTA, the initial state is (T1T1) and final state is (S1S0). Computational method for singlet fission coupling is applicable to TTA. In the present work, we use Fragment Spin Difference (FSD) and direct coupling with natural transition orbitals (DC-NTO) methods to calculate the singlet fission/TTA coupling. Based on 4 orbitals model, we found that there exists a unique symmetry for coupling and it implies a different selection rule from that of electron transfer or energy transfer. Moreover, we found that the distance dependence with the electronic coupling is exponential decay and decay rate is larger than 2 angstrom^-1. It indicates that the singlet fission is the short range interaction. Moreover, we performed the singlet fission coupling and rate with the pentacene molecules and found that the theoretical rate would be approached to the experimental value (~100 fs), when the initial or final state is mixed with the CT component. Thus we conclude that the CT-mediated state plays an important role in singlet fission.

並列關鍵字

electron transfer ； singlet fission ； coupling

參考文獻

[71] Chow, T. J.; Chiu, N.-R.; Chen, H.-C.; Chen, C.-Y.; Yu, W.-S.; Cheng, Y.-M.; Cheng,

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[20] Chen, H.-C.; Hung, C.-Y.; Wang, K.-H.; Chen, H.-L.; Fann, W. S.; Chien, F.-C.;

[78] Zimmerman, P. M.; Bell, F.; Casanova, D.; Head-Gordon, M. J. Am. Chem. Soc.

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電子轉移和單重態裂變之耦合值

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