葉綠素內轉換之理論研究

A Theoretical Study on the Internal Conversion Dynamics of Chlorophylls

施欣妤(Petra Shih)

指導教授 ： 鄭原忠

國立臺灣大學/理學院/化學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201803678

摘要

分子激發態的非絕熱躍遷在許多光物理和光化學過程中起著重要作用。特別是，葉綠素(chlorophyll)中Qx→Qy的超快內轉換(internal conversion)對於光合作用中光捕獲(light harvesting)的高效率至關重要，但其機制尚未明確闡明。在本工作中，我們通過評估振動耦合(vibronic coupling)來建構研究非絕熱內轉換的有效哈密頓算符(effective Hamiltonian)，從理論上探索了葉綠素a和細菌葉綠素a之Qx→Qy動力學。通過結合含時密度泛函理論(time-dependent density functional theory)和透熱化(diabatization)方法，最大化兩個低激發態的組態(configuration)均勻性，實現了無輻射弛豫(non-radiative relaxation)的第一原理研究。使用費米黃金律(Fermi’s Golden rule)計算的弛豫速率表明，快於100飛秒Qx→Qy過程可以用具有弱振動耦合的透熱模型充分描述。此外，我們還確定了一些支配Qx→Qy弛豫的關鍵振動模式(vibrational normal mode)，高度溶劑和取代基相關的弛豫速率可歸因於Qy/Qx能階間隙(energy gap)的變化。預計所開發的方法能廣泛應用於分子系統中能量弛豫的詳細動力學研究，並且還使我們能夠深入了解自然界光合作用中最重要的發色團的設計。

關鍵字

內轉換 ； 葉綠素 ； 非絕熱動力學 ； 透熱化 ； 震動耦合

並列摘要

Non-adiabatic transitions in molecular excited states play significant roles in many photophysical and photochemical processes. In particular, the ultrafast internal conversion of Qx→Qy in chlorophylls is crucial to the high efficiency of light harvesting in photosynthesis, yet the mechanism has not been clearly elucidated. In this work, we explored the internal conversion processes of chlorophyll a and bacteriochlorophyll a theoretically by evaluating the vibronic couplings and electronic couplings to construct effective Hamiltonians for the non-adiabatic Qx→Q¬y dynamics. The first principle study of the radiationless relaxation was achieved by combining time dependent density functional theory (TD-DFT) and diabatization method through enforcement of configuration uniformity for the two low lying excited states. The relaxation rates calculated using Fermi’s Golden rule suggest that the sub-100 fs Qx→Qy process can be fully described by a diabatic model with weak vibronic couplings. In addition, we also identified a few key vibrational modes that dominate the Qx→Qy relaxation, and the highly solvent and substituent dependent relaxation rates can be attribute to variations in Qy/Qx energy gaps. The methodology developed is expected to enable detailed dynamical study of energy relaxation in general molecular systems, and it also allows us to gain insights into the natural design of the most important chromophores in photosynthesis.

並列關鍵字

diabatization ； internal conversion ； chlorophyll ； non-adiabatic dynamics ； vibronic coupling

參考文獻

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