A Computational Study of Functional Materials: Ligand Exchange Mechanisms in MOF Synthesis and Charge Transfer in D-A-π-A Organic Sensitizers in DSSCs

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题名:	A Computational Study of Functional Materials: Ligand Exchange Mechanisms in MOF Synthesis and Charge Transfer in D-A-π-A Organic Sensitizers in DSSCs
作者:	邱志強;Chiu, Chihchiang
贡献者:	化學學系
关键词:	金屬有機架構;配位基交換;染料敏化太陽能電池;D-A-π-A;MOFs;Ligand Exchange;DSSCs;D-A-π-A
日期:	2020-01-18
上传时间:	2020-06-05 17:04:41 (UTC+8)
出版者:	國立中央大學
摘要:	本論文由「酸催化的結合取代機制應用於金屬有機框架合成中的配位基交換反應」和「染料敏化太陽能電池電荷轉移的計算研究：內部拉電子基對於D-A-π-A有機染料分子物理性質的影響」兩項研究組成酸催化的結合取代機制應用於金屬有機框架合成中的配位基交換反應金屬有機框架（metal-organic framework, MOFs）的合成可以透過下列方式來改善：添加調節劑合成（modulated synthesis）、使用前驅物進行合成（synthesis employing precursors）以及合成後交換（post-synthesis exchange, PSE），上述方式都有一個共同而關鍵的反應，也就是配位基交換的反應。然而，迄今為止，配位基交換的機制和控制交換的基本原理仍然難以捉摸。在此我們報告了二種羧酸1,4-苯二酸和2,3,5,6-四氟-1,4-苯二酸分別與前驅物Zr6O4(OH)4(OMc)12 (OMc = methacrylate，是鋯氧團簇Zr6O4(OH)4的配位基) 進行配位基交換過程的能量級距圖(energy landscapes)，該能量級距圖是使用密度泛函理論(density function theory, DFT)計算得出的。配位基交換的速率決定步驟遵循質子催化的結合取代(associative-substitution)反應機構，與先前文獻報導的動力學數據一致。我們的計算顯示，質子催化的結合取代反應取決於輸入與輸出這二個配位基的相對鹼性。這個開創性的發現使得許多以前用宏觀方法解釋的MOFs合成，可以合理地在分子層次上得到理論的支持。此研究結果為合成MOFs提供了新的見識，並為合成MOFs從一開始就提供了合理的新線索。染料敏化太陽能電池電荷轉移的計算研究：內部拉電子基對於D-A-π-A有機染料分子物理性質的影響 D-A-π-A染料不同於傳統的D-π-A骨架，在染料敏化太陽能電池（DSSC）應用中具有多個優點。相對於D-π-A染料，D-A-π-A染料紅移吸收光譜並顯示出特別的光穩定性。但是，內部的拉電子基對電荷轉移（CT）機率的影響尚不清楚。我們採用密度泛函理論（DFT），含時DFT（TD-DFT）和TD-DFT分子動力學（MD）模擬來研究內部拉電子基對D-A-π-A染料應用於DSSC時的光物理性質影響。我們的計算結果顯示，具有內部強拉電子基的D-A-π-A染料，其吸收帶具有雙重CT的顯著特徵；激發的電子密度同時轉移到內部和末端的拉電子基上。特別地，內部拉電子基在光激發時捕獲大量電子密度。在300K時的TD-DFT MD模擬表明，只有少量的激發電子密度在內部和末端拉電子基之間推拉。熱能不夠高，無法將電子密度從內部拉電子基傳遞到末端拉電子基。我們的研究揭示了D-A-π-A染料CT本質，為進一步合理的工程設計提供了理論基礎。 ;The dissertation is composed of the two studies, “Ligand Exchange in the Synthesis of Metal-Organic Frameworks Occurs Through Acid-Catalyzed Associative-Substitution”, and “A Computational Study of Charge Transfer in DSSCs: Effects of Internal Electron Acceptor in D-A-π-A Organic Dyes on Photo-physical Properties”. Ligand Exchange in the Synthesis of Metal-Organic Frameworks Occurs Through Acid-Catalyzed Associative-Substitution The syntheses of metal–organic frameworks (MOFs) can be improved through modulated synthesis, synthesis employing precursors, and post-synthetic exchange (PSE) modifications, all of which share ligand exchange as a common and crucial reaction. To date, however, the mechanism of ligand exchange and the underlying principles governing it have remained elusive. Herein, we report energy landscapes for the ligand exchange processes of 1,4-benzene dicarboxylic acid and 2,3,5,6-tetrafluoro-1,4-benzene dicarboxylic acid with Zr6O4(OH)4(OMc)12 (OMc = methacrylate), as calculated using density functional theory (DFT). The rate-limiting step of ligand exchange follows an associative-substitution mechanism catalyzed by protons, consistent with previous kinetic data. Our calculations suggest that the acid catalysis is dependent on the relative basicities of the incoming and outgoing ligands coordinated in the complex, allowing molecular-level rationalization of many seminal MOF syntheses that had previously been interpreted macroscopically. Our results provide new insights for MOF synthesis and new clues for the rational de novo synthesis of MOFs. A Computational Study of Charge Transfer in DSSCs: Effects of Internal Electron Acceptor in D-A-π-A Organic Dyes on Photo-physical Properties D-A-π-A dyes differ from the traditional D-π-A framework having several merits in dye sensitized solar cell (DSSC) applications. In relative to D-π-A dyes, D-A-π-A dyes red-shift absorption spectra and show particular photo-stability. Nevertheless, the effects of internal acceptor on the charge transfer (CT) probability are unclear. We employed density function theory (DFT), time-dependent DFT (TD-DFT), and TD-DFT molecular dynamics (MD) simulations to investigate the effects of internal acceptor on the photo-physical properties of D-A-π-A dyes on DSSCs. Our calculations show the absorption bands of D-A-π-A dyes with strong electron-withdrawing internal acceptor exhibit significant characteristics of dual CT; the excited electron density is transferred to the internal and terminal acceptors simultaneously. Particularly, the internal acceptor traps a significant amount of electron density upon photo-excitation. The TD-DFT MD simulations at 300K show only a small amount of excited electron density is pushing and pulling between the internal acceptor and the terminal acceptor moieties; the thermal energy is not high enough to drive the electron density from the internal acceptor to the terminal acceptor. Our study reveals the nature of CT bands of D-A-π-A dyes providing a theoretical basis for further rational engineering.
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