DHP 質子轉移系統與長壽命 LEPT 分子之激發態光譜動力學研究

Abstract

近年來，質子耦合能量轉移（proton-coupled energy transfer, PCEnT）作為一種新型能量轉移機制逐漸受到關注。相較於傳統能量轉移過程，PCEnT 不僅涉及電子激發能量的傳遞，也伴隨質子轉移反應的發生，因此可在未直接激發質子轉移單元的情況下產生質子轉換後的放光。然而，目前室溫溶液中 PCEnT 的直接實驗證據僅有AbPP系列，且對於 PCEnT 形成的 local electron–proton transfer（LEPT）state 是否可進一步與 charge-separated state（CSS）發生轉換，仍缺乏明確的動力學證據。為了釐清此問題，本論文以具有激發態雙質子轉移性質的 DHP 系列分子為基礎，進一步設計 An-DHP 分子，探討其激發態質子轉移、PCEnT 與 LEPT 與 CSS 轉換動力學。 本論文第一部分針對 [2,2′-bipyridine]-3,3′-diol（DHP）及其甲基取代衍生物 DHP-Me 與 DHP-diMe 進行研究，以探討對稱性與甲基取代數量對激發態雙質子轉移動力學的影響。穩態光譜與奈秒尺度時間解析螢光結果顯示，三個分子皆具有明顯的互變異構態放光與奈秒尺度的衰減。進一步利用螢光上轉換技術觀察其超快動力學後發現，具有對稱結構的 DHP 與 DHP-diMe 在紅端放光區域呈現皮秒尺度衰減訊號，並在藍端放光區域觀察到上升訊號；相較之下，不對稱的 DHP-Me 在藍端區域的上升訊號 明顯消失。結合理論計算結果可知，DHP 系列分子中 mono-keto（MK）與 di-keto（DK）states 的相對能量差異不大，且 MK state的位能面很淺，顯示其可能不是一個完全侷限且明確的中間態。因此，本研究提出修正後的動力學模型，認為 DHP 與 DHP-diMe 中藍端上升訊號 可能來自 MK-like 與 DK states 之間的平衡過程，而 DHP-Me 則因不對稱結構造成初始單質子與雙質子轉移速率比例改變，使上升訊號不容易被觀察。 本論文第二部分進一步研究全新設計的 An-DHP 分子。An-DHP 由具有質子轉移性質的 DHP-Me core 與作為能量供應者的蒽單元組成，且兩者在吸收光譜上具有分離。當以 410 nm 選擇性激發蒽單元時，仍可觀察到來自 DHP core 的 質子轉換後的放光，且其放光波形與奈秒尺度生命週期皆與 DHP-Me 相近，顯示 An-DHP 可透過 PCEnT 形成長壽命 LEPT state。在至中中極性性溶中中，An-DHP 主要呈現奈秒尺度 LEPT decay，表示此 LEPT state 相對穩定。然而，當溶中性性提高中 ACN 時，原先對應於 DHP-Me放光的 540 ps 衰減訊號消失，並轉變為約 30 ps 的快速衰減訊號。結合穩態光譜、TCSPC、螢光上轉換與變溫實驗結果，本研究將此快速動力學指派為 LEPT state 轉換中 CSS state 的過程。此結果顯示，An-DHP 不僅能在室溫溶液中透過 PCEnT 形成 LEPT state，更因為DHP-Me具有較長的放光生命週期，使 LEPT-to-CSS 的過程得以被時間解析光譜分析。整體而言，本論文提出了 DHP 系列激發態雙質子轉移的新動力學詮釋，並讓 An-DHP 作為研究 PCEnT 後續 LEPT/CSS 動力學的有效例子。

In recent years, proton-coupled energy transfer (PCEnT) has attracted increasing attention as a new type of energy-transfer mechanism. Unlike conventional energy transfer, PCEnT involves both electronic excitation-energy transfer and proton transfer, allowing proton-transferred emission to be generated without direct excitation of the proton-transfer unit. However, direct experimental evidence for PCEnT in room-temperature solution has so far been limited to the AbPP series, and clear kinetic evidence for the conversion between the local electron–proton transfer (LEPT) state and the charge-separated state (CSS) remains lacking. To address this issue, this thesis investigates the excited-state proton-transfer dynamics of [2,2′-bipyridine]-3,3′-diol (DHP) derivatives and further develops An-DHP as a molecular system for studying PCEnT and LEPT-to-CSS conversion. In the first part, DHP, DHP-Me and DHP-diMe, were studied to examine the effects of molecular symmetry and methyl substitution on excited-state double-proton-transfer dynamics. Steady-state and nanosecond time-resolved fluorescence measurements show that all three molecules exhibit tautomeric emission and nanosecond-scale decay. Fluorescence up-conversion measurements further reveal that the symmetric molecules DHP and DHP-diMe show a picosecond decay in the red-emission region and a rise signal in the blue-emission region, whereas the asymmetric DHP-Me shows no obvious blue-region rise. Together with theoretical calculations, which indicate similar relative energies of the mono-keto (MK) and di-keto (DK) states and a shallow MK potential energy surface, these results suggest that the blue-region rise in DHP and DHP-diMe may originate from an equilibrium process between MK-like and DK states. In DHP-Me, the asymmetric structure changes the initial rate ratio between single- and double-proton-transfer processes, making the rise signal difficult to observe. In the second part, a newly designed An-DHP molecule was investigated. An-DHP consists of a proton-transfer-active DHP-Me core and an anthracene energy donor with separated absorption features. Upon selective excitation of anthracene at 410 nm, proton-transferred emission from the DHP core is observed, and its emission profile and nanosecond lifetime are similar to those of DHP-Me, indicating PCEnT formation of a long-lived LEPT state. In low- to medium-polarity solvents, An-DHP mainly shows ns-scale LEPT decay. In ACN, however, the 540 ps decay associated with DHP-Me emission disappears and is replaced by a ~30 ps decay component. Based on steady-state spectra, TCSPC, fluorescence up-conversion, and temperature-dependent measurements, this fast process is assigned to LEPT-to-CSS conversion. Overall, this thesis proposes a revised interpretation of excited-state double proton transfer in the DHP series and demonstrates An-DHP as an effective system for studying LEPT/CSS dynamics following PCEnT.