首次在溶液相三元分子中直接觀測到常溫下的質子偶合能量轉移（PCEnT）現象

Abstract

近年來，電子受體–給體型三元分子(triads)系統中相繼發現了穿空間質子耦合電子轉移（PCET）發生在極為罕見的Marcus倒轉區域中，以及一種發生在激發態的質子耦合能量轉移（PCEnT）全新機制。其中PCEnT機制在缺乏基態互變異構體的情況下仍可實現能量轉移，為光化學反應機構提供了全新觀點。然而，儘管理論研究不斷推進，目前有關 PCEnT 的實驗僅限於 77 K 玻璃態基質等極端條件下，實證仍相當有限。 本論文重新研究先前關於三元分子(triads)的報導，並首次證實三元分子 triad 1 與 5 中的電荷分離態（CSS）具有放光特性，推翻以往 CSS 經由非輻射衰變回到基態的論點。因此我們進一步透過變溫放光光譜與動力學實驗分析，重新檢視Marcus 倒轉區域中的電子轉移機制，並提出機制的補充觀點。 此外，我們設計新的三元分子AbPP，藉由限制苯酚與吡啶之間的扭轉運動，在常溫條件下首次獲得明確的PCEnT實驗證據。在非極性溶劑環己烷中，AbPP 的 PCEnT 速率常數為(125 ps)-1，比起直接的激發態質子轉移（ESIPT）速率 ((176 fs)⁻¹) 慢了三個數量級，卻仍產生相同的 645 nm 互變異構體放光，顯示此過程為由弱非絕熱耦合驅動的量子機制。 進一步分析中，我們成功解析出 PCEnT 與 PCET 產生的互變異構體與電荷分離態放光訊號，得以直接探討兩機制間的競爭關係。實驗結果顯示，隨著溶劑極性的提高，PCET 逐漸佔據主導。本研究擴展了電子、質子與能量轉移三種基本反應機制之間的交互理解，為探索量子驅動化學反應提供了新的實驗基礎。

Recent studies on donor–acceptor molecular triads have revealed through-space proton-coupled electron transfer (PCET) in the extremely rare Marcus inverted region and an unconventional excited state proton-coupled energy transfer (PCEnT). PCEnT is particularly notable for enabling energy transfer without a ground-state tautomer population, offering a new perspective on excited-state dynamics. Although significant theoretical progress has been made, experimental validation of PCEnT remains scarce. Thus far, the primary experimental evidence related to PCEnT has been obtained in a solid-state matrix and is based on indirect photophysical observations. Herein, we revisit previously reported triads and present clear experimental evidence that the charge-separated state (CSS) in triads 1 and 5 is emissive, contradicting earlier assumptions of exclusive nonradiative decay. Temperature-dependent emission and kinetic studies further clarify the electron transfer mechanism in the Marcus inverted region. We also introduce a newly designed triad, AbPP, with restricted phenol–pyridine torsion, and provide direct evidence of room-temperature PCEnT. In cyclohexane, PCEnT proceeds with a rate of (125 ps)⁻¹—about 1000 times slower than the ~176 fs intramolecular proton transfer (ESIPT) of bPP—yet both yield the same 645 nm tautomer emission, indicating weak nonadiabatic coupling. Finally, the resolved emissions from PCEnT- and PCET-derived states enable direct investigation of their interplay, with PCET increasingly favored in polar solvents. These results expand our insight into how electron motion, proton dynamics, and energy flow—three core processes in chemical reactivity—can interact and influence one another.