Swords Wesley B, Meyer Gerald J, Hammarström Leif
Department of Chemistry, Ångström Laboratories, Uppsala University Box 523 SE75120 Uppsala Sweden
Department of Chemistry, University of North Carolina at Chapel Hill Chapel Hill 27599 USA.
Chem Sci. 2020 Mar 3;11(13):3460-3473. doi: 10.1039/c9sc04941j.
The use of light to drive proton-coupled electron transfer (PCET) reactions has received growing interest, with recent focus on the direct use of excited states in PCET reactions (ES-PCET). Electrostatic ion pairs provide a scaffold to reduce reaction orders and have facilitated many discoveries in electron-transfer chemistry. Their use, however, has not translated to PCET. Herein, we show that ion pairs, formed solely through electrostatic interactions, provide a general, facile means to study an ES-PCET mechanism. These ion pairs formed readily between salicylate anions and tetracationic ruthenium complexes in acetonitrile solution. Upon light excitation, quenching of the ruthenium excited state occurred through ES-PCET oxidation of salicylate within the ion pair. Transient absorption spectroscopy identified the reduced ruthenium complex and oxidized salicylate radical as the primary photoproducts of this reaction. The reduced reaction order due to ion pairing allowed the first-order PCET rate constants to be directly measured through nanosecond photoluminescence spectroscopy. These PCET rate constants saturated at larger driving forces consistent with approaching the Marcus barrierless region. Surprisingly, a proton-transfer tautomer of salicylate, with the proton localized on the carboxylate functional group, was present in acetonitrile. A pre-equilibrium model based on this tautomerization provided non-adiabatic electron-transfer rate constants that were well described by Marcus theory. Electrostatic ion pairs were critical to our ability to investigate this PCET mechanism without the need to covalently link the donor and acceptor or introduce specific hydrogen bonding sites that could compete in alternate PCET pathways.
利用光驱动质子耦合电子转移（PCET）反应已引起越来越多的关注，近期的研究重点是在PCET反应中直接使用激发态（ES-PCET）。静电离子对提供了一个降低反应级数的框架，并推动了电子转移化学领域的许多发现。然而，它们的应用尚未转化到PCET领域。在此，我们表明，仅通过静电相互作用形成的离子对提供了一种通用、简便的方法来研究ES-PCET机制。在乙腈溶液中，水杨酸根阴离子与四价阳离子钌配合物之间很容易形成这些离子对。光激发后，离子对中的水杨酸根通过ES-PCET氧化导致钌激发态猝灭。瞬态吸收光谱确定还原态的钌配合物和氧化态的水杨酸根自由基为该反应的主要光产物。由于离子配对导致反应级数降低，使得通过纳秒光致发光光谱可以直接测量一级PCET速率常数。这些PCET速率常数在较大驱动力下达到饱和，这与接近马库斯无势垒区域一致。令人惊讶的是，在乙腈中存在水杨酸根的一种质子转移互变异构体，其质子定位于羧基官能团上。基于这种互变异构化的预平衡模型提供了非绝热电子转移速率常数，这些常数可用马库斯理论很好地描述。静电离子对对于我们在无需将供体和受体共价连接或引入可能在其他PCET途径中产生竞争的特定氢键位点的情况下研究这种PCET机制的能力至关重要。
