Troian-Gautier Ludovic, Swords Wesley B, Meyer Gerald J
Department of Chemistry , University of North Carolina at Chapel Hill , Chapel Hill , North Carolina 27599 , United States.
Acc Chem Res. 2019 Jan 15;52(1):170-179. doi: 10.1021/acs.accounts.8b00373. Epub 2018 Dec 20.
Iodide redox chemistry is intimately coupled with the formation and breaking of chemical bonds that are relevant to emerging solar energy technologies. In this Account, recent advances in dye-sensitized iodide oxidation chemistry in organic solutions are described. Here Ru sensitizers with high cationic charge, tuned reduction potentials, and specific iodide receptor site(s) are shown to self-assemble in organic solvents and yield structures that rapidly oxidize iodide and generate I-I bonds when illuminated with visible light. These studies provided new insights into the fascinating behavior of our most polarizable and easily oxidized monatomic anion. Sensitized iodide photo-oxidation in CHCN solutions consists of two mechanistic steps. In the first step, an excited-state sensitizer oxidizes iodide (I) to an iodine atom (I) through diffusional encounters. The second step involves the reaction of I with I to form the I-I bond of diiodide, I. The overall reaction converts a green photon into about 1.64 eV of free energy in the form of I and the reduced sensitizer. The free energy is only transiently available, as back-electron transfer to yield ground-state products is quantitative. Interestingly, when the free energy change is near zero, iodide photo-oxidation occurs rapidly with rate constants near the diffusion limit, i.e., >10 M s. Such rapid reactivity is in line with anecdotal knowledge that iodide is an outstanding electron donor and is indicative of adiabatic electron transfer through an inner-sphere mechanism. In low-dielectric-constant solvents, dicationic Ru sensitizers were found to form tight ion pairs with iodide. Diimine ligands with additional cationic charge, or "binding pockets" that recognize halides, have been utilized to position one or more halides at specific locations about the sensitizer before light absorption. Diverse photochemical reactions observed with these supramolecular assemblies range from the photorelease of halides to the formation of I-I bonds where both iodides present in the ground-state assembly react. Natural population analysis through density functional theory calculations accurately predicts the site(s) of iodide ion-pairing and provides information on the associated free energy change. The ability to direct light-driven bond formation in these ionic assemblies is extended to chloride and bromide ions. The structure-property relationships identified, and those that continue to emerge, may one day allow for the rational design of molecules and materials that drive desired halide transformations when illuminated with light.
碘化物氧化还原化学与新兴太阳能技术相关的化学键的形成与断裂紧密相连。在本综述中，描述了有机溶液中染料敏化碘化物氧化化学的最新进展。具有高阳离子电荷、可调还原电位和特定碘化物受体位点的钌敏化剂在有机溶剂中自组装，形成的结构在可见光照射下能迅速氧化碘化物并生成I-I键。这些研究为我们最具极化性且最易氧化的单原子阴离子的迷人行为提供了新见解。在乙腈溶液中敏化碘化物的光氧化由两个机理步骤组成。第一步，激发态敏化剂通过扩散碰撞将碘化物（I⁻）氧化为碘原子（I）。第二步涉及I与I⁻反应形成二碘化物（I₂）的I-I键。整个反应将一个绿色光子转化为约1.64电子伏特的自由能，以I₂和还原态敏化剂的形式存在。由于定量的回电子转移生成基态产物，该自由能只是短暂可用。有趣的是，当自由能变化接近零时，碘化物光氧化以接近扩散极限的速率常数快速发生，即>10⁹ M⁻¹ s⁻¹。这种快速反应性与碘化物是优秀电子供体的传闻知识一致，表明通过内球机制的绝热电子转移。在低介电常数溶剂中，发现二价钌敏化剂与碘化物形成紧密离子对。具有额外阳离子电荷的二亚胺配体或识别卤化物的“结合口袋”已被用于在光吸收前将一个或多个卤化物定位在敏化剂周围的特定位置。用这些超分子组装体观察到的各种光化学反应范围从卤化物的光释放到基态组装体中存在的两个碘化物都发生反应形成I-I键。通过密度泛函理论计算进行的自然布居分析准确预测了碘化物离子对的位点，并提供了相关自由能变化的信息。在这些离子组装体中引导光驱动键形成的能力扩展到了氯离子和溴离子。已确定以及不断涌现的结构-性质关系，也许有一天能实现对分子和材料的合理设计，使其在光照下驱动所需的卤化物转化。
