Pressure effects on thermal and photochemical proton-coupled electron transfer reactions with metal complexes

在天然和人工体系中，热、电化学和/或光化学质子耦合电子转移(PCET)控制着化学能的转换。PCET的热力学和动力学益处与电荷产生或电荷移动/增加影响产物或过渡态的事实相关。一个主要的例子是协同质子-电子转移(CPET)。因此，了解PCET以及通过支持CPET而不是逐步电子(ET)和质子转移(PT)来调制它们的方法对于提高能量转换的效率是至关重要的。我们的项目旨在进行开创性的基础研究，揭示压力作为一个重要的物理参数如何被用作工具：(I)评估和区分不同的PCET机制，(Ii)了解影响它们的因素，以及(Iii)诱导机制转换，进而使反应速度最大化。压力对动力学的影响以活化体积为特征，它对过渡状态过程中总电荷或分子内电荷分布的变化高度敏感。因此，它逐渐成为区分和操纵PCET的关键因素。此外，压力调制是一种独特的方式，一方面影响内球(PC)ET的键断裂/形成和外球(PC)ET的重组能，另一方面调节长程(PC)ET的电子给体-受体距离/取向/电子耦合。在我们基于氧化还原活性金属中心和氢键组装的一系列新合成的分子体系的可变压力动力学的模型研究中，我们将对热、电化学和光化学PCET有一个全面的了解。为此，最突出的例子是均相与非均相自交换PCET、单向与双向交叉PCET，以及共价或氢键分子组件及其组合中的PCET工艺。通过系统地增加所研究系统的复杂性，我们将能够关联不同但互补的过程之间的影响，例如分子间和分子内反应之间，或者自交换和交叉PCET反应之间。在方法上，我们将使用合成和各种实验技术之间的协同作用来进行动力学阐明。各自的选择将取决于分子体系，范围从例如吸收、荧光、IR和拉曼到高压池中的电化学和核磁共振，或者是在稳态或以时间分辨的方式，并辅之以一般的分析技术，如ESI-MS、EPR、X-射线等。除了压力变化外，温度也将是一个重要的变量，既是证实反应机理的协同工具，也是微调反应动力学的工具。

Thermal, electrochemical, and/or photochemical proton-coupled electron transfer (PCET) governs the chemical energy conversion in natural and artificial systems. Thermodynamic and kinetic benefits of PCET correlate with the fact that neither charge generation, nor charge shift/increase impact product or transition state. A leading example is a concerted proton-electron transfer (CPET). As such, understanding PCET and ways to modulate them by favoring CPET rather than stepwise electron (ET) and proton transfer (PT) is of utmost importance toward increasing efficiency of the energy conversion. Our project aims at pioneering fundamental studies that should reveal how pressure as an important physical parameter can be used as a tool to: (i) assess and distinguish between different PCET mechanisms, (ii) understand the factors that affect them, and (iii) induce mechanistic changeovers and, in turn, maximize reaction rates. A pressure effect on kinetics is characterized by a volume of activation, which is highly sensitive to changes in overall charge or intramolecular charge distribution en route to the transition state. Thus, it evolves as a key factor for differentiating and manipulating PCETs. In addition, pressure modulation is a unique way to impact bond breaking / formation in inner-sphere (PC)ET and reorganization energy in outer sphere (PC)ET, on one hand, and to tune electron-donor acceptor distance / alignment/electronic coupling in long-range (PC)ET, on the other hand. Here in our model studies regarding variably pressure kinetics of a series of newly synthesized molecular systems, based on redox-active metal centers and hydrogen bonded assemblies, we will gather a comprehensive understanding of thermal, electrochemical, and photochemical PCETs. To this end, the most prominent examples are homogeneous vs. heterogeneous self-exchange PCETs, uni- vs. bi-directional cross PCETs, as well as PCET processes within covalent or hydrogen bond molecular assemblies and their combinations. By systematically increasing the complexity of the investigated systems, we will be able to correlate the effects between different, but complementary processes such as between inter- and intramolecular reactions or between self exchange and cross PCET reactions. Methodologically, we will use a synergy between syntheses and a variety of experimental techniques for kinetic elucidations. The respective choices will depend on the molecular system and will range from, for example, absorption, fluorescence, IR, and Raman to electrochemistry and NMR in high pressure cells either in a steady-state or in a time-resolved modus, complemented by general analytical techniques such as ESI-MS, EPR, X-ray, etc.. In addition to pressure variation, temperature will also be an important variable both as synergistic tools to corroborate reaction mechanisms and to fine-tune reaction kinetics.