Intermolecular Electronic Interactions: Alternative Paths for Photochemistry

光驱动化学为化学合成、可再生能源和光电子技术提供了一种新的工具。在化学合成中，使用光化学反应，而不是更传统的热驱动化学，提供了能量较少的反应条件，但也有可能生成否则无法获得的产物。对于光化学反应或光氧化还原过程，光吸收物种和反应物之间必须存在分子间电子相互作用以促进能量转移。传统的基础光化学研究侧重于了解单个孤立分子中的光物理反应途径，而不是光化学合成和光氧化还原催化所需的多物种能量转移过程。对于多物种的光化学反应，这些反应物种之间分子间相互作用的强度和性质以及它们如何在时间上演化是分子的光诱导反应活性和反应结果的关键。多年来，我开发了许多光谱工具和分析方法，以了解分子的光学和几何性质及其产生的光物理。现在，我将应用我开发的光谱工具来研究一系列主客体络合物中存在哪些类型的分子间电子相互作用。从这些实验中获得的理解将使我们更好地理解分子间电子相互作用的机制，以及如何有意识地设计新的候选分子用于光驱动应用。我们目前对分子间相互作用如何工作的了解很大程度上局限于现象学模型。了解分子及其环境对分子间电子相互作用的电子和核贡献是高度复杂的，需要使用多种尖端光谱方法。鉴于有可能增强光诱导反应结果以提高光化学合成的产率和效率，这项工作建议通过使用特殊设计的主客体络合物和超快光学和X射线光谱来发展对分子间相互作用的必要的基础理解。拟议的宿主系统家族将包括金属-有机笼子(MOCs)，一种相当于金属-有机骨架的分子，它在溶液中形成离散的单分散单元，能够容纳与蛋白质一样大的客体，以及有机大环，如葫芦巴[n]URI。在这项工作中，主体络合物将提供一种手段来控制客体分子周围的环境，并通过改变主体的化学组成和内腔大小来调节不同的电子和核贡献对分子间电子相互作用的贡献。还将探索通过直接激发宿主本身来主动使宿主络合物参与光诱导反应，以此作为触发新的光化学的一种方式。所选择的光谱方法系列，超快光学和X射线，是实现这项工作主要目标的关键。光学光谱提供了对全球电子结构和价电子态的敏感性，这在大多数光学应用中是至关重要的。作为补充，X射线光谱仪具有高度的位置和元素选择性，可以提供仅从光学方法获取的具有挑战性的结构信息。通过表征单个主客体的光物理以及主客体复合体在形成过程中发生的变化，将有可能探索影响光化学反应结果以及主客体相互作用的基本原理和性质的实用途径。

Light-driven chemistry offers a new tool for chemical synthesis, renewable energy and photonics technologies. In chemical synthesis, the use of photochemical reactions as opposed to more traditional thermally-driven chemistry offers less energy-intensive reaction conditions but also the possibility to form otherwise inaccessible products. For a photochemical reaction or photoredox process to occur, there must be an intermolecular electronic interaction between the light-absorbing species and reactant to facilitate the energy transfer. Traditional fundamental photochemical studies have focused on understanding the photophysical reaction pathways in single, isolated molecules, rather than on the multispecies energy transfer processes necessary in photochemical synthesis and photoredox catalysis. For multispecies photochemical reactions, the strength and nature of these intermolecular interactions between reactive species and how they evolve in time are key to the photoinduced reactivity of the molecule and the reaction outcome. I have developed a number of spectroscopic tools and analysis methods over the years to understand the optical and geometric properties of a molecule and its resultant photophysics. Now, I will apply the spectroscopic tools I have developed to studying what types of intermolecular electronic interactions are present in a series of host-guest complexes. The understanding gained from these experiments will allow us to better understand the mechanisms involved in intermolecular electronic interactions and how to intentionally design new molecular candidates for light-driven applications. Our current knowledge of how intermolecular interactions work is largely limited to phenomenological models. Understanding the electronic and nuclear contributions to intermolecular electronic interactions from both the molecule and its environments is highly complex and necessitates the use of multiple cutting-edge spectroscopic methods.Given the potential to enhance photoinduced reaction outcomes for improving the yields and efficiency of photochemical synthesis, this work proposes to develop the necessary fundamental understanding of intermolecular interactions through the use of specially-designed host-guest complexes and ultrafast optical and X-ray spectroscopies. The proposed families of host systems will include metal-organic cages (MOCs), a molecular equivalent of metal-organic frameworks, that form discrete monodisperse units in solution and are capable of housing guests as large as proteins, and organic macrocycles such as cucurbit[n]urils. In this work, the host complexes will provide a means of controlling the environment around the guest molecules and by changing the chemical composition of the host and the internal cavity size, a way of tuning the contribution of different electronic and nuclear contributions to the intermolecular electronic interactions. Actively involving the host complex in photoinduced reactions by direct excitation of the host itself as a way of triggering new photochemistry will also be explored. The chosen families of spectroscopic methods, ultrafast optical and X-ray, are key to achieving the main objectives in this work. Optical spectroscopies provide the sensitivity to the global electronic structure and valence electronic states that are crucial in most optical applications. In complement to this, X-ray spectroscopies are highly site and element selective and can provide the structural information that is challenging to capture from optical methodologies alone. Through characterisation of the photophysics of the individual host and guest and what alterations occur on formation on the host-guest complex it will be possible to explore practical routes to influencing the outcome of photochemical reactions as well as the fundamentals and nature of the host-guest interactions.