Design and Application of Heavy Atom-Free and Redox-Active Organic Triplet Photosensitizers

可见光辅助化学转化已经作为一种强大的小分子活化模式与最近对可持续化学的追求相结合。无机光敏剂在这一领域的流行源于它们在容易形成的三重激发态中的长寿命和强氧化还原能力。然而，从无金属的光氧化还原有机材料形成三重态通常被认为是低效的过程，导致这些化合物很少受到相关应用的关注。由于现有的有机合成方法应该提供有机氧化还原光敏剂的良好反应性，因此在本项目中，我们希望挑战传统的想法，并提供一种通用的方法，使有机分子具有光敏能力。我们的目标是实现以下三个目标。(1)我们将通过利用未充分利用的硫代羰基光化学来扩展目前的光敏剂/光催化剂工具箱，以产生充满活力的，长寿命的有机发色团的三重激发态，用于有效的电子转移反应，并完全了解其激发态动力学。(2)我们将了解控制三重态能量，寿命和硫代羰基发色团的化学稳定性的因素，并开发方法来定制这些功能。（3）基于本项目所获得的知识，我们将发现其他新的无重原子有机氧化还原光敏剂/光催化剂的方法或机制。该项目的第一阶段将主要集中在发色团的发现上，这将得到量子化学计算的帮助。在第二阶段，随着一些三重态发色团的鉴定，研究将集中在性质调整上，这对于解决这些发色团有用的两个重要方面是必不可少的：（i）分子在反应条件下照射时应表现出优异的光和化学稳定性。(ii)其次，为了实现对特定反应的良好选择性，三重态发色团的电子结构应相应地定制。我们将通过分子取代基效应、构象控制和超分子相互作用来满足这些要求。我们的努力将集中在催化过程中，可以通过单电子转移完成。除了光氧化还原催化的背景之外，重要的是要指出，我们将开发的三重态增强策略将普遍适用于长寿命激发态和/或三重态感兴趣的广泛研究。例如，在有机光致发光中，启用光活性的高性能分子/聚合物的三重态通道将由于较长的激子寿命或由于自旋限制而缓慢复合的三重态自由基离子对的形成而增强电荷分离量子产率。

Visible-light assisted chemical transformation has come forward as a powerful mode of small-molecule activation in conjunction with the recent quest for sustainable chemistry. The popularity of inorganic photosensitizers in this field stems from the long lifetime and strong redox power in their easily formed triplet excited states. However, triplet formation from metal-free, photo-redox organic materials is often regarded as an inefficient process, leading these compounds to receive little attention from relevant applications. Since the established organic synthetic methodologies should provide a well-tailored reactivity in organic redox photosensitizers, in this project, we want to challenge the conventional believes and provide a general method to enable the photosensitizing capability in organic molecules.We aim to achieve the following three objectives. (1) We will expand the current toolbox of photosensitizer/photocatalyst by exploiting the underutilized thiocarbonyl photochemistry to generate energetic, long-lived triplet excited state of organic chromophores for efficient electron transfer reactions with a complete understanding of their excited state dynamics. (2) We will understand the factors controlling the triplet energy, lifetime, and chemical stability of thiocarbonyl chromophores and to develop methods to tailor these features. And (3) we will discover other novel methods or mechanisms for heavy atom-free organic redox photosensitizer/photocatalyst based on the knowledge acquired in this project. Phase 1 of the project will focus mostly on chromophore discovery, which will be assisted by quantum chemical calculations. With a few triplet chromophores identified, in Phase 2, the research will focus on property tuning, which is essential to address two important aspects for these chromophores to be useful: (i) The molecules should exhibit excellent photo and chemical stability upon irradiation under reaction conditions. (ii) Secondly, to achieve good selectivity toward specific reactions, the electronic structure of the triplet chromophores should be tailored accordingly. We will fulfill these requirements by molecular substituent effects, conformation control, and supramolecular interactions. Our effort will be concentrated on catalytic processes that can be accomplished by one-electron transfer. In addition to the context of photo-redox catalysis, it is important to point out that the triplet-enhancing strategy that we will develop will be general to be applicable to a broad range of research where long-lived excited state and/or triplet state is of interest. For instance, in organic photovoltaics, enabling the triplet channel of photo-active, high performing molecules/polymers would enhancing charge separation quantum yields due to longer exciton lifetime or the formation of triplet radical ion pairs that recombines slowly due to spin restriction.