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

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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.