Triplet Energy Transfer in Photon Upconversion and Photoswitching

Light-driven processes involving excited triplet states can be leveraged in numerous fields of science and technology to achieve for example more sustainable manufacturing and chemical syntheses, more efficient solar energy harvesting, safer and more potent therapies for diseases, more sensitive imaging and sensors, or more stable qubits for quantum information sciences. Knowledge of the origin and fate of triplet states is therefore fundamental in realizing these outcomes. This involves understanding triplet energy transfer (TET), an integral step in many of these processes.

In this thesis we explore TET, especially in the context of triplet fusion photon upconversion and photoswitching. Photon upconversion by triplet fusion is based on pooling the energy of photoexcitation in metastable triplet states that can fuse to generate singlet excited states with higher energy. Therefore, TET is an integral part of triplet fusion upconversion. Photoswitching or photoisomerization means control of the geometric structure and thus properties of molecules with light. In some photoswitches, such as azobenzenes that are used in this thesis, isomerization can be achieved by accessing their triplet states via TET.

After introducing the definition, generation and general properties of excited triplet states and the mechanism of TET, we focus on its role in triplet fusion upconversion and photoswitching systems. This involves investigating the properties of the engaged molecules, photosensitizers and acceptors, and how they affect the performance of these systems, allowing formulation of clear guidelines for designing them. Particularly, we consider the interplay of these properties and its effect on the thermodynamics of TET. Commonly efficient TET is ensured by employing photosensitizer-acceptor pairs with a large exothermic triplet energy gap. Here we investigate systems with small or even endothermic energy gap, how to make them efficient and uncover the possibilities they offer. This thesis deepens the understanding of TET and offers insight into controlling it in photon upconversion and photoswitching, which paves way for their implementation into practical applications.