Theoretical study on reactions of triplet excited state thioxanthone with indole

Thioxanthone: a powerful photocatalyst for organic reactions

Photoorganocatalysis has been recognised by the organic chemistry community as an important part of photochemistry and catalysis. In general, aromatic ketones constitute key players in this type of catalysis as they are involved in a plethora of examples in the literature. Among the various aromatic ketones, thioxanthone (TX) seems to play a unique role in photochemistry. In comparison with other aromatic ketones, TX has a high triplet energy and a relatively long triplet lifetime, while it has the ability to participate successfully in merger reactions with metal complexes. In this review, we will discuss the photophysical properties of this small organic molecule, as well as the numerous examples of photochemical reactions, where it is employed as a mediator and more specifically in polymerisation reactions, and organic transformations.

Mechanistic Investigation of Visible-Light-Induced Intermolecular [2 + 2] Photocycloaddition Catalyzed with Chiral Thioxanthone

The recent thioxanthone-sensitizer-catalyzed intermolecular [2 + 2] cycloaddition induced by visible-light irradiation set the stage for the future development of feasible photocycloadditions. Nonetheless, the mechanism of this reaction still remains under debate, especially on the activation mode of the thioxanthone photosensitizer (energy transfer, bielectron exchange, and hydrogen transfer are all possible mechanisms). To settle this issue, systematic density functional theory calculations have been carried out. The results indicate that the energy-transfer pathway is more favorable than the bielectron-exchange and the hydrogen-transfer pathways. Meanwhile, the overall transformations involve the complexation and excitation of photosensitizer, the first C-C bond formation, the dissociation of the sensitizer, the triplet-to-singlet electronic state crossing, and the second C-C bond formation. The first C-C bond formation is the rate- and selectivity-determining step, and synergistic energy and electron transfer from photosensitizer to substrate moieties takes place along this process. On this basis, the effect of olefin substrates (ethyl vinyl ketone vs vinyl acetate) on the stereoselectivity was finally analyzed.