In addition to utilizing Lewis acids to enable enantioselective photocatalytic processes, the use of another secondary mode of catalysis is of particular importance in this field: hydrogen bonding catalysis. This non-covalent interaction has been established as a powerful method to control the selectivity of photocatalytic transformations leading to the existence of a multitude of chiral catalysts containing a hydrogen bonding site. The first bifunctional hydrogen bonding photocatalyst for an intramolecular enantioselective [2+2] photocycloaddition of quinolones was introduced in 2002 by Krische et al. and utilized benzophenone as a sensitizing moiety (Figure ). While this example was important in demonstrating the potential of enantioselective catalysis via sensitization by a photocatalyst with a pendant chiral motif, it suffers from relatively poor enantioselectivities. Since then, the Bach group has developed modified catalysts that result in higher selectivities, first by attaching a benzophenone photosensitizer to a lactam backbone and later by improving on this structure through the incorporation of a xanthone photosensitizer (xanthone catalyst shown in Scheme 2A). Despite the success of these catalysts in chiral photocatalysis, their ability to promote competing hydrogen atom abstraction reactions results in decomposition of the catalyst that limits their long-term stability. Further, both benzophenone and xanthone absorb light in the ultraviolet region, which often has sufficiently high energy to directly excite substrates resulting in undesired background reactions. In 2014, the Bach group overcame this wavelength limitation by presenting the synthesis of a chiral thioxanthone, allowing for the use of visible light in an enantioselective [2+2] photocycloaddition (Scheme 2B). 9 Scheme 2: Chiral xanthone and thioxanthone catalysts for enantioselective photocatalysis.