Visible light induced supramolecular photocatalysis: triplet sensitized one-way geometric isomerization of encapsulated β-ionyl derivatives

This study illustrates a new strategy of visible-light supramolecular photocatalysis, where triplet sensitization and host–guest chemistry combine to enable controlled, efficient trans→cis isomerization in aqueous media. Triplet energy transfer catalysis is leveraged here to achieve geometric isomerization of β-ionyl derivatives within a supramolecular host capsule under visible light. All-trans β-ionylidene trienes (retinal analogues) were encapsulated in the water-soluble octa acid (OA) capsule, and visible-light-absorbing dyes (methylene blue, Rose Bengal, Eosin Y; MB, RB and EY) served as external triplet sensitizers. Upon green-light irradiation, the confined guests underwent clean one-way isomerization from 7-trans to 7-cis, attaining near-quantitative conversion. Surprisingly, even anionic sensitizer dyes (RB and EY) effectively induced isomerization in the anionic OA host system, evidencing an unusual like-charge attraction. NMR studies confirmed that the isomerization occurs inside the OA cavity, and that the supramolecular environment modulates the reaction's photostationary state – cis isomers are favored more in the capsule than in bulk solution, due to the capsule's influence on excited-state decay pathways. Furthermore, we demonstrate that the OA-confined photocatalysis can be driven by sunlight in water with comparable efficiency to LED irradiation.