Enabling the photochromism of 4′-aminoflavylium compounds in water by host-guest interaction with β-cyclodextrin

Aminoflavylium compounds are known to display poor photochemical properties in water precluding their application in aqueous medium. In this work, we show that the host-guest encapsulation with β -cyclodextrin ( β -CD) significantly improves the photochemical performance of 4’-aminoflavylium enabling their operation in aqueous solution. The pH-dependent thermodynamics and kinetics of two aminoflavylium compounds 4’-( N , N -dimethylamino)-7-hydroxylflavylium (4’NMe 2 7OH) and 4’-( N , N -diethylamino)-7-hydroxylflavylium (4’NEt 2 7OH) were studied in water in the absence and presence of β -CD. The association constants follow the order trans -chalcone > quinoidal base > flavylium cation, resulting in β -CD lowering both p K a and p K ’ a . All species in the 4’NEt 2 7OH multistate system interact more strongly than those in 4’NMe 2 7OH. The pH-dependent kinetics toward the equilibrium follows a bell-shaped curve at moderately acidic pH and increases in the presence of β -CD in both compounds. At highly acidic pH, protonation of trans -chalcone accelerates the kinetics, particularly for 4’NEt 2 7OH due to the easier protonation of the trans -chalcone. In the presence of β -CD this effect is reduced, because protonation of trans -chalcone becomes more difficult. Upon irradiation of trans -chalcone in both compounds in the presence of β -CD, the pale yellow trans -chalcones convert into a purple, less-bound mixture of the flavylium cation and quinoidal base, with a quantum yield of ∼ 0.01. • Host-guest encapsulation with β -cyclodextrin of 4’-aminoflavylium enables its photochemistry in aqueous solutions. • Small structural changes in the 4’-amino groups give large changes in the association constants with β -cyclodextrin. • Trans -chalcones are the network species exhibiting the higher association constants with β -cyclodextrin