Enabling the Photochromism of 4’-Aminoflavylium Compounds in Water by Host-Guest Interaction with β-Cyclodextrin.

Abstract

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’NMe27OH) and 4’-(N,N-diethylamino)-7-hydroxylflavylium (4’NEt27OH) 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 pKa and pK’a. All species in the 4’NEt27OH multistate system interact more strongly than those in 4’NMe27OH. 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’NEt27OH 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.