Micelle effect on ground and excited state proton transfer reactions involving the 4-methyl-7-hydroxyflavylium cation

Abstract

In aqueous solution, the 4-methyl-7-hydroxyflavylium ion (AH⁺) undergoes a ground state dissociation reaction to form the quinoidal base A and, when excited, an efficient excited state proton transfer (ESPT) leading to *A. The effect of micelles on the ground and excited state proton transfer reactions has been investigated. As far as the acid–base properties of the ground state system are concerned, negatively charged SDS micelles stabilize the acidic form AH⁺, while the positively charged CTAB and, to a lesser extent, the neutral Triton X 100 micelles destabilize AH⁺. Moreover, the neutral form A exhibits a strong decrease in molar absorption coefficient that cannot be explained by electrostatic reasons only, suggesting that specific interactions of A with the hydrophobic part of the surfactants may play some role. Even the excited state dissociation constant (K_a^*) is affected by the micelles, and the pK_a^* is positively or negatively shifted with respect to that in water by negatively or positively charged micelles, respectively. It was also observed that the excited states can be intrinsically affected by the micelles. In particular, the excited AH⁺ is not only stabilized by the negative SDS micelle, but also exhibits a luminescence lifetime longer than that in water, most probably because of the electrostatic interaction exerted on it by the negative charge of the micelles.