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

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 reac tions has been investigated. As far as the acid-base properties of the grou nd state system are concerned, negatively charged SDS micelles stabilize th e acidic form AH(+), while the positively charged CTAB and, to a lesser ext ent, the neutral Triton X 100 micelles destabilize AH(+). Moreover, the neu tral form A exhibits a strong decrease in molar absorption coefficient that cannot be explained by electrostatic reasons only, suggesting that specifi c interactions of A with the hydrophobic part of the surfactants may play s ome role. Even the excited state dissociation constant (K-a*) is affected b y the micelles, and the pK(a)* is positively or negatively shifted with res pect to that in water by negatively or positively charged micelles, respect ively. It was also observed that the excited states can be intrinsically af fected by the micelles. In particular, the excited AH(+) is not only stabil ized by the negative SDS micelle, but also exhibits a luminescence lifetime longer than that in water, most probably because of the electrostatic inte raction exerted on it by the negative charge of the micelles.