Dynamics of Excited-State Reactions in Reversed Micelles. 2. Proton Transfer Involving Various Fluorescent Probes according to Their Sites of Solubilization

Article abstract of DOI:10.1021/j100269a029

Reversed micelles of bis(2-ethylhexyl)sulfosuccinate (AOT) in heptane were investigated with three acid fluorescent probes: 2-naphtol (NOH), sodium 2-naphtol-6-sulfonate (NSOH), and potassium 2-naphtol-6,8-disulfonate (NDSOH).These probes, which tend to undergo protolysis in the excited state, are well suited to the investigation of the acid-base reacitvity of water molecules forming the aqueous core.The rate constants for deprotonation and back-recombination were determined by phase fluorometry as a function of the water content w=/.These rate constants together with the spectroscopic properties of the probes provide information on their localization and the corresponding ability of the microenvironment to accept a proton. (i) NDSOH is localized around the center of the water pool and, at water contents, w, greater than about 10, its behavior regarding protolysis is identical with that in bulk water. (ii) NSOH resides in the vicinity of the interface and an amount of water of w <*> 40 is required for observing the same deprotonation rate as in bulk water whereas the rate of back-recombination is still much faster. (iii) NOH is localized at the interface and does not undergo deprotonation in the excited state whatever the water content.Efficiency and kinetics of proton transfer are thus strongly dependent on localization.The ability of water to accept a proton is related to is H-bonded structure and its protolytic reactivity is an image of its structure which changes as a function of the distance with respect to the interface.