Sensing micelle hydration by proton-transfer dynamics of a 3-hydroxychromone dye: role of the surfactant headgroup and chain length.

The dynamics of the excited-state intramolecular proton-transfer (ESIPT) reaction of 2-(2′-furyl)-3-hydroxychromone (FHC) was studied in micelles by time-resolved fluorescence. The proton-transfer dynamics of FHC was found to be sensitive to the hydration and charge of the micelles, demonstrated through a decrease of the ESIPT rate constant (kPT) in the sequence cationic → nonionic → anionic micelles. A remarkably slow ESIPT with a time constant (τPT) of ∼100 ps was observed in the anionic sodium dodecyl sulfate and sodium tetradecyl sulfate micelles, whereas it was quite fast (τPT ≈ 15 ps) in the cationic cetyltrimethylammonium bromide and tetradecyltrimethylammonium bromide micelles. In the nonionic micelles of Brij-78, Brij-58, Tween-80, and Tween-20, ESIPT occurred with time constants (τPT ≈ 35–65 ps) intermediate between those of the cationic and anionic micelles. The slower ESIPT dynamics in the anionic micelles than the cationic micelles is attributed to a relatively stronger hydration of the negat...