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 (<i>k</i>_PT) in the sequence cationic → nonionic → anionic micelles. A remarkably slow ESIPT with a time constant (τ^PT) of ∼100 ps was observed in the anionic sodium dodecyl sulfate and sodium tetradecyl sulfate micelles, whereas it was quite fast (τ^PT ≈ 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 (τ^PT ≈ 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 negatively charged headgroups of the former than the positively charged headgroups of the latter, which significantly weakens the intramolecular hydrogen bond of FHC in the Stern layer of the anionic micelles compared to the latter. In addition, electrostatic attraction between the positively charged −N­(CH₃)₃⁺ headgroups and the negatively charged 4-carbonyl moiety of FHC effectively screens the intramolecular hydrogen bond from the perturbation of water molecules in the micelle–water interface of the cationic micelles, whereas in the anionic micelles, this screening of the intramolecular hydrogen bond is much less efficient due to an electrostatic repulsion between its negatively charged −OSO₃^– headgroups and the 4-carbonyl moiety. As for the nonionic micelles, a moderate level of hydration, and the absence of any charged headgroups, causes an ESIPT dynamics faster than that of the anionic but slower than that of the cationic micelles. Furthermore, the ESIPT rate decreased with a decrease of the hydrophobic chain length of the surfactants due to the stronger hydration of the micelles of shorter chain surfactants than those of longer chain surfactants, arising from a less compact packing of the former surfactants compared to the latter surfactants