Excited-state proton transfer of 1-naphthol in micelles

Abstract

The fast deprotonation of 1-naphthol, which occurs in 35 ps in aqueous solution, is studied in neutral (triton X 100, reduced, TX-100R), cationic (cetyl trimethylammonium bromide, CTAB), and anionic (sodium dodecyl sulfate, SDS) micelles. Drastically different effects on the proton transfer process and the relative emission intensities of the neutral form (360 nm) and the anion (460 nm) are observed in the three micelles. The intensities of the anion and the neutral emission of 1-naphthol exhibit a break around the reported critical micellar concentration (cmc) of the three micelles. Above cmc, intensity of the neutral emission is enhanced by a factor of nearly 90, 66, and 20 in 20 mM TX-100R, 200 mM SDS, and 96 mM CTAB, respectively. The anion emission is enhanced for CTAB and TX-100R, while for SDS its intensity decreases, compared to water. In CTAB, the rise time of the 460 nm emission (600 ± 100 ps) is similar to the lifetime of decay at 360 nm. However, for TX-100R and SDS, the rise time of the anion emission (at 460 nm) is found to be faster than the decay of the neutral emission (at 360 nm). This indicates that in TX-100R and SDS, there is no parental relation between the normal and the anion emission and they originate from the probe, 1-naphthol molecules, at distinctly different locations. The rise times at 460 nm are 1.8 ± 0.1 ns and 600 ± 100 ps for TX-100R and SDS, respectively, while the corresponding decay times at 360 nm are 2.5 ± 0.1 ns and 1.8 ± 0.1 ns.