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Effect of NaCl on ESPT-mediated FRET in a CTAC micelle: a femtosecond and FCS study.

Femtosecond upconversion, single-molecule fluorescence resonance energy transfer (sm-FRET) and fluorescence correlation spectroscopy (FCS) are applied to study the competition between excited-state proton transfer (ESPT) and FRET [to rhodamine 6G (R6G)] of 8-hydroxypyranine-1,3,6-trisulfonate (HPTS) in cetyltrimethylammonium chloride (CTAC) micelles. Pyranine exhibits dual emission at λ(em)=430 nm for ROH and 520 nm for RO(-). The absorption spectrum of R6G (acceptor) has very good overlap with the RO(-) emission and poor overlap with ROH emission. It is observed that FRET occurs readily from the RO(-)* state of HPTS (donor) to R6G (acceptor). Multiple timescales of FRET were detected from the rise time of acceptor emission. The different timescales correspond to different donor-acceptor distances. The ultrafast components (8.5 and 13 ps) are assigned to FRET at a close contact of donor and acceptor (≈20 Å). The longer components (500 and 800 ps) arise from long-distance FRET from the donor to the acceptor (≈40 Å) located in different regions of the CTAC micelle. The larger donor-acceptor distances agree with those obtained from an sm-FRET study. On addition of 4 M NaCl to CTAC, the rate of proton transfer (k(PT)) slowed by about eight and two times, respectively, for the fast and slow sites of the CTAC micelle. As a result, the intensity of the ROH emission increases and that of RO(-) decreases. The decrease in the intensity of the RO(-) emission causes a decrease in the efficiency of FRET.