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Keywords:

  • FRET;
  • micelles;
  • proton transfer;
  • salt effect;
  • single-molecule studies

Abstract

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 λem=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 (kPT) 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.