title: Effect of NaCl on ESPT-mediated FRET in a CTAC micelle: a femtosecond and FCS study.
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 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.
authors.affiliation: Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India.
authors.forename: Amit Kumar
authors.initials: AK
authors.lastname: Mandal
authors.forename: Shirsendu
authors.initials: S
authors.lastname: Ghosh
authors.forename: Atanu Kumar
authors.initials: AK
authors.lastname: Das
authors.forename: Tridib
authors.initials: T
authors.lastname: Mondal
authors.forename: Kankan
authors.initials: K
authors.lastname: Bhattacharyya
mesh_terms: D000077286:Cetrimonium; D002593:Cetrimonium Compounds / Q000737:chemistry*; D031541:Fluorescence Resonance Energy Transfer; D008823:Micelles; D015394:Molecular Structure; D011522:Protons; D012965:Sodium Chloride / Q000737:chemistry*; D013050:Spectrometry, Fluorescence; D013997:Time Factors
chemical_list: D002593:Cetrimonium Compounds; D008823:Micelles; D011522:Protons; D012965:Sodium Chloride; D000077286:Cetrimonium