Ultrafast FRET in a Room Temperature Ionic Liquid Microemulsion: A Femtosecond Excitation Wavelength Dependence Study

Fluorescence resonance energy transfer (FRET) from coumarin 480 (C480) to rhodamine 6G (R6G) is studied in a room temperature ionic liquid (RTIL) microemulsion by picosecond and femtosecond emission spectroscopy. The microemulsion is comprised of the RTIL 1-pentyl-3-methylimidazolium tetraflouroborate, [pmim][BF4], in TX-100/ benzene. We have studied the microemulsion with and without water. The time constants of FRET were obtained from the risetime of the acceptor (R6G) emission. In the RTIL microemulsion, FRET occurs on multiple time scales: 1, 250, and 3900 ps. In water containing RTIL microemulsion, the rise components are 1.5, 250, and 3900 ps. The 1 and 1.5 ps components are assigned to FRET at a close contact of donor and acceptor (R-DA approximate to 12 angstrom). This occurs within the highly polar (RTIL/water) pool of the microemulsion. With increase in the excitation wavelength (lambda(ex)) from 375 to 435 nm, the relative contribution of the ultrafast component of FRET (similar to 1 ps) increases from 4% to 100% in the RTIL microemulsion and 12% to 100% in the water containing RTIL microemulsion. It is suggested that at lambda(ex) = 435 nm, mainly the highly polar RTIL pool is probed where FRET is very fast due to the close proximity of the donor and the acceptor. The very long 3900 ps (R-DA approximate to 45 angstrom) component may arise from FRET from a donor in the outer periphery of the microemulsion to an acceptor in the polar RTIL pool. The 2:50 ps component (R-DA approximate to 29 A) is assigned to FRET. from a donor inside the surfactant chains.