Ultrafast FRET in ionic liquid-P123 mixed micelles: region and counterion dependence

Ultrafast fluorescence resonance energy transfer (FRET) in a mixed micelle containing a room-temperature ionic liquid (RTIL) is studied by picosecond and femtosecond emission spectroscopy. The mixed micelle consists of a triblock copolymer, (PEO)(20)-(PPO)(70)-(PEO)(20) (Pluronic P123), and a RTIL, 1-pentyl-3-methyl-imidazolium tetra-flouroborate, ([pmim][BF(4)]) or 1-pentyl-3-methyl-imidazolium bromide ([pmim][Br]). Coumarin 480 (C480) is used as the donor, and the acceptor is rhodamine 6G (R6G). Multiple time scales of FRET were detected-an ultrashort component of 1-3 ps and two relatively long components (300-400 ps and 2500-3500 ps). The different time scales are attributed to different donor-acceptor distances. It is proposed that the ionic acceptor (R6G) is localized in the polar corona region of the mixed micelle, while the neutral donor (C480) is distributed over both corona and hydrophobic core regions. The ultrafast (1-3 ps) components are assigned to FRET at a close contact of donor and acceptor. This occurs for the donor in the polar corona region in close proximity of the acceptor. The longer components (300-400 ps and 2500-3500 ps) arise from long-distance FRET from the donor at the core and the acceptor at the corona region. The relative contribution of the ultrafast component of FRET (∼3 ps) increases from 5% at λ(ex) = 375 nm to 30% at λ(ex) = 435 nm in the 0.3 M [pmim][BF(4)] mixed micelle and from 25 to 100% in the 0.9 M [pmim][BF(4)] mixed micelle. It is suggested that, at λ(ex) = 435 nm, mainly the donor molecules present at the corona are excited, causing ultrafast FRET due to a short donor-acceptor distance. At shorter λ(ex), the donor (C480) molecule at the core regions is excited, giving rise to a very long 3400 ps component (R(DA) ∼ 50 Å). Thus, λ(ex) variation leads to excellent spatial resolution. The counterion dependence (Br(-) vs BF(4)(-)) is attributed to the difference in the local polarity and size of the two mixed micelles.