Study of Fluorescence Resonance Energy Transfer in Zwitterionic Micelle: Ionic-Liquid-Induced Changes in FRET Parameters

The fluorescence resonance energy transfer (FRET) using Coumarin-153 (C-153) as the donor and Rhodamine 6G (R6G) as the acceptor is studied in an aqueous solution of N-hexadecyl-N,N-dimethylammonio-1-propanesulfonate (SB-16) micelles by steady-state and picosecond time-resolved fluorescence spectroscopy. We have determined the rate of FRET (k(FRET)) from the rise of the acceptor (R6G) emission. In the absence of donor (C-153), the acceptor (R6G) displays a single-exponential decay with average lifetime of 4.77 ns, whereas in presence of donor (C-153), the acceptor (R6G) exhibits a biexponential fluorescence transient having a distinct rise component of 0.94 ns and decay component of 5.16 ns. We have carried out a comparative study of changes in FRET parameters upon addition of three different ionic liquids (ILs), 1-ethyl-3-methylimidazolium ethylsulfate [C(2)mim][C2SO4], 1-ethyl-3-methylimidazolium n-butylsulfate [C(2)mim]-[C4SO4], and 1-ethyl-3-methylimidazolium n-hexylsulfate [C(2)mim][C6SO4], where each ionic liquid bears the same cationic part and the anionic parts differ in the alkyl chain length only. It has been observed that with gradual addition of the ILs [C(2)mim][C2SO4], [C(2)mim][C4SO4], and [C(2)mim][C6SO4], the rise component gradually decreases and the rate of FRET (k(FRET)) gradually increases. The k(FRET) was found to be 1.06 x 10(9) in 28 mM aqueous SB-16 micelles. With the addition of 100 mM [C(2)mim][C2SO4], the k(FRET) increases by a factor of 1.33 (1.41 x 10(9) s(-1)), whereas with the addition of 100 mM [C(2)mim][C6SO4] it increases by a factor of 3.25 (3.45 x 10(9) s(-1)). This rapid increase in kgRET in the case of [C(2)mim][C6SO4] can be explained by our earlier observation (Rao, V. G.; Ghatak, C.; Ghosh, S.; Mandal, S.; Sarkar, N. J. Phys. Chem. B 2012, 116, 3690-3698), where we have shown that with the addition of [C(2)mim][C6SO4], C-153 moves toward the outer surface of the micelle. This movement of C-153 causes reduction in donor acceptor distance and enhancement in FRET rate (k(FRET)). This is well-supported by the reduced donor acceptor distance (R-DA) observed with the addition of [C(2)mim][C6SO4]. The R-DA was found to be 29.1 angstrom in 28 mM aqueous SB-16 micelles. With the addition of 100 mM [C(2)mim][C6SO4], the R-DA decreases to 24.8 angstrom. With further increase in the concentration of [C(2)mim][C6SO4], the R-DA decreases, but the time constant for the rise of acceptor emission decreases to such an extent that we are unable to observe it by our instrumental setup.