Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India.
J Phys Chem B. 2012 Oct 4;116(39):12021-9. doi: 10.1021/jp307883r. Epub 2012 Sep 24.
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][C(2)SO(4)], 1-ethyl-3-methylimidazolium n-butylsulfate [C(2)mim][C(4)SO(4)], and 1-ethyl-3-methylimidazolium n-hexylsulfate [C(2)mim][C(6)SO(4)], 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][C(2)SO(4)], [C(2)mim][C(4)SO(4)], and [C(2)mim][C(6)SO(4)], the rise component gradually decreases and the rate of FRET (k(FRET)) gradually increases. The k(FRET) was found to be 1.06 × 10(9) s(-1) in 28 mM aqueous SB-16 micelles. With the addition of 100 mM [C(2)mim][C(2)SO(4)], the k(FRET) increases by a factor of 1.33 (1.41 × 10(9) s(-1)), whereas with the addition of 100 mM [C(2)mim][C(6)SO(4)] it increases by a factor of 3.25 (3.45 × 10(9) s(-1)). This rapid increase in k(FRET) in the case of [C(2)mim][C(6)SO(4)] can be explained by our earlier observation ( Rao, V. G.; Ghatak, C.; Ghosh, S.; Mandal, S.; Sarkar, N. J. Phys. Chem. B2012, 116, 3690-3698 ), where we have shown that with the addition of [C(2)mim][C(6)SO(4)], 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][C(6)SO(4)]. The R(DA) was found to be 29.1 Å in 28 mM aqueous SB-16 micelles. With the addition of 100 mM [C(2)mim][C(6)SO(4)], the R(DA) decreases to 24.8 Å. With further increase in the concentration of [C(2)mim][C(6)SO(4)], 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.
在 N-十六烷基-N，N-二甲基-1-丙磺酸盐（SB-16）胶束水溶液中，使用香豆素-153（C-153）作为供体和罗丹明 6G（R6G）作为受体，通过稳态和皮秒时间分辨荧光光谱研究了荧光共振能量转移（FRET）。我们已经从受体（R6G）发射的上升确定了 FRET 的速率（k（FRET））。在没有供体（C-153）的情况下，受体（R6G）显示出平均寿命为 4.77 ns 的单指数衰减，而在存在供体（C-153）的情况下，受体（R6G）表现出双指数荧光瞬变具有明显的上升分量 0.94 ns 和衰减分量 5.16 ns。我们对添加三种不同的离子液体（ILs）时 FRET 参数的变化进行了比较研究，1-乙基-3-甲基咪唑硫酸乙酯[C（2）mim][C（2）SO（4）]，1-乙基-3-甲基咪唑硫酸丁酯[C（2）mim][C（4）SO（4）]和 1-乙基-3-甲基咪唑硫酸己酯[C（2）mim][C（6）SO（4）]，其中每个离子液体都具有相同的阳离子部分，阴离子部分仅在烷基链长度上有所不同。观察到随着 ILs [C（2）mim][C（2）SO（4）]，[C（2）mim][C（4）SO（4）]和[C（2）mim][C（6）SO（4）]的逐渐添加，上升分量逐渐减小，FRET 的速率（k（FRET））逐渐增加。在 28 mM 水 SB-16 胶束中发现 k（FRET）为 1.06×10（9）s（-1）。添加 100 mM [C（2）mim][C（2）SO（4）]时，k（FRET）增加了 1.33 倍（1.41×10（9）s（-1）），而添加 100 mM [C（2）mim][C（6）SO（4）]时，k（FRET）增加了 3.25 倍（3.45×10（9）s（-1））。这种在[C（2）mim][C（6）SO（4）]情况下 k（FRET）的快速增加可以用我们之前的观察结果（Rao，VG; Ghatak，C.; Ghosh，S.; Mandal，S.; Sarkar，N. J. Phys. Chem. B2012, 116, 3690-3698）来解释，其中我们已经表明，随着[C（2）mim][C（6）SO（4）]的添加，C-153向胶束的外表面移动。C-153 的这种运动导致供体-受体距离减小，FRET 速率（k（FRET））增强。这得到了添加[C（2）mim][C（6）SO（4）]后观察到的供体-受体距离（R（DA））减小的很好支持。在 28 mM 水 SB-16 胶束中，R（DA）为 29.1 Å。添加 100 mM [C（2）mim][C（6）SO（4）]后，R（DA）降低至 24.8 Å。随着[C（2）mim][C（6）SO（4）]浓度的进一步增加，R（DA）减小，但受体发射上升的时间常数减小到我们无法通过仪器设置观察到的程度。
