Ultrafast FRET in Ionic Liquid-P123 Mixed Micelles: Region and

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J. Phys. Chem. B 2010, 114, 13159–13166

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Ultrafast FRET in Ionic Liquid-P123 Mixed Micelles: Region and Counterion Dependence Dibyendu Kumar Das, Atanu Kumar Das, Tridib Mondal, Amit Kumar Mandal, and Kankan Bhattacharyya* Department of Physical Chemistry, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700 032, India ReceiVed: July 19, 2010; ReVised Manuscript ReceiVed: September 8, 2010

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-methylimidazolium tetra-flouroborate, ([pmim][BF4]) 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 detectedsan 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 λex ) 375 nm to 30% at λex ) 435 nm in the 0.3 M [pmim][BF4] mixed micelle and from 25 to 100% in the 0.9 M [pmim][BF4] mixed micelle. It is suggested that, at λex ) 435 nm, mainly the donor molecules present at the corona are excited, causing ultrafast FRET due to a short donor-acceptor distance. At shorter λex, the donor (C480) molecule at the core regions is excited, giving rise to a very long 3400 ps component (RDA ∼ 50 Å). Thus, λex variation leads to excellent spatial resolution. The counterion dependence (Br- vs BF4-) is attributed to the difference in the local polarity and size of the two mixed micelles. 1. Introduction 1,2,4b,c

Recently, many groups have reported formation of micelles and reverse micelles3,4a,d involving room-temperature ionic liquids (RTIL). The RTILs have attracted a lot of interest as a “green” solvent and catalyst.5,6 Addition of a RTIL (1-pentyl3-methyl-imidazolium tetra-flouroborate, [pmim][BF4] or [pmim][Br], Scheme 1A) to an aqueous solution of a triblock copolymer P123 leads to the formation of a mixed micelle (Scheme 1E).4b,c In such a mixed micelle, the ions of the RTIL penetrate the P123 micelle. According to SANS studies, P123 micelle consists of a hydrophobic core (PPO block) with a radius of 4.8 nm and a hydrophilic corona (PEO block) of thickness 4.6 nm (Scheme 1D) with an overall diameter ∼ 18 nm.7 On addition of 0.3 M [pmim][BF4], the size of the mixed micelle slightly increases to 21 nm. But in 0.9 M [pmim][BF4] the diameter of the mixed micelle decreases to 13 nm.4b In the case of [pmim][Br], addition of the RTIL to 5 wt % P123 causes a gradual increase in the size of the mixed micelle from 26 nm in 0.3 M RTIL to 40 nm in the presence of 0.9 M [pmim][Br].4c In our previous work, we studied solvation dynamics in different regions of an RTIL-P123 mixed micelle by varying the excitation wavelength (λex).4b,c Because of the polarity dependence of the absorption maximum of the donor (C480), a short λex selects the probe molecules in the relatively nonpolar core of the mixed micelle, while a long λex excites the molecules in the polar (corona) region (red edge excitation shift, REES).8,9 * Corresponding author. E-mail: [email protected].

The λex dependence of solvation dynamics has been used to reveal dynamic heterogeneity in a neat RTIL,4a RTIL microemulsion,4a and RTIL mixed micelle.4b,c Most recently, we demonstrated that λex variation of FRET is complementary to λex variation of solvation dynamics in a RTIL microemulsion.4d In a RTIL microemulsion, the ionic acceptor (R6G) is localized in the polar pool (water and RTIL) of the microemulsion, while the neutral C480 is present in both the polar and nonpolar regions. At a long wavelength, the donor molecules in the polar pool are preferentially excited and this leads to ultrafast FRET because of short donor-acceptor distances (both in the pool). At a short λex, the donor molecules residing in the nonpolar region between the surfactant chains are excited, and since they are quite far from the acceptor, the rate of FRET is slow. Thus, in the RTIL microemulsion, there are multiple time scales of FRET (1.5, 250, and 3900 ps).4d With an increase in the λex, the relative contribution of the ultrafast component (1.5 ps) increases.4d In the present work, we investigated FRET in different regions of a RTIL-P123 mixed micelle. For this purpose, we have chosen two ionic liquidss[pmim][BF4] and [pmim][Br], which differ in the counterion. We have chosen an ionic dye (rhodamine 6G, R6G; Scheme 1B) as the acceptor. The ionic acceptor preferentially stays in the polar corona region of the mixed micelle. The donor is a neutral molecule (coumarin 480, C480; Scheme 1C) which is distributed over regions of different polarity (core and corona) of the mixed micelle. This implies a wide distribution of the donor-acceptor distances (RDA). According to Fo¨rster theory, the rate of FRET is inversely

10.1021/jp106689w  2010 American Chemical Society Published on Web 09/29/2010

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proportional to the sixth power of the separation between the donor and the acceptor. We studied FRET from the donors residing in different regions through variation of λex. We observed significant variation for the two counterions (Br- vs BF4-). 2. Experimental Section Laser grade dyesscoumarin 480 (C480, Exciton; Scheme 1C) and rhodamine 6G (R6G, Exciton; Scheme 1B) were used as received. Sodium tetrafluoroborate (98%, Aldrich), 1-methylimidazole (99%, Aldrich), and 1-bromopentane (99%, Aldrich) were used for the synthesis of the room-temperature ionic liquid. Acetonitrile (Merck) was distilled over P2O5, and dichloromethane (Merck) was used as received. Diethyl ether (Merck) was distilled over KOH. The RTILs were synthesized following the sonochemical route10 as described earlier.4 The steady-state absorption and emission spectra were recorded in a Shimadzu UV-2401 spectrophotometer and a Spex FluoroMax-3 spectrofluorimeter, respectively. All experiments were done at room temperature (293 K). Our femtosecond up-conversion setup (FOG 100, CDP; IRF ∼ 300 fs) is described elsewhere.4 To fit the femtosecond transient, we first determined the long-picosecond components (400-7000 ps) using a TCSPC setup (IRF ∼ 90 ps) described earlier.4b The long-picosecond components were kept fixed to fit the femtosecond data. The ultrafast components (1-3 ps) were determined from the femtosecond up-conversion setup. The rate of FRET (kFRET) was calculated by following Fo¨rster theory as11

Das et al.

kFRET )

1 A τrise

)

( )

1 R0 τD0 RDA

6

(1)

where τD0 is the lifetime of the donor in the absence of acceptor A is the rise time of acceptor emission in the presence of and τrise donor. At a donor-acceptor distance RDA ) R0, the efficiency of energy transfer is 50% and kFRET ) (1/τ0D). To calculate the Fo¨rster distance R0 (Å), we used11

R0 ) 0.211[κ2n-4QDJ(λ)]1/6

(2)

where n is the refractive index of the medium (∼1.4 for macromolecules in water),11 QD is the quantum yield of the donor in the absence of acceptor, κ2 is the orientation factor, and J(λ) is the spectral overlap between the donor emission and the acceptor absorption. J(λ) is related to the normalized fluorescence intensity (FD) of the donor in the absence of the acceptor and the extinction coefficient of the acceptor (εA) as11

J(λ) )

∫0∞ FD(λ) εA(λ)λ4 dλ ∫0∞ FD(λ) dλ

(3)

The value of κ2 may vary from 0 (mutually perpendicular transition dipoles) to 4 (collinear dipoles). For κ2 ) 0, FRET is forbidden and no ultrafast component of FRET would be observed. The ultrafast FRET detected in this work obviously

SCHEME 1: Structure of (A) [pmim][BF4], (B) R6G, (C) Coumarin-480 (C480), (D) P123 Micelle, and (E) RTIL-P123 Aggregate

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Figure 1. Emission spectrum of 40 µM C480 in the absence (black lines) and in the presence (green lines) of 80 µM R6G at λex ) 375 and 435 nm in [pmim][BF4]-P123 (A, B) and [pmim][Br]-P123 (C, D) respectively. Emission spectrum of 80 µM R6G in absence of C480 in each case has been shown (red lines).

indicates a large value of κ2. One can estimate the upper (κmax2) and lower (κmin2) limit of κ2 using the steady-state fluorescence anisotropy and the initial value of anisotropy (r0) obtained in the time-resolved anisotropy measurement as11

2

κmin

[

(dDx + dAx) 2 ) 13 2

]

2 κmax2 ) (1 + dDx + dAx + 3dDxdAx) 3

(4) (5)

where dix denotes the ratio of the square root of the steadystate fluorescence anisotropy (rSS i ) and the initial value of anisotropy (r0i ) in the anisotropy decay of the ith species (donor or acceptor). In this work, we used κ2 ) 2/3 (random orientation) for the calculation of R0.4d 3. Results 3.1. Steady State Emission Spectra and REES in [pmim][BF4]-P123 and [pmim][Br]-P123 Mixed Micelles. In the RTIL-P123 mixed micelles, the emission maximum of the acceptor (R6G) does not vary with λex. This suggests that the acceptor R6G resides in a more or less uniform environment. Since R6G is ionic, it is expected to be located in the polar corona region of the mixed micelles. In both RTIL-P123 mixed micelles, the emission maximum of C480 is blue-shifted from that (489 nm) in bulk water. In [pmim][BF4]-P123 mixed micelle, with an increase in λex the emission maximum of the donor (C480) exhibits a 15 nm REES from 463 nm at λex ) 375 nm to 478 nm at λex ) 435 nm. The REES suggests that the donor (C480) is distributed over regions of varying polarity within the mixed micelle. With an increase in the concentration of [pmim][BF4], at 0.9 M the core of the mixed micelle becomes more polar due to penetration of ionic liquid and this causes a red shift of the emission maximum of C480 (∼14 nm). In 0.9 M [pmim][BF4]-P123 mixed micelle,

we observed very small (2 nm) REES (from 477 nm at λex ) 375 nm to 479 nm at λex ) 435 nm). This indicates that penetration of [pmim][BF4] at 0.9 M decreases heterogeneity in the mixed micelle. Another reason for the marked decrease in REES is the reduction of the diameter of the mixed micelle to 13 nm.4b For [pmim][Br]-P123 mixed micelle, the emission maximum of the donor (C480) displays a 19 nm REES at 0.3 M RTIL (emission maximum shifts from 458 nm at λex ) 375 nm to 477 nm at λex ) 435 nm). With an increase in the concentration of [pmim][Br] to 0.9 M, the magnitude of REES decreases slightly to 17 nm (emission maximum of C480 at 465 nm for λex ) 375 nm and at 482 nm for λex ) 435 nm). This shows that, in the case for [pmim][Br], there is a considerable amount of heterogeneity in the [pmim][Br]-P123 mixed micelle. Note, the diameter (40 nm) of the 0.9 M [pmim][Br]-P123 mixed micelle is far larger than that for the 0.9 M [pmim][BF4]-P123 mixed micelle. The difference in the size and REES of the two RTIL-P123 mixed micelles may be ascribed to the difference in [pmim][Br] TABLE 1: Energy-Transfer Parameters for C480-R6G Pair in Different Systems system 5 wt % P123 + 0.3 M [pmim][BF4] 5 wt % P123 + 0.9 M [pmim][BF4] 5 wt % P123 + 0.3 M [pmim][Br] 5 wt % P123 + 0.9 M [pmim][Br]

a

(10%.

λex (nm)

λem (nm)

J(λ)a (M-1cm-1 nm4)

R0a (Å)

εa

375 405 435 375 405 435 375 405 435 375 405 435

463 470 478 477 478 479 458 466 477 465 474 482

2.33 × 1015 2.78 × 1015 3.51 × 1015 3.63 × 1015 3.69 × 1015 3.87 × 1015 2.02 × 1015 2.49 × 1015 3.49 × 1015 2.41 × 1015 2.97 × 1015 3.70 × 1015

51.9 53.5 55.6 54.5 54.7 55.2 49.3 51.0 54.0 50.3 52.0 54.0

0.53 0.57 0.25 0.66 0.70 0.70 0.42 0.52 0.35 0.44 0.54 0.50

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Figure 2. Spectral overlap between donor (C480) emission (s) with acceptor (R6G) absorption (- · -) in 5 wt % P123-RTIL at λex (i-iii) 375, 405, and 435 nm in (A) 0.3 M [pmim][BF4], (B) 0.9 M [pmim][BF4], (C) 0.3 M [pmim][Br], and (D) 0.9 M [pmim][Br].

and [pmim][BF4]. [pmim][BF4] is highly hydrophobic and relatively less soluble in water. This may cause expulsion of water from the mixed micelle causing contraction of size and disappearance of REES. On the contrary, [pmim][Br] is highly hydrophilic and very much soluble in water. As a result, [pmim][Br]-P123 mixed micelle contains a lot of water and retains the overall structure of P123 with clear demarcation of polar and nonpolar regions, and this results in large REES. Note, for 3 M [pmim][Br] the mixed micelle becomes really very big (3500 nm).4c 3.2. Steady-State Study of FRET in RTIL-P123 Mixed Micelle: λex Dependence. In the RTIL-P123 mixed micelle, on addition of the acceptor (R6G) there is a marked reduction (by 30-40%) in the emission intensity of the donor (C480; Figure 1). The reduction in the emission intensity arises mainly as a result of FRET from C480 to R6G. The steady-state efficiency (εs) of FRET may be calculated from εs ) 1 - (IDA/ ID), where IDA and ID denote steady state emission intensity of the donor in the presence and absence of the acceptor, respectively. The efficiencies of FRET (εs) at different excitation wavelengths are summarized in Table 1. As shown in Figure 2, the overlap between the absorption spectrum of the acceptor (R6G) with the emission spectrum of the donor (C480) in both RTIL-P123 mixed micelles depends on λex. The magnitudes of the spectral overlap integral, J(λ), between the emission spectrum of the donor (C480) and the absorption spectrum of the acceptor (R6G) at different excitation wavelengths are given in Table 1. Because of the red shift of the donor emission, the magnitude of the spectral overlap, J(λ), increases about 65% as λex increases from 375 to 435 nm (Table 1) for 0.3 M [pmim][BF4]-P123. For 0.9 M [pmim][BF4]-P123 mixed micelle the magnitude of the spectral overlap, J(λ), remains almost the same with the increase in λex from 375 to 435 nm. This is consistent with a very small REES in 0.9 M [pmim][BF4]-P123 mixed micelle. In the case of [pmim][Br]-P123 mixed micelle, at both 0.3 and 0.9 M J(λ) increases about 60% as λex increases from 375

to 435 nm. This may be attributed to the highly heterogeneous structure of [pmim][Br]-P123 mixed micelle. The steady-state efficiency of FRET does not increase appreciably with an increase in λex. This may be due to the fact that only in a few mixed micelles both the donor and the acceptor are present. In most cases, a mixed micelle contains only the donor and the non-FRET donors dominate the fluorescence intensity. In keeping with this observation, Fo¨rster distance R0 values, calculated from steady-state spectra, do not change significantly with λex in both of the mixed micelles (Table 1). We will discuss later that the donor-acceptor distances show marked λex dependence if they are calculated from the rise of the acceptor fluorescence using eq 1. 3.3. Time-Resolved Studies of FRET from C480 to R6G in RTIL-P123 Mixed Micelle. 3.3.1. Picosecond Studies. In a picosecond setup, the lifetime of the donor (C480) in both of the mixed micelles does not display any change on addition of the acceptor (R6G). This is in contrast to the decrease in the emission intensity of the donor. It seems that the donor emission is dominated by unquenched non-FRET donors, and hence, no shortening of the donor lifetime is detected in a picosecond setup. Since the picosecond decay of the donor does not accurately describe FRET, we determined the rate of FRET from the rise of the acceptor emission. In this section, we report measurement of the long component of FRET from the rise of the acceptor detected using a picosecond set up. In the next section, we will discuss detection of the ultrafast component of FRET using a femtosecond setup. Insets of Figures 3 and 4 show picosecond fluorescence transients of the acceptor (R6G) at 570 nm in both [pmim][BF4]-P123 and [pmim][Br]-P123 mixed micelles in the absence and presence of the donor (C480) at λex ) 375 nm. In this work, we have monitored the fluorescence transient of the acceptor (R6G) at an emission wavelength of 570 nm. At this wavelength, the contribution of the quenched emission of the donor is negligible. In the absence of the donor (C480), at all λex, the acceptor (R6G) exhibits a single-exponential decay

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Figure 3. Femtosecond transients of the acceptor (R6G, 80 µM) emission (λem ) 570 nm) in the absence and in the presence of the donor at 0.3 M [pmim][BF4]-P123 and 0.9 M [pmim][BF4]-P123 at λex at 375 nm. Picosecond transients are shown in the inset.

Figure 4. Femtosecond transients of the acceptor (R6G, 80 µM) emission (λem ) 570 nm) in the absence and in the presence of the donor at 0.3 M [pmim][Br]-P123 and 0.9 M [pmim][Br]-P123 at λex at 375 nm. Picosecond transients are shown in the inset.

(∼4600-5000 ps) with no rise component in both of the mixed micelles. However, in the presence of the donor (C480), picosecond transients of the acceptor (R6G) at 570 nm show distinct rise components. In the mixed micelle containing 0.3 M [pmim][BF4], for λex ) 375 and 405 nm, the rise components (i.e., FRET) are found to be 400 and 2500 ps in the [pmim][BF4]-P123 mixed micelle (inset of Figure 3 and Table 2). In 0.9 M [pmim][BF4]-P123 mixed micelle, the rise (i.e., FRET) components are 350 and 3400 ps for λex ) 375 nm and 300 and 3200 ps for λex ) 405 nm. For the [pmim][BF4]-P123 mixed micelle at long wavelength of excitation (435 nm) we did not detect any ultrafast

rise in the acceptor emission in our picosecond setup (Figure 5, Table 2). This indicates that, for λex ) 435 nm, FRET is too fast to be detected in our picosecond setup (IRF ∼ 90 ps). In the RTIL-P123 mixed micelle containing 0.3 M [pmim][Br], we obtained two rise componentss400 and 3000 ps for λex ) 375 nm and 500 and 3000 ps at λex ) 405 nm (inset of Figure 4, Table 3). At λex ) 435 nm, we found only one rise component of 300 ps. In 0.9 M [pmim][Br]-P123 mixed micelle, we detected two rise (i.e., FRET) components of 450 and 3600 ps at λex ) 375 nm and at λex ) 405 nm. For λex ) 435 nm, only one rise component of 400 ps was detected for 0.9 M [pmim][Br]-P123 mixed micelle.

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TABLE 2: Femtosecond Decay Parameters of R6G (80 µM, λem ) 570 nm) in the Presence of C480 (40 µM) at Different λex in [pmim][BF4]-P123 Mixed Micelles [pmim][BF4] (M)

λex (nm)

τ1a (ps) (a1)

τ2a (ps) (a2)

τ3a (ps) (a3)

τ4a (ps) (a4)

0.3

375 405 435 375 405 435

3 (-0.083, 5%) 3 (-0.18, 10%) 3 (-0.06, 30%) 1 (-0.047,25%) 1 (-0.25, 45%) 1 (-0.13,100%)

400 (-0.60, 25%) 400 (-0.7, 25%) 400 (-0.15, 70%) 350 (-0.02, 5%) 300 (-0.03, 5%)

2500 (-1.65, 70%) 2500 (-1.9, 65%)

6960 (3.333) 7150 (3.78) 6700 (1.21) 7450 (1.257) 7410 (1.55) 7260 (1.13)

0.9

a

3400 (-0.19, 70%) 3200 (-0.27, 50%)

(10%.

Figure 5. Comparison of picosecond rise (λem ) 570 nm) of the acceptor in RTIL-P123 mixed micelles at λex ) 375 and 435 nm.

TABLE 3: Femtosecond Decay Parameters of R6G (80 µM, λem ) 570 nm) in the Presence of C480 (40 µM) at Different λex in [pmim][Br]-P123 Mixed Micelles [pmim][Br] (M)

λex (nm)

τ1a (ps) (a1)

τ2a (ps) (a2)

τ3a (ps) (a3)

τ4a (ps) (a4)

0.3

375 405 435 375 405 435

2 (-0.15, 9%) 2 (-0.15, 7%) 2 (-0.045, 25%) 0.5 (-0.15, 20%) 0.5 (-0.18, 13%) 0.5 (-0.07, 30%)

420 (-0.30, 18%) 500 (-0.45, 20%) 300 (-0.15, 75%) 450 (-0.044, 6%) 450 (-0.12, 9%) 400 (-0.15, 70%)

3000 (-1.2, 73%) 3000 (-1.65, 73%)

7350 (2.65) 6780 (3.25) 6000 (1.195) 7240 (1.779) 7070 (2.37) 7000 (1.22)

0.9

a

3600 (-0.585, 74%) 3600 (-1.07, 78%)

(10%.

Figure 5 shows the λex dependence on the rise components (i.e., FRET) for both of the mixed micelles. It is evident that with an increase in λex from 375 to 435 nm the contribution of the long component of rise decreases. 3.3.2. Femtosecond Study of FRET: Ultrafast Rise of the Acceptor Fluorescence. The ultrafast components of FRET were detected using a femtosecond setup. The ultrafast rise of the acceptor (R6G) at an emission wavelength of 570 nm in [pmim][BF4]-P123 and [pmim][Br]-P123 for λex ) 375 nm are shown in Figures 3 and 4, respectively. For all λex, we detected an ultrafast rise component for both of the mixed micelles in addition to those detected using the picosecond setup. Note, in the absence of the donor, no rise component is observed in the acceptor emission. With an increase in λex from 375 to 435 nm, the relative contribution of the ultrafast component increases markedly (Figure S1 of the Supporting Information,

and Table 2). In the following section, we will discuss the origin of the ultrafast component of FRET and explain the λex dependence. The P123 mixed micelle containing 0.3 M [pmim][BF4] exhibits an ultrafast growth component of 3 ps at all λex. In 0.9 M [pmim][BF4]-P123 mixed micelle the time constant of the ultrafast component decreases to ∼1 ps. With an increase in λex from 375 to 435 nm, the contribution of the ultrafast component increases from 5 to 30% in the mixed micelle containing 0.3 M [pmim][BF4] and from 25 to 100% (i.e., no ultraslow component) for 0.9 M [pmim][BF4]-P123 mixed micelle (Table 2). For the [pmim][Br]-P123 mixed micelle containing 0.3 M [pmim][Br], there is an ultrafast growth component of 2 ps at all λex. With an increase in the concentration of [pmim][Br] in the mixed micelle to 0.9 M, we detected a rise component of

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TABLE 4: Donor-Acceptor Distance in the Mixed Micelle Containing [pmim][BF4] Calculated from the Rise of Acceptor Emission system

λex (nm)

mixed micelle containing 0.3 M [pmim][BF4]

375 405 435 375 405 435

mixed micelle containing 0.9 M [pmim][BF4]

a

τ 3, 3, 3, 1, 1, 1

FRET

400, 400, 400 350, 300,

(ps)

RDAa (Å)

2500 15.2, 34.5, 2500 15.7, 35.5, 16.2, 36.5 3400 13.3, 35.3, 3200 13.3, 34.5, 13.5

46.8 48.2 51.6 51.1

(10%.

TABLE 5: Donor-Acceptor Distance in the Mixed Micelle Containing [pmim][Br] Calculated from the Rise of Acceptor Emission system

λex (nm)

mixed micelle containing 0.3 M [pmim][Br]

375 405 435 375 405 435

mixed micelle containing 0.9 M [pmim][Br]

τ

FRET

(ps)

2, 420, 3000 2, 500, 3000 2, 300 0.5, 450, 3600 0.5, 450, 3600 0.5, 400

RDAa (Å) 13.3, 32.3, 44.8 13.7, 34.2, 46 14.3, 33 11, 33.3, 47 11, 34.3, 48.5 11.4, 34.6

to the acceptor. Another cause of faster FRET in 0.9 M [pmim][BF4]-P123 could be its smaller size (diameter 13 nm) compared to that (40 nm) in 0.9 M [pmim][Br]-P123 mixed micelle. 5. Conclusion This work demonstrates λex dependence of ultrafast FRET between a donor and an acceptor at close proximity confined within a RTIL-P123 micelle. The λex dependence has been utilized to spatially resolve FRET. Three different time scales of FRET corresponding to three different donor-acceptor distances were detected. The counterion dependence [Br- and BF4-] is attributed to the difference in their hydrophobicity and consequent effect on the size and heterogeneity in the mixed micelle. These results may have implications in ultrafast FRET in biological environments12 and nanoparticles.13,14 Acknowledgment. Thanks are due to Department of Science and Technology, India (Project No. IR/I1/CF-01/2002 and J. C. Bose Fellowship) and Council for Scientific and Industrial Research (CSIR) for generous research support. D.K.D., A.K.D., T.M., and A.K.M. thank CSIR for research fellowships.

0.5 ps for all λex from 375 to 435 nm. The contribution of the ultrafast component increases from 9% at λex ) 375 nm to 25% at λex ) 435 nm for mixed micelle containing 0.3 M [pmim][Br] and from 20 to 30% for 0.9 M [pmim][Br] (Table 3).

Supporting Information Available: Figures showing comparison of femtosecond transients of the acceptor in the presence of donor at 0.3 M RTIL-P123 and 0.9 M RTIL-P123 for λex ) 375 and 435 nm (Figure S1) and counterion dependence of rise of acceptor emission for λex ) 375 nm (Figure S2). This material is available free of charge via the Internet at http://pubs.acs. org.

4. Discussion

References and Notes

The main findings of this work may be summarized as follows. First, FRET between a donor and an acceptor bound to a mixed micelle occurs on multiple time scales (∼1 ps, 200-300 ps, and a very long one at ∼3000 ps). This corresponds to multiple donor-acceptor distances varying from 15 to 50 Å (Tables 4 and 5). The ionic acceptor (R6G) preferentially resides at the polar corona region. The shortest donor-acceptor distance arises from FRET between a pair of donor and acceptor both residing in the polar corona region in close proximity. The longest distance (ultraslow FRET) corresponds to the acceptor at the corona and a donor in the core. Perhaps, the most interesting result is the λex dependence of the relative contribution of the ultrafast component (1-3 ps) and ultraslow component (2400-3400 ps). With an increase in λex, the relative contribution of the donor molecules in the polar corona region increases. As a result, the contribution of the ultrafast component of FRET increases. Thus, λex variation affords a simple way of spatially resolving FRET in different regions of the mixed micelle. It is also interesting to note that REES and the relative component of the ultrafast component FRET differ for the two counterions, Br- and BF4- (Figure S2 of the Supporting Information). Note that the solvation dynamics of the donor also depends on the counterion. Solvation dynamics in the [pmim][BF4]-P123 mixed micelle4b is much faster than that of [pmim][Br]-P123 mixed micelle.4c For instance, at λex ) 435 nm the average solvation time is 5 ps for [pmim][BF4]-P123 mixed micelle4b and 75 ps for [pmim][Br]-P123 mixed micelle.4c This suggests that the local environment of the donor (C480) in [pmim][BF4]-P123 mixed micelle4b is much more polar than that in [pmim][Br]-P123 mixed micelle, and hence, in [pmim][BF4] mixed micelle the donor is closer

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