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J. Phys. Chem. B 2007, 111, 5896-5902
Excitation Wavelength Dependence of Solvation Dynamics in a Supramolecular Assembly: PEO-PPO-PEO Triblock Copolymer and SDS Ujjwal Mandal, Aniruddha Adhikari, Shantanu Dey, Subhadip Ghosh, Sudip Kumar Mondal, and Kankan Bhattacharyya* Physical Chemistry Department, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700 032, India ReceiVed: December 28, 2006; In Final Form: March 27, 2007
The triblock copolymer (PEO)20-(PPO)70-(PEO)20 (P123) forms a supramolecular aggregate with sodium dodecyl sulfate (SDS). The solvation dynamics and anisotropy decay of coumarin 480 (C480) in different regions of a P123-SDS aggregate are studied through variation of the excitation wavelength (λex) using femtosecond upconversion. In a P123 micelle, because of the drastic differences in polarity between the hydrophilic corona region (PEO block) and the hydrophobic PPO core, C480 exhibits a pronounced red edge excitation shift (REES) of emission maximum by 24 nm. In the P123-SDS aggregate, SDS penetrates the core of the P123 micelle. This increases the polarity of the core and reduces the difference in the polarity between the core and the corona region. In a P123-SDS aggregate, the REES is much smaller (5 nm) which suggests a reduced difference between the core and the corona. Solvation dynamics in a P123 micelle displays a bulklike ultrafast component (20), the DLS peak corresponds to a much smaller aggregate. This is attributed to an SDS rich aggregate with a few P123 micelles in it.6b Several groups have studied the interaction of P123 with SDS at a much higher P123 concentration (10 wt % or 17 mM).7 For 10 wt % P123, the SANS studies show that penetration of SDS causes a marked decrease in the aggregation number and the size of the core (Table 1).7b The number of P123 molecules in the micelles decreases sharply from 69 in the absence of SDS to only ∼4 at a SDS/P123 molar ratio of 5:1. At a SDS/P123 ratio of 5:1, the radius of the mixed micelle consisting of 4 P123 and 20 SDS molecules is 52 Å.7b This is much larger than the radius of a SDS micelle (30 Å).8 When five SDS molecules decorate each P123, the radius of the hydrophobic core decreases to ∼19 Å. This is about 2.5 times smaller than the core of a P123 micelle (radius ∼48 Å).7b The presence of SDS in the core increases the polarity of the core. As a result, the difference in the polarity of the core and the corona region in a P123-SDS aggregate is much less than that in a P123 micelle. In this work, we focus on how penetration of SDS affects the emission spectrum and solvation dynamics of a fluorescent probe, coumarin 480 (C480), in a P123-SDS aggregate. In bulk water, the solvation dynamics is very fast with a major component in the 0.1 ps time scale and a long ∼1 ps component.9 However, at many interfaces and organized assemblies, the solvation dynamics of water displays a slow component of solvation in the 100-1000 ps time scale.10-15 Recent large scale computer simulations also confirm the presence of an ultraslow component of solvation dynamics in many confined environments.16 In a heterogeneous medium (e.g., micelle or lipid), a solvatochromic probe exhibits a marked variation in the absorption and emission maxima in different regions. Excitation at a shorter wavelength (“blue edge”) selects the probe (e.g., C480) in a relatively nonpolar environment and gives rise to a blue shifted emission spectrum. On the other hand, excitation at a longer wavelength (“red edge”) selects the probe in a relatively polar environment and gives rise to a red shifted emission spectrum. This phenomenon of excitation wavelength dependence of emission maximum is known as red edge excitation shift (REES).17 In principle, using REES, one may spatially resolve solvation dynamics in a micro-heterogeneous medium by variation of the excitation wavelength (λex). Recently, we have studied solvation dynamics in different regions of a P123 micelle,5a a reverse micelle,18a and a lipid vesicle18b,c by varying λex. In this work, we have used femtosecond upconversion to study the excitation wavelength dependence of the solvation dynamics of C480 in a P123-SDS aggregate. 2. Experimental Section The triblock copolymer, Pluronic P123 (P123) was a gift from BASF Corp. and was used without further purification. Laser
grade coumarin 480 (Exciton, Scheme 1C) and sodium dodecyl sulfate (SDS, Aldrich) were used as received. The steady state absorption and emission spectra were recorded in a Shimadzu UV-2401 spectrophotometer and a Spex FluoroMax-3 spectrofluorimeter, respectively. The copolymer solution was prepared by mixing proper amount of the copolymer (P123) with 100 mL of water. The solution was stirred for 4-5 h using a magnetic stirrer at room temperature in a sealed container. We used 10 wt % P123 (∼17.4 mM) and 87 mM SDS so that the P123/SDS molar ratio was 1:5. All experiments were done at room temperature (∼20 °C). In our femtosecond upconversion setup (FOG 100, CDP), the sample was excited at 375, 405, and 435 nm. The sample was excited using the second harmonic of a mode-locked Tisapphire laser with an 80 MHz repetition rate (Tsunami, Spectra Physics), pumped by 5 W Millennia (Spectra Physics). The fundamental beam was frequency doubled in a nonlinear crystal (1 mm BBO, θ ) 25°, φ ) 90°). The fluorescence emitted from the sample was upconverted in a nonlinear crystal (0.5 mm BBO, θ ) 38°, φ ) 90°) using the fundamental beam as a gate pulse. The upconverted light is dispersed in a monochromator and detected using photon counting electronics. A crosscorrelation function obtained using the Raman scattering from ethanol displayed a full width at half-maximum (fwhm) of 350 fs. The femtosecond fluorescence decays were fitted using a Gaussian shape for the exciting pulse. To fit the femtosecond data, one needs to know the long decay components. These were detected using a picosecond setup in which the samples were excited at 375, 405, and 435 nm using picosecond laser diodes (IBH Nanoleds) in an IBH Fluorocube apparatus. The emission was collected at a magic angle polarization using a Hamamatsu microchannel plate (MCP) photomultiplier (5000U-09). The time correlated single photon counting (TCSPC) setup consists of an Ortec 9327 CFD and a Tennelec TC 863 TAC. The data is collected with a PCA3 card (Oxford) as a multichannel analyzer. The typical fwhm of the system response using a liquid scatterer is about 90 ps. The fluorescence decays were deconvoluted using IBH DAS6 software. In order to fit the femtosecond transient, we first determined the long picosecond components by deconvolution of the picosecond decays. Then, the long picosecond components were kept fixed to fit the femtosecond data. The time resolved emission spectra (TRES) were constructed using the parameters of best fit to the fluorescence decays and the steady state emission spectrum following the procedure described by Maroncelli and Fleming.19a The solvation dynamics is described by the decay of the solvent correlation function, C(t), defined as,
C(t) )
ν(t) - ν(∞) ν(0) - ν(∞)
(1)
where ν(0), ν(t), and ν(∞) are the peak frequencies at time 0, t, and ∞, respectively. Note, a portion of solvation dynamics is missed (at λex ) 405 and 435 nm) even in our femtosecond setup with a time resolution of 350 fs. The amount of solvation missed is calculated using the Fee-Maroncelli procedure.19b In order to study fluorescence anisotropy decay, the analyzer was rotated at regular intervals to get perpendicular (I⊥) and
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Figure 1. Absorption spectra of C480 in 10 wt % P123: in the absence of SDS (s); in the presence of 87 mM SDS (- ‚ - ‚). The excitation wavelengths are marked (O) at 375, 405, and 435 nm.
Figure 3. Plot of the emission maximum of C480 in 10 wt % P123 as a function of excitation wavelength: in the absence of SDS (O); in the presence of 87 mM SDS (b).
TABLE 2: λex Dependence of the Steady State Emission Maximum and fwhm of C480 in 10 wt % P123 Micelle in the Presence and Absence of 87 mM SDS and C480 in Water fwhma (cm-1)
λmax em (nm)
λex (nm) P123 P123 + SDS water P123 P123 + SDS water 375 405 435 a
Figure 2. Emission spectra of C480 in 10 wt % P123 when excited at (i) 375 nm, (ii) 405 nm, (iii) 435 nm, and (iv) C480 in water (‚‚‚‚‚) (λex ) 375-435 nm): (A) without SDS; (B) with 87 mM SDS.
parallel (I|) components. Then, the anisotropy function, r(t), was calculated using the formula
r(t) )
I|(t) - GI⊥(t) I|(t) + 2GI⊥(t)
(2)
The G value of the picosecond setup was determined using a probe whose rotational relaxation is very fast, e.g., coumarin 480 in methanol. The G value was found to be 1.5. 3. Results and Discussion 3.1. Steady State Absorption and Emission Spectra: λex Dependence. In a P123 micelle (10 wt %), the absorption maximum of C480 is found to be at 383 nm (Figure 1). This is blue shifted by 13 nm from the reported absorption maximum (396 nm)20 of C480 in water. The absorption maximum of C480 in P123 is intermediate between those in acetonitrile (380 nm) and those in ethanol (387 nm).20 On addition of SDS, as the polarity of the P123 micelle increases, the absorption maximum of C480 undergoes a red shift (Figure 1). At a SDS/P123 molar ratio of 5:1, the absorption maximum of C480 in P123-SDS aggregate is at 387 nm which is the same as the reported absorption maximum in ethanol.20 Hence, it is inferred that the microenvironment of C480 in P123-SDS aggregate (1:5 molar ratio) is ethanol-like. The λex dependence of the emission spectra of coumarin 480 in P123 is shown in Figure 2A. Figure 2B shows the λex dependence in P123-SDS aggregate. The emission maxima of C480 at different λex’s in P123-SDS aggregate as well as in P123 micelle are listed in Table 2. Figure 3 shows the λex dependence of the emission maximum of coumarin 480 in P123-SDS aggregate and in P123 micelle. It is readily seen that, in the absence of SDS, the emission maximum of C480 in P123 micelle shows a 24 nm shift (REES)
450 462 471
471 472 475
489 489 489
3500 3350 2800
3100 2900 2650
2650 2650 2650
( 5%.
from 447 nm at λex ) 335 nm to 471 nm at λex ) 445 nm. Evidently, excitation at a short wavelength (the blue end, 335 nm) preferentially selects the probes residing at the less polar core region and, hence, gives rise to a blue shifted emission spectrum. At a long λex (445 nm, red end), probe (C480) molecules in the hydrophilic (polar) corona region are excited almost exclusively and this leads to a red shifted emission spectrum. The λex dependence of the emission maximum of C480 in a P123-SDS aggregate is much weaker than that in a P123 micelle. In a P123-SDS aggregate, the total shift in emission maximum (REES) is only 5 nm from 470 nm at λex ) 335 to 475 nm at λex ) 445 nm. The markedly reduced REES in P123SDS aggregate may be explained in terms of penetration of the PPO core of P123 micelle by SDS, as indicated in the structural studies.6,7 Inclusion of SDS into the micellar core and the consequent presence of ionic head groups of SDS, counterions, and associated water molecules increase the polarity and hydrophilicity of the PPO core significantly. Consequently, the difference in the static polarity of the core and peripheral corona region in a P123-SDS aggregate is quite small. This may be responsible for the ∼5-fold lower REES (5 nm) in P123-SDS aggregate compared to that (24 nm) in a P123 micelle. Also, because of the increased polarity of the core in the presence of SDS, the emission maxima of C480 in P123-SDS are red shifted from those in P123 micelle at all λex values. The difference in the emission maximum of C480 in P123-SDS aggregate and P123 micelle is most significant (21 nm) at λex ) 375 nm (Table 2). This is because the emission spectrum at λex ) 375 nm is dominated by the C480 molecules in the core region and SDS modifies the core more than the corona. Excitation at the red end (435 nm) exhibits a smaller difference of 4 nm (Table 2) because, at a long λex value, the highly polar corona region contributes almost exclusively and the polarity of this region does not increase much on addition of SDS. The λex dependence of the full width at half-maximum (fwhm) of the emission spectrum of C480 is more pronounced in P123 micelle than that in P123-SDS aggregate (Table 2). The fwhm’s in P123 are 3500, 3350, and 2800 cm-1 for λex ) 375, 405, and 435 nm, respectively. The corresponding values for P123-
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Figure 4. Picosecond decay of C480 (λex ) 375 nm) in 10 wt % P123 in the absence of SDS (A-C) and in the presence of 87 mM SDS (D-F).
SDS aggregate are 3100, 2900, and 2650 cm-1. At all λex’s, the fwhm’s in P123-SDS aggregate are smaller than those in P123 micelle. Note that the fwhm of the emission spectrum of C480 in water is relatively smaller (2650 cm-1)5a and is independent of λex. In summary, steady state spectra suggest that penetration of SDS increases the static polarity of the core and thus reduces the difference between the polarity of the core and that of the corona region in a P123-SDS aggregate. It may be recalled that, compared to a P123 micelle (core radius ∼48 Å), the core size in a P123-SDS aggregate is much smaller (radius ∼19 Å). Still, in P123-SDS, the core is sufficiently large to accommodate the probe C480 (radius ∼5 Å). 3.2. Time Resolved Studies. In this section, we will compare the λex dependence of solvation dynamics and anisotropy decay in P123-SDS aggregate with those in P123 micelle. We show that, although in P123-SDS aggregate the observed REES is very small, there is a marked λex dependence of solvation dynamics and anisotropy decay. 3.2.1. SolVation Dynamics of C480 in P123 and in P123SDS Self-Assembly. At all λex’s, the emission decays of C480 in a P123 display marked wavelength dependence with a rise at the red end and decay at the blue end. This is a clear signature of solvation dynamics. Figures 4 and 5 show the picosecond decay and femtosecond transients of C480 in P123 micelle in the presence and absence of SDS at λex ) 375 nm. Addition of SDS modifies the ultrafast rise and decay components. For instance, at λex ) 375 nm, in a P123 micelle, the femtosecond fluorescence transient of C480 at λem ) 540 nm (red end) displays an ultrafast rise component of 8 ps followed by two long componentssa rise of time constant 1050 ps and a 6600 ps decay component. In a P123-SDS aggregate, the femtosecond transient at the red end (λem ) 560 nm) displays three rise components of 1, 16, and 1000 ps followed by a very long decay component of 6500 ps. In P123 micelle (λex ) 375 nm), at the blue end (λem ) 420 nm), there are two ultrafast decay components, 4.5 and 55 ps, along with a long decay component of 2500 ps. In P123-SDS aggregate, at λem ) 430
Figure 5. Femtosecond transients of C480 (λex ) 375 nm) in 10 wt % P123 in the absence of SDS (A-C) and in the presence of 87 mM SDS (D-F).
Figure 6. Time resolved emission spectra (TRES) of C480 (λex ) 375 nm) in 10 wt % P123 in the presence of 87 mM SDS at 0 ps (9), 300 ps (O), 2000 ps (2), and 12 000 ps (3).
nm, there are four decay components with time constants of 3.6, 9.2, 125, and 2000 ps (Figures 4 and 5). For λex ) 435 nm, the fluorescence decay at 450 nm (blue end) in P123 micelle exhibits one ultrafast (6 ps) and two long decay components (600 and 5100 ps). In P123-SDS aggregate, the corresponding decay components are 10, 450, and 5300 ps, respectively. For P123 micelle (λex ) 435 nm), at 580 nm (red end), one observes an ultrafast rise time of 5.7 ps and two long componentssa rise of 1000 ps and a decay of 5900 ps. In P123-SDS aggregate, two rise components (6.1 and 500 ps) precede a very long (6200 ps) decay at the same (580 nm) emission wavelength. The time resolved emission spectra (TRES) were constructed from temporal decays and steady state emission spectra.19a Figure 6 shows the TRES of C480 in a P123-SDS micellar self-assembly (λex ) 375 nm). The decays of solvent response function, C(t), for different excitation wavelengths are shown in Figures 7 and 8. Note, for λex ) 405 and 435 nm, the
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Figure 9. fwhm (Γ) of the emission spectrum against time for C480 in 10 wt % P123 in the presence of 87 mM SDS at λex ) 375 nm (0) and λex ) 435 nm (O). The points denote the actual values of Γ, and the solid line denotes the best fit to a single-exponential decay.
Figure 7. Complete decay of the solvent response function, C(t), of C480 in 10 wt % P123 for (A) λex ) 375 nm, (B) λex ) 405 nm, and (C) λex ) 435 nm, in the absence (O) and presence (4) of 87 mM SDS. The points denote the actual values of C(t), and the solid line denotes the best fit. Initial portions of the decays are shown in the insets.
Figure 8. Complete decay of the solvent response function, C(t), of C480 in 10 wt % P123 in the absence (A) and presence (B) of 87 mM SDS. λex ) 375 nm (O), 405 nm (4), and 435 nm (0). The points denote the actual values of C(t), and the solid line denotes the best fit.
solvation dynamics, i.e., decay of C(t), is very fast and a major portion of the solvation dynamics was missed even in our femtosecond setup (time resolution 350 fs). The amount of solvation missed was calculated by using the Fee-Maroncelli procedure.19b The solvation dynamics in a P123 micelle displays a very fast (∼1 ps) bulk-water-like component, a 200 ps component, and a very long component (3000-5000 ps). The ultrafast (∼1 ps) component9 is ascribed to dynamics in the highly hydrophilic PEO corona region. The very long component (5000 ps at λex ) 375 and 405 nm and 3000 ps at λex ) 435 nm) is assigned to the hydrophobic PPO core. In a computer simulation, Olander and Nitzan21a detected a 100 ps component due to the segmental motion of polymer chains. A similar component for chain
Figure 10. Fluorescence anisotropy decay of C480 in 10 wt % P123 along with a fitted curve at (A) λex ) 375 nm (λem ) 445 nm) and (B) λex ) 435 nm (λem ) 460 nm) (i) in the absence and (ii) in the presence of 87 mM SDS. Fitted curves of the decay are shown in the insets in the presence (‚‚‚‚‚) and absence (s) of SDS.
dynamics was reported by other workers.21b-e In our previous work on 1% P123, we detected a 60 ps component due to the chain dynamics.5a In the present work, we have used a 10 times higher concentration (10%) of P123 and the consequent chainchain interactions may slow down the chain dynamics. We, therefore, ascribe the 200 ps component in 10% P123 micelle to the dynamics of polymer segments. In a P123 micelle, with an increase in λex, the solvation dynamics becomes faster with a reduction in the magnitude and relative contribution of the slowest component (due to core) from 82% at λex ) 375 nm to 15% at λex ) 435 nm. Simultaneously, the contribution of the polar corona region (
