J. Phys. Chem. C 2007, 111, 8775-8780
8775
Excitation Wavelength Dependence of Solvation Dynamics in a Gel. (PEO)20-(PPO)70-(PEO)20 Triblock Copolymer† Subhadip Ghosh, Aniruddha Adhikari, Ujjwal Mandal, Shantanu Dey, and Kankan Bhattacharyya* Physical Chemistry Department, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700 032, India ReceiVed: October 27, 2006; In Final Form: December 9, 2006
In a 30 wt % aqueous solution, the triblock copolymer, (polyethylene oxide, PEO)20-(polypropylene oxide, PPO)70-(PEO)20 (Pluronic P123) forms a cubic gel phase. In the gel, the micelles associate to form an interconnected network with intermicellar distance (140 Å) less than the sum (160 Å) of the radii of the micelles. Fluorescence anisotropy decay and solvation dynamics of coumarin 480 (C480) in P123 gel have been investigated as a function of excitation wavelength (λex) using femtosecond and picosecond fluorescence spectroscopy. The anisotropy decay of C480 in the macroscopically rigid (“solid”) gel phase is quite fast and only ∼1.6 times slower than that in the sol (micellar) phase. Solvation dynamics in the P123 gel displays three components. First, even in the “solid” gel, there is a bulk water-like ultrafast component (2 ps and 100 ps) of solvation dynamics in many confined environments.13 2. Experimental The triblock copolymer, Pluronic P123 (P123) was a gift from BASF Corp. and used without further purification. Laser grade coumarin 480 (C480, Exciton, Scheme 1c) was 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 copolymer (P123) with 100 mL water. The solution was stirred for 4-5 h using a magnetic stirrer at room temperature in a sealed container. After that the solution was kept in a refrigerator for a week. In our femtosecond upconversion setup (FOG 100, CDP) the sample was excited at 375, 405, and 435 nm, respectively. Briefly, the sample was excited using the second harmonic of a mode-locked Ti-sapphire 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 a gate pulse of the fundamental beam. The upconverted light is dispersed in a monochromator and detected using photon counting electronics. A cross-correlation function obtained using the Raman scattering from ethanol displayed a full width at halfmaximum (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 set up in which, the samples were excited at 375, 405, and 435 nm using
a picosecond diode laser (IBH nanoleds) in an IBH Fluorocube apparatus. The emission was collected at a magic angle polarization using a Hamamatsu MCP photomultiplier (5000U09). 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. All experiments were done at room temperature (20 °C). To study fluorescence anisotropy decay, the analyzer was rotated at regular intervals to get perpendicular (I⊥) and parallel (I||) components. Then the anisotropy function, r(t) was calculated using the formula
r(t) )
I|(t) - GI⊥(t) I|(t) + 2GI⊥(t)
(1)
The G value of the picosecond set up was determined using a probe whose rotational relaxation is very fast (e.g., coumarin 480 in methanol) and the G value was found to be 1.5. 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 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.14a 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(∞)
(2)
where ν(0), ν(t), and ν(∞) are the peak frequencies at time 0, t, and ∞, respectively. Note, a portion of solvation dynamics is
(PEO)20-(PPO)70-(PEO)20 Triblock Copolymer
J. Phys. Chem. C, Vol. 111, No. 25, 2007 8777
Figure 1. Absorption spectra of C480 in P123 gel (30 wt %). The excitation wavelengths are marked (O) at 375, 405, and 435 nm.
Figure 3. Plot of emission maximum of C480 in gel phase (30 wt %) as a function of excitation wavelength.
TABLE 1: λex Dependence of Steady-State Emission Maximum of C480 in Gel Phase and Water
Figure 2. Emission spectra of C480 in P123 gel phase (30 wt %) when excited at (a) 375 nm, (b) 405 nm, (c) 435 nm, and (d) C480 in water (‚‚‚‚‚) (λex ) 375-435 nm).
missed (at λex ) 405 and 435 nm) even in our femtosecond set up of time resolution 350 fs. The amount of solvation missed is calculated using the Fee-Maroncelli procedure.14b In this method, the emission frequency at time zero, νpem(0), may be calculated using the absorption frequency (νpabs) in a polar medium (i.e., C480 in P123 gel) as np νpem(0) ) νpabs - (νnp abs - νem)
(3)
np where νnp em and νabs denote the steady-state frequencies of emission and absorption, respectively, of the probe (C480) in a nonpolar solvent (i.e., cyclohexane).
3. Results and Discussions 3.1. Steady-State Characteristics. Figure 1 shows the absorption spectrum of C480 in P123 gel (30 wt % at 20 °C). The absorption maximum of C480 in the P123 gel is at 384 nm, which is blue-shifted by 12 nm from that in water (396 nm)15 and is very close to that in P123 in the micellar phase (1 wt %).2d Figure 2 shows the emission spectrum of C480 in a P123 gel for various excitation wavelengths (λex). Figure 3 shows variation of the emission maximum of C480 in P123 gel as a function of λex. Note, the sigmoidal shape of Figure 3 suggests that basically there are two environments, a hydrophilic peripheral shell where λmax em ∼ 470 nm and the hydrophobic ∼ 452 nm. At a very short λex (335-375 nm), core with λmax em
medium
λex (nm)
λmax em (nm)
fwhm (cm-1)
P123 gel P123 gel P123 gel Water
375 405 435 375-425
452 462 470 489
3400 3250 2730 2630
the molecules in the core region are preferentially excited giving rise to λmax em ∼ 452 nm. At the red end (λex ) 425-455 nm), C480 molecules in the peripheral hydrophilic region are selectively excited and, hence, giving rise to λmax em ∼ 470 nm. At an intermediate λex, both these two regions are excited and emission maxima are in between 452 and 472 nm. For all λex, the emission maximum of C480 in P123 gel is found to be blueshifted from the emission maximum (489 nm)15 in bulk water and depends markedly on the excitation wavelength (Table 1). From Figure 2 and 3, it is readily seen that the emission maximum of C480 in P123 gel displays a very significant REES by 22 nm from 450 nm (at λex ) 345 nm) to 472 nm (at λex ) 445 nm). The observed REES suggests a distribution of the fluorophore in different regions of varying polarity in the gel phase. 3.2. Time-Resolved Studies. 3.2.1. Fluorescence Anisotropy Decay of C480 in P123 Gel. In bulk water, the time constant of fluorescence anisotropy decay of C480 is ∼70 ps.9a,16c The fluorescence anisotropy decay of C480 in Pluronic P123 gel is found to be much slower and is fitted to a biexponential function
r(t) ) r0 [β exp(-t/τslow) + (1 - β) exp(-t/τfast)]
(4)
For λex ) 375 and 405 nm, the anisotropy decay of C480 in P123 gel (λem ) 445 nm) is very similar and is described by two decay components of 800 ( 50 ps (30%) and 4100 ( 200 ps (70%) (Figure 4 and Table 2). At λex ) 435 nm, the anisotropy decay is faster with components 650 ( 50 ps (50%) and 2350 ( 100 ps (50%) (Figure 4 and Table 2). Using wobbling in cone model as applied to P123 micelle,2d the diffusion coefficients for wobbling (Dw) is calculated to be 0.635 × 108 and 1.22 × 108 s-1, respectively, for λex ) 375 and 435 nm and the coefficients for translational diffusions (Dt) are 1.26 × 10-9 and 4.54 × 10-9 m2 s-1. In the micellar phase of P123 (1% P123), fluorescence anisotropy decay of C480 exhibits two rotational components ∼500 and 2600 ps.2d It is interesting to note that although the gel is macroscopically rigid (“solid”), the components of anisotropy decay in P123 gel are only 1.6 times longer than those in the liquid micellar phase of P123. This indicates that the microscopic friction in the gel phase is quite small.
8778 J. Phys. Chem. C, Vol. 111, No. 25, 2007
Ghosh et al.
Figure 4. Fluorescence anisotropy decay of C480 along with the fitted curve in P123 gel (30 wt %), at λex (i) 375 nm (λem ) 445 nm) and (ii) 435 nm (λem ) 460 nm).
Figure 5. Fluorescence decays of C480 (λex ) 375 nm) in P123 gel, λem at (i) 550 nm, (ii) 450 nm, and (iii) 410 nm recorded in a picosecond set up.
TABLE 2: Parameters of Anisotropy Decay of C480 in P123 Gel at Different Excitation Wavelengths (λex) at 20 °C
a
λex(nm)
r0
375
0.32
435
0.31
τfast (ps) (afast)
τslow (ps) (aslow)a
800 (30%) 600 (30%)
4100 (70%) 2350 (70%)
(5%.
Previously, several groups reported that the microscopic friction in the gel phase is small and marginally higher than that in the micellar phase for the triblock copolymers.1d,2e Jeon et al.2c studied fluorescence depolarization in Pluronic F127 gel using two photon excitation of rhodamine 123 by temperature variation around the gel transition temperature 23.5 °C.2c With increase in temperature, the faster component decreases monotonically from 570 ps at 15 °C to 390 ps at 35 °C.2c The longer component increases from 2550 ps at 15 °C to 4390 ps around gel transition temperature and then decreases to 2590 ps at 35 °C. The relative contributions of the two components do not change appreciably around the gel transition temperature.2c For Pluronic F88, Castner and co-workers reported that the anisotropy decay of C480 is described by a biexponential decay.2a The time constants of the components decreases from 300 and 1760 ps in the micelle (at 30 °C) and then rapidly decreases in the gel phase (>40 °C). Mali et al. studied photoisomerization of a polyene in P123 gel and reported that the isomerization is at most 1.4 times slower in the gel phase compared to the micellar phase.2e The very low microscopic friction (microviscosity) against rotational and isomerization dynamics in the gel phase may be ascribed to the presence of large pores occupied by water molecules. Note, for a cubic close packing ∼52% of the total volume is occupied by the polymer and ∼48% is “void” or pores (Scheme 1b). C480 molecules in the peripheral region that are exposed to the void may experience very lower friction. 3.2.2. SolVation Dynamics of C480 in P123 Gel. Emission decays of C480 in the P123 gel display marked wavelength dependence. At the red end a rise precedes the decay while at the blue end a decay (with no rise) is observed. Such a wavelength dependence of fluorescence decays is a clear indication of solvation dynamics. Figure 5 and 6 shows a few representative decay profiles of C480 in P123 gel, recorded in a picosecond and a femtosecond set up, respectively. The timeresolved emission spectra (TRES) for λex ) 375 nm is shown in Figure 7. As noted, the solvation dynamics is described by the decay of C(t). The decay of C(t) for C480 in P123 gel at different excitation wavelengths are shown in Figure 8 and the decay parameters are listed in Table 3. It is readily seen that at all λex
Figure 6. Femtosecond transients of C480 (λex ) 375 nm) in P123 gel, λem at (i) 550 nm, (ii) 450 nm, and (iii) 410 nm.
Figure 7. TRES of C480 (λex ) 375 nm) in P123 gel at 0 ps (9), 1500 ps (O), 4000 ps (2), and 10000 ps (3).
(375-435 nm) the decays of C(t) are described by three components: 2, 500, and 4500 ps. The relative contributions of these components vary with λex. For λex ) 375 nm, solvation becomes slow and entire solvation dynamics (100%) are detected in our setup. At longer λex, a portion of the dynamics is faster than the time resolution of our setup (0.3 ps). At the longest wavelength of excitation, λex ) 435 nm, 85% of the ultrafast component went undetected in our femtosecond setup. At λex ) 405 nm, 20% of the solvation is ultrafast and was missed in our femtosecond setup. We will now try to understand the origin of the three components. Even in the “solid” gel there is a ultrafast (2 ps
(PEO)20-(PPO)70-(PEO)20 Triblock Copolymer
J. Phys. Chem. C, Vol. 111, No. 25, 2007 8779
Figure 8. Complete decay of solvent response function C(t) of C480 in P123 gel for λex ) 375 nm (O), λex ) 405 nm (0), and λex ) 435 nm (4). 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 inset.
TABLE 3: Decay Parameters of C(t) of C480 in P123 Gel at Different Excitation Wavelengths (λex) at 20 °C total solvent shift detected λex(nm) ∆ν (cm-1) 375 405 435 a
1345 1235 270
missed component τ1 (a1)a
τ2 (a2) (ps)b
τ3 (a3) (ps)b
τ4 (a4) (ps)b
2 (6%) 500 (4%) 4500 (90%)
