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Dynamic light scattering studies indicate that addition of a room temperature ionic liquid (RTIL, [pmim][Br]), to a triblock copolymer (P123) micelle leads to the formation of giant P123-RTIL clusters of size (diameter) 40 nm in 0.9 M and 3500 nm (3.5 μm) in 3 M RTIL. They are much larger than a P123 micelle (∼18 nm) or [pmim][Br] (1.3 nm). Dynamics in different regions of the P123-RTIL aggregate is probed by variation of the excitation wavelength (λex) using femtosecond up-conversion. For λex= 375 nm, the nonpolar core of the P123-RTIL aggregate is preferentially excited while λex= 435 nm selects the polar corona region. Solvation dynamics and anisotropy decay of coumarin 480 (C480) in a P123-RTIL giant aggregate are markedly different from those in either P123 micelle or those in an aqueous solution of the RTIL. For λex= 405 nm in 5 wt % P123 and 0.9 M RTIL average rotational time, ( = 1350 ps) of C480 is ∼7 times longer than that (200 ps) in an aqueous solution of the RTIL in the absence of P123 and is shorter than that (3000 ps) in a P123 micelle. In 0.9 M RTIL and 5 wt % P123, solvation dynamics in the corona region (λex= 435 nm, = 75 ps) is ∼25 times faster than that at the core region (at λex= 375 nm, = 1900 ps). The solvation dynamics in the core of the P123-RTIL aggregate is faster than that in P123 micelle (3550 ps in the core) and is much slower than that (130 ps) in an aqueous solution containing 0.9 M RTIL. In the 3.5 μm sized aggregate (3 M RTIL and P123), the solvation dynamics in the core ( = 500 ps) is ∼4 times faster than that in 0.9 M RTIL. [ABSTRACT FROM AUTHOR]

Solvation dynamics and anisotropy decay of coumarin 480 (C480) in a supramolecular assembly containing a triblock copolymer, PEO20−PPO70−PEO20(Pluronic P123) and a surfactant, CTAC (cetyl trimethylammonium chloride) are studied by femtosecond up-conversion. In a P123−CTAC complex, C480 displays a significant (22 nm) red edge excitation shift (REES) in the emission maximum as exincreases from 335 to 445 nm. This suggests that the P123−CTAC aggregate is quite heterogeneous. The average rotational relaxation time (〈rot〉) of C480 in a P123−CTAC complex decreases by a factor of 2 from 2500 ps at ex375 nm to 1200 ps at ex435 nm. For ex375 nm, the probe molecules in the buried core region of P123−CTAC are excited and the solvation dynamics displays three components, 2, 60, and 4000 ps. It is argued that insertion of CTAC in P123 micelle affects the polymer chain dynamics, and this leads to reduction of the 130 ps component of P123 micelle to 60 ps in P123−CTAC. For ex435 nm, which selects the peripheral highly polar corona region, solvation dynamics in P123−CTAC and P123 are extremely fast with a major component of <0.3 ps (∼80%) and a 2 ps (∼20%) component. [ABSTRACT FROM AUTHOR]

Femtosecond solvation dynamics of coumarin 480 (C480) in a mixed micelle is reported. The mixed micelle consists of a triblock copolymer (PEO) 20−(PPO) 70−(PEO) 20(Pluronic P123) and an ionic liquid (IL), 1-pentyl-3-methylimidazolium tetrafluoroborate ([pmim][BF 4]). At a low concentration (0.3 M), the sparingly water soluble IL ([pmim][BF 4]) penetrates the hydrophobic PPO core of the P123 micelles. Thus emission maximum of C480 in the core (accessed at λ ex= 375 nm) in 0.3 M IL is red-shifted by 8 nm from that in its absence and the red edge excitation shift (REES) is large (19 ± 1 nm). At a high concentration (0.9 M), the ionic liquid [pmim][BF 4] invades both the core and corona region and the mixed micelle exhibits very small REES (3 ± 1 nm). Anisotropy decay and solvation dynamics in different regions of the mixed micelle are studied by variation of excitation wavelength (λ ex). In P123 micelle, the average rotational time (⟨τ rot⟩) is 2800 ps in the core (at λ ex= 375 nm) and 1350 ps in the corona region (at λ ex= 435 nm). In 0.3 M [pmim][BF 4], ⟨τ rot⟩ at the core of the mixed micelle decreases to 1950 ps while that in the corona remains unaffected. In 0.9 M IL, both the core and corona (λ ex= 375 and 435 nm) exhibit similar and short ⟨τ rot⟩ ∼ 600 ps. In 0.3 M IL, solvation dynamics in the core region (λ ex= 375 nm) of P123 micelle is about 2 times faster than in its absence. In 0.3 M IL, solvation dynamics in the corona region (λ ex= 435 nm) is ∼100 times faster than that in the core. In 0.9 M IL, the solvation dynamics in the core and in the corona is, respectively, ∼9 times and 4 times faster than that in 0.3 M IL. [ABSTRACT FROM AUTHOR]

Solvation dynamics of coumarin 480 (C480) in the secondary aggregate of a bile salt (sodium deoxycholate, NaDC) is studied using femtosecond up-conversion. The secondary aggregate resembles a long (∼40 Å) hollow cylinder with a central water-filled tunnel. Different regions of the aggregate are probed by variation of the excitation wavelength (ex) from 375 to 435 nm. The emission maximum of C480 displays an 8 nm red shift as the exincreases from 345 to 435 nm. The 8 nm red edge excitation shift (REES) suggests that the probe (C480) is distributed over regions of varied polarity. Excitation at a short wavelength (375 nm) preferentially selects the probe molecule in the buried locations and exhibits slow dynamics with a major (84%) slow component (3500 ps) and a small (16%) contribution of the ultrafast component (2.5 ps). Excitation at ex 435 nm (red end) corresponds to the exposed sites where solvation dynamics is very fast with a major (73%) ultrafast component (≤2.5 ps) and relatively minor (27%) slow (2000 ps) component. In sharp contrast to solvation dynamics, the anisotropy decay becomes slower as exincreases from 375 to 435 nm. It is proposed that the buried locations (ex 375 nm) offer lower friction because of the rigid sheetlike structure of the bile salt. [ABSTRACT FROM AUTHOR]

Solvation dynamics in a neat ionic liquid, 1-pentyl-3-methyl-imidazolium tetra-flouroborate (pmimBF4) and its microemulsion in Triton X-100 (TX-100)/benzene is studied using femtosecond up-conversion. In both the neat ionic liquid and the microemulsion, the solvation dynamics is found to depend on excitation wavelength (ex). The exdependence is attributed to structural heterogeneity in neat ionic liquid (IL) and in IL microemulsion. In neat IL, the heterogeneity arises from clustering of the pentyl groups which are surrounded by a network of cation and anions. Such a nanostructural organization is predicted in many recent simulations and observed recently in an X-ray diffraction study. In an IL microemulsion, the surfactant (TX-100) molecules aggregate in form of a nonpolar peripheral shell around the polar pool of IL. The micro-environment in such an assembly varies drastically over a short distance. The dynamic solvent shift (and average solvation time) in neat IL as well as in IL microemulsions decreases markedly as exincreases from 375 to 435 nm. In a pmimBF4/water/TX-100/benzene quaternary microemulsion, the solvation dynamics is slower than that in a microemulsion without water. This is ascribed to the smaller size of the water containing microemulsion. The anisotropy decay in an IL microemulsion is found to be faster than that in neat IL. [ABSTRACT FROM AUTHOR]

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 (ex) 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 (<0.3 and 1 ps) in the PEO corona region, a 200 ps component arising from dynamics of polymer segments, and a very long component (5000 or 3000 ps) due to the highly restricted PPO core. In a P123−SDS aggregate, at ex375 and 405 nm, the solvation dynamics is found to be faster than that in P123 micelle. In this case, the component (3000 ps) arising from the core region is faster than that (5000 ps) in P123 micelle. In both P123 micelle and P123−SDS aggregate, the relative contribution of the core region decreases and that of the corona region increases with an increase in ex. At ex435 nm, which probes the hydrophilic corona, the solvation dynamics for both P123 micelle and P123−SDS aggregate are almost similar. [ABSTRACT FROM AUTHOR]

Fluorescence resonance energy transfer (FRET) from coumarin 480 (C480) to rhodamine 6G (R6G) is studied in the micelle and the gel phase of a triblock copolymer, (PEO)20−(PPO)70−(PEO)20(Pluronic P123 (P123)) by picosecond and femtosecond emission spectroscopy. The time constants of FRET were obtained from the rise time of the acceptor (R6G) emission. In a P123 micelle, FRET occurs in multiple time scales:  2.5, 100, and 1700 ps. In the gel phase, three rise components are observed:  3, 150, and 2600 ps. According to a simple Förster model, the ultrafast (2.5 and 3 ps) components of FRET correspond to donor−acceptor distance RDA13 ± 2 Å. The ultrafast FRET occurs between a donor and an acceptor residing at close contact at the corona (PEO) region of a P123 micelle. With increase in the excitation wavelength (ex) from 375 to 435 nm, the relative contribution of the ultrafast component of FRET (∼3 ps) increases from 13% to 100% in P123 micelle and from 1% to 100% in P123 gel. It is suggested that at ex435 nm, mainly the highly polar peripheral region is probed where FRET is very fast due to close proximity of the donor and the acceptor. The 100 and 150 ps components correspond to RDA25 ± 2 Å and are ascribed to FRET from C480 deep inside the micelle to an acceptor (R6G) in the peripheral region. The very long component of FRET (1700 ps in micelle and 2600 ps component in gel) may arise from diffusion of the donor from outside the micelle to the interior followed by fast FRET. [ABSTRACT FROM AUTHOR]

Discussion on the structure of the water surface relies on accurate determination of the χ (2) spectrum. For obtaining accurate χ(2) spectrum of the air/water interface in the OH stretch region, we performed heterodyne-detected vibrational sum-frequency generation measurements with a high phase accuracy, and also examined the validity of the phase and amplitude calibration using different non-resonant materials. In contrast to the previous reports, it was concluded that the imaginary part of the χ(2) spectrum of the air/water interface does not exhibit noticeable positive resonance in the low frequency region within the experimental error. This result urges us to reconsider the structure of the air/water interface based on the accurate χ(2) spectrum. [ABSTRACT FROM AUTHOR]