A femtosecond study of solvation dynamics and anisotropy decay in a catanionic vesicle: excitation-wavelength dependence

The structure and dynamics of a catanionic vesicle are studied by means of femtosecond up-conversion and dynamic light scattering (DLS). The catanionic vesicle is composed of dodecyl-trimethyl-ammonium bromide (DTAB) and sodium dodecyl sulphate (SDS). The DLS data suggest that 90 % of the vesicles have a diameter of about 400 nm, whereas the diameter of the other 10 % is about 50 nm. The dynamics in the catanionic vesicle are compared with those in pure SDS and DTAB micelles. We also study the dynamics in different regions of the micelle/vesicle by varying the excitation wavelength (λex) from 375 to 435 nm. The catanionic vesicle is found to be more heterogeneous than the SDS or DTAB micelles, and hence, the λex-dependent variation of the solvation dynamics is more prominent in the first case. The solvation dynamics in the vesicle and the micelles display an ultraslow component (2 and 300 ps, respectively), which arises from the quasibound, confined water inside the micelle, and an ultrafast component (<0.3 ps), which is due to quasifree water at the surface/exposed region. With an increase in λex, the solvation dynamics become faster. This is manifested in a decrease in the total dynamic solvent shift and an increase in the contribution of the ultrafast component (<0.3 ps). At a long λex (435 nm), the surface (exposed region) of a micelle/vesicle is probed, where the solvation dynamics of the water molecules are faster than those in a buried location of the vesicle and the micelles. The time constant of anisotropy decay becomes longer with increasing λex, in both the catanionic vesicle and the ordinary micelles (SDS and DTAB). The slow rotational dynamics (anisotropy decay) in the polar region (at long λex) may be due to the presence of ionic head groups and counter ions

Ultrafast FRET in a room temperature ionic liquid microemulsion: a femtosecond excitation wavelength dependence study

Fluorescence resonance energy transfer (FRET) from coumarin 480 (C480) to rhodamine 6G (R6G) is studied in a room temperature ionic liquid (RTIL) microemulsion by picosecond and femtosecond emission spectroscopy. The microemulsion is comprised of the RTIL 1-pentyl-3-methylimidazolium tetraflouroborate, [pmim][BF<SUB>4</SUB>], in TX-100/ benzene. We have studied the microemulsion with and without water. The time constants of FRET were obtained from the risetime of the acceptor (R6G) emission. In the RTIL microemulsion, FRET occurs on multiple time scales: 1, 250, and 3900 ps. In water containing RTIL microemulsion, the rise components are 1.5, 250, and 3900 ps. The 1 and 1.5 ps components are assigned to FRET at a close contact of donor and acceptor (R<SUB>DA</SUB>&#8776;12 &#197;). This occurs within the highly polar (RTIL/water) pool of the microemulsion. With increase in the excitation wavelength (&#955; <SUB>ex</SUB>) from 375 to 435 nm, the relative contribution of the ultrafast component of FRET (1 ps) increases from 4% to 100% in the RTIL microemulsion and 12% to 100% in the water containing RTIL microemulsion. It is suggested that at &#955; <SUB>ex</SUB>= 435 nm, mainly the highly polar RTIL pool is probed where FRET is very fast due to the close proximity of the donor and the acceptor. The very long 3900 ps (R<SUB>DA</SUB>&#8776;45 &#197;) component may arise from FRET from a donor in the outer periphery of the microemulsion to an acceptor in the polar RTIL pool. The 250 ps component (R<SUB>DA</SUB>&#8776;29&#197;) is assigned to FRET from a donor inside the surfactant chains

Femtosecond solvation dynamics in a micron-sized aggregate of an ionic liquid and P123 triblock copolymer

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 &#956;m) 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 (&#955; <SUB>ex</SUB>) using femtosecond up-conversion. For &#955; <SUB>ex</SUB> = 375 nm, the nonpolar core of the P123-RTIL aggregate is preferentially excited while &#955; <SUB>ex</SUB> = 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 &#955; <SUB>ex</SUB> = 405 nm in 5 wt% P123 and 0.9 M RTIL average rotational time, (&#964;<SUB>rot</SUB>= 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 (&#955; <SUB>ex</SUB> = 435 nm, &#964;<SUB>s</SUB>= 75 ps) is 25 times faster than that at the core region (at &#955; <SUB>ex</SUB> = 375 nm, &#964;<SUB>s</SUB>= 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 &#181;m sized aggregate (3 M RTIL and P123), the solvation dynamics in the core (&#964;<SUB>s</SUB>= 500 ps) is 4 times faster than that in 0.9 M RTIL

Femtosecond study of ultrafast fluorescence resonance energy transfer in a catanionic vesicle

Ultrafast fluorescence resonance energy transfer (FRET) in a catanionic [sodium dodecyl sulfate (SDS)-dodecyltrimethyl ammonium bromide (DTAB)] vesicle is studied by femtosecond up-conversion. The vesicles (diameter ~400 nm for SDS-rich and ~250 nm for DTAB-rich vesicles) are much larger than the SDS and DTAB micelles (diameter ~4 nm). In both micelle and vesicles, FRET occurs in multiple time scales and the time scales of FRET correspond to a donor-acceptor distance varying between 12 and 36 Å

Excited state proton transfer in ionic liquid mixed micelles

Excited state proton transfer (ESPT) of pyranine (8-hydroxypyranine-1,3,6-trisulfonate, HPTS) in room temperature ionic liquid (RTIL) mixed micelles is studied by femtosecond up-conversion. The mixed micelle consists of a triblock copolymer, (PEO)20-(PPO)70-(PEO)20 (Pluronic P123), and one of the two RTILs, 1-pentyl-3-methyl-imidazolium bromide ([pmim][Br]) and 1-pentyl-3-methyl-imidazolium tetra-fluoroborate ([pmim][BF4]). The size and structure of the mixed micelle vary with the relative amount of the RTIL. For [pmim][Br], the hydrodynamic diameter of the mixed micelle is 26 nm in 0.3 M RTIL and 3500 nm in 3.0 M RTIL. The time constant of initial proton transfer (τPT) in P123 micelle (65 ps) is 10 times slower than that (5 ps) in water, while the time constants of recombination (trec) and dissociation (τdiss) are 2-3 times slower in P123 micelle. On addition of the RTIL, the rate of ESPT is markedly modified. In 0.3 M RTIL-P123 mixed micelle, τPT is shorter than that in P123 micelle. In the mixed micelle, τPT increases with an increase in the concentration of the RTIL (230 ps in 3 M [pmim][Br] and 55 ps in 0.9 M [pmim][BF4]). This is attributed to large scale penetration of the P123 micelle by RTIL replacing water molecules. The time constants of proton transfer (τPT, τrec, and τdiss) are faster than the slowest component (200-500 ps) of solvation dynamics. It seems that the ultrafast component of solvation (&lt;0.3 ps and &lt;5 ps) is enough for inducing proton transfer. The time constant of the proton transfer (τPT) in [pmim][BF4]-P123 mixed micelle is longer (~20%) than that in [pmim][Br]-P123 mixed micelle for the same concentration of RTIL. The counterion dependence of ESPT is attributed to the difference in the structure and greater hydrophobicity of the [pmim][BF4]

Deuterium isotope effect on femtosecond solvation dynamics in an ionic liquid microemulsion: an excitation wavelength dependence study

The deuterium isotope effect on the solvation dynamics and the anisotropy decay of coumarin 480 (C480) in a room temperature ionic liquid (RTIL) microemulsion is studied by femtosecond up-conversion. The microemulsion consists of the RTIL 1-pentyl-3-methyl-imidazolium tetra-fluoroborate ([pmim][BF4]) in triton X-100 (TX-100)/benzene. Replacement of H2O by D2O in the microemulsion causes retardation of solvation dynamics. The average solvation time of C480 (&#964;s) in RTIL microemulsion with 5 wt % D2O is 1.5-1.7 times slower compared to that in the H2O containing RTIL microemulsion. This suggests that the main species in the microemulsion responsible for solvation is the water molecules. In both D2O and H2O containing RTIL microemulsion, the solvation dynamics exhibits marked dependence on the excitation wavelength (&#955; ex) and becomes about 15 times faster as &#955; ex increases from 375 to 435 nm. This is ascribed to the structural heterogeneity in the RTIL microemulsion. For &#955; ex = 375 nm, the region near the TX-100 surfactant is probed where bound water molecules cause slow solvation dynamics. At 435 nm, the RTIL pool is selected where the water molecules are more mobile and hence gives rise to faster solvation. The average time constant of anisotropy decay shows opposite dependence on &#955; ex and increases about 2.5-fold from 180 ps at &#955; ex = 375 nm to 500 ps at &#955; ex = 435 nm for D2O containing RTIL microemulsion. The slower anisotropy decay at &#955; ex = 435 nm is ascribed to the higher viscosity of RTIL which causes greater friction at the core