Solvation Dynamics of Coumarin 480 in Reverse Micelles. Slow Relaxation of Water Molecules

Abstract

The picosecond time-resolved Stokes shift of the laser dye coumarin 480 (I) is studied in Aerosol OT-heptane reverse micelles at various concentrations of water. On addition of water to a n-heptane solution of I containing 0.09 M AOT, the emission spectrum of I exhibits a prominent shoulder at 480 nm which is ascribed to the dye molecules in the water pool of the reverse micelles. In n-heptane, the fluorescence decays of I at short and long wavelengths are identical, which rules out any time-resolved Stokes shift. On addition of water to the AOT/heptane system, the decay at long wavelengths exhibit a distinct growth on the nanosecond time scale. This and the time-dependent Stokes shift indicate that the water molecules in the water pool of the AOT reverse micelles undergo slow relaxation on the nanosecond time scale.

Simulations of time-dependent fluorescence in nano-confined solvents

“…10 One consistent conclusion of their studies is that there is not a one-to-one correspondence between steady-state spectra and the time-dependent Stokes shift function, S(t): ''...steady-state spectroscopy may not always be a good predictor for dynamical behavior.'' 14 Bhattacharyya and co-workers have measured the timedependent fluorescence of the dye molecule, Coumarin 480, in solventless zeolites, 4 aqueous micelles 5 and reverse micelles, 6 in water pools in a sol-gel matrix, 7 and vesicles. 8 They found that the long-time solvation dynamics depend strongly on the environment with a long-time decay of ϳ0.8 ns in the sol-gel matrix, 7 0.6 -2.4 ns in micelles ͑using different surfactants͒, 5 8 -12 ns in reverse micelles, 6 11 ns in vesicles, 8 and ϳ15.4 ns in the solventless zeolites.…”

“…14 Bhattacharyya and co-workers have measured the timedependent fluorescence of the dye molecule, Coumarin 480, in solventless zeolites, 4 aqueous micelles 5 and reverse micelles, 6 in water pools in a sol-gel matrix, 7 and vesicles. 8 They found that the long-time solvation dynamics depend strongly on the environment with a long-time decay of ϳ0.8 ns in the sol-gel matrix, 7 0.6 -2.4 ns in micelles ͑using different surfactants͒, 5 8 -12 ns in reverse micelles, 6 11 ns in vesicles, 8 and ϳ15.4 ns in the solventless zeolites. 4 ͑The solvent relaxation dynamics for Coumarin 480 in bulk aqueous solution takes place in 310 fs.…”

“…20 ͒ In the case of reverse micelles they investigated two water pool sizes and found S(t) exhibits a single 8 ns decay for the smaller water pool and a biexponential decay of 1.7 and 12 ns for the larger pool. 6 ͑Their time resolution is ϳ80 ps.͒ Sarkar and co-workers have investigated the solvation dynamics of Coumarin 490 in aqueous reverse micelles and Coumarin 152A in acetonitrile and methanol reverse micelles. 22 In all cases they find biexponential decay of S(t) with the fast decay ϳ0.5-1.7 ns and the long-time decay 7.6 -15.5 ns.…”

Validation and Divergence of the Activation Energy Barrier Crossing Transition at the AOT/Lecithin Reverse Micellar Interface

“…[15][16][17][18][19] Mid-infrared ultrafast pumpprobe spectroscopy of RMs of different well-defined sizes revealed slow vibrational relaxation dynamics of water at the interface. 16 Using the picosecond-resolved fluorescence spec-…”

Nonaqueous Microemulsions Containing Ionic Liquids – Properties and Applications

“…The solvation times in the RTIL pools were found to be nearly independent of [bmim] [BF 4 ] content and size of the microemulsion droplets. By contrast, a decrease in solvation time in aqueous reverse microemulsions with increasing R values has been documented in literature (Sarkar et al, 1996;Riter et al, 1998). Using isothermal titration microcalorimetry Li et al determined the heat of dilution and extracted the second virial coefficient, b 2 of these microemulsions (Li et al, 2008).…”