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

Abstract

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 (τ_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 (λ _ex) and becomes about 15 times faster as λ _ex increases from 375 to 435 nm. This is ascribed to the structural heterogeneity in the RTIL microemulsion. For λ _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 λ _ex and increases about 2.5-fold from 180 ps at λ _ex = 375 nm to 500 ps at λ _ex = 435 nm for D₂O containing RTIL microemulsion. The slower anisotropy decay at λ _ex = 435 nm is ascribed to the higher viscosity of RTIL which causes greater friction at the core.