Solvation dynamics in ionic liquid swollen P123 triblock copolymer micelle: a femtosecond excitation wavelength dependence study

Abstract

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][BF₄]). At a low concentration (0.3 M), the sparingly water soluble IL ([pmim][BF₄]) 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][BF₄] 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][BF₄], 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.