Solvation Dynamics in Ionic Liquid Swollen P123 Triblock Copolymer Micelle: A Femtosecond Excitation Wavelength Dependence Study.

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]