A femtosecond study of excitation wavelength dependence of solvation dynamics in a PEO-PPO-PEO triblock copolymer micelle

Abstract

Excitation wavelength (λ_ex) dependence of solvation dynamics of coumarin 480 (C480) in the micellar core of a water soluble triblock copolymer, PEO₂₀-PPO₇₀-PEO₂₀ (Pluronic P123), is studied by femtosecond and picosecond time resolved emission spectroscopies. In the P123 micelle, the width of the emission spectrum of C480 is found to be much larger than that in bulk water. This suggests that the P123 micelle is more heterogeneous than bulk water. The steady state emission maximum of C480 in P123 micelle shows a significant red edge excitation shift by 25 nm from 453 nm at λ_ex = 345 nm to 478 nm at λ_ex = 435 nm. The solvation dynamics in the interior of the triblock copolymer micelle is found to depend strongly on the excitation wavelength. The excitation wavelength dependence is ascribed to a wide distribution of locations of C480 molecules in the P123 micelle with two extreme environments-a bulklike peripheral region with very fast solvent response and a very slow core region. With increase in λ_ex, contribution of the bulklike region having an ultrafast component (≤2 ps) increases from 7% at λ_ex = 375 nm to 78% at λ_ex = 425 nm while the contribution of the ultraslow component (4500 ps) decreases from 79% to 17%.