Excitation wavelength dependence of solvation dynamics in a gel. (PEO)₂₀-(PPO)₇₀-(PEO)₂₀ triblock copolymer

Abstract

In a 30 wt % aqueous solution, the triblock copolymer, (polyethylene oxide, PEO)₂₀-(polypropylene oxide, PPO)₇₀-(PEO)₂₀ (Pluronic P123) forms a cubic gel phase. In the gel, the micelles associate to form an interconnected network with intermicellar distance (140 Å) less than the sum (160 Å) of the radii of the micelles. Fluorescence anisotropy decay and solvation dynamics of coumarin 480 (C480) in P123 gel have been investigated as a function of excitation wavelength (λ _ex) using femtosecond and picosecond fluorescence spectroscopy. The anisotropy decay of C480 in the macroscopically rigid ("solid") gel phase is quite fast and only 1.6 times slower than that in the sol (micellar) phase. Solvation dynamics in the P123 gel displays three components. First, even in the "solid" gel, there is a bulk water-like ultrafast component (2 ps and <0.3 ps), which represents response of the "free" water molecules in the pores of the gel. Second, there is an ultraslow 4500 ps component, which is ascribed to the hydrophobic PPO core. Finally, a 500 ps component arises from the interpenetrating region of the micelles. With increase in λ _ex from 375 to 435 nm, the contribution of the ultrafast bulk-water like region increases from 6 to 87% while that of the ultraslow component (4500 ps) due to the PPO core region decreases from 90 to 7%. The 500 ps component, which arises from a region of intermediate polarity, displays a maximum contribution at λ _ex = 405 nm.