A femtosecond study of excitation wavelength dependence of a triblock copolymer-surfactant supramolecular assembly: (PEO)₂₀-(PPO)₇₀-(PEO)₂₀ and CTAC

Abstract

Solvation dynamics and anisotropy decay of coumarin 480 (C480) in a supramolecular assembly containing a triblock copolymer, PEO₂₀-PPO₇₀-PEO₂₀ (Pluronic P123) and a surfactant, CTAC (cetyl trimethylammonium chloride) are studied by femtosecond up-conversion. In a P123-CTAC complex, C480 displays a significant (22 nm) red edge excitation shift (REES) in the emission maximum as λ _ex increases from 335 to 445 nm. This suggests that the P123-CTAC aggregate is quite heterogeneous. The average rotational relaxation time (τ_rot) of C480 in a P123-CTAC complex decreases by a factor of 2 from 2500 ps at λ _ex = 375 nm to 1200 ps at λ _ex = 435 nm. For λ _ex = 375 nm, the probe molecules in the buried core region of P123-CTAC are excited and the solvation dynamics displays three components, 2, 60, and 4000 ps. It is argued that insertion of CTAC in P123 micelle affects the polymer chain dynamics, and this leads to reduction of the 130 ps component of P123 micelle to 60 ps in P123-CTAC. For λ _ex = 435 nm, which selects the peripheral highly polar corona region, solvation dynamics in P123-CTAC and P123 are extremely fast with a major component of <0.3 ps (~80%) and a 2 ps (~20%) component.