Femtosecond Solvation Dynamics in a Micron-Sized Aggregate of an Ionic Liquid and P123 Triblock Copolymer

Abstract

Dynamic light scattering studies indicate that addition of a room temperature ionic liquid (RTIL, [pmim][Br]), to a triblock copolymer (P123) micelle leads to the formation of giant P123-RTIL clusters of size (diameter) 40 nm in 0.9 M and 3500 nm (3.5 microm) in 3 M RTIL. They are much larger than a P123 micelle ( approximately 18 nm) or [pmim][Br] (1.3 nm). Dynamics in different regions of the P123-RTIL aggregate is probed by variation of the excitation wavelength (lambda(ex)) using femtosecond up-conversion. For lambda(ex) = 375 nm, the nonpolar core of the P123-RTIL aggregate is preferentially excited while lambda(ex) = 435 nm selects the polar corona region. Solvation dynamics and anisotropy decay of coumarin 480 (C480) in a P123-RTIL giant aggregate are markedly different from those in either P123 micelle or those in an aqueous solution of the RTIL. For lambda(ex) = 405 nm in 5 wt % P123 and 0.9 M RTIL average rotational time, (<tau(rot)> = 1350 ps) of C480 is approximately 7 times longer than that (200 ps) in an aqueous solution of the RTIL in the absence of P123 and is shorter than that (3000 ps) in a P123 micelle. In 0.9 M RTIL and 5 wt % P123, solvation dynamics in the corona region (lambda(ex) = 435 nm, <tau(s)> = 75 ps) is approximately 25 times faster than that at the core region (at lambda(ex) = 375 nm, <tau(s)> = 1900 ps). The solvation dynamics in the core of the P123-RTIL aggregate is faster than that in P123 micelle (3550 ps in the core) and is much slower than that (130 ps) in an aqueous solution containing 0.9 M RTIL. In the 3.5 microm sized aggregate (3 M RTIL and P123), the solvation dynamics in the core (<tau(s)> = 500 ps) is approximately 4 times faster than that in 0.9 M RTIL.