In microstructured environments, probe molecules are often used to transduce local physical properties into an optical signal. However, when the probe kinetics have rate dispersion (nonexponential decays), the interpretation of the signal becomes ambiguous. For example, the nonexponential anisotropy decay of probes in the Stern layer of micelles has been attributed to either local anisotropy in the Stern layer (wobble-in-a-cone model) or to variation in the local viscosity of the Stern layer. These two mechanisms—the former homogeneous and the latter heterogeneous—cannot be distinguished by standard, one-dimensional measurements. Multiple population-period transient spectroscopy (MUPPETS) is a two-dimensional anisotropy experiment that can quantify the contribution of each mechanism. MUPPETS measurements of pyrromethene 597 in sodium-dodecyl-sulfate (SDS) micelles (Figure 1) show that the rate dispersion in this system is primarily caused by heterogeneity in local viscosity. This heterogeneity can arise from either fluctuations in the structure of the micelles or variation in the positioning of the probe molecule within that structure.

Polarized MUPPETS (multiple population-period transient spectroscopy) distinguishes between heterogeneous local viscosities and local anisotropy in SDS micelles

2019 Fall National Meeting

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