In contrast with conventional liquids, ionic liquids have solvation dynamics with more rate dispersion and with average times that do not agree with dielectric measurements. A kinetic analog of multidimensional spectroscopy is introduced and used to look for heterogeneity in simulations of coumarin 153 in [Im12][BF4]. Strong heterogeneity is found in the diffusive solvation rate. An unanticipated heterogeneity in the amplitude of the inertial solvation is also seen. Both heterogeneities exchange at the same rate. This rate is similar to the mean diffusive solvation time, putting it in the intermediate-exchange region. Overall, there are multiple violations of the assumptions usually invoked in the theory of reaction dynamics.

In polymers, the rotation of a small solute is nonexponential. Either heterogeneity in the local friction or local anisotropy—a homogeneous process—may be responsible. A new, two-dimensional anisotropy experiment is demonstrated on this problem. In poly(dimethylsiloxane), the rotation of individual solute molecules is found to be exponential, and the observed rate dispersion is primarily due to variation in the local friction. This sample is far from its glass transition. Studies of rate heterogeneity associated with the glass transition must account for the contribution from this polymer-related mechanism.Keywords: multidimensional; MUPPETS; PDMS; polarization; reorientation;

Measurements of biexciton decays in semiconductor nanoparticles are easily contaminated by contributions from photoproducts or higher excitons. Theoretical work has shown that multiple population-period transient spectroscopy (MUPPETS) can measure biexciton decays free from these interferences. In this communication, the biexciton decay of CdSe/ZnS core–shell nanoparticles is measured with MUPPETS. The decay is strongly dispersed (nonexponential) with a more than 5-fold range of rates. This large dispersion must be accounted for in the decay mechanism and in the measurement of biexciton dynamics by more conventional methods. The success of MUPPETS in this context lays the foundation for using it to study exciton–exciton interactions in a variety of materials.

Multiple population-period transient spectroscopy (MUPPETS) is a six-pulse experiment with two time dimensions that is capable of adding information about systems with complicated kinetics. The core theory for MUPPETS focuses on the χ(5) response of the chromophores. This theory was used to analyze the dynamics of excitons and biexcitons in CdSe/ZnS core–shell nanoparticles in part I of this paper [J. Phys. Chem. B 2013, DOI:10.1021/jp405785a]. In real experiments, the potential role of additional processes must also be considered, in particular, the χ(7), “saturation” of the MUPPETS signal and nonresonant signals from heating of the solvent. A pathway method for calculating fluence effects in MUPPETS is developed. The fluence dependence of the biexciton signal and its sign reversal, as found in part I, are explained without invoking higher excitons or unexpected species. A method is presented for quantitatively predicting the magnitude of signals from solvent heating using an external standard. Thermal effects in this system are found to be too small to affect the conclusions in part I. Their small size, combined with small, systematic errors in the data, also makes it difficult to measure the yield of solvent heat in these experiments.

The nonradiative relaxation of both the exciton and biexciton in CdSe/ZnS core–shell nanoparticles have complicated, nonexponential kinetics. This paper presents data on this system from multiple population-period transient spectroscopy (MUPPETS), a method for two-dimensional kinetics. An initial report of a dispersed (nonexponential) biexciton decay [J. Am. Chem. Soc. 2013, 135, 1002] is confirmed in a more rigorous analysis. Additional transient-grating data allow a quantitative treatment of the full, complex MUPPETS data set. The MUPPETS signal has a strong fluence dependence. With extrapolation to the low fluence limit, the ratio of cross sections for ground-to-exciton and exciton-to-biexciton absorption is found to be close to the predictions of the uncorrelated-electron model. The full two-dimensional MUPPETS data set is reported for the first time and is analyzed to detect heterogeneity in the exciton decay. The exciton has a substantial (>40%) nonradiative decay, but it is not due to a subset of defective particles. A surface relaxation in response to formation of the exciton is suggested. This data set is the first capable of detecting correlations between the biexciton and exciton decay mechanism. None is found.