Materials with complex structure at the molecular level often show equally complex kinetics. The decay curves are nonexponential, that is, they show rate dispersion. Such rate dispersion may be due to molecule-to-molecule rate heterogeneity or due to complex reaction mechanisms that are the same for each molecule (i.e., a homogeneous mechanism). Previous multidimensional experiments, called multiple population-period transient spectroscopy (MUPPETS), have resolved these causes of rate dispersion in ultrafast electronic-state decay. In this talk, MUPPETS is extended to rotational-rate dispersion on the picosecond timescale by introducing 2D polarization anisotropy measurements. Multidimensional kinetics are also extended to single-molecule kinetics on the second timescale. In both cases, rotation of a solute in a polymer melt are measured. Pyrromethene 597 in poly(dimethylsiloxane) (MW 5970 g/mol) at room temperature (2.1Tg) rotates with a mean time of 400 ps and a stretched exponential dispersion of � = 0.7. Polarized MUPPETS measurements show that the dispersion is primarily due to local variation in the rotational friction and not due to the formation of locally anisotropic structures in the polymer. Rhodamine 6G in poly(cyclohexylacetate) at Tg + 2 K has a rotation time of 150 s and � = 0.5. 2D analysis of single-molecule dichroism trajectories [J. Phys. Chem. B, 113  2253 (2009)] shows that the dispersion is again dominated by heterogeneity. 3D analysis shows little effect of rate exchange out to 1000 s.

Multidimensional measurements of rotation-rate heterogeneity in polymers on the picosecond and second timescales

248th National Meeting (2014)

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