With support from the Chemical Measurement and Imaging program in the Division of Chemistry, Prof. Mark Berg and his group at the University of South Carolina are developing and applying new methods to assess the rates of chemical processes (including peptide conformational dynamics) in heterogeneous materials. Traditional "one-dimensional" spectroscopic methods use a single excitation source and measure rates over a single time period. Prof. Berg is developing multidimensional methods that use more than one excitation source and measure rates over multiple periods. By using three excitation pulses, both the distribution and rates of conformational change will be measured. Multidimensional methods are also being broadened to include light pulses of variable polarization and thereby measure the distribution of local viscosities within bulk polymers.
Kinetic measurements are used throughout chemistry, biology, physics and engineering. The fundamentally new approaches to kinetics developed in this program will provide new tools to all these areas. Training in advanced spectroscopy will be done at the University of South Carolina, which is both geographically underserved and has a large population of students from underrepresented groups. In addition, Prof. Berg is collaborating with Prof. Peter Chen of Spelman College, an HBCU for women, to adapt the technological development from this project to an undergraduate research environment.
In simple materials, the individual steps in a chemical process have a single, well defined rate that is measured by standard, one-dimensional (1D) experiments. In more complex materials, these experiments indicate a multiplicity of rates, but give little information on why more than one rate is seen. This project has advanced a new approach using multidimensional kinetics to understand complex dynamics. It has shown that 2D experiments can determine if materials are kinetically homogeneous or heterogeneous at a molecular level. In heterogeneous materials, 3D experiments can measure how long a molecule stays in a particular environment. Both theoretical and experimental work has expanded the range of processes and timescales were multidimensional kinetics can be used. As these abilities have developed, they have been applied to a variety of systems in which important questions have not been answered by standard approaches: (1) Multidimensional ultrafast spectroscopy was extended to multilevel, excitonic system, and a new ability to isolate biexciton decay signals was demonstrated. Controlling biexciton decay rates is critical for using semiconductor nanoparticles in optoelectronics. The new measurements showed that existing models of biexciton decay do not correctly explain the shape of the decay and point the way to improved theories. (2) Multidimensional ultrafast spectroscopy was extended to use polarized laser pulses and, thereby, measure rotational rates of individual molecules. It is known that rotation has a single rate in simple liquids, but that multiple rates develop in a polymer. Measurements made in this project showed that polymers develop molecular-sized regions of differing local viscosity. A competing hypothesis of local alignment of the polymer chains was disproved. (3) In collaboration with the NSF sponsored group of David Vanden Bout (Univ. Texas), we extended multidimensional kinetics to single-molecule experiments and to timescales nine orders-of-magnitude slower than in previous experiments. The formation of glasses is widely hypothesized to be accompanied by local kinetic heterogeneity. Use multidimensional analysis, the first quantitative measurement of this heterogeneity was made using single-molecule data and an lower limit on the exchange between different spatial regions was found. (4) In collaboration with the NSF sponsored group of Steven Corcelli (Univ. Notre Dame), we extended ideas from the analysis of multidimensional experiments to the analysis of computer simulations and applied them to solvation in an ionic liquid. In addition to many of the favorable features of ionic liquids, they also have exceptionally slow and complex responses to the movement of charge, which can limit the rate of fast reactions. Our multidimensional analysis established that different regions in the ionic liquid have solvation rates spanning a factor of ~100. We also found an unexpected heterogeneity in the total polarity of the different regions. All these projects contributed to the training of students in important technical areas. Cross-disciplinary training was enhanced by participation in an NSF funded IGERT (DGE-1250052) and by recruiting students from statistics and computer science to participate. Two undergraduate students from underrepresented groups were recruited to participate in this project. The PI of this project also instituted the first formal program in the Chemistry department to recruit underrepresented graduate students. By establishing contacts with the GEM organization, targeted and personalized recruiting of underrepresented students from the GEM database was instituted.
