Exciton Dynamics in Disordered Molecular Semiconductors

Molecular semiconducting materials are comprised of many individual molecules whose individual electronic properties combine to yeild material-wide semiconducting properties. Unlike traditional crystalline semiconductors, the electronic structure of these materials is often spatially and temporally heterogeneous due to the relatively weak van der Waals forces that drive molecular association. Predicting how this heterogeneity affects the electronic properties of these materials is a significant theoretical challenge because the characteristic length scales involved are beyond the capabilities of most quantum chemistry techniques.

Our research combines methods of statistical mechanics and quantum dynamics in order to explore how nanoscale disorder affects the emergent electronic properties in molecular semiconducting materials. This relationship is mediated by the static and dynamic properties of excitons, Coulombically bound excited electron-hole pairs. These energetic quasiparticles mediate molecular-scale energy transport in a wide range of systems ranging from modern organic light-emitting diodes (OLEDs) to the light-harvesting machinery in photosynthetic systems. Our approach is focused on developing models that describe the subtle dependence of electronic structure on molecular configuration while simultaneously accounting for the nanoscale correlations that arise through collective molecular fluctuations.

The Structure and Dynamics of Self-Assembled Polymer Gels

This research project is aimed at enabling the rational design of a new class of metal-coordinated polymer materials. These materials utilize the strong but reversible bonds of coordination chemistry as a cross-linking motif. Because these coordination bonds are tunable through molecular design these materials can be engineered to exhibit novel structural and chemical properties. Recent advances in synthetic techniques, pioneered by our collaborators in the lab of Jeremiah Johnson (MIT Chemistry), have enabled precise control over the molecular precursors that comprise these materials. However, predicting the emergent properties of these materials based on the molecular details of their precursors continues to be a significant challenge.

Our approach utilizes a hierarchy of simulation techniques ranging from all-atom molecular dynamics to abstract models for simulating the dynamic topology of polymer networks. We focus on exploring how heterogeneity in microscopic network structure affects macroscopic material properties such as the viscoelasticity and how these properties can be controlled through systematic variations in molecular structure.

The Molecular Structure of Disordered Hydrophilic Water Interfaces

Water plays a fundamental role in processes that cut across all branches of science. Irrespective of its prevalence, water is particularly interesting because of its unique microscopic structure, which consists of a disordered array of molecules engaged in a three-dimensional hydrogen bonding network. Resolving this picture and how it relates to macroscopic properties has been a problem of long-standing scientific interest. At the interface between liquid water and a disordered substrate (such as the surface of a protein) the microscopic details of interfacial structure reflect a competition between water-water and water-substrate interactions. Variations in the balance between these interactions, along with their effect on the termination of water's three-dimensional hydrogen bonding network account for the large array of observed hydrophilic interfacial structures. This forms the basis for a rich and challenging problem in statistical mechanics.

Our research is aimed at providing a statistical mechanical framework for analyzing the molecular structure and dynamics of liquid water interfaces. Specifically, we apply tools from information theory to the analysis of atomistic simulation data as a novel and minimally subjective methodology for quantifying the molecular details of interfacial structure. We apply these tools to investigate protein hydration to explore how heterogeneous surface chemistry and conformational dynamics affect the microscopic properties of the protein's hydration shell.

Chemical Dynamics at the Aqueous Electrode Interface

The process of electrochemical water splitting is of general scientific interest due to its potentially major role in future renewable energy solutions. It is among a large number of aqueous electrode processes that intimately involve the dynamics of protons and/or hydroxide ions, the products of water auto-ionization. The aqueous dynamics of these ions (and many other reactive species) in the vicinity of the water-electrode interface and their dependence on applied voltage represent an important yet poorly understood aspect of aqueous electrochemistry.

We are interested in using molecular simulation in order to elucidate the dynamics of hydroxide and hydronium ions at the aqueous electrode interface. We study the interplay between the dynamics of electrode-adsorbed molecules and solvent fluctuations, along with the concomitant influence on electrochemical dynamics near the electrode.

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