George Schatz of Northwestern University is supported by an award from the Chemical Theory, Models and Computational Methods program in the Chemistry division to develop theoretical and computational approaches to study the rate of energy flow in molecular assemblies and other optical devices that depend on the flow of energy through an array of molecules. This research is part of an important effort by chemists to develop new leaf-like materials in which assemblies of molecules act as antenna structures to absorb sunlight and then transport this energy to locations where other molecules convert it into electrical energy. Schatz and his coworkers are developing a new theoretical approach to describe this process, one which makes it possible to include the coupling between light, and electrons and vibrational motions of the molecules and surroundings, so that the rate of transport can be quantitatively determined for structures that are much larger than have previously been accessible. The research also includes studies of the assembly of the molecules into structures that optimize energy flow, and there are extensive collaboration with experimental groups who are making the structures. This project is carried out in collaboration with undergraduates, graduate students and postdocs, Results from this research are used in developing teaching material for college courses.
In this research project, a new theoretical method is developed for characterizing the excited state dynamics of aggregates and assemblies of dye chromophores that are of interest in photonic devices. In this method, the traditional Förster approach for determining the rate of energy transfer associated with interacting excitons is replaced by a time domain approach in which the emitted radiation associated with the donor species is described by classical electromagnetic theory and the resulting electric field at the acceptor position is used to calculate a response function that describes the energy transfer rate. This approach, which is borrowed from the optical physics community, includes several factors that go beyond the Förster approach, including the incorporation of retardation and complex electromagnetic boundary conditions associated with the photonic structure. The research involves generalizing this approach to describe the physical situation associated with molecular chromophores in which point dipole emitters are replaced by oscillating currents that act as antennas, and the dielectric response of the surrounding medium is described by Lorentz oscillator models that enable the description of spectral shifts due to exciton coupling. Extensions of the theory include phonon motion, dephasing and relaxation, both incoherent and coherent energy transfer, and nonlinear effects. Numerical implementation employs the finite-difference time-domain method and other methods. The capabilities of the proposed theory is being demonstrated by applications to chromophore aggregates whose structures are determined from molecular dynamics simulations of the self-assembly process. Examples of this type include peptide amphiphiles (with embedded chromophores) that self-assemble to give micelle aggregates in which the chromophores are stacked into organized structures. Also of interest are periodic arrays of chromophores that are produced in metal organic framework materials (MOFs) and DNA-linked nanoparticle superlattices. Emphasis in these studies is on exciton transport properties that are useful in device applications.
