Structures and excited state dynamics of self-assembled photonic structures.

Overview: This proposal develops a new theoretical method 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 for several factors that go beyond the Förster approach, including the incorporation of retardation and complex electromagnetic boundary conditions associated with the photonic structure. The theory is generalized in this proposal 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. We show how the parameters needed to describe the currents can be obtained from atomic transition densities as determined by conventional electronic structure calculations for the dyes. Extensions of the theory to include phonon motion in the donor, the acceptor, or in the surrounding dielectric medium are described, as well the description of dephasing and relaxation, and both incoherent and coherent energy transfer. The treatment of two-photon and other nonlinear effects in both the excitation and energy transfer steps is developed. Numerical implementation using the finite-difference time-domain method and other methods is described. Intellectual Merit: The capabilities of the proposed theory will be 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. We also consider periodic arrays of chromophores that are produced in metal organic framework materials (MOFs) and in DNA-linked nanoparticle superlattices. Emphasis in these studies will be on exciton transport properties that are useful in device applications. Broader Impacts: The proposed research projects will be used for the training of undergraduate and graduate students, and they will be used in a wide variety of outreach activities involving broader audiences. Plans for dissemination of software and related documentation are described, as well as the development of teaching materials for General Chemistry concerned with self-assembly and photonic structures. Additional activities include: (1) Outreach to primarily minority elementary schools through Phi Lambda Upsilon, a graduate chemistry organization which I advise; (2) Teaching of an ethics course to chemistry graduate students and postdocs, as well as outreach activities related to publication ethics; (3) Summer REU projects, including minority students, that my group works on each year; (4) Activities related to exciton transport and self-assembly processes associated with the Journal of Physical Chemistry, of which I am the Editor-in-Chief; (5) Presentations to audiences, including the g