Optical Excitations in Supramolecular Assemblies of Conjugated Oligomers and Polymers

Temple University, Philadelphia PA

Investigators

Abstract

Thin films of pi-conjugated polymers and their oligomer progeny continue to receive widespread attention as the active materials for organic-based field effect transistors, light emitting diodes (OLEDs) and solar cells. Despite the many experiments designed to reveal the mechanism(s) by which oligomer/polymer films interact with light, a fundamental appreciation of the relationship between solid-state morphology and photophysical behavior is lacking. In this renewal proposal a strategy is outlined for theoretically investigating an emerging class of supramolecular assemblies based on oligo-p-phenylenevinylene (OPVn) and oligo-thiophene (OTn) derivitives. Side- and end-group functionalization induces chirality and hydrogen bonding in self-assembled nanostructures including helical rod aggregates, rosette shaped aggregates and two-dimensional lamellae. Many of these structures employ p-stacking motifs which complement the more common herringbone packing found in crystals and thin films of the unsubstituted OPVn and OTn molecules. Specific goals include: A detailed understanding of the impact of exciton delocalization, exciton-phonon coupling and disorder on the photo-excited and photo-emissive states in chiral aggregates. Abundant spectral data for helical rod aggregates including steady-state absorption, photoluminescence, circular dichroism (CD) and circularly polarized luminescence (CPL), allows rigorous testing of theoretical models featuring various scenarios for static disorder. A global account for the unusual demise of the CPL dissymmetry magnitude with increasing wavelength observed in OPVn-based helical rod aggregates as well as chiral polythiophene thin films is sought. A robust theoretical account of the deleterious effect of increasing oligomer length on the exciton bandwidth observed in a series of ethylene oxide end-capped OTn fibrillar aggregates (with n=5,6,7). How the bandwidth behaves into the polymer limit is crucial in understanding energy transport and optical properties in thin films of the technologically important regioregular poly(3-alkylthiophenes). Accurate modeling of rapid picosecond energy transport in helical rod aggregates as well as the temporal evolution of the emission spectral line shape following excitation above the optical gap. The emission spectral line shape provides valuable information about high energy emissive states. The analysis will be based on the Holstein Hamiltonian, treating excitonic interactions, linear exciton-vibrational coupling, and the influence of point and structural defects on equal footing. The two-particle approximation will be employed to reduce the basis set to a tractable size for numerical analysis without sacrificing accuracy. Excitonic polarons and their steady-state spectral signatures will be evaluated using standard numerical matrix techniques. A Pauli Master Equation will be employed for the exciton transport studies. The broader impact of the proposed activities will be felt primarily through an enhanced understanding of a technologically important class of materials, possibly resulting in novel design strategies for improved OLEDs and organic-based solar cells. The commercial impact of soft electronic devices is expected to dramatically increase over the next several years, through products like flexible displays, electronic labels and even solid-state lighting. In addition, the proposed activities will enhance research infrastructure through international collaborations. (D. Beljonne at the Universite de Mons, Belgium; S. Meskers at Eindoven University, The Netherlands; and C. Silva at the University of Montreal.) Non-Technical Abstract: This grant will support theoretical research on the properties of thin films of electrically active polymers. These materials have potential applications as components of electronic devices ("electronic plastics"). However, much needs to be done to understand their fundamental behavior. The work proposed here involves international collaborations with others working on these problems and provides wonderful learning opportunities for students and postdoctoral associates working on this project.