Using Circularly Polarized Light to Probe Electronic Excitations in Organic Supramolecular Assemblies

Temple University, Philadelphia PA

Investigators

Abstract

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). TECHNICAL ABSTRACT This award is funded by the Division of Materials Research and the Chemistry Division. It supports theoretical research and education on how oligomer/polymer films interact with and emit light. The PI aims to investigate fundamental excitations in organic assemblies by focusing on chiral supramolecular structures which are amenable to circular dichroism and circularly polarized luminescence spectroscopies. Compared with their unpolarized counterparts, these spectroscopies are far more sensitive to extended intermolecular interactions as well as the structure of the fundamental electronic excitations. Of particular interest to this study are pi-stacked helical rod aggregates consisting of functionalized conjugated molecular chromophores which self-assemble in solution or with the aid of a DNA template. Specific goals include: 1. A detailed understanding of the excitonic signatures in the circularly polarized luminescence dissymmetry spectrum. The unusual demise of the spectrum with increasing wavelength observed in helical rod aggregates and chiral polymer films contains vital information on the anatomy of the emitting species. This spectrum is unique amongst spectral probes in its high sensitivity to the inner-sphere distortion field surrounding the core vibronic excitation. The objective is to understand how the polaron radius, the reorganization energy, and the coherence length impact the shape and magnitude of the circularly polarized luminescence dissymmetry spectrum. International collaboration with experimentalists will enable applications to oligothiophene nanofibers. 2. A rigorous analysis of intermolecular interactions in helical rod N-mers. Aggregates with a controllable number of chromophores are ideal systems with which to rigorously evaluate theoretical models for extended intermolecular interactions and disorder. The circularly polarized luminescence dissymmetry spectrum and its absorption analog are sensitive probes of long-range interactions even in strongly disordered systems where excitons are localized to only a few chromophores. The PI aims to perform a detailed theoretical analysis of the absorption spectrum to help resolve these issues. 3. An appreciation of the exciton Cotton effect in systems with strong exciton-phonon coupling. In assemblies with strong exciton-phonon coupling the circular dichroism spectrum is far more complex than the simple Cotton couplet found for weak coupling. The PI aims to develop a comprehensive theory for the circular dichroism response of excitonic polarons to understand how information regarding intermolecular interactions and the aggregate morphology can be extracted from the spectral line shape. Applications to the ubiquitous and biologically-important carotenoid aggregates will be made. The above analyses will be based on a Holstein Hamiltonian. Excitonic interactions, linear exciton-phonon coupling, and disorder will be treated on equal footing under the two-particle approximation, which reduces the basis set to a tractable size without sacrificing accuracy. 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 organic light-emitting diodes and 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 solid-state lighting. In addition, the proposed activities will enhance research infrastructure through international collaborations. NONTECHNICAL ABSTRACT This award is funded by the Division of Materials Research and the Chemistry Division. It supports theoretical research and education on how thin films composed of a class of long chain molecules, polymers, interact with and emit light. The research is focused on key issues that impede understanding of the mechanisms by which these films interact with light, how light is absorbed by these materials and the nature of the electronic states after absorbing light. A key feature of the PI?s approach is to account for the interaction of electronic charge with vibrations of the molecular chains. Thin films of particular kinds of polymers may be useful as active materials for organic-based electronic devices such as transistors, light emitting diodes, and solar cells. This research project contributes to the intellectual foundations that will enable the use of these materials for lighting, solar energy conversion, and other electronic devices. 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 solid-state lighting. In addition, the proposed activities will enhance research infrastructure through international collaborations.