This award is funded by the Division of Materials Research and the Chemistry Division. It supports theoretical research and education on the interaction of thin films of pi-conjugated polymers and related oligomers with light. Despite the many experiments designed to uncover the mechanisms by which polymer films interact with light, a global appreciation of the relationship between solid-state morphology and photophysical behavior is still lacking. The H-aggregate model, which has been successful in describing the photophysics of spin-cast poly (3-hexylthiophene) films, nevertheless fails to account for several key photophysical properties of phenylene vinylene- and fluorene-based polymer films, such as the temperature-dependence of the photoluminescence line shape. This award supports research to investigate a more sophisticated polymer aggregate model, the HJ-aggregate model. Unlike the H-aggregate model, the HJ-aggregate model accounts for multidimensional exciton motion, both along and normal to the polymer backbone. Specific goals of this project include:
1) A detailed understanding of how photophysical properties are affected by the competition between interchain interactions, which lead to delocalized Frenkel type excitations and H-aggregate-like behavior, and intrachain interactions, which lead to Wannier-type excitations and J-aggregate-like behavior. The interchain vs. intrachain competition can be understood from the way the vibronic progressions in the absorption and photoluminescence spectra are altered as a function of the exciton coupling along and normal to the polymer chains, the temperature, and the degree of spatially-correlated disorder along and normal to the polymer chains. The vibronic progressions, sourced primarily by the ubiquitous vinyl-stretching mode common to virtually all pi-conjugated molecules, therefore serve as a probe of the exciton bandwidth, exciton coherence length and generalized morphology. Specific applications will be made to poly (3-hexylthiophene) assemblies, which behave as H-aggregates when spin-cast from various solvents, but also as J-aggregates, when self-assembled in a slowly-cooled toluene solution. The possibility of a morphology-driven transition from H- to J-aggregate behavior is not only a fundamental novelty but can be exploited for device optimization.
2) A more complete understanding of exciton coherence in polymer films, with an aim at optimizing coherent transport by taking advantage of the large coherence lengths recently measured in single-chain polydiacetylene.
3) A detailed analysis of conformationally disordered polymer chains, with an aim at understanding how disorder and vibronic coupling conspire to create conformational units and affect photophysical properties.
Analyses will be based on Holstein-variety Hamiltonians, treating through-bond and through-space excitonic interactions, linear exciton-vibrational coupling, and disorder on equal footing. Multi-particle approximations will be employed to reduce the basis set to a tractable size for numerical analysis without sacrificing accuracy. Fundamental excitations and their steady-state spectral signatures will be evaluated using standard numerical matrix techniques.
NON-TECHNICAL SUMMARY
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. A thrust of this project is to investigate a new model to describe the behavior of the electrons along molecular chains and between 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.
