An effective particle approach to the photophysics of conjugated polymers

This thesis develops a computational technique, called “the effective particle approach,” that enables structure-property relationships of conjugated polymers to be extracted from quantum chemistry calculations. This approach views an excited state as containing one or more “effective particles” that move on an energy landscape with a position-dependent “effective mass.” For the 1Bu state of conjugated polymers, the effective particle is an exciton, or bound electron-hole pair. The form of the particle is defined by the relative motion of the electron and hole, and its delocalization is described by its “center-of-mass” motion. This technique yields computational savings as well as interpretive advantages.; This approach relies on the ability to form orbitals that are localized on molecular segments. A technique is developed that uses sub-blocks of the Fock matrix in a hybrid atomic orbital basis to generate reasonable trial functions for each segment, and the local orbitals are then obtained by projecting these trial functions into the proper orbital space. This method can localize occupied/unoccupied molecular orbitals and can include both sigma and pi electrons. This robust technique enables the inclusion of solid-state dielectric effects and yields computational savings in post Hartree-Fock methods.; Effective particles are used to generate energy landscapes and effective mass profiles that provide insight into how the structure of a material relates to its photophysical properties. Calculations on a carbonyl defect in polyacetylene and poly(p-phenylenevinylene) (PPV) quantify the degree to which the carbonyl attracts an electron and repels a hole, thereby promoting charge separation that is strongly influenced by dielectric solvation. Landscapes and effective masses for a meta-linkage defect and torsional disorder in PPV reveal that even relatively small torsional defects have fairly large effects on the energy and reduced mass landscapes.; This approach is combined with a scattering formalism to study the photophysics of long polymer chains. This methodology is used to investigate whether biexcitons are stable and can be observed in two-photon spectroscopy. The results indicate that biexciton states are not stable in the limit of long chains but could be stabilized on short chains by confinement effects.