Electronic excitations in branched conjugated oligomers: A quasiparticle view and tight-binding models.

摘要

This dissertation focuses on the theoretical understanding and simulation of the excited state electronic structures of organic conjugated molecules. The exciton scattering (ES) approach has been extended for efficient calculation of optical spectra of large branched conjugated oligomers. The methodology of tight-binding (lattice) model, originally developed in condensed matter theory, has been extended to the building block structure of conjugated molecules for explicit description of the electronic excitations.;Within the ES approach, the electronic excitations in quasi-1D molecular structures are attributed to standing waves that represent quantum quasiparticles (excitons) scattered at the molecular vertices. Excitation energies can be found by solving a generalized "particle in the box" problem on the graph that represents the molecule. The transition dipoles can be calculated by counting dipole contribution of each molecular building block, which is proportional to the amplitude of the excitation. The ES methodology is also extended to analyze the electronic excitations in oligomers with electron donor and acceptor substituents, which are known to modify the electronic and optical properties. The energy-dependent ES parameters that characterize the exciton properties on building blocks can be extracted from quantum chemical computations in simple molecular segments and tabulated for further applications. This methodology greatly simplifies the spectroscopic calculation for any branched conjugated molecule that consists of characterized building blocks. The optical spectra predicted by the ES approach accurately reproduce the results of the corresponding quantum chemistry.;We introduce effective tight-binding models to describe exciton scattering in the imperfect geometries where the translational symmetry is violated by conformational distortions. The geometry-dependent parameters of lattice models including on-site energies and hopping constants are determined from the exciton scattering analysis. The tight-binding representation provides immediate analytic continuations of the scattering matrix, allows for the identification of the excitation, with complex wavenumbers, bound at the scattering center. The approach of tight-binding models, which simplify the description of the excited state electronic structure using a small number of constants, is useful to characterize the effect of geometric distortions on the excitations.