| Organic materials have great potential for use in electronic and photophysical devices. A detailed understanding of the electronic structure of these materials will allow us to better understand the structure-property relationships of relevance to device design. This thesis develops and applies methods for studying the electronic structure of three classes of organic electronic materials: nonlinear optical chromophores, conjugated polymers, and carbon nanotubes.; The first part of this thesis examines the nonlinear optical properties of multi-polar organic chromophores. This is done by extending the internal field model of Oudar, which is applicable only to pseudo one-dimensional systems such as push-pull polyenes, to chromophores with general multi-dimensional acceptor-donor substitution patterns. The model developed here takes full account of the multipolar nature of the internal potential applied by the acceptors and donors, and the tensor properties of the resulting nonlinear optical susceptibility. The model is tested against quantum chemical calculations on representative systems.; The second part of this thesis concerns the development of a method that allows intermediate neglect of differential overlap (INDO) calculations to be performed under periodic boundary conditions. This allows the results of calculations on conjugated polymers to be expressed in the language of solid state physics. The method is used to study the absorption spectrum of poly (para-phenylene vinylene) (PPV). INDO calculations on oligomers of PPV as well as long chains with periodic boundary conditions are reported. The long-chain calculations are used to assign the spectral features to transitions between bands, and these assignments are transferred to oligomers by examining how the calculated oligomer spectra evolve with chain length. The combination of periodic calculations and oligomer calculations enables a more complete description of the experimental absorption spectra of PPV and its derivatives.; The second part of this thesis also examines electron-hole symmetry breaking in carbon nanotubes. This study is facilitated by an analytical approach for including the effects of next-nearest-neighbor interactions in Hückel calculations on carbon nanotubes. These next-nearest-neighbor interactions break electron-hole symmetry. While this symmetry breaking does not alter the band gap, it does alter the thermoelectric power which is zero in a system that has electron-hole symmetry.; The third and final part of this thesis extends the dynamic dielectric model of Moore et al. Inclusion of dielectric effects is necessary to obtain reliable exciton binding energies for conjugated polymers. The original dynamic dielectric model used Hückel theory to describe the polarization induced in the dielectric medium. Here, this Hückel model is replaced by a more accurate Pariser-Parr-Pople model. The results indicate that the use of Hückel theory is valid, provided the parameters are chosen correctly. |
