INDO within periodic boundary conditions: A new tool for modeling the photophysics of conjugated polymers

Much of our current theoretical understanding of the electronic structure of conjugated polymers is based on two-band systems, such as the pi-electron model of polyacetylene. But PPV and many other systems exhibit a number of bands, and this raises new and interesting questions that are beginning to be addressed. Such studies are complicated by the large number of model parameters, and the sensitivity of the predictions to the values used. While INDO provides a systematic and tested approach for deriving these parameters from the chemical structure, previous tools could be applied only to oligomers, making it difficult to put the results in the context of other well-studied models. Here, we report INDO calculations on PPV and other polymers, performed within periodic boundary conditions. This allows us to derive the band-structure, Wannier functions, and exciton wave functions. The results obtained for PPV will be discussed within the context of the current debate on the assignment of the second peak in the absorption spectrum.