Intermediate neglect of differential overlap excited state calculations in periodic boundary conditions: The absorption spectrum of poly(para-phenylene vinylene)

Abstract

Much of our current theoretical understanding of the electronic structure of conjugated polymers is based on two-band systems, such as the π-electron model of polyacetylene. But poly(para-phenylene vinylene) (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 the intermediate neglect of differential overlap (INDO) method 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 oligomers of PPV as well as long chains with periodic boundary conditions. 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 effects of various electron–hole symmetry breaking mechanisms on the intensity of peak II (at 3.7 eV) relative to peak III (at 4.7 eV) in the optical absorption spectrum are examined. INDO theory on planar unsubstituted PPV breaks electron–hole symmetry due to inclusion of next-nearest neighbor transfer integrals, giving peak II about 4% of the intensity observed for peak III in the long-chain limit. Alkoxy substitution at the 2 and 5 positions of the phenyl ring is modeled using hydroxy groups (OH-PPV). (This approach is confirmed by replacing the hydroxy substituents with methoxy substituents.) For a planar OH-PPV structure, peak II has 8% of the intensity of peak III. AM1 calculations indicate that in OH-PPV, the vinylene rotates out of the plane of the phenyl rings by 20°, and this raises the intensity of peak II to 15% that of peak III. Finally, finite-chain effects cause oligomers of OH-PPV with between 4 and 8 phenylene units to have a peak II intensity that is 25% that of peak III. Oligomers exhibit additional transitions in the vicinity of peak II, but these have a strong dependence on the length of the oligomer and merge with the lowest-energy peak in the long-chain limit.