Electronic structure of pyridine-based polymers

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Abstract
We present the results of semiemiprical quantum chemical calculations on oligomers of poly(p‐pyridyl vinylene) (PPyV) and poly(p‐pyridine) (PPy). The presence of a nitrogen heteroatom in the conjugated backbone of these polymers presents a potentially severe breaking of both spatial and charge‐conjugation symmetry (CCS), and the addition of nonbonding (n) orbitals has potentially major effects on the photophysics of these systems. Geometries are optimized at the PM3 Hartree–Fock level for neutral, singly charged and doubly charged oligomers. We find that the geometric distortions associated with polaronformation are centered on the vinylene linkages in PPyV‐based systems and on the interring bonds in the PPy‐based systems. We discuss the electronic structure at the PM3 level applying configuration interaction between singly excited states (SCI), and we demonstrate that the lowest‐lying (n→π*) states of the ideal polymer chain are well above the lowest (π→π*) states, leading to strong fluorescence in these systems. Nonplanarity, however, leads to substantial mixing of the (π→π*) and (n→π*) manifolds, thereby altering this conclusion. We calculate absorption spectra for neutral, singly charged(polaron), doubly charged(bipolaron), and triplet‐state oligomers using the intermediate neglect of differential overlap/single‐excitation configuration interaction (INDO/SCI) technique. For PPyV, comparison of oligomers with differing spatial symmetry allows the isolation of the effects of CCS breaking. All calculated spectra are in good agreement with experimental results and indicate that the symmetry breaking due to the nitrogen heteroatom is weak. In particular, the polaron induces a two‐peak in‐gap feature into the absorptionspectrum and the bipolaron a single‐peak feature, as is seen in the analogous all‐hydrocarbon polymers.