ELECTRONIC-STRUCTURE OF PYRIDINE-BASED POLYMERS

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 spat ial and charge-conjugation symmetry (CCS), and the addition of nonbond ing (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 polaron formation are cente red on the vinylene linkages in PPyV-based systems and on the interrin g bonds in the PPy-based systems. We discuss the electronic structure at the PM3 level applying configuration interaction between singly exc ited states (SCI), and we demonstrate that the lowest-lying (n-->pi) states of the ideal polymer chain are well above the lowest (pi-->pi) states, leading to strong fluorescence in these systems. Nonplanarity , however, leads to substantial mixing of the (pi-->pi) and (n-->pi*) manifolds, thereby altering this conclusion. We calculate absorption spectra for neutral, singly charged (polaron), doubly charged (bipolar on), and triplet-state oligomers using the intermediate neglect of dif ferential overlap/single-excitation configuration interaction (INDO/SC I) technique. For PPyV, comparison of oligomers with differing spatial symmetry allows the isolation of the effects of CCS breaking. All cal culated spectra are in good agreement with experimental results and in dicate that the symmetry breaking due to the nitrogen heteroatom is we ak. In particular, the polaron induces a two-peak in-gap feature into the absorption spectrum and the bipolaron a single-peak feature, as is seen in the analogous all-hydrocarbon polymers. (C) 1996 American Ins titute of Physics.