Electronic Structure of Pyridine-Based Polymers.
OHIO STATE UNIV COLUMBUS DEPT OF PHYSICS
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We present the results of semiempirical quantum chemical calculations on oligomers of poly p-pyridyl vinylene PPyV and poly p-pyridine PPy. The presence op a nitrogen heteroatom in the conjugated backbone of these polymers presents a 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 polaron formation 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 approaches pi states of the ideal polymer chain are well above the lowest states, predicting strong fluorescence in these systems. Deviations from ideal geometry, however, can lead to substantial mixing of the manifolds, thereby altering this conclusion. We calculate absorption spectra for neutral, singly charged polaron doubly charged bi-polaron and triplet state oligomers using the INDOSCI 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.
- Polymer Chemistry
- Quantum Theory and Relativity