Phenyl-disubstituted polyacetylene (PDPA) is an organic semiconductor which has been studied during recent years for its efficient photoluminescence. In contrast, the molecular geometry, providing the basis for the electronic and optical properties has been hardly investigated. In this paper, we apply a density-functionaltheory based molecular-dynamics approach to reveal the molecular structure of PDPA in detail. We find that oligomers of this material are limited in length, being stable only up to eight repeat units, while the polymer is energetically unfavorable. These facts, which are in excellent agreement with experimental findings, are explained through a detailed analysis of the bond lengths. A consequence of the latter is the appearance of pronounced torsion angles of the phenyl rings with respect to the plane of the polyene backbone, ranging from 55 degrees up to 95 degrees. We point out that such large torsion angles do not destroy the conjugation of the pi electrons from the backbone to the side phenyl rings, as is evident from the electronic charge density.
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