Model systems for rod‐like polyheteroarylethynylenes

Abstract

AbstractB3LYP density functional theory (DFT) and MM3 molecular mechanics/dynamics are used to model the flexibility of the building blocks of poly(phenyleneethynylene)s, which are rod‐like segmented polymers usually assumed to be strongly rigid, with well‐defined, highly fluorescent fragments and beneficial material properties like electroluminescence and large nonlinear optical hyperpolarizabilities. Whereas the DFT‐calculated torsional barriers of the building block molecules 2,2′‐bipyridine and some tolane derivatives give insight into weak intramolecular effects in these molecules, the much less detailed MM3 barriers are nevertheless of sufficient quality to use MM3 for polymer modeling. From MM3 molecular dynamics gas phase and water box simulations at different temperatures, insight into the time‐dependent behavior of the model compounds was obtained as well as the conditions at which cis–trans isomerizations occur. It turns out that these formally rigid polymers are quite flexible and do not at all possess strictly coplanar aryl rings over time, significantly decreasing the effective conjugation length. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 84: 86–98, 2001