Sunday, 24 March 2013

Conjugated Organic Polymers

There is another recent paper by us: "Electronically Excited States in Poly(p-phenylenevinylene): Vertical Excitations and Torsional Potentials from High-Level Ab Initio Calculations." We looked at Poly(p-phenylenevinylene) (PPV), which is a prototypical conjugated organic polymer to understand the behaviour of excitation energies with respect to different chain lengths, to analyze the structure of the exciton, and to evaluate the effect of torsions.
In the following picture you can see our excited state analysis methods (as described here) when applied to PPV. The model to understand this structure is to think of a hydrogen atom (or a "quasi-particle") in a one-dimensional box. First there are different translation states for the ground state of the hydrogen atom. These should be differentiated by a different number of nodal planes (shown in the first three cases). Then a different state of the "hydrogen atom" comes into play and there is a somewhat different shape. Unfortunately this method does not show any phases or signs but I still think one can recognize the nodal structure.

If there is a torsion in the center of the molecule, the situation changes. Now there are two equivalent, effectively decoupled, units. Therefore the excited states come in pairs, corresponding to the "+" and "-" linear combinations of the fragment states.

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