Tuesday, 22 December 2015

Conjugated Polymers

If you compute excited states in conjugated polymers, all you see is orbitals. Lots of orbitals, π and π* orbitals. So how do you describe the excited states? Are they all just ππ* states? Well, not really ... but the interesting part lies in correlation effects and quantum superpositions, things you don't see from the orbitals directly.

A while back I had the chance to play around with my wavefunction analysis tools on the polypara-phenylene-vinylene molecule in a first paper. It was fun, but it did not really satisfy me. That's why I convinced Steffi, a student in our group, to do a follow-up. Now, two years later, we finally submitted the material to PCCP where it can be found as our new article: "Excitons in poly(para phenylene vinylene): A quantum-chemical perspective based on high-level ab initio calculations".

The part, which I find most appealing is shown below. The figure shows the electron-hole correlation plots for the singlet excited states of the (PV)7P oligomer. Here, the hole position is plotted along the x-axis and the electron position along the y-axis and the probability that the electron and hole reside at a specified position is coded in grey scale. Points along the main diagonal (going from lower left to upper right) are local excitations while off-diagonal contributions code for charge transfer. In these plots one can discern two types of features. Nodal planes perpendicular to the main diagonal indicate higher translational quasi-momentum, and nodal planes along the main diagonal correspond to different hydrogenic states of the exciton. The interesting thing is that the quasi-particle picture emerges directly from standard quantum chemical calculations. All you have to do, is look.

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