Wednesday, 6 January 2016

More Conjugated Polymers

It looks like the exciton models we developed over the last years are actually reaching a level where "applied" people are becoming interested in them. I was very happy to have a joint project with S. Kraner from Dresden who wanted to examine universal properties of excitons in conjugated organic polymers. This collaboration lead to a new paper "Exciton size and binding energy limitations in one-dimensional organic materials" that just appeared in JCP.

The results showed that there is a quite universal relationship between molecular size and exciton size. The important point is, however, that the exciton size does not grow indefinitly with the molecular size. If the size of the π-system reaches about 40 Ang, no further enhancement of the exciton size occurs and a bound exciton remains.

Another point (and a reason why we had the pleasure to revise the manuscript quite a few times) is the fact that the results are strongly dependent on the exchange correlation functional. While the general trend of an exciton size limitation is present whenever you have some amount of Hartree-Fock exchange in the functional, the precise value differs, as discussed also in another recent paper by us. The reason is a correlation effect and ensuing dynamic charge transfer.

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