Limitations of the Förster Description of Singlet Exciton Migration: The Illustrative Example of Energy Transfer to Ketonic Defects in Ladder‐type Poly( para‐ phenylenes)

Energy‐transfer processes in phenylene‐based materials are studied via two different approaches: i) the original Förster model, which relies on a simple point‐dipole approximation; and ii) an improved Förster model accounting for an atomistic description of the interacting chromophores. Here, to illustrate the impact of excited‐state localization and the failure of the point‐dipole approximation, we consider a simple model system which consists of two interacting chains, the first a pristine ladder‐type poly( para ‐phenylene) (LPPP) chain and the second an LPPP‐chain bearing a ketonic defect. The latter chain displays both localized electronic excitations close to the ketonic sites as well as excited states that are delocalized over the whole conjugated chain. Singlet hopping rates have been computed for energy transfer pathways involving these two types of excitations. A generalized Förster critical distance is introduced to account for the errors associated with averaging out the actual molecular structures in the original Förster model.