Limitations of the Forster 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 Forster model, which relies on a simple point-dipole approximation: and ii) an improved Forster model accounting foran automatic description of the interacting chromophores. Here, to illustrate the impact of the excited-state localization and the failure of the point-dipole approximation, we consider a simple model system which consists of two interacting chains, th 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 kenotic 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 Forster critical distance is introduced to account for the errors associated with averaging out the actual molecular structures in the original Forster model.