Triplet Excitation Energy Transfer through Fluorene π Stack

Abstract

Recently synthesized π-stacked polyfluorenes with cofacially oriented fluorene moieties are promising materials for molecular electronics. In the paper, I consider triplet excitation energy transfer (TEET) through the polyfluorene π stack. A key factor that controls the TEET rate is the electronic coupling of triplet excited states localized on neighboring moieties. Using Hartree−Fock configuration interaction singles calculations (HF/CIS-6-31G*), I study the conformational dependence of the TEET transfer integral and estimate the effective coupling between stacked fluorenes. To include the effects of conformational motion, the coupling is computed for 400 structures extracted from a molecular dynamics trajectory of the fluorene dimer. Structural fluctuations of the π stack are shown to cause large variations of the transfer integral. On the basis of the computed values of the TEET reorganization energy and the effective coupling, I estimate the absolute rate for hopping between neighboring fluorenes in the π stack. Conformational gating appears to play an important role in the energy transfer process. The triplet excitons in the π stack are found to be localized on single fluorene moieties.