Energy Transfer in Bichromophoric Molecules: The Effect of Symmetry and Donor/Acceptor Energy Gap

Abstract

The dependence of the rate of singlet excitation transfer on the donor−acceptor energy gap was investigated in bichromophoric spiranes with symmetry-forbidden zero-order electronic coupling. The fluorescence measurements were performed in a supersonic jet in order to avoid collisional and inhomogeneous line broadening. Fluorescence excitation spectra and single-vibronic-level emission spectra of the model chromophores cyclopentaphenanthrene and 1,8-dimethylnaphthalene and the bichromophores spirofluorenephenanthrene and spirofluorenenaphthalene are presented and analyzed. Although the transition moments of the linked chromophores are rigorously perpendicular and the exchange coupling between the v‘ = 0 states is computationally shown to be zero, all spiranes with energy gaps larger than ∼1000 cm-1 exhibited complete electronic energy transfer from all vibrational states of the electronically excited donor, including the undistorted v‘ = 0 state. This behavior is explained in terms of vibronic coupling bet...