Energy transfer in bichromophoric molecules: The effect of symmetry and donor/acceptor energy gap

The dependence of the rate of singlet excitation transfer on the donor-acce ptor energy gap was investigated in bichromophoric spiranes with symmetry-f orbidden zero-order electronic coupling. The fluorescence measurements were performed in a supersonic jet in order to avoid collisional and inhomogene ous line broadening. Fluorescence excitation spectra and single-vibronic-le vel emission spectra of the model chromophores cyclopentaphenanthrene and 1 ,8-dimethylnaphthalene and the bichromophores spirofluorenephenanthrene and spirofluorenenaphthalene are presented and analyzed. Although the transiti on 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 similar to 1000 cm(-1) exhi bited complete electronic energy transfer from all vibrational states of th e electronically excited donor, including the undistorted v' = 0 state. Thi s behavior is explained in terms of vibronic coupling between the sparse st ates of the donor and the dense manifold (pseudocontinuum) of the acceptor states. The electronic energy transfer was sufficiently fast to result in m easurable lifetime broadening of the donor absorption lines, from which the k(EET) was estimated. The results demonstrate that the zero-order picture overestimates the degree of the molecular orbital symmetry control over ele ctronic energy transfer and charge-transfer rates and that at sufficiently high driving forces the vibronically mediated "symmetry-forbiddcn" electron ic energy transfer can be very rapid (similar to 1 x 10(12) s(-1)).