Intramolecular electronic energy transfer between rigidly linked naphthalene and anthracene chromophores

The rate of intramolecular singlet-singlet electronic energy transfer (EET) between naphthalene and anthracene chromophores that are held in a well-defined orientation and separation by a rigid bis(norbornyl)bicyclo-[2.2.0]hexane bridge, six σ-bonds in length, is measured to be 2.9 × 10¹⁰ s^-1 (n-hexane). EET between the lowest-energy naphthalene ¹L_b and anthracene ¹L_a states (via a direct Coulombic mechanism) is forbidden in this molecule because of the orthogonality of donor and acceptor transition dipoles. EET involving a Coulombic interaction between the naphthalene ¹L_b and the nonfluorescent anthracene ¹L_b electronic states is also considered. Although Coulombic EET is vibronically allowed, to account for the high measured rate, it is proposed that relayed mechanisms, involving the bridge, play a dominant role. The significance of these results to EET in aromatic polymers and other multichromophoric systems is discussed.