Manipulation of the distance of light-induced electron transfer within a semi-rigid donor(amine)/acceptor(terpyridine) assembly via complexation of di-positive and tri-positive metal ions

Electric field-effect spectroscopy techniques (electronic absorption and emission) have been applied to the problem of light-induced electron transfer (ET) within a covalently linked organic donor/acceptor pair. The spectroscopy measurements report on the change in the assembly's dipole moment upon ET. This quantity (the dipole-moment change) represents a direct measure of the distance over which the electron is transferred. The experiments show that the true charge transfer distance is much less than the geometric separation distance between the nominal electron donor and acceptor centers. The experiments additionally show that the transfer distance can be changed: binding of both open-shell and closed-shell metal cations with the acceptor portion of the assembly causes the ET distance to increase, with a tri-positive ion inducing a greater increase than di-positive ions. Electronic structure calculations qualitatively reproduce the experimental observations. From the calculations, the lengthening of the transfer distance is an electrostatic effect that appears to be associated primarily with a change in the shape of the orbital occupied by the transferred electron in the redox product state.