Electron transfer in flexible molecules and molecular ions

Electron transfer (ET) occurring in or after electronic excitation in flexibly bonded donor-acceptor (D-A) molecules are accompanied by structural relaxation. In singly bonded D-A molecules often the "twisted" intramolecular charge transfer (TICT) structures are formed. Evidence for the TICT hypothesis is presented. In some compounds behaviour deviating from the original TICT model is observed; strict applicability of that hypothesis seems to be limited to rather simple D-A molecules. Most deactivation processes involve back ET. Kinetic data on the fluorescence rise and decay seem not always to correspond to the real population kinetics in the case of forbidden transitions in emission; a hot luminescence emitted along the reaction path may distort the curves at short delay times. Contrary to neutral D-A molecules, photophysics of ET in the molecular ions, D-A+, are much less understood: most often, unexpectedly fast radiationless processes (back ET ?) quench the emission.