Fluorescence measurements on the same set of individual crystals show a decrease in the life-time for the donor as a function of the amount of the acceptor (1.33 ns to 0.06 ns, as one would expect for a FRET-based mechanism (34-36) (Figure ). If energy transfer would solely occur via re-absorption events, the lifetime of the donor would not be affected . The lifetime of the acceptor is changing for the crystals with the highest concentration of this dye, which is in agreement with different stacking interactions.