Experimental verification of the through-bond mechanisms of electron transfer in bridged donor-acceptor complexes

Two rigid bischromophoric systems featuring identical donor and acceptor mo ieties have been studied in order to directly determine the relative import ance of through-space and through-bond mechanisms for electron-transfer rea ctions. The two molecular systems studied have the unique feature that the through-bond distance between any pair of atoms in the two molecules is hel d constant while the spatial distance between the donor and acceptor is cha nged. Time-resolved laser measurements reveal that back-electron-transfer d ynamics following photoexcitation of the ground-state charge-transfer absor ption band are the same in the two systems. The results provide direct evid ence for the through-bond mechanism of electron transfer in bridged organic donor-acceptor systems.