Excited state electron transfer in systems with a well-defined geometry. Progress report, May 1, 1979-April 30, 1980

Abstract

To determine what are the important criteria governing efficient light-induced electron transfer, we have been studying cyclophanes formed by joining two chlorophyll macrocycles together with two covalent linkages. The properties of the cofacial dimer can be altered by adding one or two magnesium atoms. When a cyclophane containing only one metal atom is excited electron transfer takes place in under ten picoseconds. Evidence for the formation of a state having a great deal of charge transfer interaction or perhaps even a radical pair is found in the time resolved difference spectrum which shows a contribution due to a cation of the magnesium containing macrocycle and an anion of the metal free portion of the molecule. The lifetime of the charge transfer state is 2.4 nsec. This would indicate that we have stabilized the charge separation for a relatively long time. The metal free dimer has an excited state product which does not appear to be a triplet state. The lifetime of this product is very long. The identity of the product has not as yet been obtained. It too could be a charge transfer state. If this is true, we have succeeded in stabilizing the charge separation for an extremelymore » long time. Finally, we observed that the photophysical properties of the chlorophyll macrocycle are modified by reduction of the vinyl group on ring I.« less