Light-induced giant dipoles in simple model compounds for photosynthesis

The primary steps in photosynthesis involve very rapid (sub-nanosecond) electron transfer between molecular entities that are rigidly embedded within a lipid membrane and separated from each other by well-defined distances on the order of 10 Å. In an attempt to simulate such systems we have studied photon-induced electron transfer within specially synthesized molecular assemblies in which a donor moiety is separated from an electron acceptor by a rigid, saturated hydrocarbon framework of variable length, from 5 to 13 Å. We find charge separation to occur on a sub-nanosecond timescale with close to unit quantum efficiency in all cases. The lifetimes of the resulting charge-transfer states, with dipole moments approaching 70 debye units, can extend to several hundred nanoseconds. Non-conjugated hydrocarbon bridges may be important in determining the rate and direction of electron transfer in photo-excited natural or artificial molecular systems.