Ultrafast energy transfer within cyclic self-assembled chlorophyll tetramers

We have prepared a zinc chlorophyll (ZC) derivative that self-assembles into a cyclic tetramer as evidenced by small-angle X-ray scattering studies in solution using a synchrotron source. This cyclic tetramer exhibits intramolecular energy transfer rates determined from singlet-singlet annihilation and transient absorption anisotropy studies that are comparable to those observed previously only for covalent ring structures. The larger transition dipole moment for the lowest energy electronic transition of ZC compared to that of metalloporphyrins increases the rate of Forster (through-space) energy transfer between the chlorophylls. Our synthetic and self-assembly strategy makes it possible to design larger monodisperse chlorophyll rings for energy transfer in artificial photosynthetic systems.