Self-assembly of a chlorophyll-based cyclic trimer: Structure and intramolecular energy transfer

Small- and wide-angle X-ray scattering (SAXS/WAXS) and DOSY 1H NMR reveal that a zinc methyl pyrochlorophyllide a (Chl) derivative with a 3-pyridyl ligand at the Chl 20-position self-assembles in toluene solution into a cyclic trimer having a radius of 8.2 ± 0.1 Å. Femtosecond transient absorption spectroscopy shows that singlet–singlet annihilation occurs within the cyclic trimer as a result of energy transfer between the Chls that occurs with an (11 ± 1 ps)−1 rate constant, which agrees with the predictions of Förster theory, even though the Chl–Chl distances are comparable to the length of the Chl transition dipole.