| Abstract
| - Directly meso−meso linked porphyrin rings CZ4, CZ6, and CZ8 that respectively comprise four,six, and eight porphyrins have been synthesized in a stepwise manner from a 5,10-diaryl zinc(II) porphyrinbuilding block. Symmetric cyclic structures have been indicated by their very simple 1H NMR spectra thatexhibit only a single set of porphyrin and their absorption spectra that display a characteristic broad nonsplitSoret band around 460 nm. Energy minimized structures calculated at the B3LYP/6-31G* level indicatethat a dihedral angle between neighboring porphyrins decreases in order of CZ6> CZ8> CZ4, which isconsistent with the 1H NMR data. Photophysical properties of these molecules have been examined bythe steady-state absorption, fluorescence, fluorescence lifetime, fluorescence anisotropy decay, and transientabsorption measurements. Both the pump-power dependence on the femtosecond transient absorptionand the transient absorption anisotropy decay profiles are directly related with the excitation energy migrationprocesses within the porphyrin rings, where the exciton−exciton annihilation time and the polarizationanisotropy rise time are well described in terms of the Förster-type incoherent energy hopping model.Consequently, the excitation energy hopping rates have been estimated for CZ4 (119 ± 2 fs)-1, CZ6 (342± 59 fs)-1, and CZ8 (236 ± 31 fs)-1, which reflect the magnitude of the electronic coupling between theneighboring porphyrins. Overall, these porphyrin rings serve as a well-defined wheel-shaped light harvestingantenna model in light of very efficient excitation energy hopping along the ring.
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