Directly meso-meso linked porphyrin rings:: Synthesis, characterization, and efficient excitation energy hopping

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) porphyrin building block. Symmetric cyclic structures have been indicated by their very simple H-1 NMR spectra that exhibit only a single set of porphyrin and their absorption spectra that display a characteristic broad nonsplit Soret band around 460 nm. Energy minimized structures calculated at the B3LYP/6-31G* level indicate that a dihedral angle between neighboring porphyrins decreases in order of CZ6 > CZ8 > CZ4, which is consistent with the 1H NMR data. Photophysical properties of these molecules have been examined by the steady-state absorption, fluorescence, fluorescence lifetime, fluorescence anisotropy decay, and transient absorption measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly related with the excitation energy migration processes within the porphyrin rings, where the exciton-exciton annihilation time and the polarization anisotropy rise time are well described in terms of the Forster-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 the neighboring porphyrins. Overall, these porphyrin rings serve as a well-defined wheel-shaped light harvesting antenna model in light of very efficient excitation energy hopping along the ring.