Modified windmill porphyrin arrays: Coupled light-harvesting and charge separation, conformational relaxation in the S-1 state, and S-2-S-2 energy transfer

The architecture of windmill hexameric zinc(II) -porphyrin array 1 is attra ctive as a light-harvesting functional unit in view of its three-dimensiona lly extended geometry that is favorable for a large cross-section of incide nt light as well as for a suitable energy gradient from the peripheral porp hyrins to the meso-meso-linked diporphyrin core. Three core-modified windmi ll porphyrin arrays 2-4 were prepared for the purpose of enhancing the intr amolecular energy-transfer rate and coupling these arrays with a charge-sep aration functional unit. Bisphenylethynylation at the meso and meso positio ns of the diporphyrin core indeed resulted in a remarkable enhancement in t he intramolecular S-1-S-1 energy transfer in 2 with tau =2 similar to 3ps, as revealed by femtosecond time-resolved transient absorption spectroscopy. The fluorescence lifetime of the S-2 state of the peripheral porphyrin ene rgy donor determined by the fluorescence up-conversion method was 68 fs, an d thus considerably shorter than that of the reference monomer (150 fs), su ggesting the presence of the intramolecular energy-transfer channel in the S-2 state manifold. Such a rapid energy transfer can be understood in terms of large Coulombic interactions associated with the strong Soret transitio ns of the donor and acceptor. Picosecond time-resolved fluorescence spectra and transient absorption spectra revealed conformational relaxation of the S-1 state of the diporphyrin core with tau =25ps. Upon photoexcitation of models 3 and 4, which bear a naphthalenetetracarboxylic diimide or a meso-n itrated free-base porphyrin attached to the modified diporphyrin core as an electron acceptor, a series of photochemical processes proceeded, such as the collection of the excitation energy at the diporphyrin core, the electr on transfer from the S-1 state of the diporphyrin to the electron acceptor, and the electron transfer from the peripheral porphyrins to the diporphyri n cation radical, which are coupled to provide a fully charge-separated sta te such as that in the natural photosynthetic reaction center. The overall quantum yield for the full charge separation is better in 4 than in 3 owing to the slower charge recombination associated with smaller reorganization energy of the porphyrin acceptor.