1,2-phenylene-bridged diporphyrin linked with porphyrin monomer and pyromellitimide as a model for a photosynthetic reaction center: Synthesis and photoinduced charge separation

Article abstract of DOI:10.1021/ja00064a021

The synthesis and excited-state dynamics are described for fixed-distance zinc diporphyrin-zinc porphyrin-pyromellitimide molecules 1-3 (D-M-Im) and zinc porphyrin-pyromellitimide molecule 7 (M-Im). In molecules 1-3, D and M-Im moieties are bridged by aromatic spacers such as 4,4′-biphenylylene-, 1,4-phenylene-, and methylenebis(1,4-phenylene) groups, respectively. The rates of charge separation (CS) and charge recombination (CR), k_cs and k_CR, of 7 were determined in C₆H₆, THF, and DMF. k_cs in 7 is essentially solvent polarity independent, while k_CR increases with solvent polarity. A small k_CR in C₆H₆ has been interpreted in terms of the small electron-transfer probability, which arises from the small solvent reorganization energy and large reaction exothermicity in the inverted region. In molecules 1-3, ¹(M)* is competitively quenched by D through the intramolecular singlet-singlet energy transfer to give ¹(D)*-M-Im and by Im through intramolecular CS to give D-(M)⁺-(Im)^-. The ¹(D)*-M-Im state decays to the ground state with a lifetime to that of the reference ¹(D)* and does not undergo any intramolecular electron-transfer reactions. On the other hand, from the initially formed ion pair (IP) state, D-(M)⁺-(Im)^-, a secondary IP state, (D)⁺-M-(Im)^-, is formed by hole transfer from (M)⁺ to D in THF or DMF. Such hole transfer does not occur in less polar C₆H₆. The lifetimes of the secondary IP states formed from 1, 2, and 3, which range from 0.25 to 23 μs, depend upon both the spacer connecting the D and M and the polarity of solvent. In contrast to the case of (M)⁺-(Im)^-, the lifetime of (D)⁺-M-(Im)^- increases with increasing solvent polarity. Marked differences in solvent polarity effects observed on the lifetime of the (M)⁺-(Im)^- and (D)⁺-M-(Im)^- states may have important practical implications for the design of artificial photosynthetic models.