1,2-PHENYLENE-BRIDGED DIPORPHYRIN LINKED WITH PORPHYRIN MONOMER AND PYROMELLITIMIDE AS A MODEL FOR A PHOTOSYNTHETIC REACTION CENTER - SYNTHESIS AND PHOTOINDUCED CHARGE SEPARATION

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 mus, 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.