A Stepwise Electron-Transfer Relay Mimicking the Primary Charge Separation in Bacterial Photosynthetic Reaction Center
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Abstract
The synthesis and excited-state dynamics are described for zinc-methylenechlorin−porphyrin−pyromellitimide triads, ZC−HP−I and ZC−ZP−I, and related dyads ZC−HP, ZC−ZP, HP−I, and ZP−I, where ZC, HP, ZP, and I indicate a zinc-methylenechlorin, a free-base porphyrin, a zinc-porphyrin, and pyromellitimide, respectively. In the steady-state fluorescence spectra of ZC−HP, ZC−HP−I, ZC−ZP, and ZC−ZP−I, only the emission from ZC is commonly observed, indicating efficient intramolecular singlet−singlet excitation energy transfer from HP or ZP to ZC. ZC−HP−I undergoes a stepwise electron-transfer relay: 1(ZC)*−HP−1 → (ZC)+−(HP)-−I → (ZC)+−HP−(I)- with overall quantum yields of 0.70 and 0.07 in THF and DMF, respectively. In ZC−ZP−I, a pre-formed equilibrium between 1(ZC)*−ZP−I and (ZC)+−(ZP)-−I is followed by a rapid charge-shift reaction to provide a secondary ion-pair state (ZC)+−ZP−(I)- with the overall quantum yield of 0.90 and 0.47 in THF and DMF, respectively. The intermediate ion-pair state (ZC)+−(ZP)-−I is stabilized enough so as to be clearly detected in DMF, while it is slightly higher in energy than 1(ZC)* in THF, rendering its detection difficult.
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