A stepwise electron-transfer relay mimicking the primary charge separation in bacterial photosynthetic reaction center

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.