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-methyl enechlorin-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 in dicate a zinc-methylenechlorin, a free-base porphyrin, a zinc-porphyri n, 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 underg oes 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 cha rge-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, r espectively. The intermediate ion-pair state (ZC)(+)-(ZP)(-)-I is stab ilized enough so as to be clearly detected in DMF, while it is slightl y higher in energy than (1)(ZC) in THF, rendering its detection diffi cult.