Synthesis and Intramolecular Charge Separation of Fixed-Distance Triads Consisting of Zinc Porphyrin, Metal-Free Porphyrin, and Electron-Accepting Diimide Moiety

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Abstract
Synthesis of fixed-distance triads consisting of zinc porphyrin (ZnP), metal-free porphyrin (H2P), and pyromellitimide (PIm) or 1,4 : 5,8-naphthalenetetracarboximide (NIm) is described. The ZnP and H2P moieties are bridged by aromatic spacers such as 1,4-phenylene, 4,4′-biphenylylene, methylenebis(1,4-phenylene), and bicyclo[2.2.2]octane-1,4-diylbis(1,4-phenylene) groups. The steady-state fluorescence spectra indicated the occurrence of intramolecular singlet-singlet energy transfer form the ZnP to the H2P. Formation of long-lived charge-separated states (ZnP)+–H2P–(PIm)− and (ZnP)+–H2P–(NIm)− with lifetimes of 0.14—80 μs in THF was observed by nanosecond to microsecond transient absorption spectroscopy. The lifetimes of the (ZnP)+–H2P–(Im)− states depend on the distances between the charged sites as well as the energy gaps between the ion pair and the ground state.