Sequential Electron Transfer Leading to Long-Lived Charge Separated State in a Porphyrin–Oxochlorin–Pyromellitdimide Triad

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Abstract
The synthesis and excited-state dynamics are described for fixed-distance porphyrin–oxochlorin–pyromellitdimide triads (P–C–Im) and related reference compounds. In zinc-oxochlorin–pyromellitdimide (ZnP–Im), the 1(ZnC)* is quenched by the attached Im through intramolecular charge separation (CS) in benzene, THF, and DMF, while the 1(H2C)* in the corresponding free base is not significantly quenched by the Im even in polar DMF. In the steady-state fluorescence emission spectra, only the emission from the 1(C)* is commonly observed, indicating an efficient intramolecular singlet–singlet excitation energy transfer from P to C. Of these, the fluorescence intensities of the 1(H2C)* in ZnP–H2C and ZnP–H2C–Im are significantly reduced in polar DMF solution and this is attributed to the intramolecular CS that gives (ZnP)+–(H2C)−–Im and (ZnP)+–(H2C)−, respectively. The (ZnP)+–(H2C)−–Im ion pair is clearly shown, by picosecond absorption spectroscopy, to be converted into a secondary, longer-lived charge separated state (ZnP)+–H2C–(Im)− via charge-shift reaction in competition with wasteful charge recombination to the ground state. The (ZnP)+–H2C–(Im)− state is formed in 0.09 quantum yield from ZnP–1(H2C)*–Im and has a lifetime of 0.24 μs in DMF.