SEQUENTIAL ELECTRON-TRANSFER LEADING TO LONG-LIVED CHARGE SEPARATED STATE IN A PORPHYRIN OXOCHLORIN PYROMELLITDIMIDE TRIAD

The synthesis and excited-state dynamics are described for fixed-distance porphyrin-oxo-chlorin-pyromellitdimide triads (P-C-Im) and related reference compounds. In zinc-oxochlorin-pyromellit-dimide (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 life time of 0.24 mu s in DMF.