Control of Photoinduced Electron Transfer in Zinc Phthalocyanine-Perylenediimide Dyad and Triad by the Magnesium Ion

Photoexcitation of a zinc phthalocyanine-peryienediimide (ZnPc-PDI) dyad and a bis(zinc phthalocyanine)perylenediimide [(ZnPc)(2)-PDI] triad results in formation of the triplet excited state of the PDI moiety without the fluorescence emission, whereas addition of Mg2+ ions to the dyad and triad results in formation of long-lived charge-separated (CS) states (ZnPc center dot+-PDI center dot-/Mg2+ and (ZnPc)(2)(center dot+)-PDI center dot-/Mg2+ in which PDI center dot- forms a complex with Mg2+. Formation of the CS states in the presence of Mg2+ was confirmed by appearance of the absorption bands due to ZnPc center dot+ and PDV center dot-/Mg2+ complex in the time-resolved transient absorption spectra of the dyad and triad. The one-electron reduction potential (E-red) of the PDI moiety in the presence of a metal ion is shifted to a positive direction due to the binding of Mg2+ to PDI center dot-, whereas the one-electron oxidation potential of the ZnPc moiety remains the same. The binding of Mg2+ to PDI center dot- was confirmed by the ESR spectrum, which is different from that of PDI center dot- without Mg2+. The energy of the CS state (ZnPc center dot+-PDI center dot-/Mg2+) is determined to be 0.79 eV, which becomes lower that of the triplet excited state (ZnPc-3PDI*; 1.07 eV). This is the reason why the long-lived CS states were attained in the presence of Mg2+ instead of the triplet excited state of the PDI moiety.