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

Photoexcitation of a zinc phthalocyanine−perylenediimide (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 Mg 2+ions to the dyad and triad results in formation of long-lived charge-separated (CS) states (ZnPc •+−PDI •−/Mg 2+and (ZnPc) 2•+−PDI •−/Mg 2+) in which PDI •−forms a complex with Mg 2+. Formation of the CS states in the presence of Mg 2+was confirmed by appearance of the absorption bands due to ZnPc •+and PDI •−/Mg 2+complex in the time-resolved transient absorption spectra of the dyad and triad. The one-electron reduction potential ( Ered) of the PDI moiety in the presence of a metal ion is shifted to a positive direction due to the binding of Mg 2+to PDI •−, whereas the one-electron oxidation potential of the ZnPc moiety remains the same. The binding of Mg 2+to PDI •−was confirmed by the ESR spectrum, which is different from that of PDI •−without Mg 2+. The energy of the CS state (ZnPc •+−PDI •−/Mg 2+) 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 Mg 2+instead of the triplet excited state of the PDI moiety. [ABSTRACT FROM AUTHOR]