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

Abstract

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 Mg2+ ions to the dyad and triad results in formation of longlived charge-separated (CS) states (ZnPc•+-PDI•-/Mg2+ and (ZnPc)2 •+-PDI•-/Mg2+) in which PDI•- 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•+ and PDI•-/Mg2+ 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 Mg2+ to PDI•-, whereas the one-electron oxidation potential of the ZnPc moiety remains the same. The binding of Mg2+ to PDI•- was confirmed by the ESR spectrum, which is different from that of PDI•- without Mg2+. The energy of the CS state (ZnPc•+-PDI•-/ 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.