Effects of Metal Ions on Photoinduced Electron Transfer in Zinc Porphyrin-Naphthalenediimide Linked Systems

Zinc porphyrin-naphthalenediimide (ZnP-NIm) dyads and zinc porphyrin-pyromellitdiimide-naphthalenediimide (ZnP-Im-NIm) triad have been employed to examine the effects of metal ions on photoinduced charge-separation (CS) and charge-recombination (CR) processes in the presence of metal ions (scandium triflate (Sc(OTf)₃) or lutetium triflate (Lu(OTf) ₃), both of which can bind with the radical anion of NIm). Formation of the charge-separated states in the absence and in the presence of Sc ³⁺ was confirmed by the appearance of absorption bands due to ZnP^.+ and NIm^.- in the absence of metal ions and of those due to ZnP^.+ and the NIm^.- Sc³⁺ complex in the presence of Sc³⁺ in the time-resolved transient absorption spectra of dyads and triad. The lifetimes of the charge-separated states in the presence of 1.0×10^-3M Sc³⁺ (14 μs for ZnP-NIm, 8.3 μs for ZnP-Im-NIm) are more than ten times longer than those in the absence of metal ions (1.3 μs for ZnP-NIm, 0.33 μs for ZnP-Im-NIm). In contrast, the rate constants of the CS step determined by the fluorescence lifetime measurements are the same, irrespective of the presence or absence of metal ions. This indicates that photoinduced electron transfer from ¹ZnP* to NIm in the presence of Sc³⁺ occurs without involvement of the metal ion to produce ZnP^.+-NIm^.-, followed by complexation with Sc³⁺ to afford the ZnP ^.+-NIm^.-/Sc³⁺ complex. The one-electron reduction potential (E_red) of the NIm moiety in the presence of a metal ion is shifted in a positive direction with increasing metal ion concentration, obeying the Nernst equation, whereas the one-electron oxidation potential of the ZnP moiety remains the same. The driving force dependence of the observed rate constants (k_ET) of CS and CR processes in the absence and in the presence of metal ions is well evaluated in terms of the Marcus theory of electron transfer. In the presence of metal ions, the driving force of the CS process is the same as that in the absence of metal ions, whereas the driving force of the CR process decreases with increasing metal ion concentration. The reorganization energy of the CR process also decreases with increasing metal ion concentration, when the CR rate constant becomes independent of the metal ion concentration.