Binding of Scandium Ions to Metalloporphyrin–Flavin Complexes for Long‐Lived Charge Separation

Abstract

A porphyrin–flavin‐linked dyad and its zinc and palladium complexes (MPorFl: 2M, M=2 H, Zn, and Pd) were newly synthesized and the X‐ray crystal structure of 2Pd was determined. The photodynamics of 2M were examined by femto‐ and nanosecond laser flash photolysis measurements. Photoinduced electron transfer (ET) in 2H₂ occurred from the singlet excited state of the porphyrin moiety (H₂Por) to the flavin (Fl) moiety to produce the singlet charge‐separated (CS) state ¹(H₂Por^.+Fl^.−), which decayed through back ET (BET) to form ³[H₂Por]*Fl with rate constants of 1.2×10¹⁰ and 1.2×10⁹ s⁻¹, respectively. Similarly, photoinduced ET in 2Pd afforded the singlet CS state, which decayed through BET to form ³[PdPor]*Fl with rate constants of 2.1×10¹¹ and 6.0×10¹⁰ s⁻¹, respectively. The rate constant of photoinduced ET and BET of 2M were related to the ET and BET driving forces by using the Marcus theory of ET. One and two Sc³⁺ ions bind to the flavin moiety to form the FlSc³⁺ and Fl(Sc³⁺)₂ complexes with binding constants of K₁=2.2×10⁵ M⁻¹ and K₂=1.8×10³ M⁻¹, respectively. Other metal ions, such as Y³⁺, Zn²⁺, and Mg²⁺, form only 1:1 complexes with flavin. In contrast to 2M and the 1:1 complexes with metal ions, which afforded the short‐lived singlet CS state, photoinduced ET in 2Pd⋅⋅⋅Sc³⁺ complexes afforded the triplet CS state (³[PdPor^.+Fl^.−(Sc³⁺)₂]), which exhibited a remarkably long lifetime of τ=110 ms (k_BET=9.1 s⁻¹).