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

摘要

A porphyrin-flavin-linked dyad and its zinc and palladium complexes (MPor-Fl:2-M, M = 2H, 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-H-2 occurred from the singlet excited state of the porphyrin moiety (H(2)Por) to the flavin (Fl) moiety to produce the singlet charge-separated (CS) state (1)(H(2)Por(center dot+)-Fl(center dot-)), which decayed through back ET (BET) to form (3)[H(2)Por]*-Fl with rate constants of 1.2-10(10) and 1.2-10(9) 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 2.1-10(11) and 6.0-10(10) 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 K-1 = 2.2-10(5)m(-1) and K-2 = 1.8-10(3)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 center dot center dot center dot Sc3+ complexes afforded the triplet CS state (3[PdPorC(center dot+)-Fl center dot--(Sc3+)(2)]), which exhibited a remarkably long lifetime of tau = 110 ms (k(BET) = 9.1 s(-1)).