title: 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 (H2 Por) to the flavin (Fl) moiety to produce the singlet charge-separated (CS) state (1) (H2 Por(.+) -Fl(.-) ), which decayed through back ET (BET) to form (3) [H2 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 Sc(3+) ions bind to the flavin moiety to form the Fl-Sc(3+) and Fl-(Sc(3+) )2 complexes with binding constants of K1 =2.2×10(5)  M(-1) and K2 =1.8×10(3)  M(-1) , respectively. Other metal ions, such as Y(3+) , Zn(2+) , and Mg(2+) , 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⋅⋅⋅Sc(3+) complexes afforded the triplet CS state ((3) [PdPor(.+) -Fl(.-) (Sc(3+) )2 ]), which exhibited a remarkably long lifetime of τ=110 ms (kBET =9.1 s(-1) ).
authors.affiliation: Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571 (Japan). kojima@chem.tsukuba.ac.jp.
authors.forename: Takahiko
authors.initials: T
authors.lastname: Kojima
authors.forename: Ryosuke
authors.initials: R
authors.lastname: Kobayashi
authors.forename: Tomoya
authors.initials: T
authors.lastname: Ishizuka
authors.forename: Shinya
authors.initials: S
authors.lastname: Yamakawa
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Kotani
authors.forename: Tatsuaki
authors.initials: T
authors.lastname: Nakanishi
authors.forename: Kei
authors.initials: K
authors.lastname: Ohkubo
authors.forename: Yoshihito
authors.initials: Y
authors.lastname: Shiota
authors.forename: Kazunari
authors.initials: K
authors.lastname: Yoshizawa
authors.forename: Shunichi
authors.initials: S
authors.lastname: Fukuzumi
mesh_terms: D056831:Coordination Complexes / Q000138:chemical synthesis / Q000737:chemistry; D018360:Crystallography, X-Ray; D004579:Electron Transport; D005415:Flavins / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D007700:Kinetics; D008665:Metalloporphyrins / Q000737:chemistry*; D008968:Molecular Conformation; D010165:Palladium / Q000737:chemistry; D010782:Photolysis; D012538:Scandium / Q000737:chemistry*; D013816:Thermodynamics; D015032:Zinc / Q000737:chemistry
chemical_list: D056831:Coordination Complexes; D005415:Flavins; D007477:Ions; D008665:Metalloporphyrins; D010165:Palladium; D015032:Zinc; D012538:Scandium
keywords: electron transfer; photochemistry; porphyrinoids; radicals; transition metals