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) ).

Thermodynamics and Photodynamics of a Monoprotonated Porphyrin Directly Stabilized by Hydrogen Bonding with Polar Protic Solvents

“…H 2 DPP, [47,83] Me 5 Fc [84] and TBABPh 4 [85] were synthesized according to the reported procedure. TFAa nd Fc derivatives (Fc, Me 2 Fc, Me 8 FC, and Me 10 Fc) were purchased from commercial sources and were used without further purification.…”

“…TFAa nd Fc derivatives (Fc, Me 2 Fc, Me 8 FC, and Me 10 Fc) were purchased from commercial sources and were used without further purification. H 2 DPP, [47,83] Me 5 Fc [84] and TBABPh 4 [85] were synthesized according to the reported procedure. All measurements were performed at 120 Ko naBruker APEXII Ultra diffractometer.T he structures were solved by ad irect method (SIR-97) and expanded with differential Fourier techniques.…”

“…TFA and Fc derivatives (Fc, Me 2 Fc, Me 8 FC, and Me 10 Fc) were purchased from commercial sources and were used without further purification. H 2 DPP, Me 5 Fc and TBABPh 4 were synthesized according to the reported procedure.…”

Effects of Lewis Acids on Photoredox Catalysis

“…A Pd porphyrin (PdP)‐flavin‐linked dyad also has the large positive shift in the one‐electron reduction potential of the Fl moiety from −0.77 V vs. SCE to +0.21 by binding two Sc 3+ ions to the Fl moiety . The photoinduced electron transfer in the PdP‐Fl‐(Sc 3+ ) 2 complex afforded the triplet‐charge‐separated state ( 3 [PdP .+ ‐Fl .− ‐(Sc 3+ ) 2 ]), which has the remarkably long lifetime of 110 ms …”

Acceleration and Stabilization of Electron Transfer Products with Improved Quantum Yields upon Cation Binding to a Fused Bis-Zinc Porphyrin-Quinone Donor–Acceptor Conjugate

“…In a number of electron transfer systems, it has been shown that the driving force of electron transfer is tuned by complexation of radical anions, produced in electron transfer reactions, with metal ions that act as Lewis acids. 11,12 In the area of light energy harvesting, the generation of longlived electron transfer products during the process of photoinduced electron transfer (PET) is indispensable for converting light energy into other forms as observed in photosynthesis, 13,14 solar cells, 15,16 and photocatalysis for fuel production. 17−19 In plant photosynthesis, photoexcitation of "special pair" chlorophylls in a well-organized donor−acceptor (DA) sequence in photosystem II triggers a series of electron transfer (ET) reactions in which the rates and direction of ET are governed by metal ions bound to quinones.…”