Binding of scandium ions to metalloporphyrin-flavin complexes for long-lived charge separation.

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) ).

Structure and photoinduced electron transfer dynamics of a series of hydrogen-bonded supramolecular complexes composed of electron donors and a saddle-distorted diprotonated porphyrin.

The excited-state photodynamics of intrasupramolecular photoinduced electron transfer was investigated in a series of hydrogen-bonded supramolecular complexes composed of diprotonated 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin (H(4)DPP(2+)) and electron donors bearing a carboxylate group. The formation of supramolecular complexes was examined by spectroscopic measurements. The binding constants obtained by spectroscopic titration indicate the strong binding (10(8)-10(10) M(-2)) even in a polar and coordinating solvent, benzonitrile (PhCN). The crystal structure of the supramolecular assembly using ferrocenecarboxylate (FcCOO(-)) was determined to reveal a new structural motif involving two-point and single-point hydrogen bonding among saddle-distorted H(4)DPP(2+) dication and two FcCOO(-) anions. Femtosecond laser flash photolysis was applied to investigate the photodynamics in the hydrogen-bonded supramolecular complexes. Rate constants obtained were evaluated in light of the Marcus theory of electron transfer, allowing us to determine the reorganization energy and the electronic coupling matrix constant of photoinduced electron transfer and back electron transfer to be 0.68 eV and 43 cm(-1), respectively. The distance dependence of electron transfer was also examined by using a series of ferrocenecarboxylate derivatives connected by linear phenylene linkers, and the distance dependence of the rate constant of electron transfer (k(ET)) was determined to be k(ET) = k(0) exp(-beta r), in which beta = 0.64 A(-1).

Reorganization energies of diprotonated and saddle-distorted porphyrins in photoinduced electron-transfer reduction controlled by conformational distortion.

Kinetics of photoinduced electron transfer from a series of electron donors to the triplet excited states of a series of nonplanar porphyrins, hydrochloride salts of saddle-distorted dodecaphenylporphyrin ([H(4)DPP]Cl(2)), tetrakis(2,4,6-trimethylphyenyl)porphyrin ([H(4)TMP]Cl(2)), tetraphenylporphyrin ([H(4)TPP]Cl(2)), and octaphenylporphyrin ([H(4)OPP]Cl(2)), were investigated in comparison with those of a planar porphyrin, zinc [tetrakis(pentafluorophenyl)]porphyrin [Zn(F(20)TPP)(CH(3)CN)], in deaerated acetonitrile by laser flash photolysis. The resulting data were evaluated in light of the Marcus theory of electron transfer, allowing us to determine reorganization energies of electron transfer to be 1.21 eV for [H(4)TMP]Cl(2), 1.29 eV for [H(4)TPP]Cl(2), 1.45 eV for [H(4)OPP]Cl(2), 1.69 eV for [H(4)DPP]Cl(2), and 0.84 eV for [Zn(F(20)TPP)(CH(3)CN)]. The reorganization energies exhibited a linear correlation relative to the out-of-plane displacements, which represent the degree of nonplanarity. The rate of electron-transfer reduction of diprotonated porphyrins is significantly slowed down by conformational distortions of the porphyrin ring. This indicates that the reorganization energy of electron transfer is governed by structural change, giving a larger contribution of inner-sphere bond reorganization energy rather than outer-sphere solvent reorganization energy.

Selective inclusion of electron-donating molecules into porphyrin nanochannels derived from the self-assembly of saddle-distorted, protonated porphyrins and photoinduced electron transfer from guest molecules to porphyrin dications.

A doubly protonated hydrochloride salt of a saddle-distorted dodecaphenylporphyrin (H2DPP), [H4DPPP]Cl2, forms a porphyrin nanochannel (PNC). X-ray crystallography was used to determine the structure of the molecule, which revealed the inclusion of guest molecules within the PNC. Electron-donating molecules, such as p-hydroquinone and p-xylene, were selectively included within the PNC in sharp contrast to electron acceptors, such as the corresponding quinones, which were not encapsulated. This result indicates that the PNC can recognize the electronic character and steric hindrance of the guest molecules during the course of inclusion. ESR measurements (photoirradiation at lambda>340 nm at room temperature) of the PNC that contains p-hydroquinone, catechol, and tetrafluorohydroquinone guest molecules gave well-resolved signals, which were assigned to cation radicals formed without deprotonation based on results from computer simulations of the ESR spectra and density functional theory (DFT) calculations. The radicals are derived from photoinduced electron transfer from the guest molecules to the singlet state of H4DPP2+. Transient absorption spectroscopy by femtosecond laser flash photolysis allowed us to observe the formation of 1(H4DPP2+)*, which is converted to H4DPP+. by electron transfer from the guest molecules to 1(H4DPP2+)*, followed by fast disproportionation of H4DPP+., and charge recombination to give diamagnetic species and the triplet excited state 3(H4DPP2+)*, respectively.