Resonance Raman Characterization of (Oxoferryl)tetraphenylporphyrin Formed during Photodisproportionation of (Fe(TPP))2O in a Detergent Micelle at Room Temperature

Low quantum efficiency of μ-oxo iron bisporphyrin photocatalysts explained with femtosecond M-edge XANES

Transient optical absorption and M-edge XANES identifies the predominant formation of an iron(iii) ion pair state over the desired iron(ii)/iron(iv)-oxo state as the source of poor photon-to-product yield in μ-oxo iron bisporphyrin photocatalysts.

(Hydr)oxo-bridged heme complexes: From structure to reactivity

Iron–porphyrins ([Formula: see text] hemes) are present throughout the biosphere and perform a wide range of functions, particularly those that involve complex multiple-electron redox processes. Some common heme enzymes involved in these processes include cytochrome P450, heme/copper oxidase or heme/non-heme diiron nitric oxide reductase. Consequently, the (hydr)oxo-bridged heme species have been studied for the important roles that they play in many life processes or for their application for catalysis and preparation of new functional materials. This review encompasses important synthetic, structural and reactivity aspects of the (hydr)oxo-bridged heme constructs that govern their function and application. The properties and reactivity of the bridging (hydr)oxo moieties are directly dictated by the coordination environment of the heme core, the nature and ligation of the second metal center attached to the (hydr)oxo group, the presence or absence of a linker, and the degree of flexibility around that linker within the scaffold. Here, we summarize the structural features of all known (hydr)oxo-bridged heme constructs and use those to categorize and thus, provide a more comprehensive picture of structure–function relationships.

Z-scan studies and quantum chemical calculations of meso-tetrakis(p-sulfonatophenyl)porphyrin and meso-tetrakis(4-N-methyl-pyridiniumyl)porphyrin and their Fe(III) and Mn(III) complexes

Optical self-defocusing, characterized by the non-linear refractive index n2, was investigated by the Z-scan technique in water solutions of two porphyrins (PPhs), negatively charged meso-tetrakis(p-sulfonatophenyl)porphyrin ( TPPS 4) and positively charged meso-tetrakis(4-N-methyl-pyridiniumyl)porphyrin ( TMPyP ), in their free base forms and as Fe (III) and Mn (III) complexes. Significant n2 values were observed only for the TMPyP metal complexes, while for the other porphyrins the n2 values were negligible. The effect is explained by the reorientation of the porphyrin molecule due to interaction of its permanent dipole moment perpendicular to the molecular ring plane with the electromagnetic field of the exciting light pulse. The permanent dipole moment is due to the shift of the metal atom out of the molecular plane. The electrostatic interaction between the metal atom and charged substituents increases (repulsion) or decreases (attraction) the shift of the metal atom and consequently affects the dipole moment value. Ligand binding to the metal atoms also increases the out-of-plane metal shift and hence the dipole moment and n2 value. pH changes were shown to modify the Fe (III)-ligand structure, thus changing n2. The experimental data correlate well with the metal shift and dipole moment values calculated for simplified PPh structures by the ZINDO method.

Application of Badger's Rule to Heme and Non-Heme Iron−Oxygen Bonds: An Examination of Ferryl Protonation States

To gain insight into the protonation state of enzymatic ferryl species we have examined the applicability of Badger's rule to heme and non-heme iron-oxygen bonds. Using density functional theory we have calculated r(e) and nu(e) for the Fe-O bonds of complexes with different axial ligands, iron-oxidation, oxygen-protonation, and spin states. Our results indicate that Badger's rule holds for heme and non-heme oxo and hydroxo complexes. We find that the long Fe-O bonds that have been reported in the crystal structures of the ferryl forms of myoglobin, horseradish peroxidase, cytochrome c peroxidase, and catalase deviate substantially from the values predicted by Badger's rule, while the short Fe-O bonds obtained from X-ray absorption measurements are in good agreement with Badger's rule. In light of our analysis we conclude that the ferryl forms of myoglobin, horseradish peroxidase, and cytochrome c peroxidase are authentic iron(IV)oxos with Fe-O bonds on the order of 1.66 A and pKa's < 4.

Transient Absorption Studies of the Pacman Effect in Spring-Loaded Diiron(III) μ-Oxo Bisporphyrins

Picosecond transient absorption spectroscopy of diiron(III) mu-oxo bisporphyrins appended to xanthene, (DPX)Fe2O and (DPXM)Fe2O, and dibenzofuran (DPD)Fe2O have been investigated in order to decipher the effect of a spring-loaded cleft on their photophysics and attendant oxidation photocatalysis. The tension of the cofacial pocket is systematically tuned with the bridge span and meso-substitution opposite to the bridge; the distances of the relaxed cofacial pockets and clamped Fe-O-Fe pockets are known from X-ray crystallography (Deltad(M-M)(relaxed-clamped)=4.271 A (DPD), 2.424 A (DPXM), 0.208 A (DPX)). The photophysical and chemical properties of these cofacial platforms are compared to the unbridged diiron(III) mu-oxo analogue, (Etio)2Fe2O. Photon absorption by the diiron(III) mu-oxo chromophore prompts Fe-O-Fe photocleavage to release the spring and present a PFeIVO/PFeII pair (P=porphyrin subunit); net photooxidation is observed when oxygen atom transfer to substrate occurs before the spring can reclamp to form the mu-oxo species. The inherent lifetimes of the PFeIVO/PFeII pairs for the four compounds are surprisingly similar (tau[(DPD)Fe2O]=1.36(3) ns, tau[(DPX)Fe2O]=1.26(5) ns, tau[(DPXM)Fe2O]=1.27(9) ns, and tau[(Etio)2Fe2O]=0.97(3) ns), considering the structural differences arising from tensely clamped (DPD and DPXM), relaxed (DPX), and unbridged (Etio) cofacial architectures. However, the rates of net oxygen atom transfer for (DPD)Fe2O and (Etio)2Fe2O are found to be 4 orders of magnitude greater than that of (DPX)Fe2O and 2 orders of magnitude greater than that of (DPXM)Fe2O. These results show that the spring action of the cleft, known as the Pacman effect, does little to impede reclamping to form the mu-oxo species but rather is manifest to opening the cofacial cleft to allow substrate access to the photogenerated oxidant. Consistent with this finding, photooxidation efficiencies decrease as the steric demand of substrates increase.

Exploitation of the Z-scan technique as a method to optically probe pK(a) in organic materials: application to porphyrin derivatives

We demonstrate the use of the Z-scan technique as a method to measure a chemical parameter, the pK(a) value, that characterizes the equilibrium constant in acid-base reactions. The measurements were performed with picosecond pulses at 532 nm in Fe(III)-meso-tetrakis(4-N-methyl-pyridyl) porphyrin. The results were compared with linear absorption measurements and electron paramagnetic resonance, the usual techniques employed to determine pK(a) values in aqueous solutions, and showed excellent agreement.