Iron(III) fluorinated porphyrins play a central role in the biomimetics of heme enzymes and enable cleaner routes to the oxidation of organic compounds. The present work reports significant ...improvements in the eco-compatibility of the synthesis of 5,10,15,20-tetrakis-pentafluorophenylporphyrin (H₂TPFPP) and the corresponding iron complex Fe(TPFPP)Cl, and the use of Fe(TPFPP)Cl as an oxidation catalyst in green conditions. The preparations of H₂TPFPP and Fe(TPFPP)Cl typically use toxic solvents and can be made significantly greener and simpler using microwave heating and optimization of the reaction conditions. In the optimized procedure it was possible to eliminate nitrobenzene from the porphyrin synthesis and replace DMF by acetonitrile in the metalation reaction, concomitant with a significant reduction of reaction time and simplification of the purification procedure. The Fe(III)porphyrin is then tested as catalyst in the selective oxidation of aromatics at room temperature using a green oxidant (hydrogen peroxide) and green solvent (ethanol). Efficient epoxidation of indene and selective oxidation of 3,5-dimethylphenol and naphthalene to the corresponding quinones is observed.
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Porphyrin nanotubes represent a new class of nanostructures for which the molecular building blocks can be altered to control their structural and functional properties. Nanotubes containing tin(IV) ...porphyrins are photocatalytically active and can reduce metal ions from aqueous solution. The metal is deposited selectively onto tube surfaces, producing novel composite nanostructures that have potential applications as nanodevices. Two examples presented here are nanotubes with a continuous gold wire in the core and a gold ball at the end and nanotubes coated with platinum nanoparticles mainly on their outer surfaces. The latter are capable of photocatalytic reduction of water to hydrogen.
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Square porphyrin nanosheets with uniform thickness (7−12 nm) and aspect ratios up to 100 have been synthesized for the first time using a reprecipitation method. They retain the photocatalytic ...activity of the precursor molecules and can be self-metallized with Pt, Pd, and Au. Their unique morphology, photocatalytic properties, and large surface areas suggest potential applications in photo- and electrocatalysis, sensing, as components for optoelectronic nanodevices, and solar energy capture and conversion.
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4.
Porphyrin Nanotubes by Ionic Self-Assembly Wang, Zhongchun; Medforth, Craig J; Shelnutt, John A
Journal of the American Chemical Society,
12/2004, Volume:
126, Issue:
49
Journal Article
Peer reviewed
Porphyrin nanotubes are made by ionic self-assembly of two oppositely charged synthetic porphyrin molecules. The diameter of the nanotubes can be altered by modifying the structure of one of the ...porphyrin tectons. The nanotubes are photocatalytically active, mechanically responsive to light, and are composed of J-aggregates that exhibit resonance light scattering.
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Seeding and autocatalytic reduction of platinum salts in aqueous surfactant solution using ascorbic acid as the reductant leads to remarkable dendritic metal nanostructures. In micellar surfactant ...solutions, spherical dendritic metal nanostructures are obtained, and the smallest of these nanodendrites resemble assemblies of joined nanoparticles and the nanodendrites are single crystals. With liposomes as the template, dendritic platinum sheets in the form of thin circular disks or solid foamlike nanomaterials can be made. Synthetic control over the morphology of these nanodendrites, nanosheets, and nanostructured foams is realized by using a tin-porphyrin photocatalyst to conveniently and effectively produce a large initial population of catalytic growth centers. The concentration of seed particles determines the ultimate average size and uniformity of these novel two- and three-dimensional platinum nanostructures.
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The view that the large red shifts seen in the UV−visible absorption bands of peripherally crowded nonplanar porphyrins are the result of nonplanar deformations of the macrocycle has recently been ...challenged by the suggestion that the red shifts arise from substituent-induced changes in the macrocycle bond lengths and bond angles, termed in-plane nuclear reorganization (IPNR). We have analyzed the contributions to the UV−visible band shifts in a series of nickel or zinc meso-tetraalkylporphyrins to establish the origins of the red shifts in these ruffled porphyrins. Structures were obtained using a molecular mechanics force field optimized for porphyrins, and the nonplanar deformations were quantified by using normal-coordinate structural decomposition (NSD). Transition energies were calculated by the INDO/S semiempirical method. These computational studies demonstrate conclusively that the large Soret band red shifts (∼40 nm) seen for very nonplanar meso-tetra(tert-butyl)porphyrin compared to meso-tetra(methyl)porphyrin are primarily the result of nonplanar deformations and not IPNR. Strikingly, nonplanar deformations along the high-frequency 2B1u and 3B1u normal coordinates of the macrocycle are shown to contribute significantly to the observed red shifts, even though these deformations are an order of magnitude smaller than the observed ruffling (1B1u) deformation. Other structural and electronic influences on the UV−visible band shifts are discussed and problems with the recent studies are examined (e.g., the systematic underestimation of the 2B1u and 3B1u modes in artificially constrained porphyrin structures that leads to a mistaken attribution of the red shift to IPNR). The effect of nonplanar deformations on the UV−visible absorption bands is then probed experimentally with a series of novel bridled nickel chiroporphyrins. In these compounds, the substituent effect is essentially invariant and the amount of nonplanar deformation decreases as the length of the straps connecting adjacent meso-cyclopropyl substituents decreases (the opposite of the effect observed for conventional strapped porphyrins). Several spectroscopic markers for nonplanarity (UV−visible bands, resonance Raman lines, and 1H NMR resonances) are found to correlate with time-averaged deformations obtained from an NSD analysis of molecular dynamics snapshot structures. These results suggest that UV−visible band shifts of tetrapyrroles in proteins are potentially useful indicators of changes in nonplanarity provided other structural and electronic factors can be eliminated.
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Electrochemical studies of the oxidation of dodecasubstituted and highly nonplanar nickel porphyrins in a noncoordinating solvent have previously revealed the first nickel(III) porphyrin dication. ...Herein, we investigate if these nonplanar porphyrins can also be used to detect the so far unobserved copper(III) porphyrin dication. Electrochemical studies of the oxidation of (DPP)Cu and (OETPP)Cu show three processes, the first two of which are macrocycle-centered to give the porphyrin dication followed by a Cu(II)/Cu(III) process at more positive potential. Support for the assignment of the Cu(II)/Cu(III) process comes from the linear relationships observed between E1/2 and the third ionization potential of the central metal ions for iron, cobalt, nickel, and copper complexes of (DPP)M and (OETPP)M. In addition, the oxidation behavior of additional nonplanar nickel porphyrins is investigated in a noncoordinating solvent, with nickel meso-tetraalkylporphyrins also being found to form nickel(III) porphyrin dications. Finally, examination of the nickel meso-tetraalkylporphyrins in a coordinating solvent (pyridine) reveals that the first oxidation becomes metal-centered under these conditions, as was previously noted for a range of nominally planar porphyrins.
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Nanotubes prepared by the self-assembly of D-Phe-D-Phe molecules are investigated by electron microscopy and Monte Carlo simulations; the nanotubes appear to be porous and are capable of forming ...novel peptide-nanotube platinum-nanoparticle composites.
Microscale four-leaf clover-shaped structures are formed by self-assembly of anionic and cationic porphyrins. Depending on the metal complexed in the porphyrin macrocycle (Zn or Sn), the porphyrin ...cores are either electron donors or electron acceptors. All four combinations of these two metals in cationic tetra(N-ethanol-4-pyridinium)porphyrin and anionic tetra(sulfonatophenyl)porphyrin result in related cloverlike structures with similar crystalline packing indicated by X-ray diffraction patterns. The clover morphology transforms as the ionic strength and temperature of the self-assembly reaction are increased, but the structures maintain 4-fold symmetry. The ability to alter the electronic and photophysical properties of these solids (e.g., by altering the metals in the porphyrins) and to vary cooperative interactions between the porphyrin subunits raises the possibility of producing binary solids with tunable functionality. For example, we show that the clovers derived from anionic Zn porphyrins (electron donors) and cationic Sn porphyrins (electron acceptors) are photoconductors, but when the metals are reversed in the two porphyrins, the resulting clovers are insulators.
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Herein, we report the first calorimetric study of the protonation of planar and nonplanar free-base porphyrins: H2OETPP (strongly saddled by its substituents), H2T(tBu)P (strongly ruffled by its ...substituents), and the nominally planar porphyrins (npPs) H2OEP, H2TPP, H2T(nPe)P, and H2T(iPr)P. The observed enthalpies of protonation in solution (ΔH protsoln) for formation of the dications in 1,1,2,2-tetrachloroethane with 2% trifluoroacetic acid are −45 ± 1 kcal mol–1 for the npPs, −52.0 kcal mol–1 for H2T(tBu)P, and −70.9 kcal mol–1 for H2OETPP. The corresponding enthalpies of protonation (ΔH DFT) obtained from DFT calculations (−27 ± 5, −42, and −63 kcal mol–1, respectively) reproduce this trend. The much more negative enthalpy of protonation seen for H2OETPP is consistent with this molecule being pre-deformed into the saddle structure favored by porphyrin dications. Except for OETPP, the calculated enthalpies of the first protonations (ΔH 1) are significantly more positive than the enthalpies of the second protonations (ΔH 2). In addition, the structural strain energies for the first protonations (ΔE st(1)) are also significantly more positive than ΔE st(2). According to the calculations, the monocations thus have higher proton affinities than the corresponding free-base porphyrins due to a structural strain effect, which is consistent with the generally elusive nature of the porphyrin monocation. The recent observations of monocations for free-base porphyrins with a high degree of saddling can be rationalized in terms of ΔH 1 and ΔH 2 being similar; so, the monocation is no longer an unstable intermediate.
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