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PubMed2001

Modified windmill porphyrin arrays: coupled light-harvesting and charge separattion, conformational relaxation in the S1 state, and S2-S2 energy transfer.

The architecture of windmill hexameric zinc(II) -porphyrin array 1 is attractive as a light-harvesting functional unit in view of its three-dimensionally extended geometry that is favorable for a large cross-section of incident light as well as for a suitable energy gradient from the peripheral porphyrins to the meso-meso-linked diporphyrin core. Three core-modified windmill porphyrin arrays 2-4 were prepared for the purpose of enhancing the intramolecular energy-transfer rate and coupling these arrays with a charge-separation functional unit. Bisphenylethynylation at the meso and meso' positions of the diporphyrin core indeed resulted in a remarkable enhancement in the intramolecular S1-S1 energy transfer in 2 with tau=2 approximately 3 ps, as revealed by femtosecond time-resolved transient absorption spectroscopy. The fluorescence lifetime of the S2 state of the peripheral porphyrin energy donor determined by the fluorescence up-conversion method was 68 fs, and thus considerably shorter than that of the reference monomer (150 fs), suggesting the presence of the intramolecular energy-transfer channel in the S2 state manifold. Such a rapid energy transfer can be understood in terms of large Coulombic interactions associated with the strong Soret transitions of the donor and acceptor. Picosecond time-resolved fluorescence spectra and transient absorption spectra revealed conformational relaxation of the S1 state of the diporphyrin core with tau = 25 ps. Upon photoexcitation of models 3 and 4, which bear a naphthalenetetracarboxylic diimide or a meso-nitrated free-base porphyrin attached to the modified diporphyrin core as an electron acceptor, a series of photochemical processes proceeded, such as the collection of the excitation energy at the diporphyrin core, the electron transfer from the S1 state of the diporphyrin to the electron acceptor, and the electron transfer from the peripheral porphyrins to the diporphyrin cation radical, which are coupled to provide a fully charge-separated state such as that in the natural photosynthetic reaction center. The overall quantum yield for the full charge separation is better in 4 than in 3 owing to the slower charge recombination associated with smaller reorganization energy of the porphyrin acceptor.

Journal Article↗
PubMed2006

Triply fused Zn(II)-porphyrin oligomers: synthesis, properties, and supramolecular interactions with single-walled carbon nanotubes (SWNTs).

The photophysical, electrochemical, and self-assembly properties of a novel triply fused Zn(II)-porphyrin trimer were investigated and compared to the properties of a triply fused porphyrin dimer and the analogous monomer. The trimer exhibited significantly red-shifted absorption bands relative to the corresponding monomer and dimer. Electrochemical investigations indicated a clear trend in redox properties amongst the three porphyrin structures, with the lowest oxidation potential and the lowest HOMO-LUMO gap exhibited by the triply fused trimer. This electrochemical behavior is attributed to the extensive pi-electron delocalization in the trimeric structure relative to the monomer and dimer. Additionally, it was found that the trimer forms extremely strong and nearly irreversible supramolecular interactions with single-walled carbon nanotubes (SWNTs), resulting in stable solutions of porphyrin-nanotube complexes in THF. Formation of these complexes required the addition of trifluoroacetic acid (TFA) to the solvent. This allowed the oligomers to make close contact with the nanotubes, enabling the formation of stable supramolecular assemblies. Atomic force microscopy (AFM) was used to observe the supramolecular porphyrin-nanotube complexes and revealed that the porphyrin trimer formed a uniform coating on the SWNTs. Height profiles indicated that nanotube bundles could be exfoliated into either individual tubes or very small bundles by exposure to the porphyrin trimer during sonication.

Journal Article↗
PubMed1994

Photophysical and photobiological activities of a porphyrin peptide fraction derived from haemoglobin.

In a previous study, we described the preparation of a porphyrin peptide hydrolysate from haemoglobin, its isolation and its analysis by high performance liquid chromatography (HPLC) and fast atom bombardment (FAB) mass spectrometry. The purpose of the present paper is to test the photosensitizing activity of this fraction. We determined the singlet oxygen quantum yield (phi delta) in order to quantify the efficiency of the porphyrin peptide fraction. The quantum yield is about phi(1O2)=0.06. An analysis of the phototoxic effect on tumour cells in culture was performed and compared with haematoporphyrin derivative (HpD), the only photosensitizer in clinical use at present. The phototoxicity of the porphyrin peptide fraction is weaker than that of HpD. However, for a porphyrin dose of 50 micrograms ml-1, the difference in phototoxicity is low, and in the absence of irradiation porphyrin peptides are less toxic than HpD. These results suggest that porphyrin peptides could be potent photosensitizers; moreover, they are of great interest since they allow the solubilization of hydrophobic porphyrins and could be applied in the future as insoluble photosensitizer carriers.

Cell Line↗
PubMed2005

Spectroscopic and optical characterization of porphyrin chromophores incorporated into ultrathin polyimide films.

Polyamic acid (PAA) containing free-base porphyrin and zinc(II) porphyrin chromophores was synthesized by copolymerization of diphenylether-type tetracarboxylic dianhydride and diamines. The monolayer of the alkylamine salts of PAA (PAASs) at the air/water interface was deposited on solid substrates by the Langmuir-Blodgett (LB) technique. The PAAS LB films thus obtained were converted to polyimide (PI) LB films by chemical treatment. The fluorescence of porphyrin moieties in the PI LB film was observed, because of the weak electron-accepting properties of the diphenylether unit. Therefore, the photophysically important processes, such as photoinduced electron transfer, excitation energy transfer, and excitation energy migration could be investigated in relation to the layered nanostructures of the ultrathin PI films. The fluorescence spectrum suggested that the aggregation of porphyrin moieties in the PI LB films was effectively prevented by the use of polymeric films. The surface plasmon (SP) measurement showed that the thickness of the monolayers was 0.9-1.0 nm for PAAS films and 0.32-0.40 nm on average for PI LB films. The absorption dichroism of the Soret band of porphyrin indicated that porphyrin moieties in the PAAS and PI LB films are oriented in parallel with the substrate. These results showed that the orientation and the spatial distribution of porphyrin units can be efficiently regulated in the PI LB films in a nanometer dimension.

Journal Article↗
PubMed2006

Electronic and fluorescence spectral studies of a novel porphyrin-polypyridyl ruthenium(II) hybrid linked by a butyl chain.

The electronic and fluorescence spectroscopic properties of a novel porphyrin-polypyridyl ruthenium(II) hybrid, [C(4)-TPP-(ip)Ru(phen)(2)](ClO(4))(2) (TPP=5,10,15,20-tetraphenylporphyrin, ip=imidazo[4,5-f][1,10]phenanthroline and phen=1,10-Phenanthroline), in which a polypyridyl ruthenium(II) moiety is linked to a porphyrin moiety by a butyl chain have been investigated and compared to its corresponding reference compounds. The studies of electronic absorption spectra have shown that there is an electronic interaction between the porphyrin moiety and the polypyridyl ruthenium(II) moiety in the hybrid. It can be found that intramolecular photoinduced electron and energy transfer processes may occur in the hybrid from the fluorescence spectra. When exciting in Soret band and Q band of porphyrin, the fluorescence quenching of the porphyrin moiety of the hybrid takes place due to electron transfer from the lowest singlet excited state (S(1)) to the appended polypyridyl rutherium(II) moiety, while the decay of S(2) (the second-excited singlet state) of the porphyrin moiety is mainly contributed to internal conversion to S(1). When exciting in MLCT band of the polypyridyl ruthenium(II) moiety, fluorescence corresponding to the polypyridyl ruthenium(II) moiety is quenched by intramolecular energy transfer from (3)MLCT of the ruthenium moiety to the lowest-energy triplet state localized on the porphyrin moiety.

Absorption↗
PubMed2003

Synthesis and in vitro antioxidant properties of manganese(III) beta-octabromo-meso-tetrakis(4-carboxyphenyl)porphyrin.

Manganese(III) meso-tetrakis(4-carboxypheny)porphyrin (MnTBAP) is a readily available and widely used agent to scavenge reactive oxygen species. A major limitation of MnTBAP is its relatively weak potency due to its low metal centered redox potential. The goal of these studies was to prepare a more potent analog of MnTBAP by increasing its redox potential through beta-substitution on the porphyrin ring by bromination. Manganese(III) beta-octabromo-meso-tetrakis(4-carboxyphenyl)porphyrin (MnBr(8)TBAP) was prepared in three steps starting from the methyl ester of the free ligand meso-tetrakis(4-carboxyphenyl)porphyrin, with an overall yield of 50%. The superoxide dismutase (SOD)-like activity of MnBr(8)TBAP (IC(50)=0.7 microM) was the same as manganese(III) meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (MnTM-4-PyP(5+)), while the metal-centered redox potential of the first was considerably higher than the second (E(1/2)=+128 and 0 mV vs. normal hydrogen electrode, respectively). However, a number of these cationic Mn-porphyrins (such as MnTM-4-PyP(5+)) redox-cycle with cytochrome P450 reductase in the presence of oxygen and NADPH whereas MnTBAP and its halogenated analog, MnBr(8)TBAP do not. The enhanced ability of MnBr(8)TBAP to inhibit paraquat- and hypoxia-induced injuries in vitro is also reported. In these in vitro models, in which cationic Mn-porphyrins exhibit very low activity, MnBr(8)TBAP appears to be at least eightfold more active than the non-brominated analog MnTBAP.

Animals↗
PubMed2003

Oxidative cleavage of plasmid bluescript by water-soluble Mn-porphyrins and artificial oxidants or molecular oxygen.

A set of eight Mn(III)-porphyrins were used as catalysts in oxidative demolition of Plasmid Bluescript, to the nicked and linear forms, in the presence of different oxygen donors (NaOCl, H(2)O(2), AcOOH, t-BuOOH). The efficiency of the catalytic system is related to a combination of factors such as porphyrin structures, pH of the aqueous phase and nature of the primary oxidant. The highest catalytic activity was observed when ionic porphyrins were used as catalyst (the cationic being more active than the anionic) and NaOCl was used as primary oxidant at pH 9.5; in contrast, lipophilic catalysts proved to be completely unreactive towards the DNA, whichever oxidant used. The plasmid demolition was also achieved by irradiating the reaction mixture, containing Zinc porphyrins, with a white lamp; under these conditions, the highest efficiency was again observed with meso-tetra(1-methyl-4-pyridyl)porphyrin. However, preliminary experiments of photo activation applied on tumour cells (HCT 116) showed no dead cells with cationic porphyrin, while the amphiphilic Zn-tetra(4-hydroxyphenyl)porphyrin gave IC(50) values at 5 x 10(-2) microM concentration (37.1 ng/mL).

Catalysis↗
PubMed1998

Porphyrin-induced protein structural alternations of heme enzymes. II: Protection of 5-aminolevulinic acid dehydratase and porphobilinogen deaminase from the photodynamic and non-photodynamic effects of URO and PROTO.

BACKGROUND AND AIMS: Uroporphyrin and protoporphyrin produce alterations on 5-aminolevulinic acid dehydratase and porphobilinogen deaminase, as a result of a direct effect of porphyrins on the protein structure. With the aim of assessing the possible protection from the porphyrins effect on the proteins, some chemicals and the enzyme substrates were assayed. METHODS: Enzymes were pre-incubated with the protecting agents (beta-mercaptoethanol, dithiotreitol, hydroxylamine, succinic anhydride) or the corresponding substrates (delta-aminolevulinic acid and porphobilinogen), and then exposed to the porphyrins. All experiments were performed in the enzyme solutions after removing the porphyrins. RESULTS: The presence of sulfhydryl reagents partially protected both the enzyme activities and the content of total SH and free amino groups, but they did not prevent the appearance of molecular aggregates in the electrophoresis. Similar results were obtained in the presence of the corresponding substrates. Nucleophilic addition of hydroxylamine to the aromatic amino acids on the enzymes and blockage of their free amino groups did not prevent the direct effect of porphyrins, but these agents protected the enzyme activities from the photodynamic action of the tetrapyrroles, and also prevented the formation of molecular aggregates. However, an increased amount of free amino groups was observed, probably due to protein fragmentation. CONCLUSIONS: Porphyrins mainly affected the SH groups at or near the active site of the enzymes. Most of the free amino groups on the treated enzymes were involved in the formation of cross-links among the protein molecules. Protein fragmentation induced by porphyrins under UV light, and the consequent increased amount of free amino groups, were observed.

Animals↗
PubMed2001

New molecular arrays based on a tin(IV) porphyrin scaffold.

Two new porphyrin arrays-a hexamer and a nonamer-have been synthesized and characterized by elemental analysis as well as mass, (1)H NMR, and UV-vis spectroscopic methods. The scheme of construction of these arrays employs a synthetic protocol involving sequential "organic" and "inorganic" reactions conducted, respectively, at the peripheral meso-phenyl ring and the central tin(IV) ion of the porphyrin scaffold. The architecture of the hexamer is such that it is based on a covalently linked tin(IV) porphyrin dimer, with each of the two tin(IV) centers trans-axially ligated to two free-base porphyrins, while the higher homologue features a tin(IV) porphyrin trimer as the basal unit, with its central metalloid ions having two free-base porphyrins as axial ligands. This extended, "axial-bonding"-type architecture of the new arrays has been investigated by the (1)H NMR method, which reveals characteristic ring-current-induced shifts and coupling patterns for the resonances due to protons of the axial free-base porphyrin subunits. The presence of any ring-ring (basal-basal, basal-axial, or axial-axial) interaction in these arrays is not obvious from their UV-vis and redox potential data, which are close to those of the corresponding constituent monomeric species. On the other hand, their singlet-state activities are quite different from those of the precursor reference compounds as probed by steady-state fluorescence. The results of the detailed investigations carried out on these hybrid, "bichromophoric" arrays have been interpreted in terms of the occurrence of intraarray, interchromophore energy- and electron-transfer reactions.

Journal Article↗
PubMed2002

Synthesis, characterization, and preliminary intramolecular energy transfer studies of rigid, emissive, rhenium-linked porphyrin dimers.

The reaction of pyridyl functionalized porphyrins with Re(CO)(5)Cl in THF results in the formation of porphyrin dimers which, despite incorporation of rhenium into the assemblies, remain fluorescent. The rigid compounds provide an efficient geometry and/or orbital pathway for singlet energy transfer, rendering these compounds suitable, in principle, for the study of both through-bond and through-space energy transfer. Derivatives containing both metallated and freebase porphyrins connected via the metal corners display efficient porphyrin-porphyrin energy transfer. The photophysical properties of the assemblies have been studied by both steady-state and time-resolved fluorescence techniques, yielding approximate rates and efficiencies for porphyrin-porphyrin energy transfer.

Journal Article↗
PubMed2004

Alkyl- and aryl-substituted corroles. 5. Synthesis, physicochemical properties, and X-ray structural characterization of copper biscorroles and porphyrin-corrole dyads.

The synthesis and characterization of cofacial copper biscorroles and porphyrin-corroles linked by a biphenylenyl or anthracenyl spacer are described. The investigated compounds are represented as (BCA)Cu(2) and (BCB)Cu(2) in the case of the biscorrole (BC) derivatives and (PCA)Cu(2) and (PCB)Cu(2) in the case of porphyrin (P)-corrole (C) dyads, where A and B represent the anthracenyl and biphenylenyl bridges, respectively. A related monomeric corrole (Me(4)Ph(5)Cor)Cu and monomeric porphyrin (Me(2)Et(6)PhP)Cu that comprise the two halves of the porphyrin-corrole dyads were also studied. Electron spin resonance (ESR), (1)H NMR, and magnetic measurements data demonstrate that the copper corrole macrocycle, when linked to another copper corrole or copper(II) porphyrin, can be considered to be a Cu(III) complex in equilibrium with a Cu(II) radical species, copper(III) corrole being the main oxidation state of the corrole species at all temperatures. The cofacial orientation of (BCB)Cu(2), (BCA)Cu(2), and (PCB)Cu(2) was confirmed by X-ray crystallography. Structural data: (BCB)Cu(2)(C(110)H(82)N(8)Cu(2).3CDCl(3)), triclinic, space group P, a = 10.2550(2) A, b = 16.3890(3) A, c = 29.7910(8) A, alpha = 74.792(1) degrees , beta = 81.681(1) degrees , gamma = 72.504(2) degrees , Z = 2; (BCA)Cu(2)(C(112)H(84)N(8)Cu(2).C(7)H(8).1.5H(2)O), monoclinic, space group P 2(1)/c, a = 16.0870(4) A, b = 35.109(2) A, c = 19.1390(8) A, beta = 95.183(3) degrees , Z = 4; (PCB)Cu(2)(C(89)H(71)N(8)Cu(2).CHCl(3)), monoclinic, space group P2(1)/n, a = 16.7071(3) A, b = 10.6719(2) A, c = 40.8555(8) A, beta = 100.870(1) degrees , Z = 4. The two cofacial biscorroles, (BCA)Cu(2) and (BCB)Cu(2), both show three electrooxidations under the same solution conditions. The reduction of (BCA)Cu(2) involves a reversible electron addition to each macrocycle at the same potential of E(1/2) = -0.20 V although (BCB)Cu(2) is reversibly reduced in two steps to give first [(BCB)Cu(2)](-) and then [(BCB)Cu(2)](2)(-), each of which was characterized by ESR spectroscopy as containing a Cu(II) center. These latter electrode reactions occur at E(1/2) = -0.36 and -0.51 V versus a saturated calomel reference electrode. The half-reduced and fully reduced (BCB)Cu(2) show similar Cu(II) ESR spectra, and no evidence of a triplet signal is observed. The two well-separated reductions of (BCB)Cu(2) to give [(BCB)Cu(2)](2)(-) can be attributed to a stronger pi-pi interaction between the two macrocycles of this dimer as compared to those of (BCA)Cu(2). The copper porphyrin-corrole dyads, (PCA)Cu(2) and (PCB)Cu(2), show five reversible oxidations and two reversible reductions, and these potentials are compared with corresponding values for electrochemical reactions of the porphyrin and corrole monomers under the same solution conditions.

Journal Article↗
PubMed1996

Aryloxo Derivatives of Phosphorus(V) Porphyrins. Synthesis, Spectroscopy, Electrochemistry, and Singlet State Properties.

Aryloxo derivatives of phosphorus(V) porphyrins of the type [(TpTP)P(OR)(2)](+)OH(-) where TpTP is the dianion of tetra-p-tolylporphyrin and OR is an axial aryloxo (2,4-dimethylphenoxo, 4-methylphenoxo, phenoxo, 4-nitrophenoxo, 4-(4-nitrophenoxy)phenoxo, or 4-(2,4-dinitrophenoxy)phenoxo) ligand have been synthesized and fully characterized by FAB-mass, UV-vis, fluorescence, infrared, and nuclear magnetic resonance ((1)H and (31)P) spectroscopies and cyclic voltammetric methods. Each new porphyrin shows a typical "normal UV-vis absorption spectrum" indicating the presence of a P(V) ion in the porphyrin cavity. The proton-decoupled (31)P NMR signal observed for these compounds, between -194 and -200 ppm, suggests that there exists an octahedral coordination around the phosphorus atom, and this supposition is further substantiated by the porphyrin ring-current-induced upfield shifts observed for protons on the two axial aryloxo ligands in the (1)H NMR spectra. Cyclic voltammetric studies reveal that each porphyrin undergoes two successive, one-electron reductions with the site of electron transfer being the porphyrin ring. The fluorescence quantum yield values of these porphyrins are found to be sensitive to the nature of the aryloxo ligand and also to the solvent polarity. The singlet state properties of these systems have been discussed in light of both the fluorescence and the redox potential data.

Journal Article↗
PubMed1997

Reactions of Hydroxylamine with Metal Porphyrins.

The reaction of hydroxylamine with a series of metal porphyrins was examined in methanol/chloroform media. The reductive nitrosylation reaction was observed for the manganese and iron porphyrins, leading to a nitrosyl complex that precipitated out of the solution in good isolatable yield (80-90%). This reaction could be used synthetically for the generation of iron and manganese porphyrin nitrosyl complexes and was particularly useful for making isotopically labeled nitrosyl complexes. On the other hand, Co(II)(TPP) and Cr(TPP)(Cl) did not react with hydroxylamine under anaerobic conditions. With trace amounts of oxygen, the reaction of Co(II)(TPP) with hydroxylamine led to the formation of a stable cobalt(III)-bis(hydroxylamine) complex. The infrared, resonance Raman, and proton NMR spectra were consistent with a cobalt(III)-bis(hydroxylamine) complex. The cyclic voltammetry and visible spectroelectrochemistry of this complex were examined. The one-electron reduction of Co(III)(TPP)(NH(2)OH)(2)(+) formed Co(II)(TPP), for which there was no evidence for the coordination of hydroxylamine. Further reduction led to Co(I)(TPP)(-), which reacted with the halogenated solvent to form a cobalt-alkyl complex. The difference in the reactivity of these four metal porphyrins with hydroxylamine correlated well with their E(1/2) values. Iron(III) and manganese(III) porphyrins were relatively easy to reduce and readily underwent the reductive nitrosylation reaction, while cobalt(II) and chromium(III) porphyrins are unreactive. The one-electron oxidation of the hydroxylamine complex with a M(III) porphyrin would be expected to oxidize the N-atom in the coordinated hydroxylamine. The oxidation of M(III)(NH(2)OH) with the loss of a proton would form M(II)(N(I)H(2)O)(+) by an internal electron transfer, which will eventually lead to M(NO). The relationship between the reductive nitrosyl reaction and the enzymatic interconversion of NO and hydroxylamine was discussed.

Journal Article↗
PubMed2003

Nanowells on silica particles in water containing long-distance porphyrin heterodimers.

Smooth and nonswelling spherical silica particles with a diameter of 100 nm and an aminopropyl coating are soluble in water at pH 11, coagulate quickly at pH 3, and redissolve at pH 9. Electron microscopy as well as visible spectra of covalently attached porphyrins indicate the aggregation state of the particles. Long-chain alpha,omega-dicarboxylic acids with a terminal oligoethyleneglycol (=OEG)-amide group were attached in a second self-assembly step to the remaining amine groups around the porphyrins. Form-stable 2-nm wells were thus obtained and were characterized by fluorescence quenching experiments using the bottom porphyrin as a target. The one-dimensional diffusion of fitting quencher molecules along the 2-nm pathway took several minutes. Porphyrins with a diameter above 2 nm could not enter the form-stable gaps at all. Added tyrosine stuck irreversibly to the walls of the nanowells and prevented the entrance of quencher molecules, the OEG-headgroups fixated 2,6-diaminoanthraquinone. A ring of methylammonium groups was then fixed at the walls of the wells at a distance of 5 or 10 A with respect to the bottom porphyrin. 2,6-Disulfonatoanthraquinone was attached only loosely to this ring, but the exactly fitting manganese(III) meso-(tetraphenyl-4-sulfonato)porphyrinate (Mn(III) TPPS) was tightly bound. Transient fluorescence experiments showed a fast decay time of 0.2 ns for the bottom porphyrin, when the Mn(III) TPPS was fixated at a distance of 5 A. Two different dyes have thus been immobilized at a defined subnanometer distance in an aqueous medium.

Journal Article↗
PubMed2005

Surface patterning with two-dimensional porphyrin supramolecular arrays.

Monolayer arrays of a series of meso-tetra-substituted porphyrins containing octadecyloxy and carboxyl (or pyridyl) groups were prepared on the highly oriented pyrolytic graphite surface at the liquid/solid interface. It was found by means of scanning tunneling microscopy that some porphyrins from this family assemble into various patterns. Specifically, slightly undulated rows are obtained from 5,10,15-tris(4-octadecyloxyphenyl)-20-(4-pyridyl)porphyrin. Meanwhile, rows with more pronounced kinks result from 5-(4-carboxyphenyl)-10,15,20-tris(4-octadecyloxyphenyl)porphyrin. The occurrence of the kinks is dependent on the arrangement of surrounding porphyrin molecules and is determined by intricate interplay between directional hydrogen-bonding interactions and packing forces, including molecule-molecule and molecule-substrate interactions. A double-layer structure is obtained from 5,10-bis(4-carboxyphenyl)-15,20-bis(4-octadecyloxyphenyl)porphyrin, probably through cyclic hydrogen bond formation. This work proves the concept that programmed surface patterning is possible by using porphyrins incorporating directional intermolecular interaction sites.

Journal Article↗
PubMed2000

Investigation of tightly coupled porphyrin arrays comprised of identical monomers for multibit information storage

Our prior designs for molecular-based information storage devices have employed multiple redox-active units organized in weakly coupled, covalently linked arrays. To explore a simpler design, we report here the synthesis of porphyrin arrays where porphyrins with identical oxidation potentials are directly linked to one another instead of joined via a molecular linker. Oxidative coupling with AgPF(6) of zinc(II)-5,15-bis(4-tert-butylphenyl)-10-phenylporphyrin, obtained by a rational synthesis, afforded the expected dimer joined by a meso-meso linkage and an unexpected trimer joined by meso-meso linkages. For attachment to an electroactive surface we synthesized a meso-linked porphyrin dimer with a thiol-linker in one of the meso positions. The S-acetyl protecting group was used to avoid handling free thiol groups. Coupling of zinc(II)-5,10,15-tris(3, 5-di-tert-butylphenyl)porphyrin ("upper half") and zinc(II)-5-[4-(S-acetylthio)phenyl]-10,20-bis(3, 5-di-tert-butylphenyl)porphyrin ("lower half") afforded three different meso-linked dimers with the desired dimer as the main product. Electrochemical examination of the meso-linked dimer in solution shows that the first two oxidation potentials of the array differ by approximately 0.15 V and straddle the value exhibited by the monomeric constituents. The third and fourth oxidation potentials of the array are also split although to a lesser extent ( approximately 0.08 V) than the first and second. For the meso-linked trimer, the first three oxidation waves are also split; however, these waves are severely overlapped. The electrochemical behavior of the dimers and trimer is indicative of strong electronic interactions among the porphyrins. The thiol-derivatized meso-linked dimers form self-assembled monolayers (SAMs) on gold via in situ cleavage of the S-acetylthio protecting group. The porphyrin SAM exhibits four well-resolved oxidation waves. Regardless, the meso-meso linkage is relatively unstable upon formation of the pi-cation radical(s). This characteristic indicates that the structural motif is of limited utility for molecular information storage elements.

Journal Article↗
PubMed2003

Efficient synthesis of porphyrin-containing, benzoquinone-terminated, rigid polyphenylene dendrimers.

A series of rigid polyphenylene, free-base porphyrin-containing dendrimers terminated with either dimethoxybenzene or benzoquinone end-groups were prepared by a combined divergent and convergent synthesis. Unlike previous routes for preparing polyphenylene dendrimers that are incompatible with end-groups bearing certain functional moieties, the synthetic methodology chosen for this work enables incorporation of functional groups on the dendrimer end-groups during preparation of the dendrimer wedges and during synthesis of the final dendrimer. The basic strategy utilized a convergent preparation of dendrimer wedges using Suzuki coupling conditions, which were then either attached to a porphyrin core in a divergent coupling step or cyclized to form the porphyrin dendrimer in a convergent step. The latter approach was found to be more general and resulted in higher yields and more readily separated products. Steady-state absorption measurements for these dendrimers showed Soret and Q-band absorptions typical of free-base porphyrins. Preliminary steady-state fluorescence measurements of these dendrimers indicate quenching of the S1 state of the free-base porphyrin in all benzoquinone-containing dendrimers that is attributed to efficient electron-transfer from the excited porphyrin to the benzoquinone end-groups. The amount of fluorescence quenching was in good agreement with the number of benzoquinone groups at the dendrimer periphery and the distance between the porphyrin and benzoquinone groups as calculated by semiempirical (AM1) molecular orbital calculations.

Journal Article↗
PubMed2005

Direct synthesis of palladium porphyrins from acyldipyrromethanes.

[reaction: see text] Palladium porphyrins are valuable photosensitizers and luminescent agents in biology and materials chemistry. New methodology is described wherein a 1-acyldipyrromethane is converted into the palladium chelate of a trans-A(2)B(2) porphyrin via a one-flask reaction. The reaction entails self-condensation of the 1-acyldipyrromethane in refluxing ethanol containing KOH (5-10 mol equiv) and Pd(CH(3)CN)(2)Cl(2) (0.6 mol equiv) exposed to air. This direct route to palladium porphyrins is more expedient than the four steps of the traditional synthesis: (1) reduction of the 1-acyldipyrromethane; (2) acid-catalyzed condensation; (3) oxidation of the porphyrinogen intermediate; and (4) metal insertion. The new synthesis requires neither acid nor DDQ and formally entails only a 2e(-) + 2H(+) oxidation overall versus the traditional multistep synthesis which requires a 2e(-) + 2H(+) reduction per each 1-acyldipyrromethane (4e(-) + 4H(+) overall) followed by a 6e(-) + 6H(+) oxidation. The analogous reaction of a 1,9-diacyldipyrromethane and a dipyrromethane also gives the palladium porphyrin. Seven palladium porphyrins have been prepared in yields of 25-57%. The direct route also can be used with Cu(OAc)(2).H(2)O to give the copper porphyrin albeit in low yield. In summary, this methodology readily affords palladium porphyrins directly from acyldipyrromethanes.

Catalysis↗