Showing papers on "Porphyrin published in 1995"

Phosphorescent Complexes of Porphyrin Ketones: Optical Properties and Application to Oxygen Sensing

Abstract: A new class of dyes, platinum(II) and palladium(II) complexes of the porphyrin ketones (or oxochlorins), exhibiting strong phosphorescence at room temperature is described. Several representative compounds were prepared and studied by spectral luminescence methods in solution. Compared to the related porphyrin and chlorin complexes, the new dyes display high photochemical stability, long wave spectral characteristics, and good compatibility with semiconductor optoelectronics (e.g., excitation by light-emitting diodes). These properties make the new dyes promising for a number of relevant applications, such as quenched phosphorescence sensing and phosphorescence probing (e.g., in binding assays). Analytical application of the porphyrin ketone complexes to phosphorescence lifetime-based sensing of molecular oxygen is described. Platinum(II) octaethylporphine ketone was dissolved in a polystyrene layer to give an oxygen-sensitive film. Oxygen measurements were performed with a prototype fiber-optic instrument based on solid-state components, such as light-emitting diodes and photodiodes. The instrument measured phosphorescence lifetime via changes in phase shift. The phosphorescence lifetime was determined to change from about 61.4 μs at zero oxygen to 16.3 μs in air (210 hPa of oxygen) at 22 °C. The analytically useful range of the sensor was 0-210 hPa of oxygen partial pressure, with a detection limit of 1.5 hPa. The precision of the device was 1.0 hPa at 210 hPa of oxygen and 0.5 hPa at zero oxygen.

Ruffling in a Series of Nickel(II) meso-Tetrasubstituted Porphyrins as a Model for the Conserved Ruffling of the Heme of Cytochromes c.

Abstract: Metalloporphyrins undergo remarkable nonplanar distortions of the macrocycle that perturb the chemical and photochemical properties of these important protein cofactors. Further, the tertiary structure of the surrounding protein can manipulate these distortions as a means of regulating biological function. For cytochromes c, for example, an energetically unfavorable, conserved nonplanar distortion of the heme exists and likely plays a role in its electron- transfer function. The heme distortion is primarily of the ruffling (ruf, type (corresponding to the lowest frequency BI,-symmetry normal mode) in which the pyrroles are twisted about the metal-Npyrrole bond. This BI.-symmetry nonplanar distortion is commonly observed in metalloporphyrin crystal structures, as are the saddling (sad) B2,- symmetry distortion, waving (wau) E,-symmetry distortions, and doming (dom) Az,-symmetry distortion. Each of these nonplanar distortions is expected to result in unique alterations of the chemical and physical properties of the nominally planar porphyrin macrocycle. Symmetrical porphyrin substitution with tetrahedrally bonded atoms at the four meso bridging carbons generally results in the BI, ruffling distortion; therefore, we investigated a series of meso-tetrasubstituted porphyrins for which the substituents vary in size (methyl, ethyl, propyl, pentyl, isopropyl, cyclopropyl derivative lla, cyclohexyl, apopinenyl (lo), tert-butyl, adamantyl), increasing the steric crowding at the periphery. Molecular mechanics calculations show increasing degree of ruffling (CaNNCa angle for opposite pyrroles varies from 0 to 57") for this series of porphyrins, generally agreeing with the X-ray structures that are available. In addition, the frequencies of the structure-sensitive Raman lines decrease nonlinearly with increasing ruffling angle. The localization of the Bl, nonplanar distortion in only the C,-C, bond torsion (not the case for the B2" sad distortion) suggests a means by which the B1, distortion might be distinguished from other types of nonplanar distortion by using resonance Raman spectroscopy. Also, the size of the red shifts in the n - n* absorption bands depends on Ca-Cm torsion angle in a nonlinear fashion and the shift is accurately predicted by INDO/s molecular orbital calculations when the nonplanar structures obtained from molecular mechanics are used.

Photophysical properties of porphyrins in biological membranes.

Abstract: This review illustrates the photophysical properties of some porphyrins, especially those used for biomedical applications, in relation to their photosensitizing efficiency in biological membranes. Porphyrin absorption and luminescence properties are mainly examined. The factors influencing the affinity of porphyrins for biological membranes, including the dye hydrophobicity, the charge and aggregation state, the pH of the medium and the physicochemical properties of the dye environment, are discussed. These factors determine the differences in the photophysical properties of porphyrins in biological membranes. Particular attention is paid to the porphyrin aggregation state: only monomeric species and possibly planar end-to-end aggregates are endowed with significant photosensitizing ability. Many conclusions presented are based on data obtained on membrane model systems such as micelles or liposomes which can mimic specific situations occurring in cells.

The Role of Porphyrin‐to‐Porphyrin Linkage Topology in the Extensive Modulation of the Absorptive and Emissive Properties of a Series of Ethynyl‐ and Butadiynyl‐Bridged Bis‐ and Tris(porphinato)zinc Chromophores

Abstract: A new method for the regulation of the photophysical properties of highly conjugated porphyrin arrays is described. The absorptive and emissive signatures of such supramolecular structures can be modulated to an impressive degree by regulation of: i) the extent of the steric interactions that define the barrier to rotation about the conjugated bridge between the porphyrin chromophores, and ii) the magnitude of ground state chromophore–chromophore electronic communication within the supermolecule. The power of this approach is illustrated by the straightforward synthesis as well as the electronic and emission spectra of eight different porphyrin arrays in which (5,10,15,20-tetraphenylporphinato)zinc(11) and (10,20-diphenylporphinato)zinc(11) complexes are joined by ethyne or butadiyne groups. The points of connectivity of these bridges between the chromophoric building blocks are systematically varied to produce a series of supramolecular structures with meso-to-meso, meso-to-β, or β-to-β linkage topologies. These variations allow excellent control of the ground- and excited-state characteristics of the arrays in the series by regulating the degree of both excitonic and electronic porphyrin-to-porphyrin coupling. Our approach shows the precision with which photophysical properties can be engineered in appropriately designed supramolecular systems.

Self-Organized Porphyrin Array on Iodine-Modified Au(111) in Electrolyte Solutions: In Situ Scanning Tunneling Microscopy Study

Abstract: The visualization of highly ordered adlayers of 5,10,15,20-tetrakis(N-methylpyridinium-4-yi)-21H,23H-porphine tetrakis(p-toluenesulfonate)(TMPyP) on the iodine-modified Au(111) in 0.1 M perchloric acid solutions was achieved by in situ scanning tunneling microscopy (STM). TMPyP was found to form highly ordered molecular layers on the iodine-modified Au(111) substrate. High-resolution images of individual TMPyP molecules showed a typical square porphyrin ring with additional four bright spots around the ring. These bright spots are attributed to the pyridinium units.

Intramolecular interactions in the ground and excited state of tetrakis(N-methylpyridyl)porphyrins.

Abstract: The fluorescent properties of the cationic free base tetrakis(4-N-methylpyridy1)porphyrin (H2TMPyP(4)) in aqueous solution have been the subject of considerable discussion. Conclusions by various authors on the presence of homo-aggregation of H2TMPyP in these solutions are contradictory. The present work reports spectroscopic data for three isomers of H2TMPyP(n) (n = 2,3, or 4) at varying concentrations, solvent polarity, and temperature. 'H NMR spectra of H2TMPyP(4) show no ground-state monomers below M in water. Fluorescence spectra of the three isomers in aqueous solutions indicate the absence of aggregates both in the ground and excited state. Fluorescence lifetimes of the three isomers both in solution as well as adsorbed on solid surfaces can be explained by taking into account their dependence on the steric hindrance for rotation of the pyridinium groups of the three isomers with respect to the porphyrin macrocycle. Molecular mechanics confirms a higher degree of steric hindrance of H2TMPyP(2) as compared to the two other isomers. From the experimental results it is concluded that the first excited singlet state SI of the porphyrin mixes with a nearby CT state slightly above this SI state. In this CT state an electron is transferred from the porphyrin macrocycle to the pyridinium group. The amount of SI-CT mixing, responsible for the spectroscopic differences, is determined by the degree of coplanarity and resonance interaction of the porphyrin and the pyridinium n-systems, and by the solvent polarity which determines the energy difference between the two states.

Hexaalkyl Terpyrrole: A New Building Block for the Preparation of Expanded Porphyrins

Abstract: A new, general synthesis of the first β-substituted tetra- and hexaalkyl terpyrroles is described. Also described are two new classes of expanded porphyrins derived from the hexaalkyl terpyrrole. The key step in the terpyrrole formation is the copper(II)-mediated oxidative coupling of the LDA-derived enolates of α-keto pyrroles. The first new expanded porphyrin reported here, the so-called “orangarin”, contains five pyrrolic subunits and two bridging carbon atoms, and is formally a 20π-electron nonaromatic macrocycle. The second new class of expanded porphyrins, the “amethyrins”, are 24π-electron nonaromatic macrocycles containing six pyrrole units. Both of these new macrocycles, as well as one of the new terpyrrolic precursors have been structurally characterized by single crystal X-ray diffraction analysis.

Intramolecular Interactions in the Ground and Excited State of

Abstract: The fluorescent properties of the cationic free base tetrakis(4-N-methylpyridy1)porphyrin (H2TMPyP(4)) in aqueous solution have been the subject of considerable discussion. Conclusions by various authors on the presence of homo-aggregation of H2TMPyP in these solutions are contradictory. The present work reports spectroscopic data for three isomers of H2TMPyP(n) (n = 2,3, or 4) at varying concentrations, solvent polarity, and temperature. 'H NMR spectra of H2TMPyP(4) show no ground-state monomers below M in water. Fluorescence spectra of the three isomers in aqueous solutions indicate the absence of aggregates both in the ground and excited state. Fluorescence lifetimes of the three isomers both in solution as well as adsorbed on solid surfaces can be explained by taking into account their dependence on the steric hindrance for rotation of the pyridinium groups of the three isomers with respect to the porphyrin macrocycle. Molecular mechanics confirms a higher degree of steric hindrance of H2TMPyP(2) as compared to the two other isomers. From the experimental results it is concluded that the first excited singlet state SI of the porphyrin mixes with a nearby CT state slightly above this SI state. In this CT state an electron is transferred from the porphyrin macrocycle to the pyridinium group. The amount of SI-CT mixing, responsible for the spectroscopic differences, is determined by the degree of coplanarity and resonance interaction of the porphyrin and the pyridinium n-systems, and by the solvent polarity which determines the energy difference between the two states.

Effectiveness of delta-aminolevulinic acid-induced protoporphyrin as a photosensitizer for photodynamic therapy in vivo.

Abstract: We examined the effectiveness of systemic administration of delta-aminolevulinic acid (delta-ALA) to induce endogenous protoporphyrin as a regimen for use in photodynamic therapy (PDT) of transplanted R3230AC rat mammary adenocarcinomas in vivo. Levels of porphyrins synthesized in various tissues after systemic administration of delta-ALA differed, with their accumulation in tumor tissue being dependent on both the dose and the time after delta-ALA administration. Tumor, liver, and intestine contained greater than 3.0 micrograms porphyrin/g tissue at 3 h after delta-ALA injection, whereas porphyrin levels in rat skin and muscle at that time were an order of magnitude lower. Analysis of tissue by HPLC revealed that the predominant porphyrin synthesized in tumors was protoporphyrin IX, whereas in liver, 18% of the total porphyrin detected was protoporphyrin IX, and in muscle, it was undetectable. Time-dependent studies of the uptake of 14C label from delta-ALA into the various tissues were not predictive of either the total amount of porphyrin or which porphyrin species would be present at 3 h after delta-ALA injection. Additionally, no simple relationship was apparent between the activities of certain selected enzymes involved in heme biosynthesis and the concentrations of porphyrins in the different tissues. High levels of tumor protoporphyrin IX were sustained by administration of two sequential doses of delta-ALA, at 3.0 and 1.5 h prior to irradiation. Using these treatment conditions, we inhibited R3230AC growth to an extent that was comparable to that obtained for Photofrin-induced PDT. High energy phosphate metabolism, measured by nuclear magnetic resonance spectroscopy in vivo, was dramatically impaired after delta-ALA-based PDT, with tumor ATP levels reduced to near zero by 4 h after irradiation. Our results demonstrated that delta-ALA-based PDT may be an alternative to current treatment protocols that use exogenously administered photosensitizers.

Multistep electron transfer between porphyrin modules assembled around a ruthenium center

Abstract: A new strategy has been devised for the construction of photoactive multicomponent arrays based on metal ion chelation whereby bisporphyrins have been assembled around a central ruthenium(II) bis(terpyridyl) complex. One of the terminal subunits is a gold (III) porphyrin while the second terminus is selected from a gold (III), zinc(II), or free-base porphyrin. Photophysical properties have been measured for each of the tripartitic compounds using ultrafast transient absorption and emission spectroscopy. Excitation into the central ruthenium(II) bis(terpyridyl) complex is followed by rapid intramolecular triplet energy transfer to one of the appended porphyrins. Direct excitation into the gold(III) porphyrin subunit generates the corresponding triplet excited state which is unreactive toward energy- or electron-transfer processes. In contrast, excitation into the zinc(II) or free-base porphyrin produces the corresponding excited singlet state which transfers an electron to the adjacent ruthenium(II) bis(terpyridyl) complex. Secondary electron transfer to the appended gold(III) porphyrin competes with reverse electron transfer such that the redox equivalents become separated by about 30 A. The original ground-state system is restored by relatively slow interporphyrin electron transfer. The energetics for each of these electron-transfer steps have been evaluated. 51 refs., 10 figs., 1 tab.

Ligand binding by butadiyne-linked porphyrin dimers, trimers and tetramers

Abstract: The binding of oligopyridyl ligands to butadiyne-linked zinc porphyrin dimers, trimers and tetramers has been investigated in detail using NMR and electronic spectroscopy. Pyridine binds to zinc porphyrin monomers in CH2Cl2 solution at 300 K with binding constants of ca. 103 mol–1 dm3, while 4,4′-bipyridyl binds to the cyclic zinc porphyrin dimer with a binding constant of 1 × 109 mol–1 dm3, giving an effective molarity, or chelation factor, of 76 mol dm–3. The analogous linear dimer binds to this ligand 100 times less strongly, but adopts a similar conformation when bound. s-Tri(4-pyridyl)triazine has an affinity of ca. 1010 mol–1 dm3 for the cyclic zinc trimer, reflecting reasonably good host–guest complementarity. The affinity of 4,4′-bipyridyl for the trimer is ca. 105 mol–1 dm3, implying two-point binding accompanied by host distortion and strain which reduce the binding constant; the trimer is therefore an elastic host, able to respond to the geometrical demands of rigid guests. The cyclic tetramer is a relatively flexible molecule, but its complexes with both bidentate and tetradentate ligands have more highly defined geometries. The cyclic dimer and trimer have open pre-organised cavities, with no conformational barrier to ligand binding inside the cavity, whereas the cyclic tetramer can adopt many conformations in free solution.

Conserved nonplanar heme distortions in cytochromes c

Abstract: A nonplanar distortion of the heme ofc-type cytochromes is conserved in the proteins isolated from diverse species based upon a comprehensive analysis of available highresolution X-ray crystal structures. This distortion is induced through the cysteine thioether linkages between the porphyrin pyrrole groups and the polypeptide and results in an asymmetric pyrrole distortion. This asymmetry in the heme distortion is also conserved. For other heme proteins which lack these covalent bonds, nearly planar porphyrins are observed. Resonance Raman evidence indicates that nonplanar distortion of porphyrins containing metals, like iron, with large core sizes (≳2.00 A) is energetically unfavorable and can occur only in the presence of significant environmental perturbations. Further, energy minimization and dynamics calculations on the ferric form of yeast iso-1-cytochromec, starting from the crystallographic coordinates and using a molecular mechanics force field which accurately reproduces nonplanar distortions in metalloporphyrins, suggest that this distortion is indeed maintained by the protein tertiary structure. It is proposed that this protein-linked heme distortion modulates electron transfer function through modification of redox potentials of the porphyrin ring and the protein binding properties ofc-type cytochromes.

Optical, Electrochemical, and Electrocatalytic Properties of Self-Assembled Thiol-Derivatized Porphyrins on Transparent Gold Films

Abstract: Visible spectroscopy and electrochemistry have been employed to study monolayer films of 5,10,15,-20-tetrakis[o-(2-mercaptoethoxy)phenyl]porphyrin in free base, H 2 (o-TMEPP), and cobalt-metalated Co-(o-TMEPP) forms chemisorbed onto transparent gold electrodes. Transmission spectra of these films are consistent with approximately monolayer ((4.8-6.5) x 10 11 mol/cm 2 ) coverages and edge-edge porphyrin-porphyrin interactions within the monolayers as revealed by red shifts and broadening in the Soret bands compared to solution species. Chemical modifications of H 2 (o-TMEPP) monolayers, namely, cobalt metalation and protonation/deprotonation, are described. The pK a of the free base porphyrin monolayer is ca. 2.5. Spectroelectrochemical experiments of Co(o-TMEPP) monolayers allow optical tracking of the redox state of the porphyrin, which was applied in electrocatalytic dioxygen reduction to show that the onset of dioxygen reduction is coincident with the Co(III) → Co(II) process in the monolayer and that the lower limit of the two-electron catalysis rate constant is 6 x 10 4 M -1 s -1 .