Showing papers on "Porphyrin published in 1999"

Cooperative Self-Assembly of Double-Strand Conjugated Porphyrin Ladders

Abstract: A series of conjugated zinc porphyrin oligomers, from the dimer through to the hexamer, form stable ladder complexes with linear bidentate ligands such as 1,4-diazabicyclo[2.2.2]octane (DABCO) and 4,4‘-bipyridyl (Bipy). The 1H NMR spectra of these ladders confirm their structures and show how they fray and dissociate in the presence of excess ligand. The thermodynamics of these equilibria were elucidated by spectrophotometric titration, using multivariate global factor analysis, in two different solvents (chloroform and toluene). Ladder formation and dissociation exhibit many indications of positive cooperativity, including all-or-nothing two-state assembly, sigmoidal isotherms, large Hill coefficients, and narcissistic self-sorting. Ladder formation increases the planarity and conjugation, resulting in a reduction in the gap between the highest occupied and lowest unoccupied molecular orbitals.

Selective supramolecular porphyrin/fullerene interactions

Abstract: Naturally assembling cocrystallates of C60 and C70 fullerenes with tetraphenylporphyrins (H2TPP·C60·3 toluene, 1; H2T3,5-dibutylPP·C60, 2; H2T3,5-dimethylPP·1.5C60·2 toluene, 3; H2TpivPP·C60, 4; H2T3,5-dimethylPP·C70·4 toluene, 5; ZnTPP·C70, 6; NiT4-methylPP·2C70·2 toluene, 7) show unusually short porphyrin/fullerene contacts (2.7−3.0 A) compared with typical π−π interactions (3.0−3.5 A). In the C60 structures, an electron-rich, 6:6 ring juncture, C−C bond lies over the center of the porphyrin ring. In the C70 structures, the ellipsoidal fullerene makes porphyrin contact at its equator rather than its poles; a carbon atom from three fused six-membered rings lies closest to the center of the porphyrin. These structures provide an explanation for the manner in which tetraphenylporphyrin-appended silica stationary phases effect the chromatographic separation of fullerenes. The interaction of the curved π surface of a fullerene with the planar π surface of a porphyrin, without the need for matching convex wit...

Interaction of Curved and Flat Molecular Surfaces. The Structures of Crystalline Compounds Composed of Fullerene (C60, C60O, C70, and C120O) and Metal Octaethylporphyrin Units

Abstract: Solutions of C60, C60O, or C70 and metal complexes of octaethylporphyrin (OEPH2) yield crystals that contain both the fullerene and the porphyrin. The structures of C60·2CoII(OEP)·CHCl3, C60·2ZnII(OEP)·CHCl3, and C60O·2CoII(OEP)·CHCl3 are isomorphous and contain an ordered C60 cage surrounded by two MII(OEP) units. Although there is no covalent bond between the fullerene and porphyrin components, the separation between these units is shorter than normal van der Waals contact. Crystals of C70·CoII(OEP)·C6H6·CHCl3, C70·NiII(OEP)·C6H6·CHCl3, and C70·CuII(OEP)·C6H6·CHCl3 are also isomorphous with an ordered fullerene, but have only one porphyrin/fullerene contact. Crystalline C60·ClFeIII(OEP)·CHCl3 lacks the close face-to-face porphyrin/porphyrin contact that is common to all of the other structures reported here but retains the intimate contact between the porphyrin and the fullerene. In (C120O)·CoII(OEP)·0.6C6H6·0.4CHCl3 the fullerene dimer is enclosed by two CoII(OEP) moieties. Unfortunately disorder in th...

An Artificial Photosynthetic Antenna-Reaction Center Complex

Abstract: A model photosynthetic antenna consisting of four covalently linked zinc tetraarylporphyrins, (PZP)3−PZC, has been joined to a free base porphyrin-fullerene artificial photosynthetic reaction center, P−C60, to form a (PZP)3−PZC−PC60 hexad. As revealed by time-resolved absorption and emission studies, excitation of any peripheral zinc porphyrin moiety (PZP) in 2-methyltetrahydrofuran solution is followed by singlet−singlet energy transfer to the central zinc porphyrin to give (PZP)3−1PZC−P−C60 with a time constant of ∼50 ps. The excitation is passed on to the free base porphyrin in 240 ps to produce (PZP)3−PZC−1P−C60, which decays by electron transfer to the fullerene with a time constant of 3 ps. The (PZP)3−PZC−P•+−C60•- charge-separated state thus formed has a lifetime of 1330 ps, and is generated with a quantum yield of 0.70 based on light absorbed by the zinc porphyrin antenna. The complex thus mimics the basic functions of natural photosynthetic antenna systems and reaction center complexes.

Efficient, Saturated Red Organic Light Emitting Devices Based on Phosphorescent Platinum(II) Porphyrins

Abstract: Two new platinum(II) porphyrins have been synthesized and their luminescent properties have been studied. The platinum porphyrins exhibited strong phosphorescence in the red with narrow line widths. When they were doped into aluminum(III) tris(8-hydroxyquinolate) (AlQ3) in the electron-transporting and -emitting layer of an organic light-emitting device, energy transfer occurred between the host AlQ3 and the platinum porphyrin. Bright saturated red emission with high efficiency at low to moderate current density has been achieved. In the high current regime, the electroluminescence efficiency decreased and the perceived emission color blue shifted as a result of mixed emission from the platinum porphyrin and AlQ3. This current dependence was due to the saturation of triplet emissive sites, because of the long-lived phosphorescence state of the platinum porphyrin complex.

Photoinduced energy and electron transfer reactions in lamellar polyanion/polycation thin films: Toward an inorganic 'leaf'

Abstract: Sequential adsorption of polyanions and polycations was used to make a five-component energy/electron-transfer cascade, which mimics some of the functions of natural photosynthetic assemblies. The photon antenna part of the system consists of coumarin- and fluorescein-derivatized poly(allylamine hydrochloride) (Coum-PAH and Fl-PAH), palladium(II)tetrakis(4-N,N,N-trimethylanilinium) porphyrin (PdTAPP4+) or palladium(II)tetrakis(4-sulfonatophenyl) porphyrin (PdTSPP4-) layers, interleaved with anionic Zr(HPO4)2·H2O (α-ZrP) sheets. α-ZrP or HTiNbO5 sheets separate the porphyrin electron donor from a polyviologen electron acceptor layer. Layer-by-layer growth of these thin film assemblies was characterized by atomic force microscopy (AFM) and ellipsometry on planar supports, and by elemental analysis, surface area measurements, and transmission electron microscopy high on surface area silica supports. UV−vis absorption and steady-state emission spectroscopies showed that the overall energy/electron-transfer re...

Self-Assembly of Porphyrin Arrays via Coordination to Transition Metal Bisphosphine Complexes and the Unique Spectral Properties of the Product Metallacyclic Ensembles

Abstract: Self-assembly of predesigned angular and linear dipyridylporphyrin modules with bisphosphine-coordinated Pd(II) and Pt(II) angular and linear modules leads to cyclic porphyrin arrays containing two or four units and ranging in size from 15 to 39 A. Multinuclear NMR spectra indicate high symmetry for these macrocycles. Restriction in rotation of trans-DPyDPP groups around the axis defined by the terminal metal−nitrogen bonds distorts the symmetry of the tetramers, but the rotation is unrestricted at elevated temperatures. Chiral metal triflates containing R(+)- or S(−)-BINAP phosphines promote formation of enantiomeric macrocycles with a puckered geometry. CD spectra of the chiral macrocycles reveal a strong exciton coupling between the porphyrin chromophores in the tetramers. Emission spectra reveal moderate fluorescence quenching of the dipyridylporphyrin fluorophores upon treatment with metal triflates and concomitant incorporation into the macrocycles.

Photoluminescence and Transient Spectroscopy of Free Base Porphyrin Aggregates

Abstract: Solutions of free base meso-tetraphenylporphyrin (H2TPP), meso-tetra(4-carboxyphenyl)porphyrin (H2TPPC), and meso-tetra(4-pyridyl)porphyrin (H2TPyP) under various conditions generate aggregates whose absorption spectra are characterized by invariant Soret bands with bandwidths that are independent of the preparative method. One of the Soret bands is blue-shifted (H-aggregate) relative to the monomeric porphyrin band; other Soret bands are red-shifted (J-aggregates). The aggregates are characterized by different nonradiative rate constants for excited singlet-state decay and by different efficiencies of singlet−singlet annihilation at the high energies of laser excitation. The quantum yields of fluorescence vary between 10-5 and 10-2, and the corresponding fluorescence lifetimes vary in the range from 10-12 to 10-9 s; they are more than 1 order of magnitude smaller than those of the corresponding monomeric porphyrins. Lifetimes (τ) correlate with the characteristic ground-state absorption recovery times of...

Theoretical Study of the Q and B Bands of Free-Base, Magnesium, and Zinc Porphyrins, and Their Derivatives

Abstract: The multireference Moller-Plesset perturbation (MRMP) theory with complete active space self-consistent field (CASSCF) reference functions is applied to the study of the Q and B bands of free-base, magnesium, and zinc porphyrins and their derivatives. The Q band of free-base porphyrin is so weak because the transition is pseudoparity-forbidden because of the alternancy symmetry. The Q band in the visible region can be intensified if the pairing property is broken. This is achieved by various chemical modifications to the basic structure of free-base porphyrin such as external substituents, changes in conjugation pathway, and change in central substituent. The intensity and color of the Q band as well as the macrocyclic porphyrin skeleton are perturbed to a greater or lesser extent by various chemical modifications. Insertion of aza groups in meso position leads to the shrinking of the size of the central hole, whereas hydrogenation to the exo double bond of pyrrole rings expands the central hole. Substitution of nitrogen atoms for meso carbon atoms significantly intensifies the oscillator strength of the higher energy side transition of the Q band with the peak position unchanged. Ring reduction widens the splitting of two peaks of the Q band, giving rise to the characteristically strong far-red band and the weaker visible band. Changes from metal-free to metalloporphyrins have a less dramatic effect on the spectra. Porphyrins and their derivatives play an important role in the biochemistry of all living systems. They form the backbone of pigments such as chlorophyll and heme, which have a universal biological distribution and were involved in the oldest metabolic phenomena on earth. Thus, knowledge of these systems and their excited states is essential in understanding a wide variety of biological processes, including oxygen binding, electron transfer, catalysis, and the initial photochemical step in photosynthesis. It was recognized early that the intensity and color of porphyrins are derived from the highly conjugated ﷿-electron systems. The electronic spectra of porphyrin compounds are characterized by three basic regions. The so-called Q band is relatively weak and occurs in the visible region. The Q band consists of a degenerate electronic transition for divalent metalloporphyrins and two separate electronic transitions for free-base porphyrins (FBP). The intense Soret or B band occurs in the near-UV and is often accompanied by a closely related N band of lower intensity. The B band for divalent metalloporphyrins is assumed to be a degenerate pair. The higher UV bands in the third region are broad and are often of near uniform

Interaction of the Tetra(4-sulfonatophenyl)porphyrin with Ionic Surfactants: Aggregation and Location in Micelles

Abstract: Interactions of the water-soluble tetra(4-sulfonatophenyl)porphyrin (TPPS4) with ionic micelles in aqueous solutions have been studied by optical absorption, fluorescence, resonance light-scattering (RLS), and 1H NMR spectroscopies The presence of three different species of TPPS4 in cationic cetyltrimethylammonium chloride (CTAC) solution has been unequivocally demonstrated: free porphyrin monomers, monomers bound to micelles, and nonmicellar porphyrin/surfactant aggregates This result is similar to our previous findings for TPPS4 interactions with biomacromolecules (serum albumin and DNA) The surfactant:porphyrin ratio for maximum aggregate formation is around 4:1−5:1 and 14:1 at pH 30 and pH 75, respectively In the case of zwitterionic N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) the aggregates were not observed Binding constants estimated from these data were of the order of 104 M-1 for CTAC and HPS Our data show that solubilization of porphyrins within nonpolar regions of micel

Self-assembled porphyrin-c60 and porphycene-c60 complexes via metal axial coordination

Abstract: Noncovalently linked electron donor−acceptor complexes consisting of either zinc tetraphenylporphyrin, (TPP)Zn, or zinc octaethylporphycene, (OEPc)Zn, as donor and pyridine appended C60, py∼C60, as...

Dendritic polyglutamic porphyrins : probing porphyrin protection by oxygen-dependent quenching of phosphorescence

Abstract: Phosphorescence quenching of the porphyrin core by O2 has been studied in a series of dendritic polyglutamic porphyrins. As the generation number increases, in water the dendritic ligands make it increasingly difficult for oxygen to reach the Pd–porphyrin core (figure); they provide no protection in DMF, since there the structure of the dendrimer is more open.

Self-assembly in water of the sodium salts of meso-sulfonatophenyl substituted porphyrins

Abstract: The aggregation in water of the free bases and the diprotonated forms of p-sulfonatophenyl and phenyl meso-substituted porphyrins [sodium salts of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4), 5,10,15-tris(4-sulfonatophenyl)-20-phenylporphyrin (TPPS3), 5,10-bis(4-sulfonatophenyl)-15,20-diphenylporphyrin (TPPS2A), 5,15-bis(4-sulfonatophenyl)-10,20-diphenylporphyrin (TPPS2O), 5-(4-sulfonatophenyl)-10,15,20-diphenylporphyrin (TPPS1) and 5,15-bis(4-sulfonatophenyl)porphine (DPPS2O)] was studied by UV/Vis, 1H-NMR, fluorescence, polarized fluorescence and resonance light scattering. The presence of hydrophobic phenyl groups favours the aggregation. The free bases TPPS4 and TPPS3 aggregate into stacks of ill-defined geometry. TPPS2A and DPPS2O give H-aggregates. TPPS2O and TPPS1 give J-aggregates through interaction of the hydrophobic phenyl groups, that is their geometry corresponds to edge-to-edge arrangements. All these porphyrins give J-aggregates upon diprotonation, through electrostatic interactions between the positively charged porphyrin ring and the sulfonato group. These J-aggregates give different arrays depending on the meso-substitution pattern.

Intramolecular Electron Transfer of Diporphyrins Comprised of Electron-Deficient Porphyrin and Electron-Rich Porphyrin with Photocontrolled Isomerization

Abstract: New azobenzene-linked diporphyrin Zn complexes and free base analogues were synthesized in an attempt to develop new materials for molecular electronics. These diporphyrin Zn complexes and the corresponding diporphyrins consist of electron-deficient porphyrin Zn complex and electron-rich porphyrin Zn complex or electron-deficient porphyrin and electron-rich porphyrin. These compounds, which are a new family of diporphyrins, exhibited the photoresponsive property with a structual change in the antenna; the trans−cis photoisomerization and cis−trans thermal recovery occurred and this process was reversible. A particularly noteworthy characteristic of the diporphyrin Zn complex is that its fluorescence spectrum changes with isomerization; the fluorescence intensity of the cis-isomer produced by photoirradiation is smaller than that of the trans-isomer. This fluorescence quenching of the cis-isomer arises from the intramolecular electron transfer from electron-rich porphyrin Zn complex to electron-deficient p...

Significant Electronic Effect of Porphyrin Ligand on the Reactivities of High-Valent Iron(IV) Oxo Porphyrin Cation Radical Complexes.

Abstract: High-valent iron(IV) oxo porphyrin cation radical complexes containing a series of substituents at the meso position of the porphyrin ring (i.e., electron-donating and -withdrawing substituents on phenyl groups) were prepared and used in oxygen atom transfer reactions to elucidate the electronic effect of porphyrin ligands on the reactivities of iron porphyrin complexes. The reactions that we studied with the in situ generated high-valent iron oxo porphyrins were (1) the relative reactivities of the intermediates toward oxygen atom transfer and ROOH disproportionation (ROOH = hydrogen peroxide and tert-butyl hydroperoxide), (2) the mechanism of heterolytic versus homolytic O−O bond cleavage of hydroperoxides, (3) the dependence of oxidizing power of the intermediates on the electronic nature of porphyrin ligands, and (4) the relative rates between oxygen atom transfer and oxygen exchange with labeled H218O. We found from these reactivity studies that (1) a high-valent iron oxo porphyrin complex containing...

Thiol-Derivatized Porphyrins for Attachment to Electroactive Surfaces

Abstract: The attachment of porphyrin monomers and multiporphyrin arrays in controlled architectures on electroactive surfaces opens many opportunities for electrochemical studies. Toward this goal, we have developed routes for the preparation of thiol-derivatized porphyrin monomers and porphyrin building blocks that require minimal or no handling of free thiols. Routes to S-protected p-thiobenzaldehydes and m-(thiomethyl)benzaldehydes have been developed. Two sets of meso-substituted porphyrins with variation in electrochemical potentials have been prepared for vertical or horizontal orientation with respect to the electroactive surface. In one set, each porphyrin bears one S-protected p-thiophenyl unit and substituents at the three remaining meso-positions. In the other set each porphyrin possesses four S-protected m-(thiomethyl)phenyl units. Tuning the electrochemical potential in the former set has been achieved by variation of the meso substituents (mesityl, 2,4,6-trimethoxyphenyl, n-pentyl, pentafluorophenyl)...

Photodynamics of a constrained parachute-shaped fullerene-porphyrin dyad

Abstract: The pronounced ability of fullerene C60 to act as an electron and energy acceptor has led to the synthesis of a large number of compounds in which C60 is covalently linked to photoactivatable groups which can serve as potential donors. Such compounds are of interest as model systems for photosynthetic reaction centers and also have potential applications in photodynamic therapy because of the highly efficient photosensitization of singlet molecular oxygen formation by C60 and C60 derivatives. By far the largest number of such systems studied to date utilize porphyrins as antennas for efficient light capture in the visible region of the spectrum, and a variety of linkers. Photophysical studies as well as molecular modeling indicate that in conformationally flexible dyads the porphyrin (P) and C60 moieties are in close proximity, due to -stacking interactions, thus facilitating through-space interactions, as demonstrated by quenching of 1P* fluorescence and generation of fullerene-excited states (by energy transfer) or P+-C60- ion-pair states (by electron transfer).2,4a,b,f-h These ion-pair states can be relatively long-lived, due to the small reorganization energy and strong thermodynamic driving force for back-electron transfer, which places this process within the Marcus inverted region.4g, Recently attention has focused on rigidly linked systems in which the porphyrin (P) and (C60) moieties are in enforced close proximity or are forced apart by a saturated norbornylogous or steroid linker. As part of a program to understand the nature of the dialogue between P and C60 chromophores as the topology of P-C60 dyads is systematically varied, we now report photophysical data for the parachute-shaped dyad 1 and the corresponding zinc complex 1-Zn. We have reported previously the synthesis of 1 by Bingel-Hirsch addition of a strapped porphyrin malonate to C60.

Dendritic Iron Porphyrins with Tethered Axial Ligands: New Model Compounds for Cytochromes.

Abstract: The quantitative evaluation of the effect of an insulating dendritic shell on the redox properties of an embedded iron porphyrin core has been achieved for the first time using water-soluble dendritic iron porphyrins with tethered axial ligands (shown schematically). Thus, these complexes, whose redox properties have been determined by chemical and electrochemical methods in solvents of different polarity (CH(2)Cl(2), MeCN, H(2)O), are valid mimics for cytochromes.

Completely Regioselective Synthesis of Directly Linkedmeso,meso andmeso,β Porphyrin Dimers by One-Pot Electrochemical Oxidation of Metalloporphyrins

Abstract: The metal ion determines the reactivity upon electrochemical oxidation of metal porphyrins that are unsubstituted at the meso position. Thus, with Cu2+, Pd2+, and Ni+2 as the central ion (and for the free porphyrin base), directly linked meso,β porphyrin dimers 2 were formed, while Mg and Zn complexes afforded the corresponding directly linked meso,meso porphyrin dimers 1. Ar=3,4-tBu2C6H3.

Cationic 5,10,15,20-Tetrakis(N-methylpyridinium-4-yl)porphyrin Fully Intercalates at 5‘-CG-3‘ Steps of Duplex DNA in Solution†

Abstract: The interaction of 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin (T4MPyP4+) with the oligonucleotide DNA duplex [d(GCACGTGC)]2 was studied by two-dimensional 1H NMR spectroscopy, optical absorbance, circular dichroism, and molecular dynamics simulation employing particle mesh Ewald methods. T4MPyP4+ is one of the largest aromatic molecules for which intercalative binding to DNA has been proposed, although this has been called into question by recent X-ray crystallographic work [Lipscomb et al. (1996) Biochemistry 35, 2818−2823]. T4MPyP4+ binding to [d(GCACGTGC)]2 produced a single set of (mostly) upfield-shifted DNA resonances in slow exchange with the resonances of the free DNA. Intra- and intermolecular NOEs observed in the complex showed that the porphyrin intercalates at the central 5‘-CG-3‘ step of the DNA duplex without disrupting the flanking base pairs. Absorption and circular dichroism spectra of the complex also support intercalative binding. Molecular dynamics simulations (using explici...

Novel Core-Modified Expanded Porphyrins with meso-Aryl Substituents: Synthesis, Spectral and Structural Characterization

Abstract: The synthesis, spectral and structural characterization of meso-aryl sapphyrins and rubyrins containing heteroatoms such as S, O, Se in addition to pyrrole nitrogens are reported. The synthesis of the desired expanded porphyrins has been achieved using a single precursor, the modified tripyrranes containing heteroatoms, through an unprecedented oxidative coupling reaction in moderately good yields. The product distribution and the isolated yields were found to be dependent on the nature of the acid catalyst and its concentration. Use of 0.1 equiv of acid exclusively gave 26π rubyrins while a higher concentration of acid gave a mixture of 18π porphyrin, 22π sapphyrin, and 26π rubyrin. Two additional products, 22π oxasmaragdyrin and 18π oxacorrole, were isolated in the reaction of oxatripyrrane. All of the sapphyrins and rubyrins exhibit well-defined intense Soret and Q-bands in the visible region, and the intensity and the position of the absorption maxima were dependent on the number and the nature of the...

"Axial-Bonding"-Type Hybrid Porphyrin Arrays: Synthesis, Spectroscopy, Electrochemistry, and Singlet State Properties.

Abstract: A series of phosphorus(V), germanium(IV), and tin(IV) porphyrin-based, "axial-bonding"-type hybrid trimers have been readily constructed by employing a new "building-block" approach. The approach is modular in nature, and it involves simple "inorganic" reactions such as axial bond formation of main group element containing porphyrins and insertion of metal/"metalloid" ions into the porphyrin cavity. The architecture of these arrays is such that, while a phosphorus(V), germanium(IV), or tin(IV) complex of meso-5,10,15,20-(tetratolyl)porphyrin forms the basal scaffolding unit, the free-base, vanadyl, cobalt(II), nickel(II), copper(II), or zinc(II) porphyrins occupy the two axial sites via an aryloxy bridge. Synthesis of an "all-phosphorus" array containing three phosphorus(V) subunits has also been accomplished. Each new porphyrin array investigated in this study has been fully characterized by various physical methods that include mass (FAB), UV-visible, infrared, fluorescence, electron spin resonance (ESR), and (1)H and (31)P nuclear magnetic resonance (NMR; 1D and 2D) spectroscopies and cyclic voltammetry. The UV-visible and ESR spectral parameters and also the redox potential data suggest that there exists no interaction between the pi-planes of the constituent monomeric porphyrins in these arrays. Detailed (1)H NMR investigations carried out with the trimers containing diamagnetic porphyrins reveal characteristic shielding/deshielding effects for the various protons on the axial porphyrin subunits. The ground state data, as probed by the spectroscopic and electrochemical techniques, collectively indicate that there exists a symmetric but nonparallel disposition of the two axial porphyrins with respect to plane of the central porphyrin. Singlet state activity of the photoactive trimers has been probed by the steady state fluorescence method with selective excitation into the bands corresponding to the two constituent monomeric species. Analysis of the fluorescence emission and excitation spectral data suggests the occurrence of electronic energy transfer as well as photoinduced electron transfer reactions in trimers endowed with free-base or zinc(II) porphyrin axial subunits. Efficiencies of the excited state processes of these trimeric arrays are shown to be dependent on the type of metal/metalloid ions present in the porphyrin crevice.