Pyrrole Units Research Articles

Electron-Transfer and Acid−Base Properties of a Two-Electron Oxidized Form of Quaterpyrrole that Acts as Both an Electron Donor and an Acceptor

Electron-transfer interconversion between the four-electron oxidized form of a quaterpyrrole (abbreviated as P4 for four pyrroles) and the two-electron oxidized form (P4H2) as well as between P4H2 and its fully reduced form (P4H4) bearing analogous substituents in the alpha- and beta-pyrrolic positions was studied by means of cyclic voltammetry and UV-visible spectroelectrochemistry combined with ESR and laser flash photolysis measurements. The two-electron oxidized form, P4H2, acts as both an electron donor and an electron acceptor. The radical cation (P4H2*+) and radical anion (P4H2*-) are both produced by photoinduced electron transfer from dimeric 1-benzyl-1,4-dihydronicotinamide to P4H2, whereas the cation radical form of the compound is also produced by electron-transfer oxidation of P4H2 with [Ru(bpy)3]3+. The ESR spectra of P4H2*+ and P4H2*- were recorded at low temperature and exhibit spin delocalization over all four pyrrole units. Thus, the two-electron oxidized form of the quaterpyrrole (P4H2) displays redox and electronic features analogous to those seen in the case of porphyrins and may be considered as a simple, open-chain model of this well-studied tetrapyrrolic macrocycle. The dynamics of deprotonation from P4H2*+ and disproportionation of P4H2 were examined by laser flash photolysis measurements of photoinduced electron-transfer oxidation and reduction of P4H2, respectively.

Novel Modified Pyrrole Monomers, 2: Behaviour in Water Solutions

Aggregate formation of modified pyrroles was investigated in water and water/ethanol mixtures by means of light scattering for novel reactive single-chain alkyl phosphonate containing pyrrole units. The influence of concentration, pH, temperature and organic solvents on the evolution of the vesicle size is discussed. Long-chain alkyl phosphonates functionalized with pyrrole units form stable vesicles in water solutions. Lowering of the pH value results in a partial protonation of the phosphonate groups and in less electrostatic repulsion between the anionic groups, leading to larger vesicle diameters and higher tendency to aggregate. Addition of ethanol, as well as increase in temperature cause an increase in the aggregate size due to better solubility of alkyl phosphonates. The reactive vesicles are extremely interesting templates for oxidative polymerisation of pyrrole and they can provide the formation of novel particles based on conjugated polymers.

Syntheses, Structures, and Coordination Chemistry of Phosphole-Containing Hybrid Calixphyrins: Promising Macrocyclic P,N2,X-Mixed Donor Ligands for Designing Reactive Transition-Metal Complexes

The syntheses, structures, and coordination chemistry of phosphole-containing hybrid calixphyrins (P,N2,X-hybrid calixphyrins) and the catalytic activities of their transition-metal complexes are reported. The 5,10-porphodimethene type 14pi-P,(NH)2,X- and 16pi-P,N2,X-hybrid calixphyrins (X = O, S, NH) are prepared via acid-promoted dehydrative condensation between a sigma4-phosphatripyrrane and the corresponding 2,5-bis[hydroxy(phenyl)methyl]heteroles followed by DDQ oxidation. Both spectroscopic and crystallographic data of the hybrid calixphyrins have revealed that the conformation and size of the macrocyclic platforms as well as the oxidation state of the -conjugated pyrrole-heterole-pyrrole (N-X-N) units vary considerably depending on the combination of heteroles. The sigma3-P,(NH)2,S- and sigma3-P,N2,S-hybrids react with Pd(OAc)2 and Pd(dba)2, respectively, to afford the same Pd(II)-P,N2,S-hybrid complex, in which the calixphyrin platform is regarded as a dianionic ligand. In the complexation with [RhCl(CO)2]2 in dichloromethane, the sigma3-P,N2,S-hybrid behaves as a neutral ligand to afford an ionic Rh(I)-P,N2,S-hybrid complex, whereas the sigma3-P,N2,NH-hybrid behaves as an anionic ligand to produce Rh(III)-P,N3-hybrid complexes. In the latter reaction, it is likely that a neutral Rh(I)-P,N3-hybrid complex, generated as a highly nucleophilic intermediate, undergoes C-Cl bond activation of the solvent. The complexation of AuCl(SMe2) with the sigma3-P,N2,X-hybrids (X = S, NH) leads to the formation of the corresponding Au(I)-monophosphine complexes. The spectral data and crystal structures of these metal complexes exhibit the hemilabile nature of the phosphole-containing hybrid calixphyrin platforms derived from the flexible phosphole unit and the redox active N-X-N units. The hybrid calixphyrin-palladium and -rhodium complexes catalyze the Heck reaction and hydrosilylations, respectively, implying that the metal center in the core is capable of activating the substrates under appropriate reaction conditions. The present results demonstrate the potential utility of the phosphole-containing hybrid calixphyrins as a new class of macrocyclic P,N2,X-mixed donor ligands for designing highly reactive transition-metal complexes.

Oligopyrrole Synthesis by 1,3‐Dipolar Cycloaddition of Azomethine Ylides with Bissulfonyl Ethylenes

One by one or two by two: In a general approach to the iterative construction of oligopyrroles, the cycloaddition of azomethine ylides derived from pyrrolyl α-iminoesters with 1,2-bis(phenylsulfonyl)ethylene is followed by the elimination of the sulfonyl groups in situ under basic conditions. This strategy is amenable to the introduction of one or two pyrrole units in each iterative cycle.

Thermodynamic Analysis Of Quadruplex Dna-Drug Interaction

This work studies the binding properties of distamycin and its carbamoyl analog, containing four pyrrole units, with the [d(TGGGGT)] 4 quadruplex by means of isothermal titration calorimetry (ITC). Analysis of the ITC data reveals that drug/quadruplex binding stoichiometry is 1:1 for both interactions and that distamycin analog gives approximately a 10-fold increase in the quadruplex affinity.

Ansa-Metallocene-Based Cyclic[2]pyrroles

Syntheses and X-ray structural analyses of ansa-metallocene-based cyclic[2]pyrroles are described. The pyrrole units in the macrocycles adopt partial 1,2-alternate conformation in the solid state.

Synthesis of 1,3-disubstituted benzo[ c]thiophene analogs containing benzo[ b]thiophene/benzo[ b]pyrrole

The synthesis of 1,3-disubstituted benzo[ c]thiophene analogs incorporating benzo[ b]thiophene/benzo[ b]pyrrole units is described.

Highly Luminescent 1,4-Diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole- (DPP-) Based Conjugated Polymers Prepared Upon Suzuki Coupling

Five new soluble conjugated polymers are described, which were prepared upon Suzuki polycondensation reactions. They alternately consist of dialkylated 1,4-diketo-3,6-diphenyl-pyrrolo[3,4-c]pyrrole (DPP) units and carbazole, triphenylamine, benzo[2,1,3]thiadiazole, anthracene, and fluorene units. The polymers were prepared from 1,4-diketo-2,5-dihexyl-3,6-bis(4-bromophenyl)pyrrolo[3,4-c]pyrrole (1a), 1,4-diketo-2,5-di(2-ethylhexyl)-3,6-bis(4-bromophenyl)pyrrolo[3,4-c]pyrrole (1b), or 1,4-diketo-2,5-dihexyl-3,6-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)pyrrolo[3,4-c]pyrrole (1c), and 3,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-ethylhexylcarbazole (2), 4,4‘-dibromotriphenylamine (3), 4,4‘-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)triphenylamine (4), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dihexylfluorene (5), 9,10-anthracenebispinacolatoboron ester (6), and 4,7-dibromo-2,1,3-benzothiadiazole (7). The polymers exhibit brilliant red colors. They are soluble in ...

Comparison of Benzene, Nitrobenzene, and Dinitrobenzene 2-Arylsulfenylpyrroles

The effectiveness of the 2,4-dinitrobenzenesulfenyl and 4-nitrobenzenesulfenyl groups as masking and directing groups at the 2-position of pyrrole has been investigated and compared to that of 2-phenylthiopyrrole. The presence of the nitro group(s) enhances stability of the corresponding pyrrole toward acid and does not significantly decrease the ability of the pyrrolic unit to undergo electrophilic aromatic substitution reactions in the form of formylation, nitration, and condensation with aldehydes. The synthetic utility of 2-(2,4-dinitrobenzenesulfenyl)pyrrole was demonstrated through the synthesis of meso-substituted dipyrromethanes. The sulfoxides 2-(2,4-dinitrobenzenesulfinyl)pyrrole and 2-(4-nitrobenzenesulfinyl)pyrrole underwent neither formylation nor nitration, and the increasing presence of nitro groups within the moiety at the 2-position resulted in decreased stability under acidic conditions.

Experimental and theoretical study of the pyrrole cluster photochemistry: Closing the πσ* dissociation pathway by complexation

Photolysis of size selected pyrrole clusters has been investigated and compared to the photolysis of an isolated pyrrole molecule. Experimentally, size distributions of different mean cluster sizes (n=3 and n>>5) have been prepared in supersonic expansions and the clusters were photolyzed at 243 and 193 nm. The kinetic energy distributions of the H photofragments have been measured. The distributions exhibit a bimodal character with fast and slow H-fragment peaks similar to the spectra of the bare molecule. However, with increasing cluster size the slow component gains intensity with respect to the fast one. A similar effect is observed with increasing the excitation energy from 243 to 193 nm. Theoretical calculations at the CASSCF/CASPT2 level have been performed for bare and complexed pyrroles (pyrrole is complexed with an argon atom and with another pyrrole unit). Combination of theoretical and experimental approaches leads to the conclusion that the direct dissociative pathway along the pisigma* potential energy surface in the N-H stretch coordinate is closed by the presence of the solvent molecule. This pathway is an important channel leading to the fast H atoms in the dissociation of the bare molecule. The solvent molecule influences significantly the electronic structure in the Rydberg-type pisigma* state while it has little influence on the valence states. The slow channel is mostly populated by the out-of-plane deformation mode which is also not influenced by solvation. We have also studied other possible reaction channels in pyrrole clusters (hydrogen transfer, dimerization). The present study shows that more insight into the bulk behavior of biologically relevant molecules can be gained from cluster studies.

Intermolecular and intramolecular coupling in charged monosubstituted hexapyrrolylbenzenes

Cyclic voltammetric and UV/vis and EPR spectroelectrochemical studies of new synthesized series of pyrrolylbenzenes indicate different oxidation products depending on the number of pyrrole units on the phenyl ring and on the pyrrole substitution. Both intra- and intermolecular coupling of charged monosubstituted hexapyrrolylbenzenes is evidenced from cyclovoltammetric and spectroelectrochemical studies. The optical and electrochemical properties as well as the extent of the intermolecular coupling strongly depend on the α and β substitution of 1- N-pyrrolyl group.

Poly( N-methyl pyrrole) and its copolymer with pyrrole for mild steel protection

In this study, it was aimed to overcome disadvantages of polypyrrole films like water up taking and resulting low stability, via copolymerization of pyrrole and N-methyl pyrrole and preparing their bilayer films. Poly( N-methyl pyrrole) coating was synthesized electrochemically on mild steel and its corrosion performance has been investigated in 3.5% NaCl aqueous solution, by using electrochemical impedance spectroscopy (EIS), anodic polarization curves and open circuit potential-time relation. This coating was found to have lower protection efficiency than single poly(pyrrole) coating. This case was related to weak adsorption behavior of poly( N-methyl pyrrole) film on mild steel. On the other hand, when this film was applied as a top coat on polypyrrole coated sample; the obtained coating system hindered the attack of corrosive environment and protected mild steel efficiently. The underlying poly(pyrrole) film was strongly adherent and the top poly( N-methyl pyrrole) coat improved barrier property by decreasing permeability and water mobility. Poly(pyrrole-co- N-methyl pyrrole) has also been synthesized from aqueous oxalic acid solutions containing various ratios of these two monomers. The copolymerization rate and protective behavior of copolymer were strongly affected by the monomer concentration ratio. The ratio of 1:3 ( N-methyl pyrrole/pyrrole) gave the most protective copolymer coating and it exhibited better barrier property than single PPy. This was related to introduction of –CH 3 group which creates hydrophobic effect, when compared to –NH group of pyrrole unit. However, the solution with the ratio of 1:1 yielded a coating which had lower protection efficiency than single poly(pyrrole) film. This behavior was attributed to significant decrease in adsorption strength as the ratio of n-methyl pyrrole increased.

Efficient Synthesis and Properties of Isomeric Photochromic Diarylethenes Having a Pyrrole Unit

Two new isomeric photochromic diarylethenes with a pyrrole unit have been developed. Their properties, including photochromic behavior, crystal structures, and fluorescent properties, have been investigated. The two isomeric compounds show distinctly different photochromism, both in solution and in the single-crystalline phase: one turns red upon photocyclization whereas the other turns blue, which may be attributed to the different substituent effects.

Sulfur-Containing Hetero-Calix[4]pyrroles as Mercury(II) Cation-Selective Receptors: Thermodynamic Aspects

Two sulfur-containing hybrid calix[4]pyrrole derivatives (III and IV) have been synthesized and fully characterized. Several analytical techniques (1H NMR, conductance measurements, UV-vis spectrophotometry, titration potentiometry, and titration calorimetry) have been used to assess the interaction between these hybrid calixpyrrole receptors and metal cations in acetonitrile and dimethylsulfoxide. The partition constants of calix[4]pyrrole, I, II, and IV in the acetonitrile-hexane solvent system and the solubilities of the ligands in various solvents at 298.15 K were determined. 1H NMR measurements reveal the sites of interaction of calixpyrrole ligands with metal cations in CD3CN. Conductance and UV-vis spectrophotometric measurements were performed to establish the composition of mercury(II) calixpyrrole complexes in acetonitrile at 298.15 K. Titration calorimetry was used to quantitatively assess Hg(II)-calixpyrrole interactions. Thus the thermodynamics of complexation of calixpyrrole ligands with the mercury(II) cation in acetonitrile at 298.15 K are reported. Potentiometric titrations were also used to establish the stepwise stability constants for the complexation of calix[3]thieno[1]pyrrole with the Hg(II) cation in acetonitrile at 298.15 K. The results show that replacement of one or more pyrrole units by thiophene rings in calix[4]pyrrole has tuned significantly the discrimination ability of these ligands between anions and enables the produced hybrid calixpyrroles to bind selectively with Hg(II) in acetonitrile. No interaction was observed between these ligands and other metal cations in acetonitrile.

Chlorin-Type Photosensitizers Photochemically Derived from Vinyl Porphyrins¶

The reaction of singlet oxygen with four vinyl-substituted dicarboxylic porphyrins, vinyldeuteroporphyrin (VD), ethylvinyldeuteroporphyrin (EVD), hydroxyethylvinyldeuteroporphyrin (HVD) and protoporphyrin (PP) in organic solutions is investigated. The main products, the “hydroxyaldehyde” chlorin-type derivatives, are formed with a concentration-dependent photochemical quantum yield that reaches a maximum of 7.4 ± 1.6 × 10−3. However, owing to the high turnover of singlet-oxygen production, these chlorin-type compounds are easily prepared photochemically with a chemical yield of 70% and little side product formation. In chemical ionization mass spectrometry, these compounds display an unusual fragmentation with a loss of 16 mass units. This is attributed to the loss of the oxygen bound to the saturated carbon of the modified pyrrole unit. All these compounds sensitize the formation of singlet oxygen with a yield around 0.8. They interact with singlet oxygen with rate constants of 5 × 106–9 × 106M−1 s−1, lower than those measured for vinyl porphyrins. These data are likely to help in the characterization of photoproducts of vinyl porphyrins relevant to photodynamic therapy (PP, HVD). As exemplified with VD and EVD, they also point out the reaction of singlet oxygen as an efficient route to chlorin-type photosensitizers.

Bipyrrole- and Dipyrromethane-Based Amido-imine Hybrid Macrocycles. New Receptors for Oxoanions

Three new amido-imine-type hybrid macrocycles based on substituted pyrrole units have been synthesized and shown to act as effective receptors for oxoanions in the solid state and in acetonitrile solution. One of the macrocycles in question, compound 15, was characterized by X-ray diffraction analysis as the free macrocycle and as a complex with sulfuric acid. A comparison of the resulting structures reveals that this receptor is capable of undergoing a conformational change and, as a consequence, varying the number of donor sites that can interact with a bound substrate. This system and the other two new receptors described in this work (macrocycles 14 and 16, respectively) display a high affinity toward oxoanions (studied as their tetrabutylammonium (TBA) salts), with association constants on the order of 10(7) M-1 being determined in acetonitrile solution using standard UV-vis spectroscopic titration methods. A competitive titration method was used to determine affinity constants in excess of ca. 10(6) M-1. Two of the receptors (14 and 15) were found to bind acetate, hydrogen sulfate, and dihydrogen phosphate anion well, and the bipyrrole-based receptor (14) was also found to bind the perrhenate anion. In contrast, the bis-dipyrromethane-derived receptor (16) was found to bind chloride anion preferentially. The unusual selectivity displayed by 16 for this spherical anion was rationalized on the basis of single-crystal X-ray diffraction data and DFT modeling calculations, which revealed a rigid structure appropriately suited for chloride anion recognition.

Efficient Synthesis of Medium‐Sized Rings Incorporating Indole or Pyrrole Units by Samarium Diiodide Induced Cyclizations.

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Cytochrome c in a Dry Trehalose Matrix: Structural and Dynamical Effects Probed by X-Ray Absorption Spectroscopy

We report on the structure and dynamics of the Fe ligand cluster of reduced horse heart cytochrome c in solution, in a dried polyvinyl alcohol (PVA) film, and in two trehalose matrices characterized by different contents of residual water. The effect of the solvent/matrix environment was studied at room temperature using Fe K-edge x-ray absorption fine structure (XAFS) spectroscopy. XAFS data were analyzed by combining ab initio simulations and multi-parameter fitting in an attempt to disentangle structural from disorder parameters. Essentially the same structural and disorder parameters account adequately for the XAFS spectra measured in solution, both in the absence and in the presence of glycerol, and in the PVA film, showing that this polymer interacts weakly with the embedded protein. Instead, incorporation in trehalose leads to severe structural changes, more prominent in the more dried matrix, consisting of 1), an increase up to 0.2 Å of the distance between Fe and the imidazole N atom of the coordinating histidine residue and 2), an elongation up to 0.16 Å of the distance between Fe and the fourth-shell C atoms of the heme pyrrolic units. These structural distortions are accompanied by a substantial decrease of the relative mean-square displacements of the first ligands. In the extensively dried trehalose matrix, extremely low values of the Debye Waller factors are obtained for the pyrrolic and for the imidazole N atoms. This finding is interpreted as reflecting a drastic hindering in the relative motions of the Fe ligand cluster atoms and an impressive decrease in the static disorder of the local Fe structure. It appears, therefore, that the dried trehalose matrix dramatically perturbs the energy landscape of cyrochrome c, giving rise, at the level of local structure, to well-resolved structural distortions and restricting the ensemble of accessible conformational substates.

Electrochemical quartz crystal impedance study on the overoxidation of polypyrrole–carbon nanotubes composite film for amperometric detection of dopamine

The electrochemical quartz crystal impedance (EQCI) method was used to study the overoxidation of polypyrrole (PPy)–multiwalled carbon nanotubes (MWCNT) nanocomposite film in neutral and alkaline solutions. The values of molar mass per electron transferred (M/n) obtained during the overoxidation of PPy in 0.10molL−1 Na2SO4 and 0.20molL−1 NaOH aqueous solutions were estimated to be ca. 17 and 22gmol−1, respectively, suggesting the nucleophilic attack of solution OH− to the pyrrole units during the overoxidation, and the possible partial formation of carboxylic groups after the overoxidation in the NaOH solution. Also, the overoxidized PPy–MWCNT composite film prepared in the NaOH solution showed a notably larger affinity to dopamine (DA) dissolved in a neutral phosphate buffer than that prepared in the Na2SO4 solution. The modification of the overoxidized nanocomposite film improved substantially the sensitivity for DA assay in a neutral phosphate buffer, as compared with the modification of overoxidized PPy or MWCNT alone. At a −6kHz (201-nm thickness) nanocomposite film prepared in a polymerization bath containing 1.0mgmL−1 MWCNT and overoxidized in 0.20molL−1 aqueous NaOH, the peak current response from differential pulse voltammetric assay of DA was linear with DA concentration from 4.0×10−8 to 1.4×10−6molL−1, with a lower limit of detection of 1.7nmolL−1, good anti-interferent ability, as well as good stability and reproducibility.

Red Emitting Diphenylpyrrolopyrrole (DPP)‐Based Polymers Prepared by Stille and Heck Coupling

AbstractSummary: Six new soluble conjugated polymers are described, which were prepared by Stille and Heck polycondensation reactions. They alternately consist of dialkylated 1,4‐diketo‐3,6‐diphenyl‐pyrrolo[3,4‐c]pyrrole (DPP) units and divinylbenzene or thiophene‐based units. The polymers were prepared from 1,4‐diketo‐2,5‐dialkyl‐3,6‐di(4‐bromophenyl)‐pyrrolo[3,4‐c]pyrrole with alkyl being hexyl (1a), methyl (1b), and 2‐ethylhexyl (1c) and 2,5‐bis(tributylstannyl)thiophene (2), 5,5′‐bis(tributylstannyl)bithiophene (3), 3,4‐ethylenedioxythiophene (4) and divinylbenzene (5). The new polymers exhibit brilliant red purple color and strong photoluminescence in common organic solvents. The DPP‐phenylenevinylene copolymers exhibit molecular weights up to 41 000 Da, and the poly(DPP‐thienylene)s up to 12 200 Da. In chloroform, the poly(DPP‐thienylene)s exhibit absorption and photoluminescence maxima up to 560 and 624 nm, respectively, the corresponding maxima of the poly(DPP‐phenylenevinylene)s are at 529 and 640 nm, respectively. Quantum yields of fluorescence up to 36% were determined. Cyclic voltammetric studies indicate quasi‐reversible oxidation (p‐doping) of the poly(DPP‐thienylene)s and irreversible reduction (n‐doping) of all polymers.Molecular structure of polymers.magnified imageMolecular structure of polymers.