Pyrrole Positions Research Articles

Synthesis and characterization of two new triads with ferrocene and C60 connected by triple bonds to the beta-positions of meso-tetraphenylporphyrin

The 2,12 pyrrole positions of meso-tetraphenylporphyrin were functionalized through triple carbon–carbon bonds by C60 and ferrocene giving new electron donor–acceptor triads which have been characterized and studied by photophysical methods. The fluorescence spectra of the new compounds have been compared to those previously reported for similar compounds giving as a result an increase of the quantum efficiency based on the larger separation of fullerene from the porphyrin ring. On the contrary the efficiency decreases with the presence of a phenylene spacer between ferrocene and the macrocycle.

Structural elucidation and study of intermolecular interactions inmeso-tetratolylporphyrins

Synthesis and crystal structure analysis of meso-tetratolylporphyrins, 1–5 combined with computational Hirshfeld surface analysis were investigated. The crystal packing of porphyrins 1, 3 and 4 are arranged in an “orthogonal fashion” whereas 2 and 5 are in a “slip-stack or off-set fashion” through various intermolecular interactions. Compound 2 exhibits saddle geometry whereas 5 showed a domed geometry as evident from the single crystal X-ray diffraction studies. The enhancement of non-planarity in 2 is probably due to the presence of numerous intermolecular interactions caused by the presence of trifluoroacetate anions on both faces of the porphyrin in addition to the bulky bromine groups at the [Formula: see text]-pyrrole positions. In 5, the non-planarity is merely due to the metal coordination at the porphyrin core as pentacoordinated Mn[Formula: see text] center with a chloro ligand in the axial position. Hirshfeld surface analysis was performed in order to analyze the various intermolecular interactions present in these porphyrins and the result was discussed.

Lewis Acid Catalyzed Regioselective Hydroheteroarylation of Pentafulvenes.

A diverse approach toward the catalytic regioselective nucleophilic addition of nitrogen heterocycles to Lewis acid activated pentafulvenes has been established. The developed protocol introduces pentafulvenes as nonsymmetrical alkenes for the hydroheteroarylation reaction, providing alkylidenecyclopentenylation at the C-3 position of indoles and the C-2 position of pyrrole.

Stereoelectronic effects of porphyrin ligand on the oxygen transfer efficiency of high valent manganese-oxo porphyrin species: A DFT study

The structure and properties of the N[Formula: see text]–Mn–O bonds of high valent manganese-oxo of the second and third generation porphyrins in the presence of imidazole have been studied by means of density functional (DFT) method with 6-31G* basis set in the gas phase as well as water solution. The geometric structures, frontier molecular orbitals, thermodynamic parameters, aromaticity indices and physical properties such as chemical potential and chemical hardness of [(TPP)(ImH)MnO][Formula: see text] and its derivatives were calculated. The obtained results showed that [(TPP)(ImH)MnO][Formula: see text] bearing halogen atoms at the [Formula: see text]-pyrrole positions had a saddle conformation with low Mn–O strength. The electron density ([Formula: see text] and Laplacian ([Formula: see text] properties at critical points of the N[Formula: see text]–Mn–O bonds, estimated by AIM calculations, indicate that Mn–O bonds in third generation porphyrins have lower [Formula: see text] and Mn–N[Formula: see text] distances have higher [Formula: see text] than the second generation ones. The calculations of aromaticity indices for chelated rings at the porphyrin center show that HOMA and NICS in third generation porphyrins are generally lower than that of second ones which is in agreement with their saddle conformation. These results are supported by natural bond orbital (NBO) analysis.

Synthesis and characterization of new ferrocene, porphyrin and C60 triads, connected by triple bonds

The 2,12 pyrrole positions of meso-tetraphenylporphyrin were functionalized through triple carbon–carbon bonds by ferrocene and C60 giving new electron donor-acceptor triads which have been characterized and studied by photophysical methods.

Porphodilactones as synthetic chlorophylls: relative orientation of β-substituents on a pyrrolic ring tunes NIR absorption.

Porphodilactones represent the porphyrin analogues, in which the peripheral bonds of two pyrrole rings are replaced by lactone moieties. They provide an opportunity to investigate how β-substituent orientation of porphyrinoids modulates the electronic structures and optical properties, in a manner similar to what is observed with naturally occurring chlorophylls. In this work, a comprehensive description of the synthesis, characterization, and optical properties of meso-tetrakispentafluorophenylporphodilactone isomers is first reported. The β-dilactone moieties are found to lie at opposite pyrrole positions (trans- and cis-configurations are defined by the relative orientations of the carbonyl group when one lactone moiety is fixed), in accordance with earlier computational predictions (Gouterman, M. J. Am. Chem. Soc. 1989, 111, 3702). The relative orientation of the β-dilactone moieties has a significant influence on the electronic structures and photophysical properties. For example, the Qy band of trans-porphodilactone is red-shifted by 19 nm relative to that of the cis-isomer, and there is a 2-fold increase in the absorption intensity, which resembles the similar trends that have been reported for natural chlorophyll f and d. An in depth analysis of magnetic circular dichroism spectral data and TD-DFT calculations at the B3LYP/6-31G(d) level of theory demonstrates that the trans- and cis-orientations of the dilactone moieties have a significant effect on the relative energies of the frontier π-molecular orbitals. Importantly, the biological behaviors of the isomers reveal their different photocytotoxicity in NIR region (>650 nm). The influence of the relative orientation of the β-substituents on the optical properties in this context provides new insights into the electronic structures of porphyrinoids which could prove useful during the development of near-infrared absorbing photosensitizers.

Gold(III) Porphyrins Containing Two, Three, or Four β,β′-Fused Quinoxalines. Synthesis, Electrochemistry, and Effect of Structure and Acidity on Electroreduction Mechanism

Gold(III) porphyrins containing two, three, or four β,β'-fused quinoxalines were synthesized and examined as to their electrochemical properties in tetrahydrofuran (THF), pyridine, CH2Cl2, and CH2Cl2 containing added acid in the form of trifluoroacetic acid (TFA). The investigated porphyrins are represented as Au(PQ2)PF6, Au(PQ3)PF6, and Au(PQ4)PF6, where P is the dianion of the 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and Q is a quinoxaline group fused to a β,β'-pyrrolic position of the porphyrin macrocycle. In the absence of added acid, all three gold(III) porphyrins undergo a reversible one-electron oxidation and several reductions. The first reduction is characterized as a Au(III)/Au(II) process which is followed by additional porphyrin- and quinoxaline-centered redox reactions at more negative potentials. However, when 3-5 equivalents of acid are added to the CH2Cl2 solution, the initial Au(III)/Au(II) process is followed by a series of internal electron transfers and protonations, leading ultimately to triply reduced and doubly protonated Au(II)(PQ2H2) in the case of Au(III)(PQ2)(+), quadruply reduced and triply protonated Au(II)(PQ3H3) in the case of Au(III)(PQ3)(+), and Au(II)(PQ4H4) after addition of five electrons and four protons in the case of Au(III)(PQ4)(+). Under these solution conditions, the initial Au(PQ2)PF6 compound is shown to undergo a total of three Au(III)/Au(II) processes while Au(PQ3)PF6 and Au(PQ4)PF6 exhibit four and five metal-centered one-electron reductions, respectively, prior to the occurrence of additional reductions at the conjugated macrocycle and fused quinoxaline rings. Each redox reaction was monitored by cyclic voltammetry and thin-layer spectroelectrochemistry, and an overall mechanism for reduction in nonaqueous media with and without added acid is proposed. The effect of the number of Q groups on half-wave potentials for reduction and UV-visible spectra of the electroreduced species are analyzed using linear free energy relationships.

Unusual Multi-Step Sequential AuIII/AuII Processes of Gold(III) Quinoxalinoporphyrins in Acidic Non-Aqueous Media

The electrochemistry of gold(III) mono- and bis-quinoxalinoporphyrins was examined in CH(2)Cl(2) or PhCN containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) before and after the addition of trifluoroacetic acid to solution. The investigated porphyrins are represented as Au(PQ)PF(6) and Au(QPQ)PF(6), where P is the dianion of the 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and Q is a quinoxaline group fused to a β,β'-pyrrolic position of the porphyrin macrocycle; in Au(QPQ)PF(6) there is a linear arrangement where the quinoxalines are fused to pyrrolic positions that are opposite each other. The porphyrin without the fused quinoxaline groups, Au(P)PF(6), was also investigated under the same solution conditions. In the absence of acid, all three gold(III) porphyrins undergo a single reversible Au(III)/Au(II) process leading to the formation of a Au(II) porphyrin which can be further reduced at more negative potentials to give stepwise the Au(II) porphyrin π-anion radical and dianion, respectively. However, in the presence of acid, the initial Au(III)/Au(II) processes of Au(PQ)PF(6) and Au(QPQ)PF(6) are followed by an internal electron transfer and protonation to regenerate new Au(III) porphyrins assigned as Au(III)(PQH)(+) and Au(III)(QPQH)(+). Both protonated gold(III) quinoxalinoporphyrins then undergo a second Au(III)/Au(II) process at more negative potentials. The electrogenerated monoprotonated monoquinoxalinoporphyrin, Au(II)(PQH), is then further reduced to its π-anion radical and dianion forms, but this is not the case for the monoprotonated bis-quinoxalinoporphyrin, Au(II)(QPQH), which accepts a second proton and is rapidly converted to Au(III)(HQPQH)(+) before undergoing a third Au(III)/Au(II) process to produce Au(II)(HQPQH) as a final product. Thus, Au(P)PF(6) undergoes one metal-centered reduction while Au(PQ)PF(6) and Au(QPQ)PF(6) exhibit two and three Au(III)/Au(II) processes, respectively. These unusual multistep sequential Au(III)/Au(II) processes were monitored by thin-layer spectroelectrochemistry and a reduction/oxidation mechanism for Au(PQ)PF(6) and Au(QPQ)PF(6) in acidic media is proposed.

Antimalarial Activity of Natural and Synthetic Prodiginines

Prodiginines are a family of linear and cyclic oligopyrrole red-pigmented compounds. Herein we describe the in vitro antimalarial activity of four natural (IC(50) = 1.7-8.0 nM) and three sets of synthetic prodiginines against Plasmodium falciparum. Set 1 compounds replaced the terminal nonalkylated pyrrole ring of natural prodiginines and had diminished activity (IC(50) > 2920 nM). Set 2 and set 3 prodiginines were monosubstituted or disubstituted at either the 3 or 5 position of the right-hand terminal pyrrole, respectively. Potent in vitro activity (IC(50) = 0.9-16.0 nM) was observed using alkyl or aryl substituents. Metacycloprodiginine and more potent synthetic analogues were evaluated in a P. yoelii murine patent infection using oral administration. Each analogue reduced parasitemia by more than 90% after 25 (mg/kg)/day dosing and in some cases provided a cure. The most favorable profile was 92% parasite reduction at 5 (mg/kg)/day, and 100% reduction at 25 (mg/kg)/day without any evident weight loses or clinical overt toxicity.

Synthesis and properties of a meso-tetraphenylporphyrin and its Zn (II) complex with a dicyanoisophorone borne by double bond in the β pyrrolic position

A novel porphyrin compound (W1) with D-π-A structure and its Zn (II) complex (Zn-W1) have been synthesized. W1 and Zn-W1 show a similar absorption and emission spectra of porphyrins. Their optical properties can be modulated by the substituent in the β pyrrolic position. W1 and Zn-W1 have preferable thermal stability, as indicated from TGA data. And Zn-W1 shows reversible redox wave in its dichloromethane solution. Also, DFT calculations were used to study these two compounds.

ChemInform Abstract: Photophysics of Halogenated Porphyrins.

A series of free-base octaalkylporphyrins having halogen substituents at different sites on the nucleus has been examined by photophysical methods. In general, halogenation decreases the fluorescence yield and lifetime, increases triplet formation and decreases the resultant triplet lifetime. The magnitude of these effects, which increases in the order F < Cl < Br < I and is dependent on the site of halogenation (benzylic position of alkyl < β position of pyrrole < meso position) can be explained in terms of a modified spin–orbit coupling theory. Multiple halogenation at meso positions, however, can cause secondary effects, due to steric crowding (bromine) or electron redistribution (fluorine). Photophysical data derived from the full set of meso-fluorinated porphyrins can be described quantitatively in terms of combined spin–orbit coupling and electronic perturbation in which a fluorine substituent deactivates adjacent meso sites but activates the site opposite the substituent.

Synthesis, Crystal Structure, and Photodynamics of π-Expanded Porphyrin−Fullerene Dyads Synthesized by Diels−Alder Reaction

Free base and zinc porphyrins are linked with fullerene (C(60)) through β,β'-pyrrolic positions rather than meso-positions by Diels-Alder reaction of monoanthraporphyrins (H(2)P-mA and ZnP-mA) and C(60) to afford a π-expanded free base porphyrin-fullerene dyad (H(2)P-C(60)) and its zinc porphyrin-fullerene dyad (ZnP-C(60)) in 86 and 51% yield, respectively. The X-ray crystallographic analysis of ZnP-C(60) showed two comma-shaped swirls-like structure in a unit cell. The intramolecular center-to-center and edge-to-edge distances between the porphyrin and fullerene moieties were 11.5 and 2.5 Å, respectively. The porphyrins and fullerenes were packed in layer-by-layer structure and the porphyrins lied down in parallel. The intermolecular center-to-center distances between the neighboring fullerenes were 10.252, 10.028, and 10.129 Å, which were less than typical van der Waals distance and the π-π interaction spread two-dimensionally. The energy of the charge-separated (CS) state of ZnP-C(60) (1.11 eV) determined by differential-pulse voltammetry measurements in benzonitrile is significantly lower than those of zinc porphyrin-C(60) dyads linked at meso positions because of the low oxidation potential of the π-expanded ZnP moiety. The CS energy of ZnP-C(60) in a nonpolar solvent such as toluene (1.40 V) is lower than the triplet excited state of C(60) ((3)C(60)*), enabling us to attain a long lived triplet CS state (8.1 μs) in toluene, detected by nanosecond laser flash photolysis experiments. The distance between unpaired electrons in the triplet CS state was determined by the EPR spectrum to be 9.1 Å, which agreed with the distance between a zinc atom of the porphyrin and a carbon edge of C(60) in the crystal structure of ZnP-C(60). The rate constants of photoinduced electron transfer and back electron transfer of H(2)P-C(60) and ZnP-C(60), which were determined by femtosecond and nanosecond laser flash photolysis measurements, were analyzed in light of the Marcus theory of electron transfer.

Halo-Wittig- and zirconium-promoted synthesis of conjugated metalloporphyrin dimers

The synthesis and spectroscopic studies of metalloporphyrin dimers having 1,3-butadiyne linker attached to the beta pyrrolic positions of the linked porphyrins are reported. Bromoethenyl metalloporphyrins were synthesized by zirconocene-promoted bromomethylenation of beta formyl metalloporphyrins. Bromoethenyl metalloporphyrins were dehydrohalogenated by potassium tert-butoxide to give beta ethynyl or 2-ethynyl metalloporphyrins. It is the first documented example of zirconocene-promoted bromomethylenation of beta formyl metalloporphyrins. Bromoethenyl metalloporphyrins were also synthesized by halo-Wittig route. 2-ethynyl metalloporphyrins were self dimerized to give metalloporphyrin dimers having 1,3-butadiyne linking bridge. FTIR, Raman spectroscopy, UV-visible, elemental analysis, 1H NMR and mass spectrometry were used to characterize the compounds.

Effect of alkyl position of pyrrole on structures and properties of conjugated polysquaraines

Poly(1-octylpyrrolyl)squaraine and poly(3-octylpyrrolyl)squaraine were synthesized from squaric acid with 1-octylpyrrole and 3-octylpyrrole under identical conditions of condensation. Visible absorption spectra, photoluminescence spectra, TGA curves, and IR spectra were recorded to characterize the structures of these polymers as synthesized; the measured optical and thermal properties are consistent with backbone structures. The polysquaraine from 3-octylpyrrole possesses mostly zwitterionic repeating units (>97%), but the polymer from 1-octylpyrrole consists of zwitterionic and covalent repeating units in a ratio 2:1. Observations from the visible absorption and photoluminescence spectra in varied solvents, TGA curves, and thermal IR measurements for these two polymers reconciled to their structures. This result indicates that the position of the alkyl group in the pyrrole derivative affects the conformation in the condensation of poly(pyrrolyl)squaraine. These polysquaraines are stable below temperature 155 °C and their visible absorption and photoluminescence properties in hydrogen-bonding solvents exhibit negative solvatochromism.

Preparation of New π-Conjugated Thiophene-Pyrrole Copolymers Having Ethynyl Substituents at the N-Position of Pyrrole

Preparation of New π-Conjugated Thiophene-Pyrrole Copolymers Having Ethynyl Substituents at the N -Position of Pyrrole

Large two photon absorption cross section of asymmetric Zn(II) porphyrin complexes substituted in the meso or β pyrrolic position by –C [tbnd]C–C 6H 4X moieties (X = NMe 2, NO 2)

A large two photon absorption (TPA) cross section of about 4000 GM is measured by Z-scan technique for a push–pull A–π–D Zn(II) porphyrin complex disubstituted in the meso position with strong donor and acceptor moieties. This result is compared to the lower TPA cross section of Zn(II) porphyrinates mono-substituted in the meso or β pyrrolic position with the same donor or acceptor substituents. The origin of such trend is discussed in terms of charge transfer processes, π delocalisation and resonance pathways.

Control of the Orbital Delocalization and Implications for Molecular Rectification in the Radical Anions of Porphyrins with Coplanar 90° and 180° β,β‘-Fused Extensions

Through-porphyrin electronic communication is investigated using "linear-type" and "corner-type" bis(quinoxalino)porphyrins in free-base form and their ZnII, CuII, NiII, and PdII derivatives. These compounds are porphyrins with quinoxalines fused on opposite or adjacent beta,beta'-pyrrolic positions; they were synthesized from 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-porphyrin-2,3,12,13- and -2,3,7,8-tetraone, respectively, by reaction with 1,2-phenylenediamine. The degree of electron spin delocalization into the fused rings in the pi-radical anions of the free-base and metal(II) bisquinoxalinoporphyrins was elucidated by electrochemistry, UV-vis absorption, and electron spin resonance (ESR) spectra of the singly reduced species and density functional theory calculations. Hyperfine splitting patterns in the ESR spectra of the pi-radical anions show that symmetric molecules have delocalized electron spin, indicating that significant inter-quinoxaline interactions are mediated through the central porphyrin unit, these interactions being sufficient to guarantee through-molecule conduction. However, when molecular symmetry is broken by tautomeric exchange of the inner nitrogen hydrogens in the free-base porphyrin with a corner-type quinoxaline substitution pattern, the pi-radical anion becomes confined so that one quinoxaline group is omitted from spin delocalization. This indicates the appearance of a unidirectional barrier to through-molecule conduction, suggesting a new motif for chemically controlled rectification.

Control of the site and potential of reduction and oxidation processes in π-expanded quinoxalinoporphyrins

Quinoxalino[2,3-b']porphyrins are pi-expanded porphyrins, having a quinoxaline fused to a beta,beta'-pyrrolic position of the porphyrin. They are used as components in systems proposed as 'molecular wires'. Knowledge of their redox properties is of value in the design of electron- or hole-conduction systems. In particular, the location of the charge density in the radical anions of quinoxalinoporphyrins can be modulated by peripheral functionalization. New theoretical treatments of electrochemical potentials are developed that identify the site of reduction in both the anions and the dianions of 33 quinoxalinoporphyrins. These molecules include free-base and metallated macrocycles substituted on the quinoxaline with electron-withdrawing groups (NO2, Cl, Br) and/or electron-donating groups (NH2, OCH3). Spectroelectrochemistry, density-functional theory calculations, and substituent-parameter models are used to verify the analysis. Five distinct patterns are observed for the locations of the first and second reductions; some of these patterns involve delocalized charges. Nitroquinoxalinoporphyrins with the nitro groups at the 5- and 6-quinoxaline positions are found to have quite different properties owing to distortions caused by peri interactions that force the nitro group of the 5-nitro regioisomer out of conjugation. Charge localization on the nitroquinoxaline fragment is found for some molecules, and this is attributed to ion-pairing with the 0.1 M tetrabutylammonium perchlorate electrolyte used, leading to the verified prediction that electron-paramagnetic resonance spectra of these molecules taken without the electrolyte yield delocalized anions. These properties enable the control of conduction through molecular wires synthesised from quinoxalinoporphyrins.

Photoinduced electron transfer in a β,β′-pyrrolic fused ferrocene–(zinc porphyrin)–fullerene

A donor-acceptor linked triad with a short spacer (Fc-ZnP-C60) 1 was designed and synthesised to attain the longest charge-separation lifetime, 630 micros, ever reported for triads at room temperature. The ferrocene electron donor and fullerene electron acceptor of triad 1 are attached to imidazole rings fused to opposite beta,beta'-pyrrolic positions of the zinc porphyrin. After excitation of the porphyrin, electron transfer to C60 occurs within 230 ps, followed by hole transfer to ferrocene after 500 ps to produce the long-lived charge-separated state.

Synthesis, Electronic Characterisation and Significant Second‐Order Non‐Linear Optical Responses of meso‐Tetraphenylporphyrins and Their ZnII Complexes Carrying a Push or Pull Group in the β Pyrrolic Position

Synthesis, Electronic Characterisation and Significant Second‐Order Non‐Linear Optical Responses of <i>meso</i>‐Tetraphenylporphyrins and Their Zn^II Complexes Carrying a Push or Pull Group in the β Pyrrolic Position