Journal of Porphyrins and Phthalocyanines

The aromatic nature of porphyrins is commonly attributed to the presence of an [18]annulene substructure. However, this viewpoint has been disputed. Drawing on a range of examples of carbaporphyrinoid systems from the author's own studies, the [18]annulene model is shown to be a self-consistent and insightful approach for considering the aromatic properties of these porphyrin analogs. Benziporphyrins provide a continuum of porphyrinoid structures that range from nonaromatic to highly aromatic species and the presence of 18π electron delocalization pathways provides an excellent explanation for the variations in their properties. The same type of analysis is applied to carbaporphyrins, tropiporphyrins, azuliporphyrins, N-confused porphyrins, pyrazoloporphyrins and dicarbaporphyrins. Nevertheless, these properties might also be explained by a 22π electron delocalization model proposed by Schleyer. The [18]annulene model gains further support from the properties of dideazaporphyrins which cannot take part in this type of 22π electron delocalization but nevertheless retain porphyrin-like aromatic properties. These results support the concept that porphyrins are bridged [18]annulenes.

5,10,15,20-tetrakis(pentafluorophenyl)porphyrin reacts with a range of nucleophiles (amines, alcohols, thiols, nitrogen heterocycles, and others) resulting in the nucleophilic aromatic substitution of the para-F atoms of the pentafluorophenyl groups. This reaction, which was fortuitously discovered by Kadish and collaborators in 1990, is now being extensively used to synthesize porphyrins bearing electron-donating substituents in the para-position of their meso-aryl groups. This mini-review highlights the methods of synthesis of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin, the use of its metal complexes in catalysis and its reaction with nucleophiles to yield new monomeric porphyrins, porphyrins supported in polymers or new polymeric porphyrin matrices useful for heterogeneous catalysis.

This review of the application of the MCD spectroscopic technique to porphyrinoids aims to illustrate the features and unique information that can be obtained and used in the assignment of the optical absorption spectrum and also the electronic structures of porphyrins and phthalocyanines. By illustrating each of the major and readily available spectral features with simple compounds we hope this review will act to guide new users in formulating the questions that room temperature MCD measurements can answer. The review is in three parts: (i) a brief introduction to the instrumental and theoretical background with an emphasis on the parameters that can be readily extracted without computational or theoretical analyses. (ii) Illustration of each MCD spectral feature observed in room temperature spectra. The MCD spectra Zn-porphyrins and phthalocyanines, and chlorophyll a are described to provide models for measurements of new compounds. (iii) The use of "finger printing" to determine the oxidation and spin states of the central iron in heme proteins, and to identify the proximal (5th) and distal (6th) iron binding ligands from the MCD spectral envelope observed between 250 nm and 1000 nm. The value of this approach is immeasurable because through room temperature measurement of the MCD spectra from dilute solutions under conditions that allow the heme protein to be stabilized it is possible to identify accurately the proximal and distal ligands. Identifying the heme binding environment for heme proteins newly purified and for heme enzymes during their reaction cycle allows function to be considered long before X-ray structural analysis is available.

This review presents the synthetic aspects and photophysical properties of trimeric systems constructed with a first unit consisting of a cofacial porphyrin and then of another porphyrin attached as a side arm. Two scenarios are dealt with. The first one is the case where the three chromophores are different, called donor 1–donor 2–acceptor, specifically where the cofacial fragment is composed of donor 1 and donor 2, and the side arm is the acceptor. They are considered as models for the apo-proteins used in the LH II (light harvesting device) in the purple photosynthetic bacteria. The second one is the case where the chromophores of the cofacial bisporphyrin residue are identical and are closely placed to each other for inter-ring π-interactions. The side arm is simply a mono-porphyrin, and therefore it is different. The cofacial bisporphyrin unit exhibits then similar characteristics to the special pairs located within the reaction center protein, and are designated as artificial special pairs. On the synthetic standpoint, the various pathways to access such models are presented fully exploiting the Suzuki methodology. On the photophysical side, a large emphasis will be placed on the singlet energy transfers. Cascade processes in the trimers donor 1–donor 2–acceptor are presented and each individual contributions donor 1 → donor 2; donor 2 → acceptor; donor → acceptor are addressed qualitatively and quantitatively. For the artificial special pairs flanked with an antenna, the effect of the spacer between the artificial special pair and the antenna will be discussed as well as the choice of substituents and metal demonstrating that one can reverse the orientation of the singlet energy transfer: artificial special pair → side arm or side arm → artificial special pair. Finally, the antenna effect are presented for one example of artificial special pair equipped with 6 semi-flexible dendritic antennas.

The synthesis and the catalytic activity of bis(μ-chloro)-bridged, phosphapalladacycle-fused diporphyrins (M-porphyrin-appended phosphapalladacycles: M = Zn, Ni, free-base), as well as the coordinating ability of their precursors, meso-phosphanylporphyrins are reported. Treatment of meso-phosphanylporphyrinatozinc(II) with an equimolar amount of PdCl₂(NCPh)₂ caused regioselective β-C–H activation of the porphyrin ring to produce bis(μ-chloro)-bridged Zn-porphyrin-appended phosphapalladacycle, which was further converted to the free-base- and Ni-porphyrin-appended analogs. The UV-vis absorption spectra of these palladium complexes showed weak exciton coupling between two porphyrin chromophores connected by the μ-chloro bridges. The M-porphyrin-appended phosphapalladacycles behaved as precatalysts for a high temperature Heck reaction between p-bromobenzaldehyde and butyl acrylate. In a range of the catalyst-loading of 0.05–0.005 mol.%, the catalytic activity was found to significantly depend on the central metal incorporated at the porphyrin core. The Ni-porphyrin-appended phosphapalladacycle showed considerably higher turnover frequency than did the Zn-porphyrin- and free-base-porphyrin-appended counterparts. This trend in the catalytic activity does not simply correlate with the σ-donating ability of the phosphorus center and the electron-donating ability of the porphyrin π-systems. Instead, it is related to the durability of the precatalysts under the reaction conditions employed. The observed results corroborate that the central metal of the meso-phosphanylporphyrin ligands plays a crucial role in the active Pd(0) catalyst-loading from the porphyrin-appended phosphapalladacycles at constant efficiency.

The synthesis of a complete series of calix[4]arene-substituted porphyrins is described. A detailed conformational analysis based on variable-temperature ¹H NMR experiments revealed dynamic phenomena arising from the restricted rotation around the calix[4]arene-porphyrin bond.

In order to check if the parallel orientation of the chlorophylls in the light harvesting antennae of the photosynthetic system plays a key role in the efficiency of the energy transfer process, we synthesized three rigid tris-porphyrins bearing di-ethynyl-anthracenic spacers. The synthesis of this family of trimers in which porphyrins are held in a cofacial orientation by an anthracenic spacer is here described, as well as the NMR comparison between the different porphyrinic wires. This constitutes a first step toward the elaboration of more sophisticated light collecting devices. More specifically, these trimers, in which the porphyrins are held in a parallel conformation, constitute an approach toward the synthesis of rigid molecular wires.

The electrochemical and spectroelectrochemical behavior of two planar binuclear naphthalocyanines containing benzene and naphthalene π-conjugated bridges was investigated for the first time. Their intense absorption in the near infrared region was observed even in a low polar solvent such as o-dichlorobenzene after electrochemical reduction at a potential of -0.4 V vs. SCE, which was not enough to reduce the complexes into their first reduction state. These data and EPR studies allow us to conclude that the electrochemically generated form is the true neutral form of the synthesized compounds whereas the suppression of the near-infrared bands is a result of the presence of their cation radicals as one-electron oxidized forms.

Thiazolyl- and bithiazolyl-substituted porphyrins have been characterized to assess their potential as new standards for singlet molecular oxygen photosensitization. Their absorption, fluorescence, phosphorescence and triplet–triplet spectra are slightly red-shifted relative to those of tetraphenylporphine (TPP), a well-established singlet molecular oxygen standard. Likewise, the singlet and triplet lifetimes, as well as the fluorescence quantum yields are roughly one order of magnitude smaller than those of TPP, while the triplet and singlet oxygen quantum yields increase concomitantly to ca. 1.

Herein, we present a straightforward method to achieve optically active films based on porphyrin derivatives. The introduction of an aminoacid functionality on the porphyrin platform confers to the macrocycle both the amphiphilic and chiral character exploited for its solvent-promoted self-aggregation leading to the formation of chiral supramolecular architectures. These ordered suprastructures have the propensity to spontaneously layer as solid films on glass surfaces. The deposited material has been characterized by means of UV-visible, fluorescence emission, circular dichroism spectroscopy and AFM. The reported studies show once more how the stereochemical information stored on a single porphyrin framework can induce the formation of supramolecular chiral architectures, in solution, as well as in solid state. Furthermore, slight modifications on the porphyrin skeleton can influence the aggregation process and the structural features of the final assemblies, leading to solid surfaces featuring different morphologies. These combined aspects can be of great importance for the achievement of solid state chemical sensors with stereoselective properties.

The UV-vis spectra of a series of subporphyrazines, SubPz(A,R), and subphthalocyanines, SubPc(A,R) (A = F, Cl; R = H, F, CH₃, C₃H₇, SCH₃, SC₂H₅ and SPh), where A is the substituent attached to the central boron atom and R is the substituent attached to the periphery of the molecule have been analyzed through the use of TD–DFT calculations in vacuum and using chloroform as a solvent. The absorption spectra depend on both, the characteristics of the substituent attached to the periphery of the molecule and the extension of the π-system on going from SubPz to the SubPc analog. These latter effects lead to a red-shift of both the Q-band and the B-band, although the effect is larger for the former, mainly due to the increase of HOMO–LUMO energy gap on going from the SubPz to the SubPc analog. The effect of the substituents R is more intricate, because the profile of the absorption spectra changes depending on whether both substituents are on the same side (uu or dd) or on opposite sides (ud) of the molecular cone. Since the three conformers are rather close in energy, the observed spectra correspond, very likely, to the sum of the spectra of all of them.

The synthesis, structural and electronic characterization of novel electroactive systems based on porphyrin-fullerene in which the chromophores are linked by an ethynylfluorene spacer unit is reported. Sonogashira couplings have been used in short and efficient sequences to give access to these new molecules on a practical scale. The absorption studies, voltamperometric measurements and theoretical calculations at DFT level reveal the push-pull behavior for these systems.

This paper presents the study of the effects of CdTe-TGA quantum dots (QDs) on optical limiting ability of different phthalocyanine (Pc) complexes (1–10) containing Zn, Ga, In central metals and substituted with benzyloxyphenoxy, phenoxy, tert-butylphenoxy and amino groups. The optical limiting parameters of Pcs were higher for tert-butylphenoxy when compared to benzyloxyphenoxy and phenoxy substituents, in DMSO. Non-peripheral substitution decreased the optical limiting parameters. Third-order susceptibility (Im[χ⁽³⁾]/α) values of Pcs in the absence and presence of CdTe QDs were in the 10^-12 to 10^-10 esu.cm range. Hyperpolarizabilities (γ) ranged from 10^-31 to 10^-29 esu L for Pc alone or in mixture with QDs. There is a general improvement in optical limiting ability of Pc complexes in the presence of CdTe TGA QDs. Due to these promising results, future work can be implemented for the creation of Pc:QD thin films, which would then be examined to ensure that their optical limiting ability is still acceptable.

Four new crystalline solids based on the zinc-5,15-bis(4′-bromophenyl)-10,20- bis(4′-pyridyl)porphyrin (Zn–DBDPyP) and zinc/copper-5,10,15-tris(4′-bromophenyl)-20-(4′-pyridyl)-porphyrin (Zn/Cu–TBMPyP) platforms as building blocks, have been prepared and structurally analyzed by X-ray diffraction in order to examine whether the Br⋯N halogen bond can be effective in directing the supramolecular assembly of this functionalized porphyrins, in a similar way observed earlier for their iodophenyl-substituted analogs. The zinc ion in the porphyrin core was protected by an external ligand (pyridyl or methanol) to prevent its possible coordination to the pyridyl-porphyrin substituents. Neither the bis-pyridyl Zn(py)–DBDPyP scaffold nor the Zn(MeOH)/Cu–TBMPyP exhibited inter-porphyrin halogen bonding in their corresponding crystals. Only the layered self-assembly of the Zn(py)–TBMPyP building block was found to be uniquely directed by Br⋯N halogen bonds, as well as by Br⋯Br and Br⋯π interactions. This observation supports our notion that asymmetric functionalization of the tetraarylporphyrin scaffold, combined with directional interporphyrin interactions (as halogen bonding), represent a promising approach to supramolecular chirality.

Organic nanoparticles of metalloporphyrins can be a versatile catalyst for the selective oxidation of alkenes and other hydrocarbons. The catalytic activity of the metalloporphyrin depends on the nature of the central metal atom, peripheral groups, and the architecture of the porphyrin macrocycle. Herein, we report the catalytic activity of organic nanoparticles of 5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)porphyrinato manganese(III), Mn(III)TPPF₂₀, for the oxidation of cyclohexene using molecular oxygen as an oxidant in aqueous solvent under ambient conditions. While the solvated metalloporphyrins catalytically oxidize alkenes to the corresponding epoxide with a modest turn-over numbers, ca. 30 nm organic nanoparticles of Mn(III)TPPF₂₀ have enhanced catalytic activity with up to a two-fold greater turn-over number and yields only allylic oxidation products. The activity of organic nanoparticles is slow compared to the solvated metalloporphyrins. These organic nanoparticles catalytic systems facilitate a greener reaction since ca. 89% of the reaction medium is water, molecular oxygen is used in place of man-made oxidants, and the ambient reaction conditions require less energy. This organic nanoparticle catalytic system also avoids using halogenated solvents commonly used in solution phase reactions. The enhanced catalytic activity of these organic nanoparticles is unexpected because the metalloporphyrins in the nanoaggregates are in the close proximity and the turn-over number should diminish by self-oxidative degradation.

Two meso-phenyl chloro-substituted copper corroles were synthesized and characterized by a variety of spectroscopic techniques. The investigated compounds are represented as ((ClPh)₃Cor)Cu and ((Cl₂Ph)₃Cor)Cu, where (ClPh)₃Cor and (Cl₂Ph)₃Cor are the trianions of the 5,10,15-tri- (4-chlorophenyl)corrole and 5,10,15-tri(2,4-dichlorophenyl)corrole, respectively. UV-visible and ESR spectroscopy revealed that the electronic ground state of each copper corrole is solvent dependent. Both compounds contain Cu(III) corroles in the solid state and in solutions of CH₂Cl₂, but an equilibrium exists between Cu(III) and its Cu(II) form in pyridine, DMSO or DMF. The Cu(II) corrole could also be generated by controlled potential reduction in a thin-layer cell, which is reversible in CH₂Cl₂ or DMF containing 0.1 M TBAP. The structure of ((ClPh)₃Cor)Cu was determined by a single-crystal X-ray study.

A series of fully β-pyrrole brominated triarylcorrole metal complexes has been prepared for investigating the changes in visible spectra and redox potentials relative to the non-brominated derivatives, as well as for comparing the effect of bromination in corroles and porphyrins. The results reveal that bromination has a much larger effect on the electrochemistry of metallocorroles relative to metalloporphyrins, for both macrocycle- and metal-centered redox processes. The HOMO–LUMO gap energy of the triarylcorrole post-transition metal complexes decreases upon bromination because the effect on the LUMO is about twice as large of as on the HOMO; and both the HOMO and the LUMO are more affected in corroles than in porphyrins. Spectroscopic examinations of the transition metal complexes reveal that the synthetic access to divalent metallocorroles becomes feasible for the brominated derivatives.

The parent compound of the trithiadodecaazahexaphyrins has been prepared for the first time. Its thermal stability has been found to be the highest reported to date among the porphyrinoid family.

Multiple photosynthetic reaction centers have successfully been constructed using a supramolecular complex of zinc porphyrin dendrimer [D(ZnP)₁₆] with pyridylnaphthalenediimide (PyNIm) in benzonitrile. The apparent formation constant determined from the fluorescence quenching of the singlet excited state of porphyrin moieties by PyNIm is significantly larger than that determined from the UV-vis spectral change. This indicates that efficient energy migration occurs between the porphyrin units of the dendrimer prior to the electron transfer from the singlet excited state of zinc porphyrin to PyNIm. The charge-separated (CS) state has been successfully detected as the transient absorption spectrum in the laser flash photolysis. The CS lifetime of in the supramolecular complex of D(ZnP)₁₆ with PyNIm was determined to be 0.83 ms at 298 K.

Influence of halide ion coordination on the basic properties and stability of indium(III) octaphenylporphyrazine complexes in acid medium have been studied. Five-coordinated halide complexes [(Hal)InPAPh₈](Hal = F, Cl, Br) are stable in dichloromethane or chloroform solutions acidified with trifluoroacetic acid up to 100%. One of four meso-nitrogen atoms is protonated in 0.1 M solution; their basicity depending of the ionic character of In–Hal bond is decreased in the order F > Cl > Br. Addition of an excess of fluoride, chloride or bromide anions leads to formation of anionic cis-dihalide complexes ^cis[(Hal)₂InPAPh₈]^- which undergo very facile demetalation with formation of the free-base H₂PAPh₈ even in a slightly acidified solutions (0.001 M of acid). Such catalytic effect of halide anions seems to be general for demetalation reactions of indium(III) complexes with tetrapyrrolic macrocycles and was also observed for the phthalocyanine complex [(Cl)InPc]. Coordination of iodide leads to reduction of porphyrazine macrocycle assisted by meso-protonation with formation of π-anionic species, which upon demetalation and subsequent deprotonation give a mixture of free-base porphyrazine H₂PAPh₈ and tetraazachlorin derivatives.

Compared to benzoporphyrin derivative-dimethyl ester (BPD-DME) and its 8-(1′-hexyloxy)ethyl analog the corresponding In(III) complexes showed enhanced in vitro photosensitizing efficacy in Colon26 tumor cells, which could be due to their higher singlet oxygen producing ability. In both organic (methanol) and aqueous Bovine Calf Serum (17% BCS) solutions the metalated analogs were significantly more stable than the parent photosensitizers. Presence of Indium as a central metal gave 13–25 nm hypsochromic shift to the long wavelength absorption band with reduced absorption and fluorescence intensity. The insertion of metal did not produce any difference in intracellular localization of the photosensitizers and were mainly localized in mitochondria.

Herein, we present a novel and facile approach to porphyrin tweezer scaffolds through copper catalyzed azide alkyne cycloaddition (CuAAC). To this end, asymmetric structures arise that are highly versatile precursors for the molecular design of multi-functional porphyrin systems. The synthesis and characterization by means of photophysical and electrochemical methods and first results on the formation of dye-sensitized solar cells (DSSC) are described.

Previously unreported octaisopropyltetraphenylporphyrin (OiPTPP) was prepared from 3,4-diisopropylpyrrole, an unstable previously unisolated heterocyclic compound. Purification of the free-base 18π porphyrin required conversion to its 18π zinc(II) complex, chromatographic separation, and acid mediated demetalation. Oxidation of the 18π porphyrin with SbCl₅ furnished metal-free 16π octaisopropyltetraphenylporphyrin (OiPTPP). X-ray structural determination revealed that the novel species is the most distorted 16π porphyrin to date. The time course of decomposition of the oxidized species monitored by UV-vis spectra implied that the introduction of high steric repulsion was effective for enhancing stability of the 16π porphyrin. The reaction of metal-free 16π OiPTPP with LiBF₄ followed by Pd₂(dba)₃ gave the corresponding 18π OiPTPP palladium(II) complex via reduction of the OiPTPP lithium(I) complex as previously reported for 16π octaisobutyltetraphenylporphyrin (OiBTPP). The new 18π porphyrin system could be characterized by the X-ray analysis of the 18π OiPTPP palladium(II) complex. The X-ray structure of the 18π metal complex also indicated that the distortion of the porphyrin ring had become larger than that of previously known 18π porphyrins

The Soret maxima of a number of metallotriarylcorroles shift sensitively in response to varying substituents on the meso-aryl groups. The effect is most pronounced for copper corroles but is not seen for silver and gold corroles. In the copper case, the effect has been attributed to a small HOMO–LUMO gap. Chromium-oxo corroles share a small HOMO–LUMO gap with copper corroles, as well as a substantially metal-based LUMO, yet the Soret maxima of chromium-oxo triarylcorroles do not shift in response to substitution on the meso-aryl groups. Molybdenum-oxo corroles are substituent-insensitive in the same sense. TDDFT calculations, focusing on chromium-oxo and molybednum-oxo triarylcorroles, are reported here in an attempt to explain the divergent spectroscopic behavior of different metallocorrole families.

The water-soluble tin(IV) tetrakis(2-N-hydroxyethyl-4-pyridinium)porphyrin 1 photocatalyst was synthesized in good yield and its structure determined by single crystal X-ray crystallography. Electrochemical measurements on tin(IV) porphyrin 1 reveal a range of complex redox processes that are highly dependent on the pH and electrode used. The cathodic processes at ca. -0.6 to -0.8 V were assigned to electrochemical processes on the pyridyl moiety following differential pulse voltammetry and spectroelectrochemical investigation into the electrochemical properties of tin(IV) porphyrin. Photocatalytic experiment on tin(IV) porphyrin 1 under anaerobic conditions using triethanolamine (TEAO) as a sacrifical donor reveal a phlorin species as the main product which rapidly disappears upon exposure to oxygen. These results suggest that tin(IV) porphyrin π-radical anion is perhaps not the species responsible for the apparent ability tin(IV) porphyrins to photocatalytically reduce various substrates, including water to hydrogen.