An Efficient Synthesis of Porphyrins with DifferentmesoSubstituents that Avoids Scrambling in Aqueous Media

Tailoring Light-Harvesting in Zn-Porphyrin and Carbon Fullerene based Donor-Acceptor Complex through Ethynyl-Extended Donor π-Conjugation

Organic photovoltaic efficiency though currently limited for practical applications, can be improved by means of various molecular-level modifications. Herein the role of extended donor π ${\pi }$ -conjugation through ethynyl-bridged meso-phenyl/pyridyl on the photoinduced charge-transfer kinetics is studied in noncovalently bound Zn-Porphyrin and carbon-fullerene based donor-acceptor complex using time-dependent optimally tuned range-separated hybrid combined with the kinetic rate theory in polar solvent. Noncovalent dispersive interaction is identified to primarily govern the complex stability. Ethynyl-extended π ${\pi }$ -conjugation results in red-shifted donor-localized Q-band with substantially increased dipole oscillator strength and smaller exciton binding energy, suggesting greater light-harvesting efficiency. However, the low-lying charge-transfer state below to the Q-band is relatively less affected by the ethynyl-extended π ${\pi }$ -conjugation, yielding reduced driving forces for the charge-transfer. Detailed kinetics analysis reveals similar order of charge-transfer rate constants (~1012 s-1) for all donor-acceptor composites studied. Importantly, enhanced light-absorption, smaller exciton binding energy and similar charge-transfer rates together with reduced charge-recombination make these complexes suitable for efficient photoinduced charge-separation. These findings will be helpful to molecularly design the advanced organic donor-acceptor blends for energy efficient photovoltaic applications.

meso-Tetrahexyl-7,8-dihydroxychlorin and Its Conversion to ß-Modified Derivatives

meso-Tetrahexylporphyrin was converted to its corresponding 7,8-dihydroxychlorin using an osmium tetroxide-mediated dihydroxylation strategy. Its diol moiety was shown to be able to undergo a number of subsequent oxidation reactions to form a chlorin dione and porpholactone, the first meso-alkylporphyrin-based porphyrinoid containing a non-pyrrolic building block. Further, the diol chlorin was shown to be susceptible to dehydration, forming the porphyrin enol that is in equilibrium with its keto-chlorin form. The meso-hexylchlorin dione could be reduced and it underwent mono- and bis-methylation reactions using methyl-Grignard reagents, and trifluoromethylation using the Ruppert-Prakash reagent. The optical and spectroscopic properties of the products are discussed and contrasted to their corresponding meso-aryl derivatives (where known). This contribution establishes meso-tetrahexyl-7,8-dihydroxychlorins as a new and versatile class of chlorins that is susceptible to a broad range of conversions to generate functionalized chlorins and a pyrrole-modified chlorin analogue.

Efficient Decrease in Corrosion of Steel in 0.1 M HCl Medium Realized by a Coating with Thin Layers of MnTa2O6 and Porphyrins Using Suitable Laser-Type Approaches

The purpose of this research is to meet current technical and ecological challenges by developing novel steel coating systems specifically designed for mechanical equipment used in aggressive acid conditions. Homogeneous sandwich-type layered films on the surface of steel electrodes were realized using a pseudo-binary oxide, MnTa₂O₆, and two different substituted porphyrin derivatives, namely: 5-(4-carboxy-phenyl)-10,15,20-tris (4-methyl-phenyl)-porphyrin and 5-(4-methyl-benzoate)-10,15,20-tris (4-methyl-phenyl)-porphyrin, which are novel investigated compound pairs. Two suitable laser strategies, pulsed laser deposition (PLD) and matrix-assisted pulsed laser evaporation (MAPLE), were applied in order to prevent porphyrin decomposition and to create smooth layers with low porosity that are extremely adherent to the surface of steel. The electrochemical measurements of corrosion-resistant coating performance revealed that in all cases in which the steel electrodes were protected, a significant value of corrosion inhibition efficiency was found, ranging from 65.6 to 83.7%, depending on the nature of the porphyrin and its position in the sandwich layer. The highest value (83.7%) was obtained for the MAPLE/PLD laser deposition of 5-(4-carboxy-phenyl)-10,15,20-tris (4-methyl-phenyl)-porphyrin/MnTa₂O₆(h), meaning that the inhibitors adsorbed and blocked the access of the acid to the active sites of the steel electrodes.

Dimeric Corrole Analogs of Chlorophyll Special Pairs

Chlorophyll special pairs in photosynthetic reaction centers function as both exciton acceptors and primary electron donors. Although the macrocyclic natural pigments contain Mg(II), the central metal in most synthetic analogs is Zn(II). Here we report that insertion of either Al(III) or Ga(III) into an imidazole-substituted corrole affords an exceptionally robust photoactive dimer. Notably, attractive electronic interactions between dimer subunits are relatively strong, as documented by signature changes in NMR and electronic absorption spectra, as well as by cyclic voltammetry, where two well-separated reversible redox couples were observed. EPR spectra of one-electron oxidized dimers closely mimic those of native special pairs, and strong through-space interactions between corrole subunits inferred from spectroscopic and electrochemical data are further supported by crystal structure analyses (3 Å interplanar distances, 5 Å lateral shifts, and 6 Å metal to metal distances).

Photostable Platinated Bacteriochlorins as Potent Photodynamic Agents

Photodynamic therapy (PDT) is used to treat various cancerous diseases. Recently, we have demonstrated that platinated pyridyl-substituted porphyrins are potent agents for PDT with very high phototoxicity (IC₅₀ down to 17 nM) and excellent phototoxic indices of higher than 5800 (p.i. = IC₅₀(dark)/IC₅₀(light)) [Rubbiani, R. et al., Chem. Commun. 2020, 56, 14373]. However, the absorption of porphyrins is not ideal for the treatment of larger tumors because they essentially do not absorb light between 650 and 850 nm. Herein, we report stable conjugates of a novel bacteriochlorin with cisplatin and transplatin. They exhibit extremely high phototoxicity (IC₅₀ values down to 6 nM, irradiated with a 750 nm LED at a fluence of 5 J/cm²), very low dark toxicity, and thereby extremely high phototoxic indices up to 8300. Based on these exciting results, we believe that platinated bacteriochlorins are promising candidates for further investigation as novel PDT anticancer agents.

Access to Corrole-Appended Persubstituted Benzofurans by a Multicomponent Reaction: The Dual Role of p-Chloranil

A multicomponent reaction among dipyrranes, aryl-propargyl aldehydes, and p-chloranil leading to 10-(benzofuran-2-yl)corroles is described. p-Chloranil was identified as a crucial reagent playing a twofold role: an oxidant taking part in the formation of the corrole macrocycle and a component undergoing heteroannulation to the incipient 10-arylethynylcorrole. A series of corroles bearing persubstituted benzofuran-2-yl moieties have been synthesized, and their fundamental electronic properties have been studied via UV-vis absorption and fluorescence spectroscopies.

Weak Interactions and Conformational Changes in Core-Protonated A2- and Ax-Type Porphyrin Dications

Individual chemical motifs are known to introduce structural distortions to the porphyrin macrocycle, be it in the core or at the periphery of the macrocycle. The interplay when introducing two or more of these known structural motifs has been scarcely explored and is not necessarily simply additive; these structural distortions have a chance to compound or negate to introduce new structural types. To this end, a series of compounds with complementary peripheral (5,15-disubstitution) and core (acidification) substitution patterns were investigated. The single-crystal X-ray structures of 18 5,15-diphenylporphyrin, 5,15-diphenylporphyrindi-ium diacid, and related compounds are reported, including the first example of a 5,15-dialkylporphyrindi-ium. Normal-coordinate structural decomposition (NSD) analysis is used for a detailed analysis of the conformation of the porphyrin subunit within the crystal structures. An elongation of porphyrin macrocycles along the C₅,C₁₅- axis (B_2g symmetry) is observed in all of the free base porphyrins and porphyrin dications; distance across the core is around 0.3 Å in the free base and diacid compounds, and more than doubled in 5,15-dipentylporphyrin and 5,15-dipentylporphyrindi-ium diacid. While the free base porphyrins are largely planar, a large out-of-plane distortion can be observed in 5,15-diphenylporphyrin diacids, with the expected “projective saddle” shape characteristic for such systems. The combination of these two distortions (B_2u and B_2g) from regular porphyrin structure results in a macrocycle best characterized in the chiral point-group D₂. A rare structural type of a cis-hydrogen bond chelate is observed for 5,15-dipentylporphyrindi-ium diacid, which adopts an achiral C_2v symmetry. Crystallographic data indicate that the protonated porphyrin core forms hydrogen bonding chelates (N-H⋯X⋯H-N) to counter-anions. Weaker interactions, such as induced intramolecular C-H⋯O interactions from the porphyrin periphery are described, with distances characteristic of charge-assisted interactions. This paper offers a conceptual framework for accessing porphyrin macrocycles with designable distortion and symmetry, useful for the selective perturbation of electronic states and a design-for-application approach to solid state porphyrin materials.

Meso-alkynyl corroles and their cobalt(III), manganese(III) and gallium(III) complexes

We report routes towards synthesis of novel [Formula: see text]-conjugated freebase cobalt, copper, gallium and manganese meso-alkynylcorroles. UV-vis spectra show that extensive peak broadening, red shifts, and changes in the oscillator strength of absorptions increase with the extension of [Formula: see text]-conjugation. Using density functional theory (DFT), we have carried out a first theoretical study of the electronic structure of these metallocorroles. Decreased energy gaps of about 0.3–0.4 eV between the HOMO and LUMO orbitals compared to the corresponding copper, gallium and manganese meso-5,10,15 triphenylcorrole are observed. In all cases, the HOMO energies are nearly unperturbed as the [Formula: see text]-conjugation is expanded. The contraction of the HOMO–LUMO energy gaps is attributed to the lowered LUMO energies.

A tris-(manganese(iii))corrole–porphyrin–corrole triad: synthesis, characterization and catalytic epoxidation

A homotrimetallic manganese(iii) corrole–porphyrin–corrole triad has been synthesized and structurally characterized.

Differential quenching of the angular momentum of the B and Q bands of a porphyrin as a result of extended ring π-conjugation

A novel porphyrin, whose [Formula: see text]-system has been extended via the presence of two additional carbon–carbon triple bonds on opposite meso-positions and by fusion of a single naphthalene unit simultaneously bridging the third meso-position and the [Formula: see text]-carbon of one of the pyrroles, has been synthesized in good yield. Absorption, magnetic circular dichroism, emission, and theoretical spectra are reported for the fused and unfused trans-naphthalene free base and zinc porphyrins. The fusing of one of the naphthalene moieties results in significant changes to the absorption spectrum and, very unusually, the bridged meso-[Formula: see text]-pyrrole fusion results in quenching of the MCD Faraday pseudo-A term in the porphyrin’s B band (S2). This unique effect was interpreted as resulting from the origin of the electronic structure of the second excited state (the B state). The [Formula: see text] and [Formula: see text] polarizations are completely mixed by the electronic effects of the non-symmetric extended conjugation of the [Formula: see text] ring. Analysis of the origin of the MCD signal indicates that the presence of this novel mixed polarization leads to negligible angular momentum in the important B state. To our knowledge, this is the first report in which the magnetic moment in a porphyrin’s intensely absorbing B band has been quenched while the angular momentum in the Q band, the first excited state, remains as normal. This implies that the photophysical properties of the B state are likely very different than those of the Q state, which has novel and significant implications for applications, especially in non-linear spectroscopy.

Synthesis and characterization of meso-substituted A2B corroles with extended π-electronic structure

Abstract

We report the chemical synthesis and characterization of the stable 5,15-bis(pentafluorophenyl)-10-(trimethylsilylethynyl)corrole which serves as a precursor for the subsequent in situ sila-Sonogashira-cross-coupling reaction and metalation with copper(II) acetate. Under ambient conditions and a common catalyst system the reaction with 1-iodopyrene occurred within five hours. Due to the direct conjugation of the 18π-electronic system of the corrole macrocycle over the alkynyl group to the pyrene moiety the optical transitions in the Soret (B-) band Q-band region are significantly altered. The copper corrole exhibited complex hyperfine and superhyperfine structure in the EPR spectrum. The assignment of the EPR spectrum reveals the existence of an axial [CuII-cor^∙+] species.

Graphical abstract

Electronic supplementary material

The online version of this article (10.1007/s00706-017-2114-6) contains supplementary material, which is available to authorized users.

Selective synthesis of 5-aryl-10-(nitromethyl) substituted 15-azatripyrrane, 15-oxatripyrrane and 15-thiatripyrrane: access to nitromethyl functionalized A3B-porphyrins

A chemical strategy toward the selective synthesis of unsymmetrical tripyrranes was developed for the building of meso-aryl and -nitromethyl substituted A₃B porphyrins.

Effect of the triazole ring in zinc porphyrin-fullerene dyads on the charge transfer processes in NiO-based devices

Three covalently linked donor–acceptor zinc porphyrin–fullerene (ZnP–C₆₀) dyads were tested as sensitizers in NiO-based dye-sensitized solar cells (DSCs).

Novel meso-substituted trans-A2B2 porphyrins: synthesis and structure of their metal-mediated supramolecular assemblies

Five novel trans-A₂B₂ porphyrins and their corresponding coordination polymers with transition metals have been synthesized. Absorption and emission studies and structural investigations define the coordination patterns of these compounds in the solid state.

Nucleophilic Aromatic Substitution on Pentafluorophenyl-Substituted Dipyrranes and Tetrapyrroles as a Route to Multifunctionalized Chromophores for Potential Application in Photodynamic Therapy

The application of porphyrinoids in biomedical fields, such as photodynamic therapy (PDT), requires the introduction of functional groups to tune their solubility for the biological environment and to allow a coupling to other active moieties or carrier systems. A valuable motif in this regard is the pentafluorophenyl (PFP) substituent, which can easily undergo a regiospecific nucleophilic replacement (S_N Ar) of its para-fluorine atom by a number of nucleophiles. Here, it is shown that, instead of amino-substitution on the final porphyrinoid or BODIPY (boron dipyrromethene), the precursor 5-(PFP)-dipyrrane can be modified with amines (or alcohols). These dipyrranes were transformed into amino-substituted BODIPYs. Condensation of these dipyrranes with aldehydes gave access to trans-A₂ B₂ -porphyrins and trans-A₂ B-corroles. By using pentafluorobenzaldehyde, it was possible to introduce another para-fluorine atom, which enabled the synthesis of multifunctionalized tetrapyrroles. Furthermore, alkoxy- and amino-substituted dipyrranes were applied to the synthesis of A₃ B₃ -hexaphyrins. The polar porphyrins that were prepared by using this method exhibited in vitro PDT activity against several tumor cell lines.

Synthesis and conformation of a novel fluorescein–Zn-porphyrin dyad and intramolecular energy transfer

Modulations of the optical properties of a new porphyrin–fluorescein dyad were elucidated using experimental and theoretical techniques, with conformational rearrangements being studied.