Synthesis and Spectroscopic Properties of Diarylethene-Subporphyrinoid Hybrids.

Two novel diarylethene-fused subporphyrinoids were prepared and characterized. A mono diarylethene derivative was obtained via a statistical condensation reaction with 2 eq. of 1,2-dicyanobenzene and 1 eq. of thiophene-disubstituted butenedinitrile. The symmetric triply diarylethene-fused subporphyrazine was synthesized via a cyclotrimerization reaction of the thiophene-disubstituted butenedinitrile derivative. These compounds were characterized by NMR spectroscopy and high-resolution mass spectrometry. The spectroscopic properties have been measured in hexane and in chloroform. The mono diarylethene-fused-type compound showed photochromism at 580 nm and >700 nm wavelength, accompanied by degradation. According to DFT calculations, photoreactivity likely depends on the contribution of aromatic feature of pyrrole ring bonded to two thiophene rings.

Stable Antiaromatic [16]Triphyrin(2.1.1) with Core Modification: Synthesis Using a 16π Electrocyclic Reaction.

Antiaromatic porphyrinoids have attracted significant attention owing to their unique electronic properties and potential applications. However, synthesis of antiaromatic contracted porphyrinoids is challenging owing to the inherent instability associated with smaller ring sizes. In this study, we report the synthesis and characterization of the first stable trioxa[16]triphyrin(2.1.1), a novel 16π antiaromatic contracted porphyrinoid. We utilized a core modification approach to stabilize the [16]triphyrin(2.1.1). X-ray crystallographic analysis revealed a nearly planar structure. Electrochemical studies demonstrated reversible oxidation behavior and a small HOMO-LUMO gap, which was consistent with its antiaromatic nature. Chemical oxidation yielded an aromatic [14]triphyrin(2.1.1) dication, highlighting the antiaromaticity-aromaticity switching capability of this system. This synthesis involved the discovery of a key intermediate, dihydrotrioxatriphyrin(2.1.1), which underwent oxidative dehydrogenation to yield the target compound. Theoretical calculations suggested that dihydrotrioxatriphyrin(2.1.1) formed via a rare 16π electrocyclic reaction. The successful synthesis and characterization of this stable trioxa[16]triphyrin(2.1.1) underscores the potential of the core modification strategies for the rational design of novel antiaromatic systems with tunable properties. Moreover, the discovery of the rare 16π electrocyclic reaction advances the understanding of high-order pericyclic processes and may inspire new synthetic strategies for complex macrocyclic compounds.

Efficient and Selective, In Vitro and In Vivo, Antimicrobial Photodynamic Therapy with a Dicationic Chlorin in Combination with KI.

Various cationic photosensitizers employed in antimicrobial photodynamic therapy (aPDT) have the ability to photoinactivate planktonic bacteria under conditions of low phototoxicity to mammalian cells and without generating antimicrobial resistance (AMR). However, the photoinactivation of biofilms requires orders-of-magnitude higher photosensitizer concentrations, which become toxic to host cells. Remarkably, the bactericidal effect of a dicationic di-imidazolyl chlorin toward planktonic S. aureus and E. coli was observed in this work for concentrations below 400 nM under illumination at 660 nm and below 50 µM for the corresponding biofilms. At the latter concentrations, the chlorin is phototoxic toward human keratinocyte cells. However, in the presence of 50 mM KI, bactericidal concentrations are reduced to less than 50 nM for planktonic bacteria and to less than 1 µM for biofilms. It is shown that the potentiation with KI involves the triiodide anion. This potentiation elicits a bactericidal effect without appreciable cytotoxicity to keratinocytes. It becomes possible to selectively inactivate biofilms with aPDT. An exploratory study treating mice with wounds infected with E. coli expressing GFP with 20 µM chlorin and 120 J cm-2 at 652 nm confirmed the potential of this chlorin to control localized infections.

Janus Reactivity of Crownphyrinoids: Alkali- vs. Transition Metal-Mediated Transformations of Crown Ether-Porphyrin Hybrids.

Crownphyrinogens and crownphyrins constitute a group of macrocycles that combine the structural facets of porphyrinoids and crown ethers. The dual-nature cavity embedded in their molecules enables reactivity involving two structurally distinct parts of the macrocyclic ligand. Upon Ni(II) and Pd(II) insertion, coordination compounds are produced wherein the metal is incorporated into the porphyrinoid-like pocket, resulting in monomeric or accordion-like dimeric products, depending on the oxidation level of the macrocycle and metal cation. The reactions with Na(I) and K(I) resulted in the formation of complexes where only the crown ether segment of the molecule is involved in metal binding, yielding remarkable dimeric species. The exploitation of a crownphyrin large enough to accommodate two metal cations allowed the synthesis of an alkali/transition metal binuclear complexes wherein the macrocycle demonstrated the Janus reactivity with one cavity acting as a porphyrinoid, and the other mimicking the crown ether.

Azolium-Porphyrin Electrosynthesis.

Electrochemical oxidation of Zn(II) 2,7,12,17-tetra-tert-butylporphyrin in the presence of a series of azole derivatives (1-methylimidazole, 1-vinyl-1H-imidazole, 2-(1H-imidazol-1-yl)pyridine, 1-methylbenzimidazole, 1-methyl-1H-1,2,4-triazole, and benzothiazole) affords the corresponding meso-substituted azolium-porphyrins in very mild conditions and good yields. It was found that these nucleophiles were strongly ligated to the zinc(II) azolium-porphyrin complexes. Thus a demetalation/remetalation procedure was performed to recover the non-azole-coordinated zinc(II) complexes. X-ray crystallographic structures of three azolium-porphyrins were solved. Cyclic voltammetry analyses provided insight into the electron-withdrawing effect of the azolium substituents.

Diphenylacetylene-Incorporating Octaphyrin: A Rigid Macrocycle with Readily Separable Conformational Isomers.

An expanded carbaporphyrinoid analogue, octaphyrin(2,1,1,1,2,1,1,1), containing two rigid diphenylacetylene moieties is reported. In contrast to traditional pyrrolic macrocycles where flexible conformers coexist in dynamic equilibrium, this macrocycle exists as two separable, conformationally stable stereoisomers, denoted as 1A and 1B. The conformational effect of both conformers, as well as their protonated forms, were thoroughly studied using NMR spectroscopy, UV-Vis, and single crystal X-ray diffraction analyses. Importantly, heating conformer 1B leads to its irreversible conversion to 1A, whereas in its protonated form, 1A·2MSA undergoes irreversible transformation to 1B·2MSA at lower temperatures. These temperature-dependent features establish a foundation for developing new accumulated heat sensors, as demonstrated by the use of the present octaphyrins as a customized thermochromic indicator in steam sterilization. The present study thus underscores how the conformational rigidity of these new polypyrrolic macrocycles imparts properties that are distinct from historically flexible expanded porphyrinoids.

Multimodal Molecular Motion in the Rotaxanes and Catenanes Incorporating Flexible Calix[n]phyrin Stations.

The synthesis of [2]rotaxanes stoppered with one or two dipyrromethane groups opened a route for the construction of mechanically interlocked molecules incorporating various porphyrinoid stations. The exploitation of those precursors allowed for the creation of [3]rotaxanes and [2]catenanes based on the calix[4]phyrin motif, presenting intriguing molecular dynamics. The intrinsic flexibility of the porphyrinoid allowed the introduction of a new type of molecular motion within the rotaxanes, termed fluttering. The latter involved a bending of the axle, interconverting two angular-shaped stereoisomers of the rotaxane through a planarised transition state. Simple chemical transformations, i.e. methylation and (de)protonation of [3]rotaxane and [2]catenane allowed for the controllable transformations within the conformationally flexible calix[4]phyrin-incorporated mechanically interlocked porphyrinoids.

Fused Hexabenzocoronene-Porphyrin Conjugates with Tailorable Excited-State Lifetimes.

A new clear-cut strategy for fusing N-heterocyclic and carbon-pure systems is introduced en route to a versatile platform of multi-purpose tetrapyrrolic chromophores. In particular, three novel C-C bond-fused porphyrin-hexabenzocoronene (HBC) conjugates were synthesized under oxidative cyclodehydrogenation conditions, starting from tailor-made nickel porphyrin precursors. The fusion of the individual aromatic systems via 5-membered rings led to highly soluble π-extended porphyrins in excellent yields. The resulting porphyrin-HBC conjugates exhibit absorption cross-sections that are of interdisciplinary interest in the ever-growing field of organic photovoltaics and near-infrared (NIR) dyes. Quantum chemical calculations show that the newly formed 5-membered rings induce biradicaloid character in the porphyrin core, which has a strong impact on excited state lifetimes. This is confirmed by a thorough optoelectronic and time-resolved characterization in order to understand these unique features better. Broadened absorption characteristics go hand-in-hand with short-lived excited states with up to six orders of magnitude faster decay rates.

A Class of Heptaphyrins with NIR Absorption Modulated by Metal Coordination and Nucleophilic Substitution.

The intensive interest in expanded porphyrins can be attributed to their appealing photoelectric and coordination behavior. In this work, an N-confused heptaphyrin 1 was synthesized by an acid-catalyzed [3+4] condensation reaction. The introduction of an N-confused pyrrolic unit into the heptaphyrin macrocycle led to the formation of a figure-eight-like conformation with nonsymmetrical "NNNN" and "NNNC" coordination cavities employable for bimetallic coordination. As a result, chelation of 1 with Zn(II) and Cu(II) afforded mono-Zn(II) complex 2 and bis-Cu(II) complex 3, respectively, with the metal atoms exhibiting distorted square-planar geometries. In complex 3, an oxygen atom is attached to the α-C atom of N-confused pyrrole D, and thus the N and C atoms of ring D participate in coordination within the two cavities. Interestingly, treatment of 1 with Cs2CO3 in MeOH resulted in regioselective substitution of all the seven para-F atoms in the meso-C6F5 groups as well as the α-H of ring D by eight methoxy moieties. Complex 3 displays a red-shifted absorption band edge of ca. 2200 nm, compared to that of ca. 1600 nm observed for 1. This work provides an example of incorporating an N-confused pyrrole to construct expanded porphyrins with distinctive coordination behavior and tunable NIR absorption.

Crystal structure of (6,9-diacetyl-5,10,15,20-tetra-phenyl-secochlorinato)nickel(II).

Title compound 1Ni, [Ni(C46H32N4O2)], a secochlorin nickel complex, was prepared by diol cleavage of a precursor trans-di-hydroxy-dimethyl-chlorin. Two crystallographically independent mol-ecules in the structure are related by pseudo-A lattice centering, with mol-ecules differing mainly by a rotation of one of the acetyls and an adjacent phenyl groups. The two mol-ecules have virtually identical conformations characterized by noticeable in-plane deformation in the A1g mode and a prominent out-of-plane deformation in the B1u (ruffling) mode. Directional inter-actions between mol-ecules are scarce, limited to just a few C-H⋯O contacts, and inter-molecular inter-actions are mostly dispersive in nature.

Phthalocyanines Synthesis: A State-of-The-Art Review of Sustainable Approaches Through Green Chemistry Metrics.

Driven by escalating environmental concerns, synthetic chemistry faces an urgent need for a green revolution. Green chemistry, with its focus on low environmental impacting chemicals and minimized waste production, emerges as a powerful tool in addressing this challenge. Metrics such as the E-factor guide the design of environmentally friendly strategies for chemical processes by quantifying the waste generated in obtaining target products, thus enabling interventions to minimize it. Phthalocyanines (Pcs), versatile molecules with exceptional physical and chemical properties, hold immense potential for technological applications. This review aims to bridge the gap between green chemistry and phthalocyanine synthesis by collecting the main examples of environmentally sustainable syntheses documented in the literature. The calculation of the E-factor of a selection of them provides insights on how crucial it is to evaluate a synthetic process in its entirety. This approach allows for a better evaluation of the actual sustainability of the phthalocyanine synthetic process and indicates possible strategies to improve it.

Improving the Photodynamic Activity of Water-Soluble Porphyrin-Polysaccharide Complexes by Folic Acid Modification.

Studies have shown that folate receptors are highly expressed in various cancer cells. Here, we synthesized folic acid-conjugated pullulan (FAPL) as a solubilizing agent to improve the photodynamic activity of porphyrin derivative-polysaccharide complexes. The porphyrin derivative-FAPL complex exhibited long-term stability in an aqueous solution, attributed to the folic acid modification. Furthermore, in vitro and in vivo experiments highlighted the enhanced photodynamic activity of the porphyrin derivative-FAPL complex toward 4T1 breast-cancer cells, compared with the activities of the porphyrin derivative-pullulan complex and Photofrin. This enhanced activity is attributed to the improvement of intracellular uptake by the folate receptor.

Photoacoustic Imaging Using Polysaccharide-Porphyrin Complexes by Photoirradiation at Long Wavelengths.

Photoacoustic (PA) imaging is a novel biological imaging technique with superior depth resolution compared to fluorescence imaging. The efficacy of PA imaging depends on contrast agents that possess considerable absorbance at longer wavelengths, coupled with high permeability in biological tissue and minimal fluorescence, achieved through mitigating aggregation-caused quenching (ACQ) that attenuates PA intensity. Despite the successful transfer of porphyrin 2 featuring amino moieties from polysaccharides to liposomes, most of 2 incorporated within λ-carrageenan (CGN-2 complex) remained in CGN under acidic lysosomal conditions (pH 5.0). Consequently, the CGN-2 complex exhibited a strong PA signal under 680 nm photoirradiation in Colon26 cells owing to the ACQ of 2. Moreover, the PA intensity of the CGN-2 complex was further enhanced under 780 nm photoirradiation owing to the increased absorbance at 780 nm facilitated by the redshift of the Q-band at pH 5.0.

Intermolecular Enantioselective Amination Reactions Mediated by Visible Light and a Chiral Iron Porphyrin Complex.

In the presence of 1 mol % of a chiral iron porphyrin catalyst, various 3-arylmethyl-substituted 2-quinolones and 2-pyridones underwent an enantioselective amination reaction (20 examples; 93-99 % ee). The substrates were used as the limiting reagents, and fluorinated aryl azides (1.5 equivalents) served as nitrene precursors. The reaction is triggered by visible light which allows a facile dediazotation at ambient temperature. The selectivity of the reaction is governed by a two-point hydrogen bond interaction between the ligand of the iron catalyst and the substrate. Hydrogen bonding directs the amination to a specific hydrogen atom within the substrate that is displaced by the nitrogen substituent either in a concerted fashion or by a rebound mechanism.

Porphyrin Atropisomerism as a Molecular Engineering Tool in Medicinal Chemistry, Molecular Recognition, Supramolecular Assembly, and Catalysis.

Porphyrin atropisomerism, which arises from restricted σ-bond rotation between the macrocycle and a sufficiently bulky substituent, was identified in 1969 by Gottwald and Ullman in 5,10,15,20-tetrakis(o-hydroxyphenyl)porphyrins. Henceforth, an entirely new field has emerged utilizing this transformative tool. This review strives to explain the consequences of atropisomerism in porphyrins, the methods which have been developed for their separation and analysis and present the diverse array of applications. Porphyrins alone possess intriguing properties and a structure which can be easily decorated and molded for a specific function. Therefore, atropisomerism serves as a transformative tool, making it possible to obtain even a specific molecular shape. Atropisomerism has been thoroughly exploited in catalysis and molecular recognition yet presents both challenges and opportunities in medicinal chemistry.

Molecular Symmetry and Art: Visualizing the Near-Symmetry of Molecules in Piet Mondrian's De Stijl.

Symmetry and shape are essential aspects of molecular structure and how we interpret molecules and their properties. We, as chemists, are comfortable with pictorial representations of structure, in which some nuance is lost-investigating molecular shape numerically by looking at how closely it fits a reference, such as a plane, or a set of vectors or coordinates, is informative, though far from engaging. Often relationships between chemical structure and derived values are obscured. Taking our inspiration from Piet Mondrian's Compositions, we have depicted the symmetry information encoded within 3D data as blocks of color, to show clearly how chemical arguments and resultant molecular distortion may contribute to symmetry. Great art gives us a new perspective on the world; as a pastiche, this art may allow us to look at familiar molecules, such as porphyrins, in a new light, understanding how their shape and properties are intertwined.

Bioinspired Self-Assembly of Metalloporphyrins and Polyelectrolytes into Hierarchical Supramolecular Nanostructures for Enhanced Photocatalytic H2 Production in Water.

Polypeptides, as natural polyelectrolytes, are assembled into tailored proteins to integrate chromophores and catalytic sites for photosynthesis. Mimicking nature to create the water-soluble nanoassemblies from synthetic polyelectrolytes and photocatalytic molecular species for artificial photosynthesis is still rare. Here, we report the enhancement of the full-spectrum solar-light-driven H2 production within a supramolecular system built by the co-assembly of anionic metalloporphyrins with cationic polyelectrolytes in water. This supramolecular photocatalytic system achieves a H2 production rate of 793 and 685 µmol h-1 g-1 over 24 h with a combination of Mg or Zn porphyrin as photosensitizers and Cu porphyrin as a catalyst, which is more than 23 times higher than that of free molecular controls. With a photosensitizer to catalyst ratio of 10000 : 1, the highest H2 production rate of >51,700 µmol h-1 g-1 with a turnover number (TON) of >1,290 per molecular catalyst was achieved over 24 h irradiation. The hierarchical self-assembly not only enhances photostability through forming ordered stackings of the metalloporphyrins but also facilitates both energy and electron transfer from antenna molecules to catalysts, and therefore promotes the photocatalysis. This study provides structural and mechanistic insights into the self-assembly enhanced photostability and catalytic performance of supramolecular photocatalytic systems.

Template-Directed Synthesis of Strained meso-meso-Linked Porphyrin Nanorings.

Strained macrocycles display interesting properties, such as conformational rigidity, often resulting in enhanced π-conjugation or enhanced affinity for non-covalent guest binding, yet they can be difficult to synthesize. Here we use computational modeling to design a template to direct the formation of an 18-porphyrin nanoring with direct meso-meso bonds between the porphyrin units. Coupling of a linear 18-porphyrin oligomer in the presence of this template gives the target nanoring, together with an unexpected 36-porphyrin ring by-product. Scanning tunneling microscopy (STM) revealed the elliptical conformations and flexibility of these nanorings on a Au(111) surface.

A Triply Linked Porphyrin-Norcorrole Hybrid with Singlet Diradical Character.

Norcorrole Ni(II) complexes have recently received considerable attention because they are readily accessible antiaromatic molecules. Their high stability under ambient conditions and ease of synthesis have enabled the exploration of the intrinsic properties of antiaromatic molecules. Here, we report the synthesis and properties of meso-meso singly linked porphyrin-norcorrole hybrids and a triply linked porphyrin-norcorrole hybrid. The singly linked and triply linked porphyrin-norcorrole hybrids were fully characterized, including an X-ray structural analysis. Due to their orthogonal conformation, the singly linked hybrids maintain the individual electronic properties of their porphyrin and norcorrole subunits, while the triply linked hybrid shows a significantly smaller electrochemical HOMO-LUMO gap (0.45 eV) than that of Ni(II) dimesitylnorcorrole (1.08 eV). Furthermore, the triply linked hybrid exhibits singlet diradical characteristics, as confirmed by VT NMR, ESR, and SQUID experiments.

Synthesis and Characterization of Bola-Amphiphilic Porphyrin-Perylenebisimide Architectures.

We report on the synthesis and characterization of a family of three water-soluble bola-amphiphilic zinc-porphyrin-perylenebisimide triads containing oligo carboxylic-acid capped Newkome dendrons in the periphery. Variations of the perylenebisimide (PBI) core geometry and dendron size (G1 and G2 dendrons with 3- and 9-carboxylic acid groups respectively) allow for tuning the supramolecular aggregation behavior with respect to variation of the molecular architecture. The triads show good solubility in basic aqueous media and aggregation to supramolecular assemblies. Theoretical investigations at the DFT level of theory accompanied by electrochemical measurements unravel the geometric and electronic structure of the amphiphiles. UV/Vis and fluorescence titrations with varying amounts of THF demonstrate disaggregation.