Development of Ion Chemosensors Based on Porphyrin Analogues

Over one century after discovery: pyrylium salt chemistry emerging as a powerful approach for the construction of complex macrocycles and metallo-supramolecules

Over one century after its discovery, pyrylium salt chemistry has been extensively applied in preparing light emitters, photocatalysts, and sensitizers. In most of these studies, pyrylium salts acted as versatile precursors for the preparation of small molecules (such as furan, pyridines, phosphines, pyridinium salts, thiopyryliums and betaine dyes) and poly(pyridinium salt)s. In recent decades, pyrylium salt chemistry has emerged as a powerful approach for constructing complex macrocycles and metallo-supramolecules. In this perspective, we attempt to summarize the representative efforts of synthesizing and self-assembling large, complex architectures using pyrylium salt chemistry. We believe that this perspective not only highlights the recent achievements in pyrylium salt chemistry, but also inspires us to revisit this chemistry to design and construct macrocycles and metallo-supramolecules with increasing complexity and desired function.

Local and macrocyclic (anti)aromaticity of porphyrinoids revealed by the topology of the induced magnetic field

The aromaticity in porphyrinoids results from the π conjugation through two different annular perimeters: the macrocyclic ring and the local heterocyclic rings appended to it. Analyses, based on aromatic stabilization energies (ASE), indicate that the local circuits (6π) are responsible for the significant aromatic stabilization of these systems. This local aromaticity can be coupled with the one from 4n + 2π macrocyclic circuit. It can either compensate for the destabilization due to a 4n π macrocyclic circuit, or be the only source of aromatic stabilization in porphyrinoids with macrocycles without π-conjugated bonds. This "multifaceted" aromatic character of porphyrinoids makes it challenging to analyze their aromaticity using magnetic descriptors because of the intricate interaction of local versus macro-cyclic circulation. In this contribution, we show that the analysis of the bifurcation of the induced magnetic field, Bind, allows clear identification and quantification of both local, and macrocyclic aromaticity, in a representative group of porphyrinioids. In porphyrin, bifurcation values accurately predict the local and macrocyclic contribution rate to overall aromatic stabilization determined by ASE.

Unique Role of Heterole-Fused Structures in Aromaticity and Physicochemical Properties of 7,8-Dehydropurpurins

Porphyrins with a fused five-membered ring, such as 7,8-dehydropurpurins, have appeared as an emerging class of unique porphyrinoids. Their altered absorption spectra, relatively short lifetimes of excited states, and small HOMO-LUMO gaps arise from the harmony of the antiaromatic 20π-circuit and the aromatic 18π-circuit. In this regard, the electronic properties of 7,8-dehydropurpurins are expected to be controlled by modulating the contribution of the antiaromatic π-circuit to the whole aromaticity. Here the comparison of pyrrole- and phosphole-fused 7,8-dehydropurpurins is reported in terms of their aromaticity and physicochemical properties. The spectroscopic investigation revealed the larger contribution of the antiaromatic 24π-circuit in pyrrole-fused 7,8-dehyrdopurpurins than in phosphole-fused 7,8-dehydropurpurins. The DFT calculations also supported the feasibility of tuning the aromaticity of 7,8-dehydropurpurins by heterole-fused structures. Thus, the introduction of heterole-fused structures into porphyrinoids is a universal strategy to get new insight into aromaticity and their intrinsic properties in cyclic π-conjugated molecules.

Recent Advances in Porphyrin-Based Materials for Metal Ions Detection

Porphyrins have planar and conjugated structures, good optical properties, and other special functional properties. Owing to these excellent properties, in recent years, porphyrins and their analogues have emerged as a multifunctional platform for chemical sensors. The rich chemistry of these molecules offers many possibilities for metal ions detection. This review mainly discusses two types of molecular porphyrin and porphyrin composite sensors for metal ions detection, because porphyrins can be functionalized to improve their functional properties, which can introduce more chemical and functional sites. According to the different application materials, the section of porphyrin composite sensors is divided into five sub-categories: (1) porphyrin film, (2) porphyrin metal complex, (3) metal–organic frameworks, (4) graphene materials, and (5) other materials, respectively.

New Sensitive and Selective Chemical Sensors for Ni2+ and Cu2+ Ions: Insights into the Sensing Mechanism through DFT Methods

We report the synthesis and theoretical study of two new colorimetric chemosensors with special selectivity and sensitivity to Ni²⁺ and Cu²⁺ ions over other metal cations in the CH₃CN/H₂O solution. Compounds (E)-4-((2-nitrophenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)aniline (A) and (E)-4-((3-nitrophenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)aniline (B) exhibited a drastic color change from yellow to colorless, which allows the detection of the mentioned metal cations through different techniques. The interaction of sensors with these metal ions induced a new absorption band with a hypsochromic shift to the characteristic signal of the free sensors. A theoretical study via time-dependent density functional theory (TD-DFT) was performed. This method has enabled us to reproduce the hypsochromic shift in the maximum UV-vis absorption band and explain the selective sensing of the ions. For all of the systems studied, the absorption band is characterized by a π → π* transition centered in the ligand. Instead of Ni²⁺ and Cu²⁺ ions, the transition is set toward the σ* molecular orbital with a strong contribution of the 3d_x²-_y² transition (π → 3d_x²-_y²). These absorptions imply a ligand-to-metal charge transfer (LMCT) mechanism that results in the hypsochromic shift in the absorption band of these systems.

Fabrication of bacteriochlorin shell/gold core nanoparticles for the sensitive determination of trichlosan using differential pulse voltammetry

The triclosan contamination in daily life has attracted great attention, and there is rare electroanalytical assay based on π-system dyes. In this work, a facile preparation and electroanalytical application of an organic dispersion containing bacteriochlorin dyes (LS11) and gold nanoparticles (AuNPs) was proposed. The organic-inorganic hybrid nanocomposites were characterized by transmission electron microscope (TEM) showing a core-shell structure with a uniform layer of dye molecules. The as-prepared nanocomposites were successfully coated onto glassy carbon electrodes, and the surface characteristics of the top most layer of the modified electrodes were examined by atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM) and water contact angle experiments. The nanocomposite film-modified electrodes exhibited good electrochemical activity towards oxidation of triclosan. The oxidation of adsorbed triclosan occurred at a reduced overpotential, and the anodic current responses under a pre-concentration step prior to the potential scan were used for quantitative analysis. A good linear relationship from 0.01 μM to 0.5 μM was obtained using differential pulse voltammetry. The sensitivity and detection limit (S/N = 3) were 23.69 μA μM^-1 and 0.03 μM, respectively. The proposed assay was applied to detect triclosan in two personal hygiene products using standard addition method, and the results showed good recoveries that ranged from 96.6% to 101.5% and from 99.3% to 103.8% for a toothpaste sample and a hand wash sample, respectively. A reference HPLC-UV method was used to evaluate the proposed electroanalytical method, and a good agreement was achieved.

Perylene diimides: will they flourish as reaction-based probes?

Perylene diimides (PDI) are a well-studied class of functional organic dyes, and in recent years, they have been accepted as promising scaffolds for the design of small molecule/polymer-based chromogenic and fluorogenic reaction-based-probes because of their strong absorption combined with high fluorescence quantum yield in organic solvents, low reduction potential, good electron-acceptor properties, and broad color range properties. Undoubtedly, the intrinsically poor solubility of PDI-based derivatives in water greatly hampers their exploitation as reaction-based probes; however, a vast array of functionalizations now offer design strategies that have resulted in >50% solubility of PDI derivatives in water. A chemodosimeter, wherein chemical transformation is achieved by specific reactions, affords naked-eye visibility, fast response time, sensitivity, ratiometric response, and low cost. The present review focuses on the progress of PDI-based chemodosimeters achieved so far since the inception of this member in the rylene diimide family. This comprehensive review may facilitate the development of more powerful chemodosimeters based on PDI for broad and exciting applications in the future.

NH Tautomerism of N-Confused Porphyrin: Solvent/Substituent Effects and Isomerization Mechanism

The effects of substituents and solvents on the NH tautomerism of N-confused porphyrin (2) were investigated. The structures, electronic states, and aromaticity of NH tautomers (2-2H and 2-3H) were studied by absorption and nuclear magnetic resonance (¹H, ¹³C, and ¹⁵N) spectroscopies, single-crystal X-ray diffraction analysis, and theoretical calculations. The relative stability of the tautomers is highly affected by solvents, with the 3H-type tautomer being more stable in nonpolar solvents, while the 2H-type tautomer being highly stabilized in polar solvents with high donor numbers such as N,N-dimethylformamide (DMF), pyridine, and acetone. Electron-withdrawing groups on the meso-aryl substituents as well as the methyl group at the ortho position also stabilize the 2H-type tautomer. Kinetically, the tautomerism rate is significantly influenced by solvent and concentration, and a particularly large activation entropy (ΔS^⧧) is obtained in pyridine. The first-order deuterium isotope effect on the reaction rates of NH tautomerism (k_H/k_D) is determined to be 2.4 at 298 K. On the basis of kinetic data, the mechanism of isomerization is identified as an intramolecular process, including the rotation of the confused pyrrole in pyridine/chloroform and DMF/chloroform mixed solvent systems, and as a pyridine-mediated process in pyridine alone.

Rhenium in the core of porphyrin and rhenium bound to the periphery of porphyrin: synthesis and applications

An overview of most of the well known rhenium porphyrins (rhenium in the core of porphyrins) is presented here reviewing their synthesis, coordination chemistry, and applications. The important features of oxorhenium(v) porphyrins are discussed elaborately taking into account their application in epoxidation reaction. Moreover, the chemistry of some recently known porphyrin-Re conjugates (rhenium bound to the periphery of porphyrin) is reported considering their applications in the photochemical carbon dioxide reduction process and photodynamic therapy. The number of well characterized rhenium porphyrinoids are limited but they show interesting diverse properties, some of which are also discussed in this review.

A novel "turn-on" fluorescent probe based on hydroxy functionalized naphthalimide as a logic platform for visual recognition of H2S in environment and living cells

A novel hydroxy functionalized naphthalimide-based fluorescent probe (denoted SP2)was successfully designed and synthesized for monitoring of H₂S in living cells and environmental. In particular, SP2 can detect the H₂S without assistance of organic solvent or surfactant. When H₂S is present, the azide group in SP2 was reduced to amine group, resulting in a turn-on fluorescence signal. This remarkable properties of SP2 enable its applications in monitoring ex/endogenous H₂S in HepG-2 cells and hydrogen sulfide release in laboratories or chemical plants through visual recognition by optical color change. The probe displays highly selective and sensitive recognition to H₂S, with a low detection limit of 50.8 nM. Futhermore, this work presents the possibility of using naphthalimide-based "logic gate" platform for monitoring H₂S in biological and environmental samples.

Porphyrins as Colorimetric and Photometric Biosensors in Modern Bioanalytical Systems

Advances in porphyrin chemistry have provided novel materials and exciting technologies for bioanalysis such as colorimetric sensor array (CSA), photo-electrochemical (PEC) biosensing, and nanocomposites as peroxidase mimetics for glucose detection. This review highlights selected recent advances in the construction of supramolecular assemblies based on the porphyrin macrocycle that provide recognition of various biologically important entities through the unique porphyrin properties associated with colorimetry, spectrophotometry, and photo-electrochemistry.

Azide functionalized porphyrin based dendritic polymers for in vivo monitoring of Hg2+ ions in living cells

A porphyrin cored azide functionalised dendritic polymer was developed as a selective sensor for in vivo monitoring of mercuric ions in living (normal and cancer) cells and in an aqueous medium. The developed sensor could sense mercuric ions even at a nanomolar concentration with a limit of detection value of 0.9 nM. This probe can be used to monitor mercuric ions in living cells due to its low cytotoxicity and high cell permeability. The hydrophilic nature of the polymer makes it a promising candidate for sensing mercuric ions in real water samples. Moreover, the reversibility of this sensing strategy helps in constructing a logic gate, which is particularly useful in smart sensor design.

N-Confused Porphyrin - A Unique "Turn-On" Chemosensor for CN- and F- ions and "Turn-Off" Sensor for ClO4 - ions

N-Confused meso-tetrakis(4-carbomethoxyphenyl)porphyrin (1) and its Ni(II) complex (1 a) have been synthesized and utilized for anion sensing studies, and the results are compared with N-confused meso-tetraphenylporphyrin (NCTPP). Anion susceptibilities of 1 and 1 a were investigated using spectroscopic, electrochemical, and DFT studies. Porphyrins 1 and 1 a were able to detect CN^- , F^- , and ClO₄ ^- ions selectively over the tested set of anions even at ppm level. Interestingly, the addition of ClO₄ ^- ions resulted in fluorescence quenching (turn off) whereas the addition of F^- or CN^- resulted in fluorescence enhancement (turn on). Notably, the TFA addition resulted in fluorescence quenching, whereas the fluorescence enhancement was observed while adding TBAOH. The higher association constant (K_a ) values with anions, lower detection limit, and shifts in redox potentials are due to the electron-withdrawing effect of the -COOCH₃ group at the para-position of the meso-phenyl ring. This electron-withdrawing nature is crucial for the higher affinity towards anions. The anion sensing description in this article may not only unveil the built-in nature of N-confused porphyrins, but may also provide a general proposal for the development of novel anion sensors based on porphyrinoids. The electron-deficient porphyrin framework, large polarisable π-system, and anion binding through the outer NH or a combination of the above factors serve as a foundation for N-confused porphyrin to act as an anion sensor.

Selective ratiometric red-emission detection of In3+ in aqueous solutions and in live cells using a fluorescent peptidyl probe and metal chelating agent

Indium has been regarded as one of the most rarely used metal ions; however, the consumption of indium has increased intensively due to its increasing use in electrodes of liquid crystal displays (LCDs). In recent years, warnings have been issued about the toxicity of indium to aquatic ecosystems and humans. Thus, the development of efficient and selective detection methods for In³⁺ in aquatic environments as well as in live cells is highly required. However, the selective and sensitive detection of In³⁺ in the presence of trivalent metal ions and other metal ions is highly challenging. In the present study, we synthesized a fluorescent probe (1) for In³⁺ and Al³⁺ based on an unnatural peptide receptor and an aggregation-induced emission fluorophore and developed a selective fluorescent detection method for In³⁺ in aqueous solutions and live cells using the probe and a metal chelating agent. 1 recognized In³⁺ and Al³⁺ selectively among 19 metal ions in aqueous solutions depending on pH by the enhancement of the red emission at 600 nm and decrease in the green emission at 530 nm. 1 sensitively detected In³⁺ and Al³⁺ by ratiometric response in a wide pH range (3.5-7.4), and the ratiometric response was complete within 20 seconds in an aqueous buffered solution at pH 5.0. Interestingly, the addition of EDTA to the complex of 1 with In³⁺ or Al³⁺ did not induce the Al³⁺-free spectrum but instead induced the In³⁺-free spectrum; thus, In³⁺ and Al³⁺ could be easily differentiated. The detection limit of 1 for In³⁺ ions was 211 nM (R² = 0.981) in purely aqueous solutions. The fluorescence ratiometric detection method using 1 could quantify low concentrations of In³⁺ in ground water and tap water. Fluorescence cell image studies revealed that the probe was cell-permeable, and low concentrations of In³⁺ inside the cells could be recognized by the enhancement of the red emission at 600 nm. The binding mode study via NMR, IR, and CD spectroscopy revealed how the peptide receptor of 1 interacted with In³⁺ and resulted in the enhancement of the red emission in an aqueous solution.

Metal-Covalent Organic Frameworks (MCOFs): A Bridge Between Metal-Organic Frameworks and Covalent Organic Frameworks

Many sophisticated chemical and physical properties of porous materials strongly rely on the presence of the metal ions within the structures. Whereas homogeneous distribution of metals is conveniently realized in metal-organic frameworks (MOFs), the limited stability potentially restricts their practical implementation. From that perspective, the development of metal-covalent organic frameworks (MCOFs) may address these shortcomings by incorporating active metal species atop highly stable COF backbones. This Minireview highlights examples of MCOFs that tackle important issues from their design, synthesis, characterization to cutting-edge applications.

Targeted Synthesis of Regioisomerically Pure Dodecasubstituted Type I Porphyrins through the Exploitation of Peri-interactions

A targeted synthesis of dodecasubstituted type I porphyrins that utilizes the reaction of unsymmetrical 3,4-difunctionalized pyrroles and sterically demanding aldehydes was developed. This way, type I porphyrins could be obtained as the only type isomers, likely due to a minimization of the steric strain arising from peri-interactions. Uniquely, this method does not depend on lengthy precursor syntheses, the separation of isomers, or impractical limitations of the scale. In addition, single-crystal X-ray analysis was used to elucidate the structural features of the macrocycles.

Two spectral QSPR models of porphyrin macromolecules for chelating heavy metals and different ligands released from industrial solvents: CH2Cl2, CHCl3 and toluene

Two simple and reliable correlations are introduced for the prediction of emission and absorption of porphyrins and their derivatives, i.e. metalloporphyrins and ligand coordinated metalloporphyrins. They can be used to sense the extracted precious metals. The proposed models require only simple structural parameters such as the number of carbon, metal and metal-free molecular fragments of desirable porphyrins or their derivatives. Since the proposed models depend on molecular structures of the desired compounds, they can be easily applied for complex molecular structures. Experimental data of 272 porphyrin derivatives were used to derive and test the novel models for the assessment of their emission (Em.) and absorption (Abs.) values in three solvents namely dichloromethane, toluene and chloroform. The values of the coefficients of determination (r ²) for the training set (183 compounds) in dichloromethane and three different test sets, corresponding to the three mentioned solvents, for the emission and absorption correlations were greater than 0.70. The calculated values of the root-mean-square error (RMSE) for the training sets of Em. and Abs. correlations were equal to 7.56 and 4.86 nm, respectively. Further statistical parameters also confirm the high reliability of the new models.

Light induced intramolecular energy and electron transfer events in carbazole–corrole and phenothiazine-corrole dyads

We report two corrole based donor–acceptor (D–A) dyads, Cbz-Cor and Ptz-Cor to understand the energy/electron transfer reactions. In these D–A systems, the donor, either carbazole (Cbz) or phenothiazine (Ptz), is covalently connected at the meso-phenyl position of 10-(phenyl)-5,15-bis-(pentafluorophenyl)corrole (Ph-Cor) by C–N linkage. Both the dyads were characterized by 1H NMR, MALDI-TOF MS, UV-vis, electrochemical, computational methods, study state fluorescence and TCSPC techniques. A comparison of absorption spectra with their reference monomeric compounds (Cbz-Ph, Ptz-Ph and Ph-Cor) revealed minimal ground-state interactions between chromophores in both dyads. Fluorescence studies suggested that singlet–singlet energy transfer from 1Cbz* to corrole is the major photochemical pathway in the Cbz-Cor dyad with a quenching efficiency of [Formula: see text]99%. Detailed analysis of the data suggests that Forster’s dipole–dipole mechanism does not adequately explain this energy transfer. However, at a 410 nm excitation, florescence quenching is detected in Ptz-Cor (49%) supporting a photo induced electron transfer (PET) process from the ground state of PTZ to the excited state of corrole macrocycle. The electron-transfer rates ([Formula: see text] of Ptz-Cor are found in the range [Formula: see text] to [Formula: see text] and are concluded to be solvent dependent.

Noninvasive In Situ Ratiometric Imaging of Biometals Based on Self-Assembled Peptide Nanoribbon

Development of probes for accurate sensing and imaging of biometals in situ is still a growing interest owing to their crucial roles in cellular metabolism, neurotransmission, and apoptosis. Among them, Zn²⁺ and Cu²⁺ are two important cooperative biometals closely related to Alzheimer's disease (AD). Herein, we developed a multifunctional probe based on self-assembling peptide nanoribbon for ratiometric sensing of Zn²⁺, Cu²⁺, or Zn²⁺ and Cu²⁺ simultaneously. Uniform peptide nanoribbon (AQZ@NR) was rationally designed by coassembling a Zn²⁺-specific ligand AQZ-modified peptide (AQZKL-7) with peptide KL-7. The nanoribbon further combined with Cu²⁺-sensitive near-infrared quantum dots (NIR QDs) and Alexa Fluor 633 as an inner reference molecule, which was endowed with the capability for ratiometric Zn²⁺ and Cu²⁺ imaging at the same time. The peptide-based probe exhibited good specificity to Zn²⁺ and Cu²⁺ without interference from other ions. Importantly, the nanoprobe was successfully applied for noninvasive Zn²⁺ and Cu²⁺ monitoring in both living cells and zebrafish via multicolor fluorescence imaging. This gives insights into the dynamic Zn²⁺ and Cu²⁺ distribution in an intracellular and in vivo mode, as well as understanding the neurotoxicity of high concentration of Zn²⁺ and Cu²⁺. Therefore, the self-assembled nanoprobe shows great promise in multiplexed detection of many other biometals and biomolecules, which will benefit the diagnosis and treatment of AD in clinical applications.

Nanozyme-Modified Metal-Organic Frameworks with Multienzymes Activity as Biomimetic Catalysts and Electrocatalytic Interfaces

Many metal-organic frameworks have been designed and synthesized for biosensors because of high surface area and porosity, suitable size, and good biocompatibility. Despite recent advances, however, most of them are only used as a nanocarrier. In this work, a new artificial nanozyme was constructed on a metalloporphyrinic metal-organic framework (PMOF(Fe)), which was formed by Fe porphyrin and Zr⁴⁺ ions. Then, ultrasmall Pt nanoparticles (Pt NPs) were loaded on the surface of PMOF(Fe) to form Pt@PMOF(Fe). Because of the high surface area and exposed Fe activity center, PMOF(Fe) works as a nanocarrier to hinder the Pt NP aggregation and exhibits high peroxidase-mimicking activity. Hence, Pt NPs decorated on the surface of PMOF(Fe) possessed high stability and exhibited high activity. Due to the synergistic effect between PMOF(Fe) and Pt NPs, Pt@PMOF(Fe) exhibits superior catalase- and peroxidase-like activities. Moreover, Pt@PMOF(Fe) possesses high electrocatalytic activity toward the reduction of H₂O₂ and the oxygen reduction reaction (ORR). This strategy may serve as a strong foundation to design MOF-based artificial nanozymes and develop an ideal platform for MOFs and nanozymes toward artificial enzymatic catalytic systems, fuel cells and new analytical applications.

Switching on the Fluorescence Emission of Polypyridine Ligands by Simultaneous Zinc(II) Binding and Protonation

The synthesis and characterization of the two new open-chain ligands 1,15-bis-[6-(2,2'-bipyridyl)]-2,5,8,11,14-pentaaza-octadecane (L1) and 1,15-bis-[2-(1,10-phenanthroline)-9-methyl]-2,5,8,11,14-pentaazaoctadecane (L2), both featuring a tetraethylenpentaamine chain linking via methylene bridges the 6 and 2 positions of two identical 2,2'-bipyridyl (bpy) and 9-methyl-1,10-phenanthroline (9-methyl-phen) moieties respectively, are reported. Their protonation and binding ability for Cu²⁺ , Zn²⁺ , Cd²⁺ and Pb²⁺ have been studied by coupling potentiometric titrations with UV-vis absorption and fluorescence emission measurements in water. L1 and L2 afford stable mono- and dinuclear complexes, in which the metal ion is bound by a single bpy or 9-methyl-phen unit and the amine groups on the aliphatic chain. However, L1 displays a greater binding ability for Cu²⁺ and Zn²⁺ with respect to L2, the stability constants of the [ML1]²⁺ complexes being 21.8 (Cu²⁺ ) and 19.4 (Zn²⁺ ) log units vs 20.34 and 16.8 log. units for the corresponding L2 species. Among all the metal ions tested, only the Zn²⁺ complex with L2 features an enhanced fluorescence emission at neutral pH, thanks to the simultaneous binding of one Zn²⁺ ion and H⁺ ion(s), that inhibits any possible photoinduced electron transfer (PET) process from the amine donors to the excited phen moiety. Binding of a second metal switches off the emission again.

Iron-porphyrin-based covalent-organic frameworks for electrochemical sensing H2O2 and pH

Here, a novel iron-porphyrin-based covalent organic framework (COF_{p-Fepor NH2-BTA}) was synthesized and applied for electrochemical sensing H₂O₂ and pH which involved in many biological processes. The COF_{p-Fepor NH2-BTA} was obtained by post-modification of porphyrin-based COF (COF_{p-por NH2-BTA}) which was firstly synthesized by aldehyde-ammonia condensation reaction between 1,3,5-benzenetricarboxaldehyde and 5,10,15,20-tetrakis(4-aminophenyl)-21H,23H- porphine. The COF_{p-por NH2-BTA} was proved to be regular and uniform spherical particles with diameter about 1 μm, as well as possessed good crystalline structure and abundant micropores of about 1.4 nm. The resulted COF_{p-Fepor NH2-BTA} after post-modification with Fe²⁺ maintained the original shape and crystalline structure of COF_{p-por NH2-BTA}, while the micropores decreased to be about 0.89 nm. Electrochemical results indicated that the synthesized COF_{p-Fepor NH2-BTA} had good electrochemical redox and proton activity owing to iron-porphyrin, enabling to simultaneously be used as mimic peroxidase to catalyze the reduction of hydrogen peroxide (H₂O₂) and evaluate pH using current and potential as signal, respectively. The prepared sensor showed good performance for H₂O₂ detection from 6.85 nM to 7 μM with the detection limit of 2.06 nM (S/N = 3), and pH test from 3.0 to 9.0. This work demonstrated that the iron-porphyrin-based COF could be used as a mimic peroxidase to apply in biological fields.

Meso-Substituted Corroles from Nitrosoalkenes and Dipyrromethanes

The synthesis of bilanes and hexapyrroles containing an oxime functionality, prepared by two and three consecutive hetero-Diels-Alder reactions (or conjugated additions) between nitrosoalkenes and dipyrromethanes, is described. Bilanes underwent oxidative macrocyclization to afford a new class of trans-A₂B-corroles. Porphyrins could also be obtained by reacting bilanes with aldehydes in the presence of trifluoroacetic acid, followed by an oxidative step.

A simple pyrimidine based colorimetric and fluorescent chemosensor for sequential detection of copper (II) and cyanide ions and its application in real samples

In this study, a new pyrimidine-based chemosensor (PyrCS) has been developed for sequential detection of copper (II) and cyanide ions. The PyrCS has revealed high sensitivity and selectivity toward copper ion over other metal ions in aqueous media. The PyrCS as an optical probe exhibited a distinct color change and a bathochromic shift in UV spectra in the presence of copper ion in a few seconds due to the formation of stable complex (PyrCS-Cu²⁺). The results confirmed that the PyrCS has a widely linear detection range of 0.3-30 μM toward Cu²⁺. The calculated limit of detection for Cu²⁺ ions was low as 0.116 μM. Moreover, the fluorescent intensity of PyrCS at 507 nm was significantly quenched in the presence of Cu²⁺ and Fe²⁺ ions. Additionally, complex PyrCS-Cu²⁺ was successfully used to detect cyanide ions via Cu²⁺ displacement approach. The free PyrCS was recovered after adding the CN‾ ions in a few seconds due to the formation of the stable copper cyanide complex Cu(CN)_x. The calculated LOD for CN‾ ions was low as 0.320 μM. The data also clarified that the other competing anions did not create a clear color change in solutions. Since the proposed method could provide a vivid colorimetric response in the presence of detected analytes within the pH range of 3-9, we can claim that the developed chemosensor can be utilized in any physical and biological conditions.

21-Carbaporphyrin: a cyclopentadiene moiety entrapped into a porphyrin scaffold

This minireview underscores the chemistry of 21-carbaporphyrins containing a plain cyclopentadiene moiety. Thus the cyclopentadiene incorporation afforded two 21-carbaporphyrin series represented by meso-tetraaryl-21-carbaporphyrin and [Formula: see text]-alkylated 21-carbaporphyrin with their properties evidently controlled by the nature of perimeter substitution. The synthetic strategy, physicochemical characterization and some insight in coordination properties of 21-carbaporphyrins have been illustrated. The formation of palladium(II), rhodium(III) and gold(III) meso-tetraaryl-21-carbaporphyrins via unprecedented intramolecular contractions of meta-benzi- or para-benziporphyrins has been also addressed.

Switching between Hückel and Möbius aromaticity: a density functional theory and information-theoretic approach study

Benziporphyrins are versatile macrocycles exhibiting aromaticity switching behaviors. The existence of both Hückel and Möbius (anti)aromaticity has been reported in these systems, whose validity is respectively governed by the [4n + 2] and [4n] π-electron rule on the macrocyclic pathway. Despite the experimental evidence on the floppiness of benziporphyrins, the switching mechanism between Hückel and Möbius structures is still not clear, as well as the factors influencing the stability of the different π-conjugation topologies. For these reasons, we performed a systematic study on A,D-di-p-benzihexaphyrins(1.1.1.1.1.1) with two redox states corresponding to [28] and [30] π-electron conjugation pathways. Whereas benzi[28]hexaphyrin obeys Möbius aromaticity, benzi[30]hexaphyrin follows Hückel aromaticity. The dynamic interconversion between Möbius and Hückel aromaticity is investigated through the rotation of a phenylene ring, which acts as the topology selector. Further analyses of the energy profiles using energy decomposition and information-theoretic approaches provide new insights into conformational stability, aromaticity and antiaromaticity for these species. Strong and opposite cross correlations between aromaticity indexes and information-theoretic quantities were found for the two macrocyclic systems with opposite global aromaticity and antiaromaticity behaviors. These results indicate that Hückel and Möbius aromaticity and antiaromaticity, though qualitatively different, are closely related and can be interchanged, and information-theoretic quantities provide a novel understanding about their relevance. Our present results should provide in-depth insights to appreciate the nature and origin about Möbius (anti)aromaticity and its close relationship with Hückel (anti)aromaticity.

Reduced texaphyrin: A ratiometric optical sensor for heavy metals in aqueous solution

We report here a water-soluble metal cation sensor system based on the as-prepared or reduced form of an expanded porphyrin, texaphyrin. Upon metal complexation, a change in the redox state of the ligand occurs that is accompanied by a color change from red to green. Although long employed for synthesis in organic media, we have now found that this complexation-driven redox behavior may be used to achieve the naked eye detectable colorimetric sensing of several number of less-common metal ions in aqueous media. Exposure to In(III), Hg(II), Cd(II), Mn(II), Bi(III), Co(II), and Pb(II) cations leads to a colorimetric response within 10 min. This process is selective for Hg(II) under conditions of competitive analysis. Furthermore, among the subset of response-producing cations, In(III) proved unique in giving rise to a ratiometric change in the ligand-based fluorescence features, including an overall increase in intensity. The cation selectivity observed in aqueous media stands in contrast to what is seen in organic solvents, where a wide range of texaphyrin metal complexes may be prepared. The formation of metal cation complexes under the present aqueous conditions was confirmed by reversed phase high-performance liquid chromatography, ultra-violet-visible absorption and fluorescence spectroscopies, and high-resolution mass spectrometry.

2D and 3D Porphyrinic Covalent Organic Frameworks: The Influence of Dimensionality on Functionality

The construction of 2D and 3D covalent organic frameworks (COFs) from functional moieties for desired properties has gained much attention. However, the influence of COFs dimensionality on their functionalities, which can further assist in COF design, has never been explored. Now, by selecting designed precursors and topology diagrams, 2D and 3D porphyrinic COFs (2D-PdPor-COF and 3D-PdPor-COF) are synthesized. By model building and Rietveld refinement of powder X-ray diffraction, 2D-PdPor-COF crystallizes as 2D sheets while 3D-PdPor-COF adopts a five-fold interpenetrated pts topology. Interestingly, compared with 2D-PdPor-COF, 3D-PdPor-COF showed interesting properties, including 1) higher CO₂ adsorption capacity; 2) better photocatalytic performance; and 3) size-selective photocatalysis. Based on this study, we believe that with the incorporation of functional moieties, the dimensionality of COFs can definitely influence their functionalities.

Increasing the complexity of oxoporphyrinogen colorimetric sensing chromophores: N-alkylation and β-substitution

Meso-5,10,15,20-tetrakis-3,5-di-tert-butyl-4-oxocyclohexadienylideneporphyrinogen, OxP, is a versatile, highly colored chromophore derived from meso-5,10,15,20-tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin. It exhibits a wide range of chromogenic responses to solvents (solvatochromism), anions and acidic media (halochromism) making it potentially useful as an analytical reagent. The chromogenic responses of OxP can be modulated by varying its chemical structure, and this is reviewed here based on the introduction of substituents at central nitrogen atoms or pyrrolic [Formula: see text]-positions. OxP and its N-alkylated derivates Bn2OxP and Bn4OxP have been used to estimate acidity in non-polar solvents. Bn2OxP can also be used to determine enantiomeric excesses of chiral substances. N-alkylation has also been used to introduce higher functional groups such as porphyrins to prepare self-assembling systems. [Formula: see text]-Substitution has been used to introduce selectivity of anion interactions including towards basic anions (fluoride, cyanide) and polyoxoanions (nitrate, perchlorate, etc.). These aspects make OxP a highly adaptable tetrapyrrole molecule for sensing and other applications.

2,3-Dicyano-5,6-dichlorobenzoquinone-Mediated and Selective C-O and C-C Cross-Couplings of Phenols and Porphyrins

A selective C-O cross-coupling reaction between porphyrins and phenols has been developed through 2,3-dicyano-5,6-dichlorobenzoquinone (DDQ)/Sc(OTf)₃ oxidation, efficiently delivering meso-etherified porphyrins in good yields (≤93%). The radical complex process was proposed and calculated as the rationalized mechanism to block the homocoupling process. In addition, the switchable selective C-C cross-coupling reaction was achieved by using bulky electron-rich phenols and naphthols under DDQ oxidation conditions.

A Hydrosilylation Approach to Silicon-Bridged Functional Dipyrromethanes: Introducing Silicon to A New Arena

Two silylene-spaced ((E)-vinylsilyl)anthracene-dipyrromethane dyads have been designed and synthesized by RhCl(PPh₃ )₃ -catalyzed hydrosilylation reactions of 5-methyl-5'-(ethynylaryl)dipyrromethanes with (9-Anthryl)-dimethylsilane. The complexation studies of dyads toward different anions have also been performed, which reveal that dyads exhibit a highly selective response towards fluoride anion attributable to both hydrogen-bonding and pentacoordination phenomena. This dual-mode fluoride recognition event is unprecedented and may pave the way for future developments in the areas of porphyrinoids, organosilicon, polymer, and supramolecular chemistry.

Tuning the excited-state intramolecular proton transfer (ESIPT) process of indole–pyrrole systems by π-conjugation and substitution effects: experimental and computational studies

A series of amino (NH)-type hydrogen-bonding (H-bonding) compounds, BNDAB-1–4, containing π-enlarged indole and β-ethoxycarbonyl-substituted pyrrole units were designed and synthesized.