Syntheses and Functionalizations of Porphyrin Macrocycles

Structural Decoration of Porphyrin/Phthalocyanine Photovoltaic Materials

Porphyrin/phthalocyanine compounds with fascinating molecular structures have attracted widespread attention in the field of solar cells in recent years. In this review, we focus on the pivotal role of porphyrin and phthalocyanine compounds in enhancing the efficiency of solar cells. The review seamlessly integrates the intricate molecular structures of porphyrins and phthalocyanines with their proficiency in absorbing visible light and facilitating electron transfer, key processes in converting sunlight into electricity. By delving into the nuances of intramolecular regulation, aggregated states, and surface/interface structure manipulation, it elucidates how various levels of molecular modifications enhance solar cell efficiency through improved charge transfer, stability, and overall performance. This comprehensive exploration provides a detailed understanding of the complex relationship between molecular design and solar cell performance, discussing current advancements and potential future applications of these molecules in solar energy technology.

Electrical Conductance and Thermopower of β-Substituted Porphyrin Molecular Junctions─Synthesis and Transport

Molecular junctions offer significant potential for enhancing thermoelectric power generation. Quantum interference effects and associated sharp features in electron transmission are expected to enable the tuning and enhancement of thermoelectric properties in molecular junctions. To systematically explore the effect of quantum interferences, we designed and synthesized two new classes of porphyrins, P1 and P2, with two methylthio anchoring groups in the 2,13- and 2,12-positions, respectively, and their Zn complexes, Zn-P1 and Zn-P2. Past theory suggests that P1 and Zn-P1 feature destructive quantum interference in single-molecule junctions with gold electrodes and may thus show high thermopower, while P2 and Zn-P2 do not. Our detailed experimental single-molecule break-junction studies of conductance and thermopower, the latter being the first ever performed on porphyrin molecular junctions, revealed that the electrical conductance of the P1 and Zn-P1 junctions is relatively close, and the same holds for P2 and Zn-P2, while there is a 6 times reduction in the electrical conductance between P1 and P2 type junctions. Further, we observed that the thermopower of P1 junctions is slightly larger than for P2 junctions, while Zn-P1 junctions show the largest thermopower and Zn-P2 junctions show the lowest. We relate the experimental results to quantum transport theory using first-principles approaches. While the conductance of P1 and Zn-P1 junctions is robustly predicted to be larger than those of P2 and Zn-P2, computed thermopowers depend sensitively on the level of theory and the single-molecule junction geometry. However, the predicted large difference in conductance and thermopower values between Zn-P1 and Zn-P2 derivatives, suggested in previous model calculations, is not supported by our experimental and theoretical findings.

Meso-Formyl, Vinyl, and Ethynyl Porphyrins-Multipotent Synthons for Obtaining a Diverse Array of Functional Derivatives

This review presents a strategy for obtaining various functional derivatives of tetrapyrrole compounds based on transformations of unsaturated carbon-oxygen and carbon-carbon bonds of the substituents at the meso position (meso-formyl, vinyl, and ethynyl porphyrins). First, synthetic approaches to the preparation of these precursors are described. Then diverse pathways for the transformations of the multipotent synthons are discussed, revealing a variety of products of such reactions. The structures, electronic, and optical properties of the compounds obtained by the methods under consideration are analyzed. In addition, there is an overview of the applications of the products obtained. Biomedical use of the compounds is among the most important. Finally, the advantages of using the reviewed synthetic strategy to obtain dyes with targeted properties are highlighted.

Development and Validation of Nerve-Targeted Bacteriochlorin Sensors

We report an innovative approach to producing bacteriochlorins (bacs) via formal cycloaddition by subjecting a porphyrin to a trimolecular reaction. Bacs are near-infrared probes with the intrinsic ability to serve in multimodal imaging. However, despite their ability to fluoresce and chelate metal ions, existing bacs have thus offered limited ability to label biomolecules for target specificity or have lacked chemical purity, limiting their use in bio-imaging. In this work, bacs allowed a precise and controlled appending of clickable linkers, lending the porphyrinoids substantially more chemical stability, clickability, and solubility, rendering them more suitable for preclinical investigation. Our bac probes enable the targeted use of biomolecules in fluorescence imaging and Cerenkov luminescence for guided intraoperative imaging. Bacs' capacity for chelation provides opportunities for use in non-invasive positron emission tomography/computed tomography. Herein, we report the labeling of bacs with Hs1a, a (NaV1.7)-sodium-channel-binding peptide derived from the Chinese tarantula Cyriopagopus schmidti to yield Bac-Hs1a and radiolabeled Hs1a, which shuttles our bac sensor(s) to mouse nerves. In vivo, the bac sensor allowed us to observe high signal-to-background ratios in the nerves of animals injected with fluorescent Bac-Hs1a and radiolabeled Hs1a in all imaging modes. This study demonstrates that Bac-Hs1a and [64Cu]Cu-Bac-Hs1a accumulate in peripheral nerves, providing contrast and utility in the preclinical space. For the chemistry and bio-imaging fields, this study represents an exciting starting point for the modular manipulation of bacs, their development and use as probes for diagnosis, and their deployment as formidable multiplex nerve-imaging agents for use in routine imaging experiments.

Porphyrin-based conjugated microporous adsorbent material for the efficient remediation of hexavalent chromium from the aquatic environment

The encapsulation and eradication of anions from water have received a lot of scrutinize and are extremely important for virtuous production and environmental treatment. To prepare extremely efficient adsorbents, a highly functionalized and conjugated microporous porphyrin-based adsorbent material (Co-4MPP) was synthesized using the Alder Longo method. Co-4MPP featured a hierarchical microporous and mesoporous layered structure containing nitrogen and oxygen-based functional groups with a specific surface area of 685.209 m2/g and a pore volume of 0.495 cm3/g. Co-4MPP demonstrated a greater Cr (VI) adsorption empathy than the pristine porphyrin-based material did. The effects of various parameters such as pH, dose, time, and temperature were explored on the Cr (VI) adsorption by Co-4MPP. The pseudo-second-order model and the Cr (VI) adsorption kinetics were in agreement (R2 = 0.999). The Langmuir isotherm model matched the Cr (VI) adsorption isotherm, demonstrating the optimum Cr (VI) adsorption capacities: 291.09, 307.42, and 339.17 mg/g at 298K, 312K, and 320K, correspondingly, with remediation effectiveness of 96.88%. The model evaluation further revealed that Cr (VI) adsorption mechanism on Co-4MPP was endothermic, spontaneous, and entropy-rising. The detailed discussion of the adsorption mechanism suggested that it could be a reduction, chelation, and electrostatic interaction, in which the protonated nitrogen and oxygen-containing functional groups on the porphyrin ring interacted with Cr (VI) anions to form a stable complex, thus remediating Cr (VI) anions efficiently. Moreover, Co-4MPP demonstrated strong reusability, maintaining 70% of its Cr (VI) elimination rate after four consecutive adsorptions.

New AB3-type porphyrins with piperidine and morpholine motifs; synthesis and photo-physicochemical and biological properties

In this study, new unsymmetrical meso-tetraaryl AB₃-type porphyrins 1 and 2 were successfully synthesized by the reaction of p-bromobenzaldehyde and p-hydroxybenzaldehyde with pyrrole in propionic acid. AB₃-type porphyrin building blocks with hydroxyl functionality (1 and 2) were further used to generate both covalently linked metal free and Zn(II) porphyrins 3-6 having piperidine and morpholine heterocyclic units. These novel compounds were characterized by using ¹H NMR, ¹³C NMR, FT-IR and MALDI-TOF spectrophotometry. The photophysical and photochemical properties of compounds 1-6 were investigated by employing UV-vis absorption and fluorescence emission spectroscopy in tetrahydrofuran (THF). From the view of biological properties, the antioxidant capacities of porphyrins were determined by using DPPH radical scavenging activity and 2 was determined as the most potent porphyrin analog with a value of 98.42% at 200 mg L^-1. All the targeted compounds displayed significant DNA nuclease activity. In addition, the antimicrobial potential of compounds 1-6 was also investigated by a micro-dilution process and 2 was found to be the most effective candidate against the tested microbial strains. The newly synthesized porphyrins also showed 100% microbial cell viability inhibition against E. coli at all examined concentrations. In terms of biofilm inhibition activity, the best results for the maximum photodynamic antimicrobial biofilm inhibition of S. aureus and P. aeruginosa were obtained by compound 2 with the values of 99.75% and 93.39%, respectively.

Synthesis and Self-Assembly of β-Octa[(4-Diethoxyphosphoryl)phenyl]porphyrins

The β-substituted porphyrinoids commonly used to form functional assembled systems in nature yet are still scarcely used in material chemistry probably due to the laborious synthesis of these compounds. In this work, β-octa[(4-diethoxyphosphoryl)phenyl]porphyrin (2HOPPP) and its metal (Zn(II), Cd(II), Cu(II), and Ni(II)) complexes were prepared in good yields. These highly soluble chromophores were characterized in solution using spectroscopic (NMR, UV-vis, fluorescence), electrochemical, and spectroelectrochemical methods. Attachment of the electron-deficient residue (ArP(O)(OEt)₂) to the porphyrin macrocycle leads to easier reductions and harder oxidations of the macrocycle for all complexes studied as compared to corresponding meso-tetra[4-(diethoxyphosphoryl)phenyl]porphyrin derivatives reported previously. We demonstrated that the strong electron-deficient character of the MOPPP porphyrins results principally from the increase in the number of electron-withdrawing groups at the periphery of the tetrapyrrolic macrocycle. Electron-deficient porphyrins are highly required in supramolecular and material chemistry in part due to their ability to form supramolecular assemblies via the coordination of axial ligands to the central metal atom. According to single-crystal X-ray data, ZnOPPP forms in the crystalline phase dimers in which each of the two tetrapyrrolic macrocycles is connected through an unusual combination of hydrogen bonding of two phosphoryl groups and the water molecules axially coordinated to the zinc atom of the partner molecule. The involvement of water molecules in porphyrin binding allows for an increase of distance between two porphyrin mean N₄ planes, up to 4.478 Å. The offset of phosphoryl groups attached to the macrocycle through a 1,4-phenylene spacer withdraws the whole porphyrin macrocycle of one molecule from spatial overlap with the macrocycle of a partner molecule and increases the Zn-Zn distance up to 10.372 Å. This still unknown type of porphyrin dimers allows one to get deeper insights into the organization of naturally occurring tetrapyrrolic macrocycles. ZnOPPP also forms a labile dimeric complex in 5.3 × 10^-7-5.8 × 10^-5 M chloroform solutions. In contrast, other complexes prepared in this work exist as monomeric species under these experimental conditions. The self-association constant of ZnOPPP has been determined by electronic absorption spectroscopy.

Facile Synthesis of β-Tetracyano Vanadyl Porphyrin from Its Tetrabromo Analogue and Its Excellent Catalytic Activity for Bromination and Epoxidation Reactions

Complex 2,3,12,13-tetracyano-5,10,15,20-tetraphenylporphyrinatooxidovanadium(IV) {[V^IVOTPP(CN)₄], 2} has been prepared by nucleophilic substitution of β-bromo groups of the corresponding 2,3,12,13-tetrabromo-5,10,15,20-tetraphenylporphyrinatooxidovanadium(IV) {[V^IVOTPP(Br)₄], 1} using CuCN in quinoline. Both complexes show biomimetic catalytic activity similar to enzyme haloperoxidases and efficiently brominate various phenol derivatives in the presence of KBr, H₂O₂, and HClO₄ in the aqueous medium. Between these two complexes, 2 exhibits excellent catalytic activity with high turnover frequency (35.5-43.3 s^-1) due to the strong electron-withdrawing nature of the cyano groups attached at β-positions and its moderate nonplanar structure as compared to 1 (TOF = 22.1-27.4 s^-1). Notably, this is the highest turnover frequency value observed for any porphyrin system. The selective epoxidation of various terminal alkenes using complex 2 has also been carried out, and the results are good, specifying the importance of electron-withdrawing cyano groups. Catalysts 1 and 2 are recyclable, and the catalytic activity proceeds through the corresponding [V^VO(OH)TPP(Br)₄] and [V^VO(OH)TPP(CN)₄] intermediates, respectively.

Enantiomeric Resolution and Enantiomer Isolation of H2 TPPS4 J-Aggregate from Aqueous Solution Is Enabled by Vortexes

Protonated achiral H₂ TPPS4 spontaneously self-arranges at acids pH and high ionic strength to build mesoscopic J-aggregates that are intrinsically chiral. According to the symmetry rule aggregation leads to a racemate that, however, can be unbalanced by chemical (chiral pollutants) or physical stimuli (as vortexing the solution). Vortexing the title racemate, in principle, might either induce chiral separation or chiral enrichment. Indeed, herein it is shown that vortices enable the resolution of this racemic solution exploiting the tendency to deposit, onto the quartz cuvette walls, of the enantiomer favored by the stirring sense. Simultaneously, over time, it was found that the opposite chiral conformation becomes prevalent in solution realizing a significant enantiomeric resolution. Therefore, after removing all stirring-favored chiral J-aggregate from the solution, the recovering and isolating of the desired enantiomers from the cuvette walls was successfully obtained without complex procedures. In this sense, it has been demonstrated that the stirring forces are executively able to fulfil the chiral separation in H₂ TPPS4 J-aggregates, employed as model of a self-assembled system in aqueous solution.

Porphyryne

Density functional theory calculations with the B3LYP*-D3 method with large STO-QZ4P basis sets unambiguously predict a singlet ground state for Zn-porphyryne. However, the calculations also predict a low singlet-triplet gap of about 0.4 eV and a high adiabatic electron affinity of 2.4 eV. Accordingly, the reactivity of porphyryne species may be dominated by electron transfer, hydrogen abstraction, and proton-coupled electron transfer processes.

Main Strategies for the Synthesis of meso-Arylporphyrins

meso-Arylporphyrins as most accessible tetrapyrrole macroheterocycles have always been the focus of attention from researchers concerned with practically useful properties of these compounds. The first syn­theses of meso-arylporphyrins date back to about 90 years ago. Up to now, the yields of these compounds have been improved from 5 to 80%. The present review analyzes different ways and strategies for the synthesis of meso-aryl-substituted porphyrins. The most efficient methods that can be scaled up to an industrial level have been identified.

Porphyrins as Chelating Agents for Molecular Imaging in Nuclear Medicine

Porphyrin ligands, showing a significant affinity for cancer cells, also have the ability to chelate metallic radioisotopes to form potential diagnostic radiopharmaceuticals. They can be applied in single-photon emission computed tomography (SPECT) and positron emission tomography (PET) to evaluate metabolic changes in the human body for tumor diagnostics. The aim of this paper is to present a short overview of the main metallic radionuclides complexed by porphyrin ligands and used in these techniques. These chelation reactions are discussed in terms of the complexation conditions and kinetics and the complex stability.

Supramolecular Chirality in Porphyrin Self-assembly Systems in Aqueous Solution

Abstract:

The self-assembly process appears as a powerful and attractive strategy for constructing complex supramolecules by the spontaneous organization of appropriate building blocks. In this scenario, water-soluble porphyrinoids lend themselves as ideal paradigms to disclose the self-assembly phenomenon by exploiting their well-known tendency to build aggregates in aqueous media via weak non-covalent forces. Nevertheless, the spontaneous organization of achiral porphyrins can result in a final chiral superstructure moving away from single-molecule behaviour to supramolecular chirality. Therefore, over the years numerous attempts have been implemented to investigate how a porphyrin aggregate, made up of achiral monomers, becomes not-symmetric and which processes govern the bias for a certain enantiomeric assembly rather than another. Thus, in this mini-review, we exclusively discuss the main strategies for designing and building chiral aggregates in water from achiral porphyrin monomers, with particular regard to their chiroptical features.

Selective epoxidation of olefins by vanadylporphyrin [VIVO(TPP)] and electron deficient nonplanar β-octabromovanadylporphyrin [VIVO(TPPBr8)]

Meso-tetraphenylporphyrinatooxidovanadium(IV) [VIVO(TPP)] (1) and 2,3,7,8,12,13, 17,18-octabromo-meso-tetraphenylporphyrinatooxidovanadium(IV) [VIVO(TPPBr[Formula: see text]] (2) were synthesized and characterized by various spectroscopic techniques. 1 and 2 were utilized as efficient catalysts for the selective epoxidation of olefins. Catalyst 2 shows very high catalytic activity (TOF = 8783–13108 h[Formula: see text] as compared to 1 (TOF = 2175–3296 h[Formula: see text]. The catalysts are recyclable, as their recyclability was checked for four cycles and show only minor reduction of their efficiency.

Regioselective Stepwise Bromination of [14]Triphyrins(2.1.1) and Their Effects on Structural, Spectral, and Redox Properties

Regioselective stepwise bromination of meso-tetraaryl [14]triphyrins(2.1.1) was explored to investigate the effect of bromine substitution at the β-pyrrole carbons of triphyrin(2.1.1) on the structural, spectral, photophysical, and redox properties. A series of β-monobromo to β-hexabromo triphyrins(2.1.1) 2-7 were synthesized by treating triphyrin(2.1.1) 1 with appropriate equivalents of N-bromosuccinimide at ambient temperature in decent yields. The regiochemistry of bromines in β-brominated triphyrins(2.1.1) 3-5 and 7 was confirmed by X-ray crystallography, and the analysis revealed the effect of bromination of triphyrin(2.1.1) on the structural framework was significant in the case of hexabromotriphyrin(2.1.1) 7 compared to other macrocycles. Absorption spectroscopy showed that stepwise substitution of bromines at β-pyrrole carbons of triphyrin(2.1.1) resulting in bathochromic shifts of absorption bands relative to triphyrin(2.1.1) 1 and hexabromotriphyrin(2.1.1) 7 exhibited absorption bands at longer wavelengths. The redox studies revealed that compounds 2-7 were easier to reduce than triphyrin(2.1.1) 1, and the first reduction potential wave shifted anodically with an increase in the number of bromine substituents at β-pyrrole carbons of triphyrin(2.1.1) 1 from one to six. These structural, spectral, and electrochemical properties were also predicted by density functional theory calculations, and the analysis was consistent with the experimental observations.

Amphiphilic Zinc Porphyrin Single-Walled Carbon Nanotube Hybrids: Efficient Formation and Excited State Charge Transfer Studies

Herein, the microscopic and spectroscopic characterization of a novel non-covalent electron donor-acceptor system, in which three different metalloporphyrins (1, 2, and 3) play the dual role of light harvester and electron donor with SWCNTs as electron acceptor, is described. To this end, microscopy, that is, atomic force microscopy (AFM) and transmission electron microscopy (TEM) corroborate the formation of 1-SWCNT, 2-SWCNT, and 3-SWCNT. Spectroscopy by means of Raman, fluorescence, and transient absorption spectroscopy confirmed efficient charge-transfer interaction from photoexcited metalloporphyrins to SWCNTs in the ground and excited state of 1-SWCNT, 2-SWCNT, and 3-SWCNT. The complementary use of spectroelectrochemical and transient absorption measurements substantiates the formation of one-electron oxidized metalloporphyrins after photoexcitation. Multiwavelength global analysis provides insights into the charge-separation and recombination processes in 1-SWCNT, 2-SWCNT, and 3-SWCNT upon photoexcitation. Notably, both the charge-separation and recombination dynamics are fastest in 2-SWCNT. Importantly, the strongest interactions in the steady-state experiments are associated with the fastest excited state decay in the time-resolved measurements.

Structural and Photophysical Characterization of All Five Constitutional Isomers of the Octaethyl-β,β'-dioxo-bacterio- and -isobacteriochlorin Series

It is well-known that treatment of β-octaethylporphyrin with H₂ O₂ /conc. H₂ SO₄ converts it to a β-oxochlorin as well as all five constitutional isomers of the corresponding β,β'-dioxo-derivatives: two bacteriochlorin-type isomers (β-oxo groups at opposite pyrrolic building blocks) and three isobacteriochlorin-type isomers (β-oxo-groups at adjacent pyrrolic building blocks). By virtue of the presence of the strongly electronically coupled β-oxo auxochromes, none of the chromophores are archetypical chlorins, bacteriochlorins, or isobacteriochlorins. Here the authors present, inter alia, the single crystal X-ray structures of all free-base diketone isomers and a comparative description of their UV-vis absorption spectra in neutral and acidic solutions, and fluorescence emission and singlet oxygen photosensitization properties, Magnetic Circular Dichroism (MCD) spectra, and singlet excited state lifetimes. DFT computations uncover underlying tautomeric equilibria and electronic interactions controlling their electronic properties, adding to the understanding of porphyrinoids carrying β-oxo functionalities. This comparative study lays the basis for their further study and utilization.

Mitochondria are specifically vulnerable to 420nm light in drosophila which undermines their function and is associated with reduced fly mobility

Increased blue light exposure has become a matter of concern as it has a range of detrimental effects, but the mechanisms remain unclear. Mitochondria absorb short wavelength light but have a specific absorbance at 420nm at the lower end of the human visual range. This 420nm absorption is probably due to the presence of porphyrin. We examine the impact of 420nm exposure on drosophila melanogaster mitochondria and its impact on fly mobility. Daily 15 mins exposures for a week significantly reduced mitochondrial complex activities and increased mitochondrial inner membrane permeability, which is a key metric of mitochondrial health. Adenosine triphosphate (ATP) levels were not significantly reduced and mobility was unchanged. There are multiple options for energy/time exposure combinations, but we then applied single 420nm exposure of 3h to increase the probability of an effect on ATP and mobility, and both were significantly reduced. ATP and mitochondrial membrane permeability recovered and over corrected at 72h post exposure. However, despite this, normal mobility did not return. Hence, the effect of short wavelengths on mitochondrial function is to reduce complex activity and increasing membrane permeability, but light exposure to reduce ATP and to translate into reduced mobility needs to be sustained.

Recent Progress in (Photo-)-Electrochemical Conversion of CO2 With Metal Porphyrinoid-Systems

Since decades, the global community has been facing an environmental crisis, resulting in the need to switch from outdated to new, more efficient energy sources and a more effective way of tackling the rising carbon dioxide emissions. The activation of small molecules such as O₂, H⁺, and CO₂ in a cost—and energy-efficient way has become one of the key topics of catalysis research. The main issue concerning the activation of these molecules is the kinetic barrier that has to be overcome in order for the catalyzed reaction to take place. Nature has already provided many pathways in which small molecules are being activated and changed into compounds with higher energy levels. One of the most famous examples would be photosynthesis in which CO₂ is transformed into glucose and O₂ through sunlight, thus turning solar energy into chemical energy. For these transformations nature mostly uses enzymes that function as catalysts among which porphyrin and porphyrin-like structures can be found. Therefore, the research focus lies on the design of novel porphyrinoid systems (e.g. corroles, porphyrins and phthalocyanines) whose metal complexes can be used for the direct electrocatalytic reduction of CO₂ to valuable chemicals like carbon monoxide, formate, methanol, ethanol, methane, ethylene, or acetate. For example the cobalt(III)triphenylphosphine corrole complex has been used as a catalyst for the electroreduction of CO₂ to ethanol and methanol. The overall goal and emphasis of this research area is to develop a method for industrial use, raising the question of whether and how to incorporate the catalyst onto supportive materials. Graphene oxide, multi-walled carbon nanotubes, carbon black, and activated carbon, to name a few examples, have become researched options. These materials also have a beneficial effect on the catalysis through for instance preventing rival reactions such as the Hydrogen Evolution Reaction (HER) during CO₂ reduction. It is very apparent that the topic of small molecule activation offers many solutions for our current energy as well as environmental crises and is becoming a thoroughly investigated research objective. This review article aims to give an overview over recently gained knowledge and should provide a glimpse into upcoming challenges relating to this subject matter.

Wet-Chemically Prepared Porphyrin Layers on Rutile TiO2(110)

Porphyrins are large organic molecules that are interesting for different applications, such as photovoltaic cells, gas sensors, or in catalysis. For many of these applications, the interactions between adsorbed molecules and surfaces play a crucial role. Studies of porphyrins on surfaces typically fall into one of two groups: (1) evaporation onto well-defined single-crystal surfaces under well-controlled ultrahigh vacuum conditions or (2) more application-oriented wet chemical deposition onto less well-defined high surface area surfaces under ambient conditions. In this study, we will investigate the wet chemical deposition of 5-(monocarboxyphenyl)-10,15,20-triphenylporphyrin (MCTPP) on well-defined rutile TiO₂(110) single crystals under ambient conditions. Prior to deposition, the TiO₂(110) crystals were also cleaned wet-chemically under ambient conditions, meaning none of the preparation steps were done in ultrahigh vacuum. However, after each preparation step, the surfaces were characterized in ultrahigh vacuum with X-ray photoelectron spectroscopy (XPS) and the result was compared with porphyrin layers prepared in ultrahigh vacuum (UHV) by evaporation. The differences of both preparations when exposed to zinc ion solutions will also be discussed.

Chiral Alkyl Groups at Position 3(1') of Pyropheophorbide-a Specify Uptake and Retention by Tumor Cells and Are Essential for Effective Photodynamic Therapy

To investigate the importance of the chirality and precise structure at position 3(1') of pyropheophorbide-a for tumor cell specificity and photodynamic therapy (PDT), a series of photosensitizers (PSs) was synthesized: (a) with and without chirality at position 3(1'), (b) alkyl ether chain with a variable number of chiral centers, (c) hexyl ether versus thioether side chain, and (d) methyl ester versus carboxylic acid group at position 17². The cellular uptake and specificity were defined in human lung and head/neck cancer cells. PSs without a chiral center and with an alkyl chain or thioether functionalities showed limited uptake and PDT efficacy. Replacing the methyl group at the chiral center with a propyl group or introducing an additional chiral center improved cellular retention and tumor cell specificity. Replacing the carboxylic acid with methyl ester at position 17² lowered cellular uptake and PDT efficacy. A direct correlation between the PS uptake in vitro and in vivo was identified.

Classic highlights in porphyrin and porphyrinoid total synthesis and biosynthesis

Porphyrins feature prominently in nature, be it as enzymatic cofactors, electron and exciton shuffles, as photoactive dyes, or as signaling substances. Their involvement in the generation, storage and use of oxygen is pivotal to life, while their photochemical properties are central to the biochemical functioning of plants. When complexed to metals, porphyrins can engage in a multitude of contemporary applications ranging from solar energy generation to serving as catalysts for important chemical reactions. They are also able to function as useful theranostic agents, and as novel materials for a wide range of applications. As such, they are widely considered to be highly valuable molecules, and it almost goes without saying that synthetic organic chemistry has dramatically underpinned all the key advances made, by providing reliable access to them. In fact, strategies for the synthesis of functionalized porphyrins have now reached a state of refinement where pretty well any desired porphyrin can successfully be synthesized with the approaches that are available, including a cornucopia of related macrocycle-modified porphyrinoids. In this review, we are going to illustrate the development of this exciting field by discussing a number of classic syntheses of porphyrins. Our coverage will encompass the natural protoporphyrins and chlorophylls, while also covering general strategies for the synthesis of unsymmetrical porphyrins and chlorins. Various industrial syntheses of porphyrins will also be discussed, as will other routes of great practical importance, and avenues to key porphyrinoids with modified macrocycles. A range of selected examples of contemporary functionalization reactions will be highlighted. The various key syntheses will be described and analyzed from a traditional mechanistic organic chemistry perspective to help student readers, and those who are new to this area. The aim will be to allow readers to mechanistically appreciate and understand how many of these fascinating ring-systems are built and further functionalized.

Porphyrin–Nanodiamond Hybrid Materials—Active, Stable and Reusable Cyclohexene Oxidation Catalysts

The quest for active, yet “green” non-toxic catalysts is a continuous challenge. In this work, covalently linked hybrid porphyrin–nanodiamonds were prepared via ipso nitro substitution reaction and characterized by X-ray photoelectron spectroscopy (XPS), fluorescence spectroscopy, infrared spectroscopy (IR) and thermogravimetry-differential scanning calorimetry (TG-DSC). The amine-functionalized nanodiamonds (ND@NH2) and 2-nitro-5,10,15,20-tetra(4-trifluoromethylphenyl)porphyrin covalently linked to nanodiamonds (ND@βNH-TPPpCF3) were tested using Allium cepa as a plant model, and showed neither phytotoxicity nor cytotoxicity. The hybrid nanodiamond–copper(II)–porphyrin material ND@βNH-TPPpCF3-Cu(II) was also evaluated as a reusable catalyst in cyclohexene allylic oxidation, and displayed a remarkable turnover number (TON) value of ≈265,000, using O2 as green oxidant, in the total absence of sacrificial additives, which is the highest activity ever reported for said allylic oxidation. Additionally, ND@βNH-TPPpCF3-Cu(II) could be easily separated from the reaction mixture by centrifugation, and reused in three consecutive catalytic cycles without major loss of activity.

Stepwise Reduction of Octaethyl-β,β'-dioxochlorin Isomers: Access to Structurally and Electronically Diverse Hydroporphyrins

Di- and tetrahydroporphyrins (chlorins, bacteriochlorins and isobacteriochlorins, respectively) are key "pigments of life." They have been a major focus of attention in synthetic tetrapyrrole chemistry. A long-known one-pot epoxidation/epoxide ring-opening/pinacol-pinacolone rearrangement of octaethylporphyrin (OEP) generates a β-ketochlorin and all five β,β'-diketone isomers. We present herein the single and double reductions of all isomers of the β,β'-diketones, generating hydroxychlorin and β-hydroxy-β-ketodihydroporphyrin isomers, generally in regioselective manner, and sets of separable stereoisomeric dihydroxytetrahydroporphyrin regioisomers. The connectivity of the regio- and stereoisomers were determined spectroscopically and, in many cases, using single-crystal X-ray crystallography. The optical properties of the chlorin-, bacteriochlorin-, and isobacteriochlorin-type chromophores are described. They highlight general observations on the regiochemical effects of the β-oxo-auxochrome. This contribution thus delineates the formation of a range of regio- and stereoisomers of a family of chromophores with broadly varying optical properties from a single and readily available starting material (OEP) in two straightforward steps, albeit requiring extensive chromatography.

Control of Porphyrin Planarity and Aggregation by Covalent Capping: Bissilyloxy Porphyrin Silanes

Porphyrins are cornerstone functional materials that are useful in a wide variety of settings, ranging from molecular electronics to biology and medicine. Their applications are often hindered, however, by poor solubilities that result from their extended, solvophobic aromatic surfaces. Attempts to counteract this problem by functionalizing their peripheries have been met with only limited success. Here, we demonstrate a versatile strategy to tune the physical and electronic properties of porphyrins using an axial functionalization approach. Porphyrin silanes (PorSils) and bissilyloxy PorSils (SOPS) are prepared from porphyrins by operationally simple κ⁴N-silylation protocols, introducing bulky silyloxy "caps" that are central and perpendicular to the planar porphyrin. While porphyrins typically form either J- or H-aggregates, SOPS do not self-associate in the same manner: the silyloxy axial substituents dramatically improve the solubility by inhibiting aggregation. Moreover, axial porphyrin functionalization offers convenient handles through which optical, electronic, and structural properties of the porphyrin core can be modulated. We observe that the identity of the silyloxy substituent impacts the degree of planarity of the porphyrin in the solid state as well as the redox potentials.

Porphyrin-Ryleneimide Hybrids: Tuning of Visible and Near-Infrared Absorption by Chromophore Desymmetrization

Unsymmetrically fused porphyrins containing one or two naphthalimide subunits were prepared in modular syntheses relying on electron-rich and electron-poor pyrrole building blocks. These new chromophores show progressive changes in their electron-deficient character, while retaining comparably small optical and electrochemical band gaps. The intrinsic curvature and extended optical absorption of these systems make them of interest as mono- and difunctional components of multichromophoric assemblies.

The Red Color of Life Transformed - Synthetic Advances and Emerging Applications of Protoporphyrin IX in Chemical Biology

Protoporphyrin IX (PPIX) is the porphyrin scaffold of heme b, a ubiquitous prosthetic group of proteins responsible for oxygen binding (hemoglobin, myoglobin), electron transfer (cytochrome c) and catalysis (cytochrome P450, catalases, peroxidases). PPIX and its metallated derivatives frequently find application as therapeutic agents, imaging tools, catalysts, sensors and in light harvesting. The vast toolkit of accessible porphyrin functionalization reactions enables easy synthetic modification of PPIX to meet the requirements for its multiple uses. In the past few years, particular interest has arisen in exploiting the interaction of PPIX and its synthetic derivatives with biomolecules such as DNA and heme-binding proteins to evolve molecular devices with new functions as well as to uncover potential therapeutic toeholds. This review strives to shine a light on the most recent developments in the synthetic chemistry of PPIX and its uses in selected fields of chemical biology.

Photoactivatable Surface-Functionalized Diatom Microalgae for Colorectal Cancer Targeted Delivery and Enhanced Cytotoxicity of Anticancer Complexes

Systemic toxicity and severe side effects are commonly associated with anticancer chemotherapies. New strategies based on enhanced drug selectivity and targeted delivery to cancer cells while leaving healthy tissue undamaged can reduce the global patient burden. Herein, we report the design, synthesis and characterization of a bio-inspired hybrid multifunctional drug delivery system based on diatom microalgae. The microalgae's surface was chemically functionalized with hybrid vitamin B12-photoactivatable molecules and the materials further loaded with highly active rhenium(I) tricarbonyl anticancer complexes. The constructs showed enhanced adherence to colorectal cancer (CRC) cells and slow release of the chemotherapeutic drugs. The overall toxicity of the hybrid multifunctional drug delivery system was further enhanced by photoactivation of the microalgae surface. Depending on the construct and anticancer drug, a 2-fold increase in the cytotoxic efficacy of the drug was observed upon light irradiation. The use of this targeted drug delivery strategy, together with selective spatial-temporal light activation, may lead to lower effective concentration of anticancer drugs, thereby reducing medication doses, possible side effects and overall burden for the patient.

Synthesis and characterization of 5-(4-carboxyphenylspermine)-10,15,20-triphenylporphyrin

We synthetized and characterized a mono spermine porphyrin derivative by NMR, UV-vis and fluorescence spectroscopy. The photophysical properties and the protonation equilibria of 5-(4-carboxyphenylspermine)-10,15,20-triphenylporphyrin have been investigated, showing that porphyrin does not aggregate in acidic solutions, differently from what occurs as soon as the core of the porphyrin is deprotonated. These aggregation processes have been detected by the rising of new fluorescence band and a significant splitting of the Soret band.

Enhanced Cellular Uptake and Photodynamic Effect with Amphiphilic Fluorinated Porphyrins: The Role of Sulfoester Groups and the Nature of Reactive Oxygen Species

A class of amphiphilic photosensitizers for photodynamic therapy (PDT) was developed. Sulfonate esters of modified porphyrins bearing-F substituents in the ortho positions of the phenyl rings have adequate properties for PDT, including absorption in the red, increased cellular uptake, favorable intracellular localization, low cytotoxicity, and high phototoxicity against A549 (human lung adenocarcinoma) and CT26 (murine colon carcinoma) cells. Moreover, the role of type I and type II photochemical processes was assessed by fluorescent probes specific for various reactive oxygen species (ROS). The photodynamic effect is improved not only by enhanced cellular uptake but also by the high generation of both singlet oxygen and oxygen-centered radicals. All of the presented results support the idea that the rational design of photosensitizers for PDT can be further improved by better understanding the determinants affecting its therapeutic efficiency and explain how smart structural modifications can make them suitable photosensitizers for application in PDT.

A new synthesis of porphyrins via a putative trans-manganese(iv)-dihydroxide intermediate

A new method for the synthesis of meso-substituted porphyrins was developed. In this two-step methodology, the first step involves the condensation of pyrroles/dipyrromethanes with aldehydes in a water-methanol mixture under acidic conditions. The second step involves manganese induced cyclization followed by oxidation via PhIO/O2. This methodology has been useful for the synthesis of a wide range of trans-A2B2 porphyrins and also symmetric porphyrins in moderate to good yields. A detailed investigation of the manganese induced cyclization reaction has allowed us to characterize a Mn-porphyrinogen complex. A series of analytical and spectroscopic techniques and DFT calculations have led us to the conclusion that the putative intermediate species are trans-manganese(iv)-dihydroxide complexes. EPR and magnetic susceptibility measurements helped us to assign the oxidation state of the manganese complexes in their native state. The assumption of trans-manganese(iv)-dihydroxide as the true intermediate for this porphyrin synthesis has been authenticated via in situ UV-Vis experiments. This new methodology is certainly different from other previously reported methodologies in many aspects and most importantly these reactions can be easily performed on a gram scale for the synthesis of porphyrins.