Porphyrin Monomers Research Articles

A Porphyrin-Based sp2 Carbon-Conjugated covalent organic polymer containing flexible chains for efficient heterogeneous photocatalysis

Porphyrin based covalent organic polymers are excellent catalysts. Herein, a sp2 carbon linked porphyrin based organic polymer H2Pp-PDAN was designed and synthesized by co-condensation of porphyrin monomer containing flexible chains H2Pp with 1, 4-phenylenediacetonitrile PDAN. As photocatalysts, H2Pp-PDAN exhibited excellent photocatalytic performance for photocatalytic oxidative coupling of amines, and was successfully used for 15 consecutive reactions with no detectable reduction of the catalytic activity. H2Pp-PDAN showed good photocatalytic activity and excellent recyclability for photocatalytic oxidation of thioanisole to methyl phenyl sulfoxide. Mechanistic studies revealed that the photocatalytic oxidation of benzylamine coupling by H2Pp-PDAN proceeds via both energy transfer and electron transfer pathways.

Research on the fabrication of nanorod-structured phorphyrin/g-C3N4 as photocatalyst materials for the removal of Cr6+ ions in an aqueous solution

Due to their potential to expand the photon energy harvesting zones and improve charge separation, two semiconductors together have been shown to be a promising technique to increase the photocatalytic activity of photocatalysts. Through the use of monomeric porphyrin molecules and g-C3N4 nanomaterials, self-assembly of the hybird g-C3N4@porrphyrin nanorod materials under CTAB surfactant-assited conditions was demonstrated in this study. SEM, EDS, FTIR, and UV-vis were used to characterize the generated hybrid material. Under simulated sunlight irradiation, the hybrid mateial’s photocatalytic behaviour was examined for the photoreduction of Cr6+ ions. After 100 minutes of reaction time under the simulated solar spectrum, the results showed that the hybrid material demonstrated high photocatalytic performance against, with clearance percentage of almost 100%. The prepared photocatalyst was also highly stable with the reduction of Cr6+ removal efficiency of less than 10% after 4 cycles of photocatalytic testing.

Self-Assembly of the Porphyrin Monomer on the Surface of Fe/Graphene Material: A Novel Sensing Material for the Detection of Chloramphenicol Antibiotic in Aqueous solution.

Currently, electrochemical sensors are being developed and widely used in various fields, and new materials are being explored to enhance the precision and selectivity of the sensors. The present investigation involved the fabrication of a Fe/graphene/porphyrin nanocomposite through self-assembly, wherein the individual porphyrin molecules were arranged on the Fe/graphene nanomaterials' surface. The Fe/graphene nanoparticles were synthesized utilizing a green approach, wherein leaf extract was employed as the reducing agent. The resulting materials underwent comprehensive characterization using a range of contemporary techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy. The study's findings revealed that the nanocomposites of Fe/graphene/porphyrin comprised zero-valent iron nanoparticles, exhibiting an average particle size ranging from 15 to 60 nm. These nanoparticles were seen to be evenly dispersed across the graphene sheets. The presence of nanostructure porphyrin nanofibers, measuring 20 nm in diameter, was also shown to exhibit strong integration with the surface of the Fe/graphene nanomaterials. The electrochemical properties of the Fe/graphene/porphyrin nanocomposite were also investigated, demonstrating that the prepared material could be effectively employed as a sensing electrode in the electrochemical sensor for detecting Chloramphenicol (CAP) through CV, EIS, and DPV techniques using a three-electrode electrochemical system. Under optimal conditions, Fe/graphene/porphyrin exhibited a high current response when detecting CAPs. Electrochemical sensors created using Fe/graphene/porphyrin nanocomposite have high stability and repeatability, and they hold promise in developing sensors capable of identifying other antibiotic residues in agriculture.

Controlled Self‐Assembly of Zn–Tetraphenylporphyrins for Efficient Photocatalytic Solar H2 Production and Simultaneous Organic Transformation to Valuable Chemicals

Herein, we have designed aqueous dispersed self‐assembled nanostructures with diverse morphologies from the zinc tetraphenyl porphyrin (ZnTPP) monomer employing a simple solution‐based coprecipitation methods. Detail morophological studies have been carried out by various electron microscopy techniques. Finally, the structural features were correlated with the underpinning photophysical processes using steady‐state and time‐resolved spectroscopy. Detailed studies suggest that control morphology and highly defined intermolecular interactions affect the overall photoinduced charge transfer process. Based on the fundamental investigations, all these different types of nanostructures have been utilized as photocatalysts for solar hydrogen production, without using any cocatalysts, and it was found that the spherical nanostructure exhibits significantly higher H2 production rates of ~1682 m mole/ g, which is few folds higher than other 1D and 2D nanostructured materials. The experimental findings were further supported by the TD‐DFT study. Furthermore, the detailed computational studies suggest that the spherical aggregates exhibited a more vital interaction between the ZnTPP molecules, causes significant electronic coupling between bright local excited and charge transfer states, which supports our experimental findings. Finally, we have selectively utilized the oxidative half‐reaction for the simultaneous transformation of glycerol to valuable chemicals along with photocatalytic H2 production through reductive half‐reaction.

Methane Formation Induced via Face-to-Face Orientation of Cyclic Fe Porphyrin Dimer in Photocatalytic CO2 Reduction.

Iron porphyrins are known to provide CH4 as an eight-electron reduction product of CO2 in a photochemical reaction. However, there are still some aspects of the reaction mechanism that remain unclear. In this study, we synthesized iron porphyrin dimers and carried out the photochemical CO2 reduction reactions in N,N-dimethylacetamide (DMA) containing a photosensitizer in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as an electron donor. We found that, despite a low catalytic turnover number, CH4 was produced only when these porphyrins were facing each other. The close proximity of the cyclic dimers, distinguishing them from a linear Fe porphyrin dimer and monomers, induced multi-electron CO2 reduction, emphasizing the unique role of their structural arrangement in CH4 formation.

Broadband Ultrafast Nonlinear Optical Limiting at a Porphyrin‐Based 2D Polymer

AbstractUltrafast lasers have significantly contributed to the advancement of research and technology; however, high‐intensity lasers bring potential risks to precision instruments and the human eye. Developing optical limiting (OL) devices capable of reasonably controlling laser energy to an acceptable energy level is imperative. Nevertheless, achieving both exceptional OL performance and broad‐spectrum laser intensity tunability proves highly challenging. In this work, the synthesis of a porphyrin‐based 2D polymer (Por‐2DP) using a template‐assisted self‐coupling polymerization, constructing a 2D framework through the carbon‐carbon single bond coupling of porphyrin monomer is presented. The resultant Por‐2DP demonstrates remarkable broadband nonlinear absorption performance, spanning the visible and near‐infrared spectral regions under an ultrafast pulse laser (35 fs) for the first time. OL thresholds at excitation wavelengths of 515, 800, and 1550 nm are determined to be 1.44, 0.48, and 0.54 mJ cm−2, respectively, surpassing reported 2D materials to date. Additionally, the Por‐2DP exhibits prominent photostability, enabling sustained operation under intense light conditions for a long time, thereby enhancing OL practical applicability. This study not only introduces a novel OL material and device but also promotes the application of innovative 2D organic polymers in NLO field.

Electrochemical synthesis of catalytically active linear and cross-linked polyporphyrins

In this work, the electrochemical method was applied to prepare linear and 3D polyporphyrins based on 5,15-di(4-aminophenyl)− 10,20-diphenylporphyrin and 5,10,15-tri(4-aminophenyl)− 20-phenylporphyrin. The formation of polyporphyrins consisted in electrooxidation of porphyrins via the amino-groups of amino-phenyl substituents. Based on the assumed structure of the polyporpyrins, UV–vis spectroscopy data, structure of the porphyrin monomers and comparison of the swelling degrees, it was established that poly-5,15-di(4-aminophenyl)− 10,20-diphenylporphyrin has a linear structure, whereas poly-5,10,15-tri(4-aminophenyl)− 20-phenylporphyrin is a cross-linked polymer. The branched cross-linked polyporphyrin obtained from 5,10,15-tri(4-aminophenyl)− 20-phenylporphyrin on conductive substrates is much more effective in accelerating the electrons, which improves the catalytic characteristics of the oxygen electroreduction catalyst.

Fabrication of g-C3N4@porphyrin nanorods hybrid material via CTAB surfactant-assisted self-assembly for photocatalytic degradation of Cr(VI) and methylene blue

Photocatalysts are an attractive solution for pollutant degradation under sunlight irradiation. One approach that has been proposed to enhance their activity is to combine two semiconductors, which can broaden the photon energy harvesting regions and improve charge separation. Herein, a facile approach to fabricating a g-C3N4@porphyrin nanorods hybrid material is presented using CTAB surfactant-assisted self-assembly of monomeric porphyrin molecules and g-C3N4 nanomaterials. Using different technical methods, the hybrid material was studied, and it was found that the porphyrin nanorods on the surface of g-C3N4 were all in the same place. The photocatalytic performance of the hybrid material was evaluated by investigating its behavior for the photo-oxidation and -degradation of Cr+6 ions and methylene blue organic dye under simulated sunlight irradiation. High photocatalytic performance towards these two pollutants was exhibited by the hybrid material with a removal percentage of nearly 100% after 100 min of reaction time under the simulated sunlight spectrum. Also, a possible photocatalytic mechanism of the C3N4@porphyrin nanorods photocatalyst was proposed. This mechanism involved the efficient separation and transfer of photo-induced electrons and holes on the surface of the hybrid material. This work offers a simple and efficient method for creating high-performance photocatalysts, and we have made progress in our understanding of their photocatalytic mechanisms. The findings have important implications for wastewater treatment and solar energy conversion. The use of this hybrid material may contribute to addressing environmental challenges and assist in building sustainable energy systems.

Preparation of a Water-Soluble Glycopolymer Bearing Porphyrin Skeletons and Its Biological Properties.

A known tetraphenyl porphyrin (TPP) having an amino functional group [5-(4-aminophenyl)-10,15,20-(triphenyl)porphyrin] was converted into the corresponding monomer by means of condensation with acryloyl chloride. Simple radical polymerization of the porphyrin monomer and a glycosyl monomer in the presence of acrylamide as a regulator monomer in order to avoid steric interference gave a water-soluble glycopolymer bearing porphyrin moieties. Spectroscopic analyses suggested incorporation of porphyrin moieties in the glycopolymer. The physical properties of the water-soluble glycopolymer bearing porphyrin moieties were examined in aqueous media, and the results also indicated the incorporation of TPP moieties in the polymer. Uptake of the polymer into HeLa cells was observed, and the cytotoxicity of the polymer was confirmed by microscopic analyses. The glycopolymer bearing porphyrin moieties is promising not only for photodynamic therapy but also as an anti-cancer reagent.

Two-dimensional iron porphyrin nanozyme mimics cytochrome P450 activity for cancer proliferation inhibition

Two-dimensional materials have excellent performance in energy catalysis and biomedicine due to their unique advantages in surface and interfacial properties. Taking into account the unique advantages of the surface interface and catalysis of two-dimensional materials, it is a very feasible way to construct two-dimensional materials with clear atomic structure to break the redox balance in tumor cells, so as to achieve efficient anti-tumor effects. Based on this, we selected the porphyrin ligands that widely exist in nature, equipped with iron metal active centers, and constructed a two-dimensional iron porphyrin nanozyme with a clear atomic structure, which simulated the redox activity of cytochrome P450 enzymes. The experimental results show that by carrying a rotatable pyridine group on the porphyrin ring, the iron porphyrin monomers can be effectively connected and expanded into a two-dimensional iron porphyrin microstructure. Enough iron centers are exposed on the surface of two-dimensional iron porphyrins as catalytic sites. In the presence of oxidants, two-dimensional iron porphyrins can efficiently catalyze the oxidation of small organic molecules, and have good P450-like enzymatic activity. At the same time, the two-dimensional iron porphyrin nanomaterial was used for anti-tumor, and it was found that it has an efficient ability to inhibit tumor proliferation, which may be due to the catalytic oxidation of various organic small molecules in tumor cells by the high concentration of hydrogen peroxide in tumor cells. At the same time, we found that the two-dimensional iron porphyrin nanozyme may have a good drug loading capacity due to the many exposed iron sites on its surface, which provides a possible option for drug loading to achieve combined high-efficiency tumor therapy.

SYNTHESIS, PROPERTIES, AND APPLICATIONS OF SUPRAMOLECULAR LIQUID CRYSTAL MATERIALS AND ACRYLAMIDE PORPHYRIN POLYMERS

A review of the work in the field of design, study of properties, and application of liquid crystal materials and acrylamide porphyrin polymers, performed at the Department of Chemistry and Technology of High-molecular Compounds, in the laboratory of Supramolecular and liquid crystal systems and in the laboratory of Polymer materials of the Research Institute of Macroheterocyclic compounds of the Ivanovo State University of Chemical Technology over the past 10 years, is presented. Synthetic methods were developed, mesogenic compounds and mixed liquid crystal compositions based on classical and supramolecular mesogens doped with additives containing polar, amphiprotic and chiral groups were obtained and studied. The mechanisms of chiral transport in the mesophase were studied. The results on the use of liquid crystals as light and heat stabilizers of polymeric materials are presented. Stationary phases for gas-liquid chromatography based on supramolecular mesogens, macroheterocyclic compounds with high structural selectivity with respect to various isomers were developed. Methods have been developed for the synthesis of water-soluble porphyrin polymers and porphyrin-containing hydrogels based on acrylamide and synthetic porphyrin monomers, realizable both under thermal heating and under microwave radiation. It is shown that a change in the ratio and nature of the components, as well as the polymerization conditions in the synthesis of porphyrin polymers provides targeted control of the properties of the resulting compounds. The molecular-mass and spectral characteristics of the obtained polymers, their surface morphology and sorption properties of porphyrin-containing acrylamide hydrogels are investigated. For citation: Burmistrov V.A., Aleksandriiskii V.V., Novikov I.V., Pechnikova N.L., Shilov I.V., Lyubimtsev A.V., Ageeva T.A., Koifman O.I. Synthesis, properties, and applications of supramolecular liquid crystal materials and acrylamide porphyrin polymers. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2023. V. 66. N 7. P. 31-51. DOI: 10.6060/ivkkt.20236607. 6832j.

Facile preparation of porphyrin@g-C3N4/Ag nanocomposite for improved photocatalytic degradation of organic dyes in aqueous solution

In the quest of improving the photocatalytic efficiency of photocatalysts, the combination of two and more semiconductors recently has garnered significant attention among scientists in the field. The doping of conductive metals is also an effective pathway to improve photocatalytic performance by avoiding electron/hole pair recombination and enhancing photon energy absorption. This work presented a design and fabrication of porphyrin@g-C3N4/Ag nanocomposite using acid-base neutralization-induced self-assembly approach from monomeric porphyrin and g-C3N4/Ag material. g-C3N4/Ag material was synthesized by a green reductant of Cleistocalyx operculatus leaf extract. Electron scanning microscopy (SEM), X-ray diffraction (XRD), FT-IR spectroscopy, and UV–vis spectrometer were utilized to analyse the properties of the prepared materials. The prepared porphyrin@g-C3N4/Ag nanocomposite showed well integration of porphyrin nanostructures on the g-C3N4/Ag's surface, in which porphyrin nanofiber was of the diameter in nanoscales and the length of several micrometers, and Ag NPs had an average particle size of less than 20 nm. The photocatalytic behavior of the resultant nanocomposite was tested for the degradation of Rhodamine B dye, which exhibited a remarkable RhB photodegrading percentage. The possible mechanism for photocatalysis of the porphyrin@g-C3N4/Ag nanocomposite toward Rhodamine B dye was also proposed and discussed.

Covalent triazine framework featuring single electron Co2+ centered in intact porphyrin units for efficient CO2 photoreduction

Covalent triazine frameworks (CTFs) are porous materials constructed by the trimerization of aromatic nitriles. They have frequently been utilized in heterogeneous catalysis due to rich nitrogen sites, excellent stability and porosity. In the past decade, many methods have been developed to synthesize CTFs, however, the roles of structural differences of CTFs synthesized by different methods in catalytic performance remain unclear. Herein, we synthesized two Co porphyrin-based CTFs using same porphyrin monomer by different approaches. CTF-TfOH-Co formed by trifluoromethanesulfonic acid vapor catalysis retains original porphyrin units, and Co (active catalytic sites) feature single electrons. In contrast, CTF-ZnCl2-Co synthesized by the ionothermal method no longer preserves the original porphyrin units, and the Co sites have no single electrons. CTF-TfOH-Co exhibits efficient photocatalytic activity for CO2 reduction with a CO yield rate of 2562.8 μmol g-1h−1, which greatly exceeds that of CTF-ZnCl2-Co. Those results highlight that the integrity of the porphyrin units and the electronic state of Co significantly are vital to their photocatalytic performance. Steady-state photoluminescence tests confirm that CTF-TfOH-Co has a better ability to separate photogenerated electron-hole pairs than CTF-ZnCl2-Co, which accounts for its superior photoreduction performance.

Indacenodithiophene Bridged Dimeric Porphyrin Donor and Absorption Complementary Indacenodithiophene Acceptor for Nonfullerene Organic Photovoltaics

Porphyrins are one of the most promising materials for organic photovoltaics (OPVs) because of their easily tunable functional groups. Herein, we present the design and synthesis of two porphyrin-based small molecule donors, IDT-2TPE and TPE, in which two porphyrin units are bridged by an indacenodithiophene (IDT) diethynylene unit for the former and a single porphyrin monomer for the latter, and 3-ethylrhodanine units are then end-capped symmetrically by phenylenethynylene π-linkers. The porphyrin dimer IDT-2TPE exhibits outstanding red-shifted absorptions with a valley in both solution (400–800 nm) and film (400–900 nm), low-lying energy levels, and excellent thermal stability compared to the porphyrin monomer TPE due to the extended π-conjugation and increased rigidity with the IDT unit. Impressively, the device based on the IDT-2TPE donor and IDTCR nonfullerene acceptor shows a broad photoelectron response up to 1000 nm and a power conversion efficiency (PCE) of 10.41% at AM 1.5G. For comparison, the device based on the TPE donor and IDTCR acceptor has a much lower PCE of 4.22%. The much better performance of the device IDT-2TPE/IDTCR is attributed to its higher photocurrent generation and more favorable surface morphology with elevated charge carrier mobilities.

Self-assembly of monomeric porphyrin molecules into nanostructures: Self-assembly pathways and applications for sensing and environmental treatment

Self-assembly of monomeric porphyrin molecules into nanostructures: Self-assembly pathways and applications for sensing and environmental treatment

Vanadyl(IV) Porphyrin Dimers with Palladium(II) and Platinum(II) Linkages: Syntheses, Electronic Properties, and Magnetic Interactions between the Two Moieties

We report the synthesis of new vanadyl porphyrins bearing peripheral coordination sites. By using palladium(II) or platinum(II) as connecting ions, porphyrin dimers were isolated and characterized by X-ray crystallography. These paramagnetic species were studied by electronic spectroscopy and continuous wave electron paramagnetic resonance (CW EPR). Large bathochromic shifts were observed by comparing porphyrin monomers and dimers showing that electronic delocalization was present as demonstrated earlier for diamagnetic compounds. Despite the large distance (>14 Å) between the two vanadyl centers in these dimers, small dipolar interactions between the two paramagnetic ions could be determined.

Construction of Aluminum‐Porphyrin‐Based Hypercrosslinked Ionic Polymers (HIPs) by Direct Knitting Approach for CO2 Capture and In‐Situ Conversion to Cyclic Carbonates

AbstractHypercrosslinked polymers (HCPs) constructed by Friedel‐Crafts reaction have drawn increasing attention in recent decades, but their multifunctionalization remains a huge challenge. Herein, a series of aluminum‐porphyrin‐based hypercrosslinked ionic polymers have been successfully synthesized by direct knitting approach without additional electrophilic comonomers, which involves the copolymerization of neutral porphyrin monomers and ionic building. By increasing the connected nodes numbers of ionic monomers, the introduction of abundant tetraphenylmethane fragements into the porphyrin backbones endows them with high‐density ionic active sites. Accordingly, the well‐matched molar ratios of aluminum sites to nucleophilic chloride anions that work synergistically for CO2/epoxide coupling can be achieved. Al‐HIP‐2 with flexible ionic pendants and a high Al/Cl ratio allows CO2 cycloaddition to be performed at ambient conditions or with diluted CO2; while Al‐HIP‐3 with rigid ionic moieties and a relatively low Al/Cl ratio exhibits higher specific surface area and stronger CO2 capture ability. Therefore, this co‐condensation strategy provides a simple and feasible pathway for adjusting the reactivity ratios of comonomers via simple structural changes in the knitting process, thus achieving the rational construction of multifunctionalized HCPs for CO2 capture and in‐situ conversion to cyclic carbonates.

Cofacial porphyrin organic cages. Metals regulating excitation electron transfer and CO2 reduction electrocatalytic properties.

Herein, we introduce a comprehensive study of the photophysical behaviors and CO2 reduction electrocatalytic properties of a series of cofacial porphyrin organic cages (CPOC-M, M = H2, Co(ii), Ni(ii), Cu(ii), Zn(ii)), which are constructed by the covalent-bonded self-assembly of 5,10,15,20-tetrakis(4-formylphenyl)porphyrin (TFPP) and chiral (2-aminocyclohexyl)-1,4,5,8-naphthalenetetraformyl diimide (ANDI), followed by post-synthetic metalation. Electronic coupling between the TFPP donor and naphthalene-1,4 : 5,8-bis(dicarboximide) (NDI) acceptor in the metal-free cage is revealed to be very weak by UV-vis spectroscopic, electrochemical, and theoretical investigations. Photoexcitation of CPOC-H2, as well as its post-synthetic Zn and Co counterparts, leads to fast energy transfer from the triplet state porphyrin to the NDI unit according to the femtosecond transient absorption spectroscopic results. In addition, CPOC-Co enables much better electrocatalytic activity for CO2 reduction reaction than the other metallic CPOC-M (M = Ni(ii), Cu(ii), Zn(ii)) and monomeric porphyrin cobalt compartment, supplying a partial current density of 18.0 mA cm-2 at -0.90 V with 90% faradaic efficiency of CO.

Acrylate-functionalized porphyrin-covalent organic framework for bacterial-targeted and reaction-enhanced synergistic phototherapy/chemotherapy toward sterilization and wound healing.

Porphyrinic covalent organic frameworks (COFs) have emerged as prospective materials in photodynamic and photothermal sterilization. However, it is still a great challenge to construct an efficient COF-based sterilizing agent with good photothermal and photodynamic properties and bacterial targeting ability. Herein, we report a multifunctional porphyrin-COF for bacterial-targeted and reaction-enhanced synergistic phototherapy/chemotherapy for sterilization and wound healing. The ordered crystal structure of the porphyrin-COF not only effectively avoids the self-aggregation-induced quenching of the porphyrin monomer, but also facilitates the storage and transport of singlet oxygen. The acrylate substituent in the other monomer serves as a bacterial targeting moiety and the in situ reaction site with the sulfhydryl group of the bacterial surface protein via a Michael addition reaction, thus fixing the bacteria on the surface of COF and making them lose the colonization ability. Furthermore, the bonding of COF and bacteria further amplifies the therapeutic efficiency of phototherapy. Therefore, the developed multifunctional sterilization platform not only provides a new strategy for the design of novel bactericidal materials but also broadens the biological applications of COF-based materials.

Remarkable Enhancement of Catalytic Activity of Cu-Complexes in the Electrochemical Hydrogen Evolution Reaction by Using Triply Fused Porphyrin.

A bimetallic triply fused copper(II) porphyrin complex (1) was prepared, comprising two monomeric porphyrin units linked through β-β, meso-meso, β'-β' triple covalent linkages and exhibiting remarkable catalytic activity for the electrochemical hydrogen evolution reaction in comparison to the analogous monomeric copper(II) porphyrin complex (2). Electrochemical investigations in the presence of a proton source (trifluoroacetic acid) confirmed that the catalytic activity of the fused metalloporphyrin occurred at a significantly lower overpotential (≈320 mV) compared to the non-fused monomer. Controlled potential electrolysis combined with kinetic analysis of catalysts 1 and 2 confirmed production of hydrogen, with 96 and 71 % faradaic efficiencies and turnover numbers of 102 and 18, respectively, with an observed rate constant of around 107 s-1 for the dicopper complex. The results thus firmly establish triply fused porphyrin ligands as outstanding candidates for generating highly stable and efficient molecular electrocatalysts in combination with earth-abundant 3d transition metals.