Porphyrin-Polyoxotungstate Molecular Hybrid as a Highly Efficient, Durable, Visible-Light-Responsive Photocatalyst for Aerobic Oxidation Reactions

Organic-polyoxometalate (POM) hybrids have recently attracted considerable interest because of their distinctive properties and wide-ranging applications. For the construction of organic-POM hybrids, porphyrins are promising building units owing to their optical properties and reactivity, including strong visible-light absorption and subsequent singlet-oxygen (1O2*) generation. However, the practical utilization of porphyrins as photocatalysts and photosensitizers is often hindered by their own degradation by 1O2*. Therefore, there is a substantial demand for the development of porphyrin-derived photocatalysts with both high efficiency and durability. Herein, we present a porphyrin-polyoxotungstate molecular hybrid featuring a face-to-face stacked porphyrin dimer (I) fastened by four lacunary polyoxotungstates. Hybrid I exhibited remarkable efficiency and durability in photocatalytic aerobic oxidation reactions, and the selective oxidation of various dienes, alkenes, sulfides, and amines proceeded using just 0.003 mol % of the catalyst. Mechanistic investigations suggested that the high activity of I stems from the efficient generation of 1O2*, resulting from the heavy-atom effect of POMs. Furthermore, despite its high efficiency in 1O2* generation compared to free porphyrins, I exhibited superior durability against 1O2*-induced degradation under photoirradiation.

Periodate-Mediated Aerobic Oxidation of Sulfides over a Bifunctional Porphyrin-polyoxometalate Catalyst: Photosensitized Singlet Oxygen Oxidation of Iodate to Periodate

Regeneration of terminal oxidants by molecular oxygen in metal-catalyzed oxidations of organic substrates has the advantage of avoiding the use of stoichiometric amounts of hazardous and/or expensive reagents to meet (some of) the green chemistry requirements. In the present study, photosensitized singlet oxygen oxidation of iodate to periodate has been used to regenerate the oxidant in polyoxometalate (POM)-catalyzed oxidation of sulfides to sulfoxides with periodate in water. To the best of our knowledge, it is the first report on singlet oxygen oxidation of iodate to periodate. In order to determine the contribution of photooxidation and oxidation pathways in the formation of sulfoxide, the oxidation of diphenyl sulfide with a very low reactivity toward aerobic photooxidation was studied; a sevenfold increase in the conversion of the sulfide to the diphenyl sulfoxide was observed for the reaction conducted in the presence of H2TMPyP-PW12O40/IO3-/O2/hν compared to that in the presence of H2TMPyP-PW12O40/O2/hν. Also, under the same conditions, a ca. 1.5-fold increase was observed in the case of methyl phenyl sulfide, which shows high reactivity toward both the oxidation and photooxidation reactions. A porphyrin-POM nanocomposite formed by the electrostatic immobilization of meso-tetra(N-methylpyridinium-4-yl)porphyrin (H2TMPyP) on PW12O40 was employed for the one-pot oxidation and photooxidation reactions. Field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), diffuse-reflectance UV-vis spectroscopy, thermal gravimetric analysis, and Fourier transform infrared were used to characterize the formation of the hybrid compound. An average particle size of 42 nm was estimated for H2TMPyP-PW12O40 from XRD peak broadening using the Scherrer equation. Also, FESEM images showed the formation of nearly spherical nanoparticles with a size of ca. 200 nm. The redshift of the Soret band of H2TMPyP upon immobilization on POM was attributed to strong N-H···O hydrogen-bond interactions between POM and porphyrin.

Synthesis of Pore-Wall-Modified Stable COF/TiO2 Heterostructures via Site-Specific Nucleation for an Enhanced Photoreduction of Carbon Dioxide

Constructing stable heterostructures with appropriate active site architectures in covalent organic frameworks (COFs) can improve the active site accessibility and facilitate charge transfer, thereby increasing the catalytic efficiency. Herein, a pore-wall modification strategy is proposed to achieve regularly arranged TiO₂ nanodots (≈1.82 nm) in the pores of COFs via site-specific nucleation. The site-specific nucleation strategy stabilizes the TiO₂ nanodots as well as enables the controlled growth of TiO₂ throughout the COFs' matrix. In a typical process, the pore wall is modified and site-specific nucleation is induced between the metal precursors and the organic walls of the COFs through a careful ligand selection, and the strongly bonded metal precursors drive the confined growth of ultrasmall TiO₂ nanodots during the subsequent hydrolysis. This will result in remarkably improved surface reactions, owing to the superior catalytic activity of TiO₂ nanodots functionalized to COFs through strong NTiO bonds. Furthermore, density functional theory studies reveal that pore-wall modification is beneficial for inducing strong interactions between the COF and TiO₂ and results in a large energy transfer via the NTiO bonds. This work highlights the feasibility of developing stable COF and metal oxide based heterostructures via organic wall modifications to produce carbon fuels by artificial photosynthesis.

A Comparison of Uric Acid Optical Detection Using as Sensitive Materials an Amino-Substituted Porphyrin and Its Nanomaterials with CuNPs, PtNPs and Pt@CuNPs

Hybrid nanomaterials consisting in 5,10,15,20-tetrakis(4-amino-phenyl)-porphyrin (TAmPP) and copper nanoparticles (CuNPs), platinum nanoparticles (PtNPs), or both types (Pt@CuNPs) were obtained and tested for their capacity to optically detect uric acid from solutions. The introduction of diverse metal nanoparticles into the hybrid material proved their capacity to improve the detection range. The detection was monitored by using UV-Vis spectrophotometry, and differences between morphology of the materials were performed using atomic force microscopy (AFM). The hybrid material formed between porphyrin and PtNPs hasthe best and most stable response for uric acid detection in the range of 6.1958 × 10−6–1.5763 × 10−5 M, even in the presence of very high concentrations of the interference species present in human environment.

Porphyrins and Polyoxometalate Scaffolds

Polyoxometalates (POMs) can act as unique reservoirs for multiple electron transfers. As POMs display only weak absorption in the visible spectrum, they can be associated with chromophores such as porphyrins and porphyrin antennae. In this Minireview, the research dedicated to the combination of porphyrins and polyoxometalates is put in context and the state of the art identifying the challenges addressed in the optimization of hybrid materials for applications is detailed.

SnSe2-Zn-Porphyrin Nanocomposite Thin Films for Threshold Methane Concentration Detection at Room Temperature

Nanocomposite thin films, sensitive to methane at the room temperature (25–30 °C), have been prepared, starting from SnSe2 powder and Zn(II)-5,10,15,20-tetrakis-(4-aminophenyl)- -porphyrin (ZnTAPP) powder, that were fully characterized by XRD, UV-VIS, FT-IR, Nuclear Magnetic Resonance (1H-NMR and 13C-NMR), Atomic Force Microscopy (AFM), SEM and Electron Paramagnetic Resonance (EPR) techniques. Film deposition was made by drop casting from a suitable solvent for the two starting materials, after mixing them in an ultrasonic bath. The thickness of these films were estimated from SEM images, and found to be around 1.3 μm. These thin films proved to be sensitive to a threshold methane (CH4) concentration as low as 1000 ppm, at a room temperature of about 25 °C, without the need for heating the sensing element. The nanocomposite material has a prompt and reproducible response to methane in the case of air, with 50% relative humidity (RH) as well. A comparison of the methane sensing performances of our new nanocomposite film with that of other recently reported methane sensitive materials is provided. It is suitable for signaling gas presence before reaching the critical lower explosion limit concentration of methane at 50,000 ppm.

Novel viscoelastic gelling agent with unique physico-chemical properties

A novel innovative viscoelastic gelling agent (novel gel, NG) has been developed by combining citric acid (CA) and disodium 5-guanylate (DG). NG has the potential to replace other gelling agents such as gelatine, which has been commonly used in foods, dietary supplements, pharmaceutical and cosmetic products including ointments and sprays. NG has unique physico-chemical properties, including a wide range of concentration-dependent, temperature-sensitive gel strengths. Based on the rheological measurement results, NG depicted similar shear thinning behaviour to gelatine, within shear rates ranging from 25.8 to 129 (s^-1). NG also significantly increased the shelf-life (by 21 days) of minced beef, as well as inhibited the growth of major spoilage pathogens, such as E. coli, S. aureus, Salmonella sp., Listeria sp., yeast and moulds, making it an ideal candidate for gelatine replacement.

An inorganic-organic hybrid MnIII{Mn}2 cluster consisting of rare Lindqvist-like Mn6 subunits with high proton conductivity

A new inorganic-organic hybrid Mn^III{Mn}₂ with rare Lindqvist-like Mn₆ subunits was synthesized. Our studies reveal the distinct advantages of the Mn^III{Mn}₂ compound as a potential material of proton exchange membranes, including its facile and cost-effective synthesis, insolubility in water, and most importantly, its maximum proton conductivity of 4.69 × 10^-3 S cm^-1 at 368 K and 97% RH.

Antimicrobial activity of different substituted meso-porphyrin derivatives

The increasing resistance against classical antibiotic treatment forces the researchers to develop novel non-toxic antimicrobial agents. The aim of this study was to determine the antimicrobial properties of seven different porphyrins having distinctive hydrophobicity/hydrophilicity: P1 meso-tetra(4-methoxy-phenyl)porphyrin, P2 Zn(II)-meso-5,10,15,20-tetrapyridylporphyrin, P3 meso-tetra(p-tolyl)porphyrin, P4 5,10,15,20-tetraphenylporphyrin; P5 (5,10,15,20-tetraphenylporphinato) dichlorophosphorus(V) chloride, P6 5,10,15,20-tetrakis-(N-methyl-4-pyridyl) porphyrin-Zn(II) tetrachloride, P7 Zn(II)-5,10,15,20-meso-tetrakis-(4-aminophenyl)porphyrin. The meso-porphyrin derivatives were screened for their antimicrobial activity against six reference strains: Streptococcus pyogenes ATCC 19615, Staphylococcus aureus ATCC 25923, Escherichia coli ATCC 25922, Klebsiella pneumoniae ATCC 700603, Pseudomonas aeruginosa ATCC27853 and Candida albicans ATCC 10231. The antimicrobial activity of these samples was evaluated by the agar disk diffusion method and dilution method, with the determination of the minimum inhibitory concentration (MIC), the minimum bactericidal concentration (MBC) and the minimum fungicidal concentration (MFC). The most significant result is provided by the water-soluble P5 manifesting an obvious antimicrobial activity against Streptococcus pyogenes. On the other hand, P6 is a moderately active derivative against Streptococcus pyogenes and Escherichia coli and P7 presents moderate activity against Streptococcus pyogenes and Staphylococcus aureus. All the tested porphyrin bases, presenting hydrophobic character, have no antimicrobial activity under the investigated conditions. The common characteristics of the porphyrins that act as promising antimicrobial agents in the non-irradiated methods are: the cationic nature, the increased hydrophilicity and the presence of both amino functional groups grafted on the porphyrin ring and the coordination with Zn or phosphorus in the inner core.

Unsymmetrical β-functionalized ‘push–pull’ porphyrins: synthesis and photophysical, electrochemical and nonlinear optical properties

A new family of unsymmetrical β-TPA appended ‘push–pull’ porphyrins has been synthesized in good yield for the first time. They exhibited red shifted broad absorption spectral features with high dipole moments and tunable redox properties.

Unsymmetrical nonplanar 'push-pull' β-octasubstituted porphyrins: facile synthesis, structural, photophysical and electrochemical redox properties

Mixed substitution at the β-position of porphyrins influences their photophysical and electrochemical redox properties. Two new series of asymmetrically mixed β-octasubstituted porphyrins viz. MTPP(Ph)2Br5X (X = NO2 or Br and M = 2H, Co(ii), Ni(ii), Cu(ii), and Zn(ii)) have been synthesized and characterized by various spectroscopic techniques. The single crystal X-ray structure of H2TPP(NO2)(Ph)2Br5 showed a nonplanar saddle shape conformation of the macrocyclic core. Furthermore, the fully optimized geometries confirmed the saddle shape conformation of H2TPP(Ph)2Br5X (X = NO2 or Br). Electronic spectra revealed a significant bathochromic shift by appending both electron donor and acceptor substituents at the β-position of the meso-tetraphenylporphyrin skeleton, which reflects the following order H2TPP < H2TPP(NO2) < H2TPP(NO2)(Ph)2 < H2TPP(Ph)2Br6 < H2TPP(NO2)(Ph)2Br5. H2TPP(Ph)2Br5X (X = NO2 or Br) exhibited a significant bathochromic shift (Δλmax = 53-61 nm) in the Soret band and (Δλmax = 90-95 nm) in the longest wavelength Qx(0,0) band as compared to H2TPP. Nonplanar conformations and electron withdrawing β-substituents induce higher protonation and deprotonation constants for H2TPP(NO2)(Ph)2Br5 and H2TPP(Ph)2Br6 as compared to precursor porphyrins viz. H2TPP, H2TPP(NO2) and H2TPP(NO2)(Ph)2. The electronic spectral properties and redox potentials of MTPP(Ph)2Br5X (X = NO2 or Br and M = 2H, Co, Ni, Cu and Zn) are affected by β-substituents at the periphery of the porphyrin core. Redox tunability was achieved by appending push-pull substituents at the β-position of the MTPP (M = 2H, CoII, NiII, CuII, and ZnII) skeleton of the macrocycle. CuTPP(Ph)2Br6 and CuTPP(NO2)(Ph)2Br5 exhibited a dramatically reduced HOMO-LUMO gap with a difference of 0.55 V and 0.62 V, respectively as compared to CuTPP due to the push-pull effect of β-substituents and nonplanarity of the porphyrin core.

Three Pd-decavanadates with a controllable molar ratio of Pd to decavanadate and their heterogeneous aerobic oxidation of benzylic C–H bonds

Three Pd-decavanadates have been synthesized and they are active in the heterogeneous aerobic oxidation of benzylic hydrocarbons.

Cesium salts of niobo-tungstate isopolyanions with intermediate group V–group VI character

The physical and electronic structures of cesium salts of niobo-tungstate Lindqvist ions vary with Nb content, elucidated experimentally and computationally.

Donor–acceptor organo-imido polyoxometalates: high transparency, high activity redox-active NLO chromophores

Organoimido polyoxometalates are a powerful new class of acceptor for non-linear optics. As a basis for donor–acceptor systems, they offer an excellent transparency/non-linearity trade-off and break through empirical performance limits for comparable organic materials.

Crystal structure, magnetic and catalytic oxidation properties of manganese(iii) tetrakis(ethoxycarbonyl)porphyrin

5,10,15,20-tetrakis(ethoxycarbonyl)porphyrin manganese(iii) chloride (Mn^IIITECPCl) existed as a Mn–O coordinated dimer. Mn^IIITECPCl gave high yield in oxidation of styrene by using TBHP or PhIO oxidant and was found recyclable.

Synthesis, spectroscopic, electrochemical redox, solvatochromism and anion binding properties of β-tetra- and -octaphenylethynyl substituted meso-tetraphenylporphyrins

β-Phenylethynyl (PE) porphyrins exhibited unique photophysical properties and solvatochromic behavior due to ICT from the porphyrin core to PE moieties. Further, Zn(ii) porphyrins were utilized for anion sensing.

Electron deficient nonplanar β-octachlorovanadylporphyrin as a highly efficient and selective epoxidation catalyst for olefins

Highly electron deficient and nonplanar β-octachlorovanadylporphyrin (VOTPPCl₈) was synthesized and utilized for selective epoxidation of olefins with very high TOF numbers (6566-9650 h⁻¹).

Porphyrin chemodosimeters: synthesis, electrochemical redox properties and selective ‘naked-eye’ detection of cyanide ions

β-Dicyanovinyl substituted porphyrins have been synthesized and utilized as chemodosimeters for ratiometric and colorimetric sensing of cyanide ions in CH₃CN and H₂O/CH₃CN mixture.

Synthesis and characterization of conjugated Dawson-type polyoxometalate-porphyrin copolymers

Hybrid polyoxometalate-porphyrin copolymeric films were obtained by the electro-oxidation of zinc octaethylporphyrin (ZnOEP) in the presence of a Dawson type polyoxophosphovanadotungstate bearing two pyridyl groups (POM(py)2). The synthesis of a series of POM(py)2 consisting of [P2W15V3O62](9-) functionalized with diol-amide or triol moieties, as well as the characterization of the copolymers are presented.

Synthesis, electrochemical and photophysical properties of covalently linked porphyrin-polyoxometalates

Two covalently linked porphyrin-polyoxometalate hybrids have been prepared: an Anderson-type hexamolybdate [N(C(4)H(9))(4)](3)[MnMo(6)O(18){(OCH(2))(3)CNHCO(ZnTPP)}(2)] with two pendant zinc(II)-tetraphenylporphyrins, and a Dawson-type vanadotungstate [N(C(4)H(9))(4)](5)H[P(2)V(3)W(15)O(59){(OCH(2))(3)CNHCO(ZnTPP)}] with one porphyrin. Electrochemical studies show independent redox processes for the organic and inorganic parts at usual potentials. Photophysical studies reveal an electron transfer from the excited porphyrin to the Dawson polyoxometalate, but not to the Anderson polyoxometalate. Time resolved absorption spectroscopy allows the identification of the electron transfer pathways and the determination of the time constants.