Journal of Porphyrins and Phthalocyanines

Water-soluble cationic cobalt(tetra(N-methyl-pyridyl)porphyrins, CoTMpyP(4) and CoTMpyP(2) and the β-pyrrole octabrominated derivative CoTMpyP(4)Br₈ have been studied by cyclic voltammetry under aqueous aerobic conditions at pH of 4.0 with and without addition of nitrite to explore their ability to catalyze the reduction of dioxygen. The porphyrins and their nitro/nitrosyl derivatives were studied in aqueous acetate buffer solution at a solid silver electrode and then also as they were immobilized in Nafion® film loaded with silver nanoparticles which provided electrical conductivity. Under these reducing conditions with nitrite in solution, coordinated nitrite appears to undergo an oxo-transfer/reduction reaction resulting in the formation of the nitrosyl ligand. The reaction of the nitrosyl complex allows a short-lived electrocatalytic reduction reaction with dioxygen before NO is lost from the complex by dissociation. The thermodynamics of the reactions and possible catalytic intermediates in the reduction of dioxygen were studied by DFT (BP/6-31G*) computations. An unusual N-bound cyclic NO₃^- structure was obtained in the optimized geometries for the product of reduced nitrosyl porphyrins and dioxygen in the model complexes and for [CoTMpyP(2)(NO-O₂)]^-. These structures are tentatively proposed to represent an intermediate in the mechanism of activation of dioxygen in catalytic reduction. DFT (BP/6-31G*) computations were also applied to probe thermodynamics and intermediates in the known catalytic reduction of NO to N²O by the reduced porphyrin under anaerobic conditions. Thermodynamics estimates suggest that reduction occurs through coordinated NO in the reduced porphyrin species similar to cytochrome P450nor (nitric oxide reductase), but the detailed mechanism is not clear.

Although a variety of conditions proved unsuccessful for gold insertion into β-unsubstituted meso-triarylcorroles, refluxing free-base β-octabromo-meso-triarylcorrole ligands, H₃[Br₈T(p-X-P)C] (X = MeO, n-Bu, CH₃, H, and CF₃), with chloroauric acid in dichloromethane containing a small amount of triethylamine for 30 min led to 54–65% yields of the corresponding gold complexes.

Photoinduced electron transfer from the electron-donating zinc tetra-tert-butylphthalocyanine, ZnTBPc, to the electron-accepting dodecafluorosubphthalocyanine, SubPcF₁₂, in the polar benzonitrile has been investigated with nanosecond laser photolysis method. The examined ZnTBPc/SubPcF₁₂ mixture absorbs the light in a wide section of the UV/vis/NIR spectra. Owing to the particular electronic properties of both entities, such combination seems to be perfectly suited for the study of intermolecular electron-transfer process in the polar solvents via the triplet-excited state of SubPcF₁₂. Upon excitation of SubPcF₁₂ with 570 nm laser light in polar benzonitrile (ε_s = 25.2), the electron transfer from ZnTBPc to the triplet-excited state of SubPcF₁₂ was confirmed by observing the transient absorption bands of ZnTBPc radical cation and SubPcF₁₂ radical anion in the visible and near-IR region. On addition of an appropriate electron acceptor with excellent electron-accepting properties, namely dicyanoperylene-3,4,9,10-bis(dicarboximide) (PDICN₂), the anion radical of SubPcF₁₂ transfers to the PDICN₂ yielding the PDICN₂ radical anion. These observations confirm the photosensitized electron-transfer/electron-mediating cycle of ZnTBPc/SubPcF₁₂/PDICN₂ system. In non-polar toluene (ε_s = 2.2), the energy-transfer process from the triplet-excited state of SubPcF₁₂ to the low-lying triplet state of ZnTBPc was confirmed by the consecutive appearance of the triplet ZnTBPc.

Singlet molecular oxygen produced in the presence of 2,2,6,6-tetramethyl-4-piperidone (TMPD) in 1,4-dioxan, by boron subphthalocyanine chloride as the sensitizer, was monitored by Electron Paramagnetic Resonance (EPR) via the oxidation of TMPD to the corresponding aminoxyl radical. This radical product is formed in low yields along the singlet oxygen pathway. A kinetic analysis of the process allows one to understand the reasons behind this, while affording more accurate quantitative insights into the mechanistic details. A crucial aspect of this reaction system, which explains the low aminoxyl radical yields along the singlet oxygen pathway, is the disappearance of the sensitizer triplet state in a reaction with TMPD, which has a rate constant of 1 × 10⁹ M^-1.s^-1.

We report successful retention of the spectroscopic properties of Chlorophyll-a molecules encapsulated in carboxymethyl cellulose acting as an "artificial membrane" material. Films were prepared on indium tin oxide substrates utilizing the spin-coating technique at different concentrations of carboxymethyl cellulose aqueous solution. Results show that the native state of the encapsulated Chlorophyll-a molecules were retained for extended periods of time enhanced further by the optimum concentration effect. Investigations also revealed that the molecules exist in nanocrystal forms with crystal size of 10 nm regardless of the carboxymethyl cellulose concentrations and can be further optimized by varying the spin rates. Chlorophyll-a molecules were found well dispersed in the carboxymethyl cellulose films suggesting the suitability of the gel-like polymer solution as a dispersion agent. It was therefore obvious that the encapsulation of Chlorophyll-a molecules by carboxymethyl cellulose provides prolonged retention of its photoactivity highlighting extended usage when incorporated into future device applications.

Effect of different reaction parameters on the catalytic activity of β-tetrabrominated meso-tetra(para-tolyl)porphyrinatomanganese(III), MnT(4-CH₃P)PBr₄(OAc), for oxidation of different sulfides and hydrocarbons with tetra-n-butylammonium hydrogen monopersulfate (TBAHS) has been studied. In oxidation of sulfides, the chemoselectivity of reaction has been significantly changed in THF as the solvent compared with the common organic solvents. Also, using nitrogenous bases bearing electron-withdrawing groups (-Cl or -CN) clearly increased the ratio of sulfoxide to sulfone relative to the electron-donating ones. Catalytic oxidation of olefins with TBAHS was conducted in protic and aprotic solvents and acetonitrile has been found as the best solvent. A significantly large difference was found between the co-catalytic activity of imidazole (ImH) and pyridine in comparison with that observed in dichloromethane. The competitive oxidation of cis- and trans-stilbene suggests the presence of a high valent manganese oxo as well as a six coordinate (ImH)MnT(4-CH₃P)PBr₄(HSO₅) species as the active oxidants in acetonitrile.

The synthesis, isomeric separation, and identification of β-sulphonic phthalocyanine zinc complexes were reported. While the sulphonic phthalocyanines have been studied extensively, the development of separation technology may enable deeper insights into their isomeric constitution. An ion-pair reversed-phase high performance liquid chromatography (IP-RP-HPLC) method was developed to separate the sulphonic phthalocyanine isomers. The results showed that the product of the condensation reaction is a mixture of all possible isomers with statistical distribution. Several isomers were obtained and structural determination was undertaken by NMR. Based on the IP-RP-HPLC elution sequence of these well-identified isomers, a relationship between the structure and efficiency was deduced: closely spaced intervals of sulphonic groups lead to higher hydrophobicity and shorter retention times on HPLC. Based on this relationship, each HPLC peak was assigned to the corresponding isomeric structure.