Tebello Nyokong

Photophysical and photochemical studies of zinc(II) phthalocyanine derivatives—effects of substituents and solvents

The effects of substituents and solvents on the photophysical and photochemical parameters of zinc(II) phthalocyanines are reported. The complexes studied are zinc phthalocyanine (ZnPc), zinc tetra(tert-butylphenoxy)phthalocyanine [ZnPc(TBPh)4], zinc octa(methylphenoxy)phthalocyanine [ZnPc(MPh)8], zinc tetranitrophthalocyanine [ZnPc(NO2)4], zinc octachlorophthalocyanine (ZnPcCl8), zinc tetrasulfophthalocyanine [ZnPc(SO3−)4], a mixture of zinc mono-, di-, tri- and tetrasulfophthalocyanine [ZnPc(SO3−)mix] and zinc naphthalocyanine (ZnNPc). It was found that the presence of peripheral substituents on the macrocycle enhances the yield of the triplet state. Among the different substituents, the sulfonated derivative, ZnPc(SO3−)mix, has the longest triplet lifetime (τT) and the highest singlet oxygen quantum yield (ϕΔ). The near infra-red absorptions of the solvents reveal that solvents that absorb around 1100 nm (triplet energy level) and around 1270 nm (singlet oxygen energy level), quench the triplet state of the ZnPc derivative as well as singlet oxygen. Although water has a high singlet oxygen quenching effect, the ϕΔ value for ZnPc(SO3−)mix in water is still reasonably high at 0.48, which may provide an explanation for the efficient photosensitization by this molecule in photodynamic studies.

Solvent effects on the photochemical and fluorescence properties of zinc phthalocyanine derivatives

The effects of solvents on the singlet oxygen, photobleaching and fluorescence quantum yields for zinc phthalocyanine (ZnPc) and its derivatives; (pyridino)zinc phthalocyanine ((py)ZnPc), zinc octaphenoxyphthalocyanine (ZnOPPc) and zinc octaestronephthalocyanine (ZnOEPc), is presented. The effects of the solvents on the ground state spectra are also discussed. The largest red shift of the Q band was observed in aromatic solvents, the highest shift being observed for 1-chloronaphthalene. Higher singlet fluorescence quantum yields were observed in THF for ZnPc and ZnOPPC. Also in the same solvent phototransformation rather than photobleaching was observed for ZnOPPc. Split Q band in the emission and excitation spectra of ZnOPPc was observed in some solvents and this is explained in terms of the lowering of symmetry following excitation.

Photochemical studies of tetra-2,3-pyridinoporphyrazines

Tetra-2,3-pyridinoporphyrazines and the corresponding water-soluble N,N′,N′′,N′′′-tetramethyl-tetra-2,3-pyridinoporphyrazine complexes, containing central metal atoms; M=Ge, Sn, Si and Zn, were synthesized and their photochemical properties were investigated. The reductive quenching of pyridinoporphyrazines excited states, enhanced relative to phthalocyanines, was considered as the first photochemical step of dyes phototransformation in dimethylformamide (DMF) and dimethylsulfoxide (DMSO) solutions under irradiation with visible light. Efficiency of singlet oxygen photosensitization decreases significantly in the row phthalocyanines, unquaternized, quaternized tetra-2,3-pyridinoporphyrazine metallocomplexes.

The interaction of melatonin and its precursors with aluminium, cadmium, copper, iron, lead, and zinc: An adsorptive voltammetric study

Limson J, Nyokong T, Day a S. The interaction of melatonin and its precursors with aluminium, cadmium, copper, iron, lead, and zinc: An adsorptive voltammetric study. J. Pineal Res. 1998; 24:15–21.

Electrocatalytic oxidation and detection of hydrazine at gold electrode modified with iron phthalocyanine complex linked to mercaptopyridine self-assembled monolayer

Electrocatalytic oxidation and detection of hydrazine in pH 7.0 conditions were studied by using gold electrode modified with self-assembled monolayer (SAM) films of iron phthalocyanine (FePc) complex axially ligated to a preformed 4-mercaptopyridine SAMs. The anodic oxidation of hydrazine in neutral pH conditions with FePc-linked-mercaptopyridine-SAM-modified gold electrode occurred at low overpotential (0.35V versus Ag|AgCl) and the treatment of the voltammetric data showed that it was a pure diffusion-controlled reaction with the involvement of one electron in the rate-determining step. The mechanism for the interaction of hydrazine with the FePc-SAM is proposed to involve the Fe((III))Pc/Fe((II))Pc redox process. Using cyclic voltammetry (CV) and Osteryoung square wave voltammetry (OSWV), hydrazine was detected over a linear concentration range of 1.3x10(-5) to 9.2x10(-5)mol/L with low limits of detection (ca. 5 and 11muM for OSWV and CV, respectively). At concentrations higher than 1.2x10(-4)mol/L the anodic peak potential shifted to 0.40V (versus Ag|AgCl), and this was interpreted to be due to kinetic limitations resulting from the saturation of hydrazine and its oxidation products onto the redox-active monolayer film. This type of metallophthalocyanine-SAM-based electrode is a highly promising electrochemical sensor given its ease of fabrication, good catalytic activity, stability, sensitivity and simplicity.

Effects of axial ligands on the photophysical properties of silicon octaphenoxyphthalocyanine

The photochemistry and photophysics of six axially substituted silicon phthalocyanines are reported and show the importance of the axial groups in the photochemistry of these compounds. The fluorescence quantum yields are especially affected by the axial ligand. A very good correlation was found for the experimentally determined fluorescence lifetimes and the theoretically determined lifetimes using the Strickler-Berg equation for the unaggregated molecules.

Silicon octaphenoxyphthalocyanines: photostability and singlet oxygen quantum yields

Abstract Photochemical properties of series of axially substituted silicon octaphenoxyphthalocyanines with aryloxy, siloxy, aminoalkoxy, esters of carboxylic acids and sulphonic acid esters residues as axial ligands were studied in dimethylsulfoxide (DMSO) solution. It was found, that under Q-band excitation axial ligands in compounds studied have the propensity to be changed by hydroxyl groups with quantum yields in the range 10 −5 to 10 −2 depending on the nature of the axial ligand. Axial substituent phototransformation was followed by slow photobleaching of dihydroxysilicon octaphenoxyphthalocyanine (photoproduct) in self-sensitized singlet oxygen mediated oxidation of the macrocycle. Singlet oxygen quantum yields were found to be in the range 0.15–0.20 for majority of the phthalocyanines (Pc) studied.

Electrocatalytic behavior of substituted cobalt phthalocyanines towards the oxidation of cysteine

Ring substituted cobalt phthalocyanine complexes of the form CoPc(X)4, (where X=NH2, NO2, C(CH3)3, SO3H and COOH) are compared for their catalytic activities towards the oxidation of cysteine. The potential for the electrocatalytic oxidation of cysteine is closely related to the CoIII/CoII couple of the CoPc(X)4 complexes in acidic media and to the CoII/CoI couple in basic media. The catalytic currents and the oxidation potential for cysteine are dependent on the pH of the solution, the potential becoming less positive as the pH increases and the catalytic currents decreasing with increase in pH, for the same concentration of cysteine. The magnitudes of the catalytic currents (after ten scans) for cysteine oxidation at pH 8.3 and on electrodes modified with the CoPc(X)4 complexes increased with the nature of the ring substituent as follows: NO2

Synthesis, electrochemical and photochemical properties of unsymmetrically substituted zinc phthalocyanine complexes

Abstract The synthesis of symmetrically and unsymmetrically substituted zinc phthalocyanine (ZnPc) derivatives are reported. The ZnPc derivatives are synthesized by ring expansion of subphthalocyanine complexes. Ring substitution is effected with tert-butyl phenol, naphthol and hydroxybenzoic acid. Comparison of the redox potentials for complexes substituted with varying numbers of tert-butyl phenol: 1 (complex 5), 2 (complex 6), 3 (complex 7), 6 (complex 8) and 8 (complex 9) shows that the complexes with the highest number of substituents are more difficult to oxidize and easier to reduce. All the reported complexes are relatively photostable, with photobleaching quantum yields of the order of 10−5. Singlet oxygen quantum yields for the complexes ranged from 0.22 to 0.68.

The renaissance in optical spectroscopy of phthalocyanines and other tetraazaporphyrins

Spectral properties of metallophthalocyanines and other tetraazaporphyrins are governed mainly by the Q band which originates from the π-π* transitions within the ring. The position and intensity of the Q band is important in tailoring new phthalocyanine derivatives for particular applications. Aggregation, the nature of the central metal, π conjugation, symmetry of the molecules, and axial, peripheral or non-peripheral substitutions affect the spectra and hence the properties of the phthalocyanine molecule. This review gives a brief outline on how optical spectroscopy provides useful informations on molecular and electronic structures, chemistry and physics of phthalocyanines and other tetraazaporphyrins.