Journal of Porphyrins and Phthalocyanines

Spectroscopic, electrochemical and fluorescence properties of five octahedral tin(IV) porphyrins ([(TpTP)Sn(O-m,p-C₆H₃(CH₃)₂)₂] (1), [(TpTP)Sn(O-p-C₆H₄(CH₃))₂] (2), [(TpTP)Sn(O-C₆H₅)₂] (3), [(TpTP)Sn(O-p-C₆H₄O-o,p-C₆H₃(NO₂)₂)₂] (4) and [(TpTP)Sn(O-p-C₆H₄(NO₂))₂] (5)) bearing electron donating/electron withdrawing aryloxo subunits as the axial ligands are investigated in detail. These complexes have been synthesized by the reaction of 5,10,15,20-tetra(4-methylphenyl)porphyrinato tin(IV) dihydroxide ([(TpTP)Sn(OH)₂]) and either 3, 4-dimethylphenol, p-cresol, phenol, 4-(2,4-dinitrophenoxy)phenol or p-nitrophenol and were isolated in good-to-moderate yields. Analysis of the spectral data (FAB mass, IR, UV-vis and ¹H NMR) of 1 - 5 suggests that the two trans axial aryloxo ligands are strongly bound in a symmetric manner at the tin center in these complexes. Specifically, the general symmetry observed in the ¹H NMR spectra and the ring current effect experienced by the protons present on the axial ligands are revealing in this regard. Each porphyrin ring undergoes two successive, reversible/quasi-reversible, one-electron reductions in CH₂Cl₂, 0.1 M TBAP. The bound nitroaromatic axial ligands of complexes 4 and 5 could also be reduced under the same set of experimental conditions. The wavelengths of maximum emission, the singlet energies and the Stokes’ shifts observed in the fluorescence spectra of 1 - 5 are close to the corresponding parameters of the reference porphyrin, [(TpTP)Sn(OH)₂]. On the other hand, fluorescence intensities of complexes 1 - 4 are quenched in comparison with that of [(TpTP)Sn(OH)₂] in three different solvents. A detailed analysis of the emission and redox potential data indicates that a photoinduced electron transfer from the axial aromatic subunit to singlet state of the basal tin(IV) porphyrin can, in principle, explain the fluorescence quenching observed in these donor-acceptor systems. The spectroscopic and redox features of these tin(IV) porphyrins are compared with those of the analogous phosphorus(V) porphyrins reported by us earlier.

Manganese and iron 2,6-disubstituted meso-tetraphenylporphyrins, bearing halogen atoms on the β-positions, have been used as catalysts for the oxidation of a cardanol derivative (hydrogenated tert-butylcardanol), using iodosilbenzene or hydrogen peroxide as oxygen atom donors. The reaction products can be correlated to the catalyst structure and to the nature of the oxidants.

This work reports on the synthesis of tin and germanium phthalocyanine complexes containing phenoxy and substituted phenoxy groups as phthalocyanine ring substituents. The compounds studied are: dichlorogermanium phthalocyanine complexes containing eight phenoxy (4a), o-methyl phenoxy (4b) or estrone (4c) groups on the ring. The corresponding dichlorotin complexes (5a, 5b and 5c) and diiodotin complex (6a) were also investigated, as well as diestrone phthalocyaninato tin (7). Germanium octaphenoxy phthalocyanine complexes undergo phototransformation rather than direct photobleaching, whereas tin octaphenoxy phthalocyanine complexes undergo a photobleaching process, which is mediated by photoreduction of the phthalocyanine ring. Tin octaphenoxy phthalocyanine complexes gave higher Φ_Δ values than the corresponding germanium complexes. Also tin phthalocyanine complexes containing an unsubstituted ring gave higher Φ_Δ values than the corresponding octaphenoxy substituted complexes. The triplet quantum yields increased with the increase in electron-donating power of the ring substituents.

Lysine-linked viologen was prepared as a substrate for the hydrogenase. By using reduced lysine-linked viologen (LysV^+•), the hydrogenase-LysV^+• complex was formed efficiently, leading to effective hydrogen evolution compared with methyl viologen. Lysine-linked viologen as an electron carrier was applied for the photoinduced hydrogen evolution system containing hydrogenase, Tetrakis(4-carboxyphenyl)porphyrin (TCPP) as a photosensitizer, and a sacrificial electron donor for TCPP. In this system, effective photoinduced hydrogen evolution was observed.

A series of meso-phenylpyridylporphyrins and their respective supermolecular species obtained by the coordination of [Ru(bipy)₂Cl]⁺ groups to the pyridyl substituents was synthesized and characterized. Their spectroscopic and electrochemical behavior were similar to that observed for the meso-tetra(4-pyridyl)porphyrin derivative, but the redox potential of the Ru(III/II) redox pair was about 70 mV more negative. The porphyrin centered fluorescence quantum yield exhibited a linear decrease as a function of the number of pyridyl substituents on the porphyrin ring. Efficient intramolecular energy transfer processes from the ³MLCT state of the ruthenium complexes to the porphyrin center were observed at 77K in ethanol glass.

The efficiency of zinc phthalocyanine and methylene blue (CI 52015) in compromise the viability of TG 180 malignant cells by photodynamic action, was compared by counting the number of killed cells. The results show that, under the observed experimental conditions, zinc phthalocyanine (ZnPc) is at least two times more efficient than methylene blue (MB), and possesses a very low citotoxicity and high capacity to penetrate the cellular membrane. Quenching experiments using β-carothene evidence the importance of a type II mechanism during cell inactivation by ZnPc.

The synthesis of 2, 3, 7, 8, 12, 13, 17, 18-octabromo-meso-tetrakis(2-pyridyl)porphyrin, H₂Br₈T2PyP, is described, including the comparison of four attempted methods for the demetallation of ZnBr₈T2PyP. One of the methods represents a strategy of demetallation based on the acid-base properties of the macrocycle, the solvent-dependent kinetics of metal insertion into porphyrins and the pH-dependent solubility of the 2-pyridylporphyrin derivatives in water. Self-coordinating aggregation of ZnBr₈T2PyP in non-coordinating solvents was verified by ¹H NMR spectroscopy. The Mn(III)/Mn(II) redox potential for MnBr₈T2PyPCl is 0.38 V higher than the reduction potential measured for its first-generation analogue, MnT2PyPCl. Cyclohexane hydroxylation by iodosylbenzene was performed in CH₃CN catalyzed by MnBr₈T2PyPCl and MnT2PyPCl. MnBr₈T2PyPCl was highly active, even at low concentration (5 × 10⁻⁵M), but perhalogenation did not account for oxidative robustness. At such a low catalyst concentration, MnT2PyPCl exhibited no activity as inferred by comparison to blank experiments.

Zinc(II) diporphyrin systems, characterised by the presence of flexible oligooxaethylene spacers of different lengths, have been synthesised and the binding properties toward a series of aliphatic and aromatic diamine derivatives characterised by different structural and coordination properties, investigated. The results obtained, compared to that for a rigid diporphyrin cleft, point out that the observed degree of selectivity is dependent both on the length of the spacer and on the nature of the nitrogen ligands.

A new class of phthalocyanine derivatives containing fused [5]helicene moieties has been synthesized. These derivatives, named as helicenocyanines, are soluble in common organic solvents and exhibit significant red shift of Q-band absorptions. Electrochemical, thermal and fluorescence characteristics of these helicenocyanines are presented.

The interaction of the free nucleic acid bases adenine, cytosine, and thymidine with meso-tri(4-sulfonatophenyl) meso 4-phenyl porphyrin cytosine amide (TPSC) trisodium salt at pH 5.9, 7.4, and 9.3 results is a decrease in the absorbance of the Soret (B-band) at 413 nm of TPSC. A decrease in Soret band absorbance is observed with guanine below pH 8; at higher pH values a new absorbance band at 424 nm is observed. The appearance of the band is consistent with hydrogen bonding between the cytosine-functionalized porphyrin and guanine and the resulting perturbation of the electron orbitals of the porphyrin.