Fábio M. Engelmann

Synthesis, electrochemistry, spectroscopy and photophysical properties of a series of meso-phenylpyridylporphyrins with one to four pyridyl rings coordinated to [Ru(bipy)2Cl]+ groups

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.

Modulation of vectorial energy transfer in the tetrakis[tris(bipyridine)ruthenium(II)]porphyrinate zinc complex

Abstract Reported are the synthesis and photophysical properties of a new supermolecular zinc porphyrin containing four tris(4,4′-dimethyl-2,2′-bipyridine)ruthenium(II) complexes as substituents at the meso position. In this system, the lowest energy singlet excited state of the zinc porphyrin is energetically close to the tris(bipyridine)ruthenium(II) triplet MLCT excited state. The fact that the zinc porphyrin states are sensitive to ligand coordination effects, in contrast with the ruthenium(II) moieties, has allowed a selective tuning of the ruthenium complex and porphyrin excited states, providing an efficient control of the vectorial energy transfer process.

Singlet oxygen quantum yields (Φd) in water using beetroot extract and an array of LEDs

It is proposed a simple and inexpensive strategy to determine singlet oxygen (1O2) quantum yields (ΦΔ) of photosensitizers (PS) in water using beetroot extract containing betacyanin (Bc) and a set of light emitting diodes (LEDs) for excitation. Bc, a cationic natural dye, was obtained by flash chromatography purification from the red beet extract (Beta vulgaris) and employed as a convenient probe for 1O2 detection. Solutions of Bc and PS were illuminated with an array of LEDs adapted in the cuvette compartment of a commercial spectrophotometer, and the decrease in Bc absorbance was followed as a function of time. Bc photobleaching decreased in de-aerated solution and increased in D2O, indicating the involvement of 1O2. The observed photobleaching rate constant (kobs) was proportional to the LED intensity, concentration and ΦΔ of the PS. By keeping the light source constant we could estimate the overlap integral (R) between the LED emission and PS absorbance for different PS concentrations. The slope of R versus kobs is the value of the photobleaching rate constant (k), which was shown to be proportional to ΦΔ. Values of ΦΔ obtained by this method were compared with those obtained by measuring NIR (near infrared) emission for a series phenothiazine dyes.

Acid–base and spectroscopic properties of a novel supramolecular porphyrin bonded to four pentacyanoferrate(II) groups

Abstract The synthesis and characterization of a new supermolecular porphyrin obtained by the coordination of four pentacyanoferrate(II) groups to the pyridyl N-atoms of meso -(3-pyridyl)porphyrin are reported. The redox potential and the 57 Fe Mossbauer isomeric shift indicated that the pyridylporphyrin behaves as an electron-withdrawing group. The binding of pentacyanoferrate(II) groups induced a remarkable increase in the basicity of the porphyrin ring, in contrast to the [Ru(bipy) 2 Cl] + groups. In the presence of Ni(II) ions, the pentacyanoferrateporphyrin complex precipitated forming a Prussian blue type material. The voltammograms of a carbon paste electrode of this material exhibited two broad redox waves around 0.43 and 0.63 V ascribed to two distinct iron sites, as confirmed by 57 Fe Mossbauer spectroscopy.

Porphyrin doped vanadium pentoxide xerogel as electrode material

Abstract The lamellar composite material, VXG-TMPyP, obtained from the combination of cationic, water-soluble meso-(tetra-4-methylpyridinium)porphyrin (TMPyP) and vanadium pentoxide gel was investigated and employed as electrode modifying material. This material was isolated as a xerogel and characterized by X-ray diffraction, UV-Vis spectroscopy, cyclic voltammetry, spectroelectrochemistry and TG analysis. According to the X-ray diffraction data, the original VXG lamellar matrix framework is kept in the composite, evidencing a topotatic reaction. UV-Vis spectra indicated a strong interaction between VXG and TMPyP leading to the protonation of the porphyrin ring. In contrast with the vanadium oxide xerogel the new material is stable in water. The presence of the cationic porphyrin species in its structure turns it able to incorporate negatively charged ions, such as ferrocyanide and I−. The presence of the I2/I− couple gives rise to a dramatic increase in the reversibility of the VV/IV process and in the charge capacity of the material.