Application of the electronic spectra of porphyrins for analytical purposes: the effects of metal ions and structural distortions

We investigated the UV-Vis absorption, singlet-1 and singlet-2 fluorescence, as well as the formation of several metalloporphyrins from equilibrial and kinetic aspects in aqueous solution. Among these complexes were numerous typical out-of-plane and several in-plane metalloporphyrins, and between the two categories, a few border-line cases. On the basis of our results, we have complemented the categorization introduced by Barnes and Dorough for the metalloporphyrins. According to our observations, also in metalloporphyrins, the distortion, i.e., the planarity or nonplanarity of the macrocycle, is basically responsible for the spectral characteristics, while the electronic structure of metal center is a secondary factor, with a considerable importance mainly in the in-plane complexes. The type of complexes can be spectrophotometrically determined on the basis of their UV-Vis absorption and fluorescence spectra. Beside the spectral and photophysical effects of metalation, also those of the structural distortions were studied, which can originate also from metalation, protonation or overcrowded peripheral substitution of the free-base porphyrins, as well as from the axial ligation of metalloporphyrins. Our observation may be useful for different spectrophotometric analytical detection and determination methods, e.g. size-selective metal detection using free-base porphyrins (or other ringed chelate ligands), as well as the determination of Lewis bases as potential axial ligands, using metalloporphyrins

Equilibrium, photophysical and photochemical examination of anionic lanthanum(III) mono- and bisporphyrins: the effects of the out-of-plane structure

Lanthanum(III) ion forms kinetically labile complexes with the 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin anion (H2TSPP4-), the compositions and formation constants of which significantly depend on the presence of potential axial ligands (at 0.01 M). Deviating from the chloride ion, acetate coordinating to the metal center hinders the formation of bisporphyrin complex. In these lanthanum(III) complexes, the metal center, due to its large ionic radius (103.2 pm), is located out of the ligand plane, distorting it. Accordingly, the absorption and fluorescence spectra of these coordination compounds display special properties characteristic of the so-called sitting-atop (SAT) or out-of-plane (OOP) porphyrin complexes. Metalation significantly decreases the quantum yield of the fluorescence from the S1 excited state. Quantum chemical calculations (DFT) confirm the considerable OOP displacement of the La(III) center (about 120 pm in the monoporphyrin complexes). The monoporphyrins display efficient fluorescence (0.03), while the bisporphyrin does not emit. Differing from the normal (in-plane) metalloporphyrins, excitation of these lanthanum(III) porphyrins leads to an irreversible ligand-to-metal charge transfer (LMCT) followed by the opening of the porphyrin ring, which is also typical of OOP complexes. Dissociation releasing free-base porphyrin can also be observed upon irradiation of the monoporphyrin in acetate solution, while in the presence of chloride ions interconversions of the mono- and bisporphyrins may also take place beside the irreversible photoredox reaction

Photoactivated chemotherapy (PACT) : the potential of excited-state d-block metals in medicine

The fields of phototherapy and of inorganic chemotherapy both have long histories. Inorganic photoactivated chemotherapy (PACT) offers both temporal and spatial control over drug activation and has remarkable potential for the treatment of cancer. Following photoexcitation, a number of different decay pathways (both photophysical and photochemical) are available to a metal complex. These pathways can result in radiative energy release, loss of ligands or transfer of energy to another species, such as triplet oxygen. We discuss the features which need to be considered when developing a metal-based anticancer drug, and the common mechanisms by which the current complexes are believed to operate. We then provide a comprehensive overview of PACT developments for complexes of the different d-block metals for the treatment of cancer, detailing the more established areas concerning Ti, V, Cr, Mn, Re, Fe, Ru, Os, Co, Rh, Pt, and Cu and also highlighting areas where there is potential for greater exploration. Nanoparticles (Ag, Au) and quantum dots (Cd) are also discussed for their photothermal destructive potential. We also discuss the potential held in particular by mixed-metal systems and Ru complexes

Photocatalytic Degradation of Rhodamine B in Heterogeneous and Homogeneous Systems

This study focuses on the photocatalytic degradation of Rhodamine B (RhB) in heterogeneous and homogeneous photo-Fenton reactions. In the heterogeneous system, iron(II) doped copper ferrite CuII(x)FeII(1-x)FeIII2O4 nanoparticles (NPs) prepared in our previous work were employed as potential catalysts. The photodegradation of RhB was carried out in a quartz cuvette located in a diode array spectrometer. The experimental conditions such as pH, NPs dosage and H2O2 dosage with regard to the photocatalytic degradation of RhB were optimized to be 7.5, 500 mg/L and 8.9x10-2 mol/L, respectively. In addition, visible light-induced photodegradation of RhB was also carried out by using \ch{H2O2} over a wide pH range in the absence of heterogeneous photocatalysts. It was observed that the reaction rate significantly increased above pH 10, resulting in a faster rate of degradation of RhB, which may be attributed to the deprotonation of hydrogen peroxide. Furthermore, the potential antibacterial property of such catalysts against the Gram-negative bacterium Vibrio fischeri in a bioluminescence assay yielded inhibition activities of more than 60% in all cases

Peculiarities of the reactions between early lanthanide(III) ions and an anionic porphyrin

Insertion of early lanthanide(III) ions into the coordination cavity of porphyrins is a slow and complicated process in aqueous solution, originating from the stability of their aqua complexes and their possible oligomerization. The presence of potential axial ligands can accelerate the coordination of the first porphyrin, but it can hinder the connection of a further porphyrin. Lanthanide ions may coordinate not only to the pyrrolic nitrogens of porphyrins but, under kinetic control, also to the peripheral substituents possessing O-donor atoms. In the case of such anionic porphyrins, the coordination position of metal ions can be influenced by the change of temperature