Effect of co-ligands on photoredox pathways in Cr(III) oxalate complexes

The photochemical behaviour of some mixed ligand chromium(III) complexes with amino acids, [Cr(C 2 O 4 ) 2 (Aa)] n − (where Aa = alanine, valine, serine, cysteine, asparagine, aspartic acid) was studied. The attention was focused on the photoredox mode, which proceeded via inner- or intramolecular path- way yielding Cr(II) species and hydrated electrons, respectively. The secondary thermal processes were dependent on the O 2 presence and solution pH: (i) in oxygen-free media the regeneration of substrate and photoaquation induced by the Cr(III) → Cr(II) reduction were observed, (ii) in the presence of O 2 both Cr(II) and ligands were oxidized and the former was transformed not only into Cr(III) but also to Cr(VI) (provided that pH > 7). Prolonged irradiation resulted in photoreduction of Cr(VI) accompanied by pho- todegradation of oxalate and/or amino-acid ligands. The photoreaction modes were independent of the co-ligand nature, but the secondary reaction rates and efficiencies were sensitive both to the co-ligand nature and its side substituent. Environmental consequences of the chromium photoreduction are dis- cussed in the paper: the parameters affecting production and consumption of Cr(VI) are analysed, and the tools of controlling the photoredox behaviour of the Cr(III) and Cr(VI) compounds are suggested

Effects of heavy central metal on the ground and excited states of chlorophyll

Chlorophylls, owing to their adjustable p-electron system and intense, well-separated electronic transitions, can serve as convenient intrinsic spectroscopic probes of ligand–metal center interactions. They are also interesting for their photosensitizing properties. In order to examine the heavy-atom effects on the chlorophyll triplet state, a key intermediate in chlorophyll–photosensitized reactions, the synthesis of a novel Pt(II)-substituted chlorophyll a was carried out, and the effects of the substitution on steady-state and transient photophysical properties of chlorophyll were studied by absorption and fluorescence spectroscopies, and by laser flash photolysis. The presence of highly electronegative platinum as the central ion increases the energies of the chlorophyll main absorption transitions. As laser flash photolysis experiments show, in air-equilibrated solutions, chlorophyll triplets are efficiently quenched by molecular oxygen. Interestingly, this quenching by oxygen is more effective with metal-containing pigments, in spite of the increased spin–orbit coupling, introduced with the central metals. This points to occurrence of nonspecific interactions of molecular oxygen with metallochlorophylls. The differences in the effects exerted on the pigment triplet by the central metal become distinct after the removal of oxygen. The lifetime of a Ptchlorophyll triplet remains very short, in the range of only a few microseconds, unlike in the free-base and Mg- and Zn-substituted chlorophylls. Such drastic shortening of the triplet lifetime can be attributed to a large heavy-atom effect, implying that strong interactions must occur between the central Pt(II) ion and the chlorophyll macrocycle, which lead to a more efficient spin–orbit coupling in Pt-chlorophyll than in Pt-porphyrins

Thermodynamics and kinetics of RuIII(edta)Ru^{III}(edta) as an efficient scavenger for nitric oxide in aqueous solution

The edta complex of Ru III reacts very rapidly with NO in aqueous solution at pH = 5 to form a stable nitrosyl complex. The results from FT-IR (ATR) and 15 N-NMR studies clearly support the NO character of coordinated NO, such that the nitrosyl product can be formulated as [Ru II (edta)NO] . A combination of UV-Vis spectroscopy and electrochemical detection of NO was used to determine the overall equilibrium constant K NO as (9.1 ± 1.2) × 10 7 M 1 at 25 C and pH = 5.0. Stopped- fl ow kinetic studies on the reaction of acetate-bu ff ered solutions of [Ru III (edta)H 2 O] with NO gave k on values two orders of magnitude lower than that reported in the literature as a result of bu ff er e ff ects. The values of k on determined at low and high pH, viz . 3.8 × 10 4 and 1.2 × 10 5 M 1 s 1 , respectively, are signi fi cantly smaller than that found at pH = 5.0, and in agreement with that observed for the substitution reactions of Ru III (edta) with other entering nucleophiles. Attempts to determine k on for the binding of NO to [Ru III (edta)H 2 O] using laser fl ash photolysis failed due to the occurrence of side reactions. Under speci fi c conditions (NO in excess and NO 2 as impurity), the formation of the disubstituted [Ru II (edta)(NO )(NO 2 )] 2 species was detected using 15 N-NMR spectroscopy. Laser fl ash photolysis of this complex leads to multiple chemical reaction steps as a result of the formation of two primary photoproducts, which decay with di ff erent rate constants to the starting complex. Possible mechanisms for these photoinduced reactions are proposed and compared to related systems reported in the literature

Verteporfin, photofrin II, and merocyanine 540 as PDT photosensitizers against melanoma cells

The efficiency of photodynamic effect (PDE) for Photofrin II (PfII), Verteporfin, and Merocyanine 540 (MC540) was compared against neoplastic cells. Triplet state lifetimes and singlet molecular oxygen quantum yields were correlated with biological effect. PfII triplet lifetime was two times longer than that of Verteporfin, however, its singlet molecular oxygen quantum yield was two times lower in comparison with Verteporfin. High singlet molecular oxygen quantum yield of Verteporfin resulted in high biological efficacy. To achieve 50% mortality of cells four times lower light dose and five times lower concentration of Verteporfin were applied in comparison with PfII. The same level of cell damage was reached using 10 times higher light dose and two times higher concentration of MC540 in comparison with PfII. Our results confirm that singlet molecular oxygen based mechanism, prevalent for Verteporfin and PfII, was highly effective against melanoma cells. Verteporfin can be used at small doses with high cellular damage efficiency