Efficiency Of Singlet Oxygen Research Articles

Singlet oxygen ( 1Δg) sensitization and quenching by vitamin B 12 derivatives

Two Co(III) corrins, cobester and pyrocobester, and a Co(II) corrinate, derived from vitamin B 12, have been investigated for their efficiencies of singlet oxygen ( O 2( 1Δ g ) ) production and quenching in polar and non-polar media. The quantum yields of singlet oxygen production have been determined by measurements of the singlet oxygen luminescence in the near-infrared region upon continuous excitation of the sensitizer. The results show that, among the three derivatives investigated, only pyrocobester produces singlet oxygen in significant amounts, with a quantum yield ( Φ Δ ) of 0.2(±0.02) in benzene and 0.3(±0.03) in methanol. Combining luminescence experiments and kinetic measurements of the vitamin B 12 derivatives disappearance, the rate constants of singlet oxygen physical quenching and of the chemical reaction of these compounds with singlet oxygen have been determined, in polar and non-polar media. The results show that the three vitamin B 12 derivatives are more efficient physical quenchers than chemical acceptors of singlet oxygen. Among them, the Co(II) corrinate is a particularly efficient physical quencher with a rate constant greater than 10 91 mol −1s −1 in methanol, most probably implying a charge transfer between the Co(II) metal center and singlet oxygen. The chemical reactivity, higher for pyrocobester, depends on the structure of the corrin ligand, as well as on the degree of oxidation of the metal.

Generation and quenching of singlet molecular oxygen by aggregated bacteriochlorophyll d in model systems and chlorosomes

Both photogeneration and quenching of singlet oxygen by monomeric and aggregated (dimeric and oligomeric) molecules of bacteriochlorophyll (BChl) d have been studied in solution and in chlorosomes isolated from the green photosynthetic bacterium Chlorobium vibrioforme f. thiosulfatophilum. The yield of singlet-oxygen photogeneration by pigment dimers was about 6 times less than for monomers. Singlet oxygen formation was not observed in oligomer-containing solutions or in chlorosomes. To estimate the efficiency of singlet oxygen quenching an effective rate constant for (1)O2 quenching by BChl molecules (kq (M)) was determined using the Stern-Volmer equation and the total concentration of BChl d in the samples. In solutions containing only monomeric BChl, the kq (M) values coincide with the real values for (1)O2 quenching rate constants by BChl molecules. Aggregation weakly influenced the kq (M) values in pigment solutions. In chlorosomes (which contain both BChl and carotenoids) the kq (M) value was less than in solutions of BChl alone and much less than in acetone extracts from chlorosomes. Thus (1)O2 quenching by BChl and carotenoids is much less efficient in chlorosomes than in solution and is likely caused primarily by BChl molecules which are close to the surface of the large chlorosome particles. The data allow a general conclusion that monomeric and dimeric chlorophyll molecules are the most likely sources of (1)O2 formation in photosynthetic systems and excitation energy trapping by the long wavelength aggregates as well as (1)O2 physical quenching by monomeric and aggregated chlorophyll can be considered as parts of the protective system against singlet oxygen formation.

Mechanism of the Triplet-State Quenching by Molecular Oxygen in Solution

Rate constants k q T ranging between 0.3×10 9 and 15×10 9 M -1 s -1 for quenching of the lowest excited triplet state of several substituted aromatic hydrocarbons and ketones by molecular oxygen were measured in toluene. The efficiencies of singlet oxygen (O 2 ( 1 Δg)) formation S Δ were determined, ranging from 0.24 to 1.00. The results can be explained by CT interaction in that k q T and S Δ correlate with the free enthalpy of electron transfer ΔG el , calculated from the redox potentials of the sensitizer and O 2

Antioxidant activity of flavonoids: Efficiency of singlet oxygen ( 1Δ g) quenching

Flavonoids, polyphenolic pigments widely present in plants, have been reported to act as scavengers of various oxidizing species. However, most often an overall antioxidant effect was measured. In this paper we report the results of a systematic study of the reactivity of 13 selected flavonoids (from the flavonol, flavone, flavanone and flavane families) with singlet oxygen ( 1O 2( 1Δ g)) in order to establish a structure—activity relationship. The rate constants of the chemical reaction of these flavonoids with 1O 2( k r) and their rate constants of 1O 2 physical quenching ( k q) have been determined by kinetic measurements and near-IR singlet oxygen luminescence. The efficiency of the physical quenching is mainly controlled by the presence of a catechol moiety on ring B, whereas the structure of ring C (particularly the presence of a hydroxyl group activating the double bond) is the main factor determining the efficiency of the chemical reactivity of these compounds with 1O 2. The total reactivity scale is dominated by k q, which is in general higher than k r. (+)-Catechin is the most efficient quencher of the series.

Singlet oxygen quenching by tertiary alkyl ethers

The efficiency of singlet oxygen O 2( 1Δ g) ( 1O 2) quenching by methyl tert-butyl ether and methyl tert-amyl ether has been quantified by steady state and time-resolved 1O 2 luminescence measurements. These ethers are very poor 1O 2 quenchers with bimolecular quenching rate constants lower than 7 x 10 3 1 mol −1 s −1. Moreover, we have shown that the presence of variable amounts of olefinic impurities (olefins being used for the synthesis of acyclic ethers) affects the 1O 2 lifetimes in these solvents, even at concentrations lower than 1%, olefins being relatively efficient 1O 2 quenchers. Discrepancies between 1O 2 lifetimes measured by different groups, as well as between experimental values and those calculated in using additivity factors, may also be explained by the presence of impurities in the solvents used.

Mechanism on the Efficient Formation of Singlet Oxygen by Energy Transfer from Excited Singlet and Triplet States of Aromatic Hydrocarbons

Abstract The formation efficiencies of singlet oxygen for the oxygen quenching in excited singlet of aromatic hydrocarbons; A and for the quenching in the triplet T1; B were estimated. The hydrocarbons were classified into three types: (i) A = 1 and B = 1 for dicyanoanthracene i.e., total 200% efficiency, (ii) A ≈ 0.3 and B = 1 for anthracene, (iii) A ≈ 0 and B = 1 for pyrene, as examples. It is suggested that the energy levels of T2 and a dissociative exciplex; (T11Δg) correlated to A and B values from the energetic considerations.

ChemInform Abstract: Microheterogeneous Photooxidation.

AbstractThe modified rose bengal (I), which is able to complex a quencher or an acceptor, enhances the efficiency of singlet oxygen quenching by β‐carotene compared with solutions containing β‐cyclodextrin and the sensitizer as independent entities.

Quenching of singlet oxygen luminescence by fatty acids and lipids: Contribution of physical and chemical mechanisms

By the use of photosensitized luminescence of singlet oxygen ( 1O 2) in CCl 4, the rate constants for quenching 1O 2 by saturated and unsaturated fatty acids were determined. The experimental data suggest that saturated fatty acids quench 1O 2 via a physical mechanism, presumably as a result of energy transfer to the vibrational sublevels of CH- and COOH-groups. Unsaturated fatty acids are predominantly chemical quenchers. The relative contribution of the physical quenching depends on the number of double bonds in fatty acid molecules. It was found that the quenching activity of egg phosphatidylcholine is approximately equal to the sum of quenching activities of the lipid fatty acids. The data obtained may be used for prediction of the efficiency of singlet oxygen quenching by any lipids whose fatty acid composition is known.