Robert Mechin

Quantum yield of singlet oxygen production by monomeric and aggregated forms of hematoporphyrin derivative.

Hematoporphyrin derivative (HpD) is a complex mixture of monoporphyrinic compounds, including hematoporphyrin, and oligomers containing up to eight porphyrin units. In methanol a sensitizer concentration-independent quantum yield of 0.64 is measured for the HpD-induced production of singlet molecular oxygen O2 (1Delta(g)). This finding is consistent with the dye components remaining unassociated in this solvent. In water pH 7.4 HpD consists of a complex mixture of non-aggregated and self- and cross-aggregated monoporphyrinic and oligomeric species, and the quantum yield of O2 (1Delta(g)) formation decreases significantly with increasing HpD concentration due to the lower quantum yield of aggregates. These variations can be quantitatively described in terms of a monomer-dimer equilibrium, with quantum yields of 0.64 and 0.11, respectively, for monomers and dimers. The yields of unassociated species are identical in methanol and water.

Mechanistic and Kinetic Aspects of Photosensitization in the Presence of Oxygen

Abstract Determining whether the first step of photooxygenation is Type I or Type II is a necessary prerequisite in order to establish the mechanism of photodynamic action. But this distinction is not sufficient, because other processes, both consecutive and competitive, commonly participate in the overall mechanism. Thus, in both Type I and Type II reactions, the initial products are often peroxides that can break down and induce free radical reactions. These aspects of photosensitization are discussed and illustrated by sensitizer/substrate systems involving (1) only radical reactions (decatungstate/alkane) and (2) reactions of singlet oxygen occurring in competitive and consecutive processes and possibly followed by radical reactions (methylene blue/2′-deoxyguanosine). Two other previously investigated systems involving, respectively, a Type II interaction followed by radical processes (methylene blue/alkene) and Type II reactions, some of which being competitive or consecutive (rose bengal/alkene), are briefly reconsidered.

REACTION AND QUENCHING OF SINGLET MOLECULAR OXYGEN WITH ESTERS OF POLYUNSATURATED FATTY ACIDS

Abstract The rate constants for the reactive (kR) and unreactive (kQ) interaction of singlet molecular oxygen with three esters of polyunsaturated fatty acids (PUFA: cis‐methyl oleate, MO; cis‐methyl linoleate, MLA and cis‐ethyl linolenate, ELN) are determined. The values of the ratio kQ/kR are 0.51, 0.26 and 0.20 for MO, MLA and ELN, respectively. This variation results principally from that of kR because the values of kQ are only slightly different (1.24 × 104M−1 s−1 for MO and ∼1.0 × 104M−1 s−1 for MLA and ELN). It is shown that the rate constant kQ characterizes mainly an interaction with the unreactive part of the molecule rather than with the double bonds (solventlike quenching).

Origin of dye bleaching and polymer degradation in the methylene blue-sensitized photo-oxygenation of polybutadiene

Abstract Polymer chain scission and bleaching of the sensitizer are observed during the photo-oxygenation of cis-1,4-polybutadiene (PH) sensitized by several dyes in solution. The methylene blue (MB) sensitized photo-oxygenation of PH in benzene:methanol 9:1 occurs via a singlet oxygen mechanism. The decreases in MB concentration and molecular weight of PH exhibit an induction period followed by an increasing rate. It is shown that these two processes are consecutive to the formation of the hydroperoxy groups. Their possible origin is examined and discussed. It is concluded that both phenomena arise from an interaction of the polymeric hydroperoxy groups with the triplet state of the dye. The results are consistent with a sensitized photolysis of hydroperoxy groups which yield radical species leading to bleaching and chain scission with an efficiency about three orders of magnitude lower than that of photo-oxygenation. It is proposed that these processes may be significant in photodynamic action.

Polyoxometalate sensitization in mechanistic studies of photochemical reactions: The decatungstate anion as a reference sensitizer for photoinduced free radical oxygenations of organic compounds

The photosensitized oxygenation of organic molecules plays a key role in numerous processes of biological and industrial significance, such as, for example, photodynamic action and photodegradation of polymers. These reactions proceed either via quenching by the substrate of photophysically generated singlet oxygen, O2(1deltag), or via addition of ground state oxygen to photochemically generated radicals derived from the substrate, or via both pathways. The evaluation of the contributions of both mechanisms to the overall process requires reference sensitizers that exclusively induce one of the corresponding reactions. Some compounds are known to produce singlet oxygen with unit efficiency, but no references to sensitizers producing free radicals but no singlet oxygen have been found so far. In this work, we propose to use the decatungstate anion, W10O32(4-), as a first reference sensitizer for free radical oxygenations of organic molecules. A combination of time-resolved and steady-state studies has been performed to compare the photo-oxygenation of simple reference compounds, including 2-methyl-2-pentene and 2,3-dimethylbutene, by W10O32(4-) and by classical O2(1deltag) sensitizers, such as methylene blue and ruthenium complexes. It is demonstrated that W10O32(4-) sensitized oxygenation of organic compounds occurs exclusively by a free radical pathway, which differs clearly from both Type I and Type II oxygenations. Comparison with Type II reactions shows that: (i) in spite of their weaker reactivity, singlet oxygen mediated reactions are associated with larger photo-oxygenation yields than W10O32(4-) induced processes, due to the longer lifetime of the reactive species; and (ii) reaction of alkenes with both singlet oxygen and decatungstate features charge transfer interactions, whose magnitude is larger in the case of O2(1deltag).

Alkyl hydroperoxides as electron donors in photochemical reactions

Alkyl hydroperoxides, which are formed as major reaction products during illumination of dyes in the presence of oxygen and an organic substrate, are shown to quench the excited triplet state of the dye by way of an electron-transfer mechanism, leading to the intermediary formation of reactive free radicals.

Mechanism and kinetics of the methylene-blue-sensitized photo-oxygenation of polydienes

Abstract The methylene-blue-sensitized photo-oxygenation of cis -1,4-polyisoprene (PI) and of cis -1,4-polybutadiene (PB) proceeds via pure type II mechanism. It is shown that these polymers not only react with singlet molecular oxygen but also deactivate O 2 ( 1 Δ). The corresponding rate constants k R and k Q are determined: k R PI = 12 × 10 4 M −1 s −1 ; k R PB = 1.7 × 10 4 M −1 s −1 ; k Q = 0.6 ± 0.3 × 10 4 M −1 s −1 for both PI and PB. The latter process is attributed mainly to the interaction of O 2 ( 1 Δ) with CH bonds (solvent-like quenching). It is also shown that the probability of singlet oxygen capture in concentrated media is close to unity even for polymers of fairly low reactivity. The interest of these results for complex biological macromolecules is outlined.

Mise en evidence de reactions d'echange halogene—lithium lors de la desactivation d'un oligo-isoprenyllithium par les halogenures d'alkyle

Abstract The termination of anionic oligomerization by alkyl halides leads to the attachment on the active chain end not only of the alkyl group of the halide but also of the alkyl group of the organolithium initiator. The distribution of the terminated oligomers depends on the alkyl groups, solvent, halogen, and monomer to initiator ratio. Different lithium-halogen interconversion reactions leading to these products are described.