Charles Tanielian

Phenalenone, a universal reference compound for the determination of quantum yields of singlet oxygen O2(1Δg) sensitization

Abstract The aromatic ketone phenalenone (PH) is shown to be a universal reference compound for the determination of quantum yields QΔ of 1O2 sensitization. Values of QΔ of PH have been determined by photochemical photoacoustic and near-IR luminescence methods. QΔ varies in 13 solvents of very different physical and chemical properties and in methanol-water mixtures only in a narrow range 0.94⪕QΔ⪕ 1.00. Extrapolations to air-saturated organic solvents and to O2-saturated micellar systems of low viscosity should be possible assuming QΔ=0.95±0.05.

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.

Production and quenching of singlet oxygen by the sensitizer in dye-sensitized photo-oxygenations

Abstract The production and quenching of singlet oxygen O2(1Δg) by the sensitizer in dye-sensitized photo-oxygenations are studied. The quantum yield γΔ of O2(1Δg) formation is equal to the intersystem crossing yield γis for dyes such as Rose Bengal, eosin and methylene blue which do not exhibit fluorescence quenching by O2(3Σ). For these dyes the efficiency of O2(1Δg) production from the O2(3Σ) quenching of the triplet state is about 100%. For dyes such as chlorophyll a and tetraphenylporphine which do exhibit fluorescence quenching, γΔ > γis and it is shown that the O2(3Σ)-catalysed intersystem crossing of their singlet state is operative. Moreover it appears that the quenching of O2(1Δg) by the dyes themselves is a rather general process. Thus the values of the rate constants for Rose Bengal and eosin in acetonitrile are 5.0 × 107 M−1 s−1 and 7.2 × 107 M−1 s−1 respectively compared with that of chlorophyll a in benzene (7.3 × 108 M−1 s−1. The consequences of these results are discussed.

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.

MECHANISM OF PHYSICAL QUENCHING OF SINGLET MOLECULAR OXYGEN BY CHLOROPHYLLS and RELATED COMPOUNDS OF BIOLOGICAL INTEREST

Abstract— The rate constant k5/ > for physical quenching of singlet oxygen O2(δ1;) by the sensitizer in dye‐sensitized photooxygenations is determined in the case of chlorophylls a and b (7.3 times 108, 4.2 times 108 M‐1 s‐1 respectively), pheophytins a and b (7.4 times 107, 3.0 times 107M‐1 s_1 respectively), tetraphenylporphyrin (4.4 times 107 M‐1 s_1), magnesium tetraphenylporphyrin (5.0 times 108M‐1 s_1), zinc tetraphenylporphyrin (1.5 times 108 M‐1 s_l) and protoporphyrin IX‐dimethylester (9.1 times 107M‐1 s_1) in benzene. These sensitizers show a linear correlation between log ksO, and their half‐wave oxidation potentials and the value of the slope is similar to that observed for various compounds such as phenols. It is concluded that (i) the interaction between chlorophylls and related compounds with singlet oxygen may involve an exciplex as for phenols, and (ii) physical quenching may be envisaged as a spin‐orbit‐induced intersystem crossing within the exciplex.

EFFECT OF AGGREGATION ON THE HEMATOPORPHYRIN-SENSITIZED PRODUCTION OF SINGLET MOLECULAR OXYGEN

Abstract— The hematoporphyrin‐sensitized production of singlet molecular oxygen, O2(1Δg), has been investigated in methanol and in aqueous solution. The quantum yield for formation of O2(1Δg) (ΦΔ) has been measured by both steady‐state (oxygen consumption) and time‐resolved (near‐infrared luminescence) methods. In methanol, both techniques indicate that ΦΔ= 0.76 and the value remains independent of sensitizer concentration over a wide range. This finding is consistent with the dye persisting in a monomelic form in methanol solution. In contrast, ΦΔ decreases markedly with increasing sensitizer concentration in water due to dimerization of the dye. Analysis of the steady‐state data indicates ΦΔ values of 0.74 and 0.12, respectively, for monomer and dimer. It is further shown that the efficiency whereby quenching of the triplet state by O2 results in generation of O2(1Δg) is substantially lower for the dimer than for the corresponding monomer. Because monomer and dimer possess quite different absorption spectral profiles, the efficacy for photodynamic action with hematoporphyrin exhibits a pronounced wavelength dependence.

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.

Photosensitization of L1210 leukaemia cells by argon laser irradiation after incubation with haematoporphyrin derivative and rhodamine 123

The selectivity and efficacy of photodynamic therapy (PDT) may be improved by the combined use of photosensitizers in a similar manner to the combined use of drugs in cancer chemotherapy. Two photosensitizers (haematoporphyrin derivative (HPD) and rhodamine 123 (Rh-123) were analysed which can be irradiated at the same wavelength (514 nm), are preferentially taken up by tumour tissue and are not specific for the same target (membrane for HPD, mitochondria for Rh-123). The analysis of the phototoxic effects in surviving fractions showed a dependence on dose for both products and a dependence on incubation time for HPD but not Rh-123. The lethal dose for 50% cell death (LD50) for HPD increased from 25 to 56 J cm-2 when the HPD dose was reduced from 2.5 to 1 micrograms ml-1 for the same incubation time. When the incubation time was increased from 15 to 45 min, the surviving fraction decreased by 37% and 17% for doses of 1 and 2.5 micrograms ml-1 respectively. For low doses (0.5 and 1 microgram ml-1), the toxicity of the two photosensitizers added simultaneously was weaker than for Rh-123 alone, whereas for high doses (2.5 micrograms ml-1) the surviving fraction was less than that obtained with Rh-123 alone. These results were compared with the light energy absorbed, the quantum yield of singlet oxygen and Rh-123 uptake as determined by flow cytometry analysis.

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).