PHOTOOXYGENATION OF METHYL LINOLEATE SENSITIZED BY PORPHYRINS AND DYES IN ACETONITRILE SOLUTION AND AQUEOUS EMULSION SYSTEMS

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Abstract
Photooxygenation reaction of an unsaturated fatty acid ester, methyl linoleate (methyl 9‐cis.12‐cis‐octadecadienoate, ML‐H), sensitized by porphyrins and several types of dyes has been studied in aqueous emulsion and acetonitrile solution under air at 40°C. The oxygen (O2) uptake proceeded slowly in the absence of sensitizers upon irradiation of an aqueous emulsion and an acetonitrile solution of ML‐H (20 mM) at ?ex > 290 nm (11.4 and 6.1 μmol h‐1, respectively). The rate of O2 uptake was enhanced by a catalytic amount (0.1 mM) of porphyrins and dyes; hematoporphyrin (HP), zinc tetrakis(N‐methyl‐4‐pyridiniumyi)porphyrin (ZnTMPyP), methylene blue (MB), rose bengal (RB), acridine orange (AO), and acriflavine (AF). In both systems, the sensitized photooxidation of ML‐H by O2 proceeded equimolarly to produce isomeric mixture of C9 and C13 hydroperoxides having the trans,cis and trans,trans conjugated diene configurations, independent of the types of the sensitizers used. The yield ratio of trans,trans/ trans,cis products in the MB‐sensitized photooxygenation in acetonitrile and aqueous emulsion were almost equal (0.32 and 0.35. respectively). The sensitizing activity of the sensitizers, as measured by the quantum yield of O2 uptake, increased in the order: MB (≃ 0) < ZnTMPyP < RB < HP < AF < AO in the aqueous emulsion and AO < AF < HP < RB = MB in the acetonitrile solution. The order in homogeneous acetonitrile solution was interpreted by the sensitizing ability of the dyes to produce singlet oxygen, while that in heterogeneous aqueous emulsion was correlated to the lipophilicity of dyes as well as the singlet‐oxygen‐producing ability.