AN EXAMPLE OF HOW PHOTOPHYSICAL PROPERTIES CAN INFLUENCE THE MAGNITUDE OF SOLVENT ISOTOPE EFFECTS UPON PHOTO‐OXIDATION REACTIONS

Abstract Substantial isotope effects have been observed for the dye sensitised photo‐oxidation of 1,3‐diphenyl‐2‐pyrazoline in both polar and non polar solvents, implicating singlet oxygen as a reactive intermediate. By way of contrast, a solvent isotope effect upon the direct photo‐oxidation of the pyrazoline was only observed when a protic solvent (methanol) was used. It was found that the photophysical properties (e.g. quantum yields and fluorescence lifetimes) of pyrazolines are sensitive to the isotopic composition of protic solvents but not aprotic solvents. The solvent isotope effect observed for the direct photo‐oxidation reaction in methanol may not therefore be a true indication of the participation of singlet oxygen. Since this reaction may not be singlet oxygen mediated, an alternative mechanism is proposed.