KINETIC-STUDIES OF SINGLET OXYGEN [4-CYCLOADDITIONS WITH CYCLIC 1,3-DIENES IN 28 SOLVENTS(2])

The [4 + 2]-cycloaddition of singlet oxygen with 1,4-dimethylnaphthale ne (DMN) and derivatives has been studied in 28 solvents by laser flas h photolysis and steady state photolysis. The bimolecular rate constan ts of singlet oxygen quenching by DMN, via a physical process (k(q)) a nd via a chemical reaction (k(r)), are solvent-dependent and increase by more than 2 orders of magnitude from cyclohexane to formamide. This significant solvent dependence is in contrast with previous investiga tions conducted in six or seven solvents. It is discussed in terms of the solvatochromic properties of the different solvents. Moreover, for seven water-soluble 1,3-dienes, the overall rate constant k(0) (= k(r ) + k(q)) is much higher in water than in methanol. Our results are co nsistent with a two-stage mechanism implying as a first step an equili brium producing an exciplex with charge transfer character. A shift to ward the formation of this exciplex leading to the cycloaddition produ ct occurs by an increase (i) in the Hildebrand solubility parameter de lta(H), (ii) in the dipolarity-polarizability parameter pi of the sol vent, and (iii) in the solvophobicity of the diene.