Comparison of the photochemical and thermal rearrangement reaction of endoperoxides

Abstract

The yields Q dec(S 1) and A dec of the photochemical and thermal rearrangement reaction have been determined for 13 endoperoxides of the anthracene and tetracene series. A strong substituent effect operates on Q dec(S 1) and A dec, which are closely correlated. The interpretation of our results leads to the postulate that the lowest excited singlet state of endoperoxides is deactivated exclusively by a chemical process, by OO bond breakage on the repulsive S 1 potential surface. Re-formation of ground state endoperoxide occurs by subsequent OO bond formation. This conclusion is confirmed by the observation that even for photochromic endoperoxides with low Q dec(S 1) neither fluorescence nor phosphorescence occur. The intermediate biradical, which is the common precursor in the photochemical and thermal rearrangements, is involved in a chemical equilibrium with ground state endoperoxide. For the photochromic endoperoxides, and most other endoperoxides, the rate constants of thermal cycloreversion and of thermal OO bond cleavage are of the same order of magnitude.