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 OO bond breakage on the repulsive S 1 potential surface. Re-formation of ground state endoperoxide occurs by subsequent OO 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 OO bond cleavage are of the same order of magnitude.
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