Subpicosecond transient absorption studies of the photocycloreversion of an aromatic endoperoxide.

The photocycloreversion of the endoperoxide of dimethylhomoeocoerdianthrone has been studied by transient absorption spectroscopy. The results are consistent with the current model which assumes a two-step process. In the first step, homolytic rupture of a single C-0 bond in the reactive S 3 (ππ ≡ 8) state of the endoperoxide takes place within the time resolution of 0.35 ps. Thus a transient biradical is formed, which decays thermally with a lifetime of 1.6±0.5 ps (at 22 o C in dichloromethane), to yield ground-state homoeocoerdianthrone and singlet oxygen