Triplet-sensitized cycloreversion of endoperoxides of aromatic compounds. An adiabatic reaction route

Abstract Efficiencies EF T c of triplet-sensitized cycloversion (T-cycloversion) of several aromatic endoperoxides (APOs) have been determined. From the dependence of EF T c on the triplet state energy of the sensitizer the energies E T (ππ*) of reactive triplet states of APO are estimated ranging from E T (ππ*)≲ 170 kJ mol −1 for the endoperoxide of mesodiphyenylhelianthrene (MDHPO) to E T (ππ*)≈250 kJ mol −1 for the endoperoxide of rubrene. Quantaum yields Q T c of T-cycloversion could be derived from [APO]- dependent measurements of EF T c Values of Q T c =0.07±0.01 for MDHPO and Q T c =0.042±0.008 for the endoperoxide of benzodixanthene resulted. T-cycloversion of the endoperoxide of homoeocoerdianthrone (HOCDPO) was found to be an adiabatic and spin-allowed reactions. S 1 -excited HOCD and O 2 ( 3 Σ − g are formed in equimolar amounts. Thus T-cycloversion of APOs parallels in an amazing way singlet state cycloreversion.