Mesodiphenylhelianthrene-the most reactive singlet oxygen acceptor

— The self-sensitized photooxidation of mesodiphenylhelianthrene in various solvents has been investigated. The involvement of 1O2 as the reactive intermediate in the formation of the endoperoxide has been demonstrated by the quenching of the photooxidation by the efficient 1O2-quencher β-carotene. The rate constant for the addition of 1O2 to mesodiphenylhelianthrene has been determined to be kR≅ 1010M-1 s-1, which is the highest value hitherto known in the literature. The probability factor p, which describes the concentration independent part of the overall quantum yield, has been measured to be p=0.17.