Rate Parameters for the Two-Step Photofragmentation of Aromatic Endoperoxides in Solution

Article abstract of DOI:10.1021/j100156a051

The formation of dimethylhomoocoerdianthrone (HOCD) following subpicosecond laser photolysis of its endoperoxide (PO) was studied by laser-induced fluorescence (LIF) in the solvents dichloromethane and toluene.The measured rise time of the LIF signal refers to a time constant of τ₂ = 1.7 +/- 0.5 ps at 22 deg C.The study of the characteristic rate constant at different temperatures gives an activation energy of E_a = 5.5 +/- 0.5 kcal/mol and a preexponential factor A = 10^{15.7 +/- 0.3} s^-1 in toluene.These parameters are consistent with our earlier model for the two-step photofragmentation of aromatic endoperoxides, which assumes a short-lived 1,6-biradical intermediate crossing an activation barrier.The photolysis study of the newly synthesized endoperoxide of anthrabisbenzothiopyran (ABTPO) reveals that photocycloreversion starting from S₃(??^*) proceeds with a time constant of τ₂ = 46 +/- 6 ps in dichloromethane (at 22 deg C).In this case the activation parameters amount to E_a = 5.9 +/- 0.6 kcal/mol and A = 10^{14.8 +/- 0.3} s^-1.The observed activation barrier on the potential energy surface can be a result from resonance effects in the chromophores of the peroxy biradical.The large A factors reflect the importance of internal degrees of freedom in the transition state.