Photochemistry and photophysics of the endoperoxide of mesodiphenylhelianthrene: a contribution to the localization of the S1(π*σ*) state of aromatic endoperoxides

Abstract

The endoperoxide of mesodiphenylhelianthrene MDHPO has been studied in detail with respect to fluorescence and photo-induced rearrangement. MDHPO proved to be non-fluorescent, although its absorption spectrum is dominated at the low energy side by a strong ππ* band with a maximum at 429.5 nm. Irradiation of that band effects rearrangement to the corresponding diepoxide MDHDO, a reaction typical for S₁(π*σ*) excited endoperoxides (EPOs). The absorption spectrum of the product MDHDO is blue shifted by only 3.5 nm. MDHDO has the same extended planar aromatic system like its precursor MDHPO, but MDHDO fluoresces strongly. These results set the excitation energy of the S₁(π*σ*) state of MDHPO to ≤23 000 cm⁻¹, which is considered to be a generally realistic value of the S₁(π*σ*) state energy of aromatic EPOs. The main reaction of S₁(π*σ*) excited MDHPO is, however, chemical deactivation to ground state MDHPO via an oxygen biradical. The sequence of O–O bond opening and closing is the general way of repopulation of the S₀ state of aromatic EPOs from S₁(π*σ*) excited states.