Electronic absorption spectra between 20 000 and 62 000 cm-1 of anthracene-9,10-endoperoxide (APO), its 9,10-dimethyl derivative (DMAPO), and perdeutero-APO isolated in Ar matrices are presented. These spectra are interpreted with the help of semiempirical (INDO/S and CNDO/S) calculations of the electronically excited states. Both experimental and theoretical results lead to an unambiguous assignment of the electronic states of APO and its derivatives. Several higher excited singlet states with significant oscillator strengths have been observed for the first time. The present assignment disagrees with the so far accepted assignment of the electronic states of these APOs, namely, that the first excited singlet state (S1) is located at 23 000 cm-1 and that the photocycloreversion occurs from the higher excited singlet state (S2) at ∼36 000 cm-1. In our new assignment, the first excited singlet state (S1) is located at 36 360 cm-1 and the low-lying triplet states that are predicted by the semiempirical calculations and measured using the heavy atom effect of C2H5I solvent are located at ∼21 000 cm-1. Thus, the present assignment shows that the photocycloreversion of these APOs is not an exception to Kasha's rule. The spectroscopic signature of APO and DMAPO is governed by the exciton-like interactions between the phenyl subunits, which is also reflected in the strikingly similar spectroscopic properties of an isoelectronic molecule of APO, namely, 9,10-dihydro-9,10-ethanoanthracene. Possible reasons for the misinterpretation and/or wrong generalizations carried out in earlier works have been discussed.

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