Identifying the low-lying electronic states of anthracene-9,10-endoperoxide

The electronic excited states of anthracene-9,10-endoperoxide giving rise to singlet oxygen generation (1O2) or homolytic O–O cleavage have been unambiguously characterized using multi-configurational second order perturbation theory MS-CASPT2/CASSCF, with inclusion of solvent interactions, femtosecond polarization resolved UV/IR pump–probe, and steady-state fluorescence experiments. The S1 state, responsible for homolytic O–O cleavage, is observed at 34 400 cm−1. Determination of the transition dipole moment direction of the 37 000 cm−1 band, confirms the assignment of the S4 state, from which 1O2 is generated, and allows assigning the other electronic states. Furthermore, it is demonstrated that APO can emit after electronic excitation.