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

(a) Absorption spectrum of APO in CHCl3. The inset shows APO and the relevant transition dipole orientations: S0 → S1, S0 → S2 (both orange), S0 → S4 (blue), and the δ(C-H) bending vibration (black). (b) Emission spectrum and excitation spectrum of APO. Sticks are the calculated transitions of APO in CHCl3 (grey).