Fluorescence of alizarin complexone and its metal complexes

Alizarin complexone (AC) in ethanol shows two fluorescence bands which originate from the lowest excited singlet before and after proton transfer. Upon addition of various metal ions (Ni2+, Zn2+, Gd3+, Th4+) to AC in ethanol the formation of well known AC complexes takes place. They are characterized by intense colours which are caused by IL (or ILCT) transitions. If low-energy LF states (Ni2+) are absent (Zn2+, Gd3+, Th4+) these complexes show a weak fluorescence. Even in the case of Gd and Th a heavy-atom effect is not apparent and fluorescence quenching is not observed. Moreover, AC complexes are not phosphorescent under any conditions.