Ligand-centered luminescence and deactivation by ligand field states in alizarin complexes

Abstract

The red/near-infrared emission spectra of alizarin (1,2-dihydroxyanthraquinone), the potassium salt of alizarin and the aluminum and zinc complexes of alizarin in the solid state are reported. The emission is assigned to an alizarin localized transition from an in plane non-bonding orbital delocalized over the anthraquinone nucleus to a predominantly π* quinone CO acceptor orbital (nπ* ligand localized excited state). The cobalt, nickel and copper complexes do not emit from their nπ* excited state. In these complexes, metal centered dd excited states which lie lower in energy than the ligand localized nπ* state effectively deactivate the luminescence.