Photophysical properties of alizarin and purpurin Al(III) complexes in solution and in solid state

The present study was undertaken to investigate the photophysical properties of the organic-metal compounds which are the main components of madder lake, one of the most commonly used and widespread organic pigments in painted artworks, from both geographic and historic points of view. Alizarin- and purpurin-Al(III) complexes were studied in solution and as powders. In solution, the chelate stoichiometry, their absorption and emission properties and the efficiency of their excited electronic state deactivation pathways have been determined. The two organic-metal compounds show relevant differences in terms of spectral features consisting of multiple peak (structured) absorption and emission spectra for the purpurin derivative and single broad bands (structureless) for the Al(III)-alizarin chelate. For both the investigated molecules, the chelation process induces a relevant increase of the emission quantum yields and lifetimes. The main differences between photophysical properties of the two metal complexes concern emission quantum yield and lifetime, which are both higher for purpurin chelate compared to alizarine chelate. Furthermore, interesting differences between the two metal complexes concerning the relative relevance of inter- and intra-molecular interaction involved in the mechanism of the excitation energy dissipation have been also highlighted. The knowledge of the determined parameters allows better understanding of the spectral behaviour in the solid state, thus providing a solid reference for the non-invasive characterisation and identification of madder lake on original artworks through its absorption and emission features.