A spectrophotometric and fluorimetric study of some anthraquinoid and indigoid colorants used in artistic paintings

The present study was undertaken to investigate the potential of spectrophotometric and fluorimetric techniques for identifying the materials used in artistic paintings. Two classes of organic colorants were examined, anthraquinoid and indigoid dyes, in their naturally occurring and synthetic forms. Absorption and fluorescence spectra were recorded in both solution and solid layer using linseed oil as a binder. Fluorescence quantum yields were determined. A non-destructive, instrumental set-up to record fluorescence spectra on painted surfaces was successfully tested. The solvent effects on the spectra of the dyes are interpreted in terms of intra-and inter-molecular hydrogen-bonding interactions. Molecular aggregation of indigo was investigated in dichloroethane solution and assigned to a dimer. The spectra recorded from the painted surfaces are broader than in solution. However, the emission spectra are still suitable to identify the colorants. (C) 1998 Elsevier Science B.V. All rights reserved.