Ultraviolet-visible absorption and luminescence properties of quinacridone-barium sulfate solid mixtures.

The absorption and emission spectral features of the dye quinacridone (QA) have been studied in solution and in the solid phase. In the solid phase, QA has been investigated as pure microcrystalline powder and mixed with barium sulfate (BaSO(4)) in different mass percentages. Two kinds of QA-BaSO(4) mixtures have been prepared: physical blends of the two microcrystalline powders, and mixtures with gum arabic as binder. The latter was used in painting mock-ups. Luminescence properties of the mixtures have been investigated using both steady-state and time-resolved techniques. The prepared samples have allowed the Kubelka-Munk correction model, formulated for the emission spectra, to be tested. Moreover, the luminescence decay profiles have been analyzed using the maximum entropy method (MEM) and the nonlinear least-squares method. All the results obtained highlight how physical (self-absorption) and chemical (composition of the microenvironment) factors can influence the spectral and kinetic properties of dyes. These factors should always be taken into account in the diagnostic activity applied to works of art.