Role of protolytic interactions in photo-aging processes of carminic acid and carminic lake in solution and painted layers

摘要

In this paper absorption and fluorescence spectra and emission quantum yields and lifetimes of the red colorants,ncarminic acid and its metal complex, carminic lake, were studied in solution and on painted surfaces. Acceleratednphoto-aging of carminic acid and lake was investigated in solution, in the presence and absence of a binder (arabicngum) commonly used in water-colour painting, while natural ageing was followed for several months on water-colournpainted paper.nThe study of carminic acid in water as a function of pH showed that the absorption spectrum changes with pH.nFour acid–base dissociation steps were detected and the corresponding pKs were determined fromnspectrophotometric and fluorimetric titrations. The fluorescence quantum yields (in the 10u00012n–10u00014n range) and thenlifetimes (on the sub-nanosecond timescale, 90–1000 ps) were markedly dependent on the pH of the medium. Excitednstate pK*s were calculated by means of the Förster cycle. The acidity decreased upon excitation for the firstndeprotonation step involving the –COOH group (∆pK* = u00011.9), but increased slightly for the successivendeprotonation steps involving three phenolic hydroxy groups (∆pK* = 0.6, 2.0 and 0.2, respectively).nThe results obtained from the aging experiments indicate that both carminic acid and lake are bleached duringnirradiation. While the binder prevents lake from fading, it destabilises the carminic acid. These findings are discussednin the light of the interactions of the dye with the solvent and matrix.