Abstract:
Red lakes are a group of historically significant pigments based on the aluminium complexes of naturally occurring hydroxyanthraquinones. They have been in consistent artistic use worldwide for more than 4000 years and are notable for their propensity to fade in light. In this thesis work, four hydroxyanthraquinones (alizarin, purpurin, carminic acid and laccaic acid A) representing the principal chromophores of three major historical red lake pigments were studied using photochemical and photophysical methods. The chromophores’ ultrafast dynamics were assessed with fs-TrA in a variety of solvent systems, in conjunction with long-term irradiation experiments, to establish their vulnerability to photodegradation. The anthraquinones were investigated in both their pure form and, where possible, in their aluminium-complex lake. The results show a clear connection between the anthraquinone’s microenvironment, its excited state lifetime and its long term photodegradation. Across the four anthraquinones studied, the general trend is of a reduction in the S1 excited state lifetime of the molecule as the availability of labile protons in the environment increases (acetone/acetonitrile > methanol > water). We assign this to an increase in the extent of intramolecular hydrogen bonding, accelerating the non-radiative deactivation of the excited state. This reduction in excited state lifetime is reflected in a concomitant decrease in long-term photodegradation. Laking of the molecules also enhances the non-radiative decay, resulting in a decrease in the S1 excited state lifetime and extent of photodegradation. These results give evidence of a correlation between the femtosecond-nanosecond dynamics of photoexcited anthraquinones and their long-term photostability. Not only are such fundamental relationships between light and pigment chromophores vital to improving the understanding and mitigation of fading in works of art, they contribute to the knowledge of light-molecule interactions that advance fields as diverse as chemical physics, industrial catalysis and cancer therapeutics.