Search

The fundamental chemistry underlying the photo-induced degradation of cochineal (carmine), a red dye extracted from South American scale insects and used as an artist colorant, and its stability in solution and as a lake pigment in different paint media exposed to light in various ambient environments is examined. The influence of oxygen on the photodegradation was examined for dye solutions and for solid paint samples containing the lake pigment, and the effect of ambient humidity was assessed for the paint samples. The degradation was followed using absorbance spectroscopy, and characterized using IR and LC-MS. Results indicate lower reaction rates of the carminic acid chromophore in solution and in oil paints during light exposure in anoxic conditions; whereas watercolor paints demonstrated a stronger dependence of the rates on the ambient humidity. A ring opening of the anthraquinone core is suggested in the breakdown of the dye and pigment. This study illustrates that solution phase systems may serve as simple models to assess the kinetics and mechanism of photochemical breakdown of organic-based pigments in solid samples.

A ruthenium polypyridyl chromophore with electronically isolated triarylamine substituents has been synthesized that models the role of tyrosine in the electron transport chain in photosystem II. When bound to the surface of a TiO2 electrode, electron injection from a Ru(II) Metal-to-Ligand Charge Transfer (MLCT) excited state occurs from the complex to the electrode to give Ru(III). Subsequent rapid electron transfer from the pendant triarylamine to Ru(III) occurs with an observed rate constant of ∼1010 s-1, which is limited by the rate of electron injection into the semiconductor. Transfer of the oxidative equivalent away from the semiconductor surface results in dramatically reduced rates of back electron transfer, and a long-lived (τ = ∼165 μs) triarylamine radical cation that has been used to oxidize hydroquinone to quinone in solution.