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Cadmium yellows (CdYs) refer to a family of cadmium sulfide pigments, which have been widely used by artists since the late 19th century. Despite being considered stable, they are suffering from discoloration in iconic paintings, such as Joy of Life by Matisse, Flowers in a blue vase by Van Gogh, and The Scream by Munch, most likely due to the formation of CdSO4 ⋅n H2 O. The driving factors of the CdYs degradation and how these affect the overall process are still unknown. Here, we study a series of oil mock-up paints made of CdYs of different stoichiometry (CdS/Cd0.76 Zn0.24 S) and crystalline structure (hexagonal/cubic) before and after aging at variable relative humidity under exposure to light and in darkness. Synchrotron radiation-based X-ray methods combined with UV-Vis and FTIR spectroscopy show that: 1) Cd0.76 Zn0.24 S is more susceptible to photooxidation than CdS; both compounds can act as photocatalysts for the oil oxidation. 2) The photooxidation of CdS/Cd0.76 Zn0.24 S to CdSO4 ⋅n H2 O is triggered by moisture. 3) The nature of alteration products depends on the aging conditions and the Cd/Zn stoichiometry. Based on our findings, we propose a scheme for the mechanism of the photocorrosion process and the photocatalytic activity of CdY pigments in the oil binder. Overall, our results form a reliable basis for understanding the degradation of CdS-based paints in artworks and contribute towards developing better ways of preserving them for future generations.

Ultraviolet and visible (UV-vis) fluorescence and subtracted shifted Raman spectroscopy have exploited to identify orchil in a fragment from the ninth century Bible de Theodulphe and a parchment fragment sampled from a 16th century map of Auvergne. Orchil is a purple lichen dye widely used since ancient times in place of the more expensive Tyrian purple and therefore known as the ‘poor person's purple’. Unlike the latter, orchil has a low lightfastness, and in ancient works, it is often faded. Orchil is a complex mixture of different coloured compounds, and they all share a common structure resulting from phenoxazone with a number of different substituents. In this work, UV-vis fluorescence combined with resonance Raman spectroscopy allowed for the identification of orchil in a fragment from the ninth century Bible de Theodulphe. Raman spectroscopy has been applied also for studying a parchment fragment sampled from a 16th century map of Auvergne. In both cases, subtracted shifted Raman spectroscopy (SSRS) has been exploited for removing the strong fluorescence background. Overall results have been confirmed by liquid chromatography with quadrupole time-of-flight mass spectrometry (LC/ QTOF-MS). Both the electronic and vibrational spectral features of ancient orchil showed some interesting differences with respect to those of the standard sample. Taking into account the poor lightfastness of the purple dye, the same investigation has been carried out on photochemically aged orchil reproducing the spectral modification observed on the ancient fragments. Copyright © 2013 John Wiley & Sons, Ltd.

Although the photophysics and photochemistry of aromatic carbonyl compounds have been extensively studied over several years, the interest in their photobehaviors is still alive and they continue to be investigated by exploiting advanced experimental and computational technologies. In this article complete series of pyridyl, di-pyridyl, thienyl, di-thienyl and thienyl-pyridyl ketones are reviewed, mainly from the spectroscopic and photochemical points of view. Properties and reactivity of their excited states have been investigated through stationary absorption and emission spectra, time resolved transient spectroscopy, dynamic luminescence techniques and computational methods. The results reported demonstrate that the photophysics and photochemistry of these molecules can be modulated by changing the rings linked to the carbonyl (pyridyl, thienyl, phenyl) and their linking position, as also the microenvironment where they are included. Of special importance are their properties of triplet photosensitizers and their ability in photogenerating free radicals. Enhancing or depressing such properties may be required in applications and this can be achieved by structure and environment changes.

Absorption and fluorescence spectra of alizarin (1,2-dihydroxyanthraquinone) and purpurin (1,2,4-trihydroxyanthraquinone) were investigated as a function of pH in water–dioxane (2:1, v/v) in the pH range 2–14. Absorbance changes with pH are accompanied by color changes; by increasing the pH, the spectra shift to the red. These molecules exhibit fluorescence emissions over the whole pH range explored; fluorescence quantum yields increase with increasing pH and are greater for purpurin (ΦF = 10−3–10−2) than for alizarin (ΦF = 10−4–10−3). The fluorescence lifetimes were measured by the phase-shift technique for the three species derived from purpurin (neutral, τ = 150 ps; mono-anion, τ = 2600 ps and di-anion, τ = 380 ps), and only for the di-anion of alizarin (τ = 80 ps). Both spectrophotometric and fluorimetric titrations gave pKs of the first and second dissociation steps in the ground state. Excited-state pK*s were calculated by the Forster cycle. In the ground state, purpurin (pK1 = 4.7; pK2 = 9.5) is a stronger acid than alizarin (pK1 = 6.6; pK2 = 12.4) in both the first and second dissociation steps; in the excited state, purpurin is a weaker acid in both dissociation steps. Copyright © 2000 John Wiley & Sons, Ltd.

The photophysics of dipyridyl ketones (DPKs) has been investigated in aqueous solutions as a function of pH using pulsed and steady-state emission spectroscopy and nanosecond laser flash photolysis. All six DPK isomers were found to emit phosphorescence in aqueous frozen solutions at 77 K, but only three of them exhibited phosphorescence emissions and showed triplet transient absorption at room temperature. From both phosphorescence and triplet–triplet absorption titrations, they revealed proton-transfer reactions in the excited state occurring at different pH/H0 values than the ground-state acid–base equilibrations. Comparing results from different techniques gave evidence that the carbonyl oxygen is involved in the triplet state protonation (pK*≈ 3.5–6) for the isomers where it conjugates with the ring nitrogen atom, while proton addition prevalently occurs on the nitrogen for the unconjugated meta–meta isomer. A second protolytic equilibrium (pK*≈–2 to –1.6) implies formation of nitrogen diprotonated ions for all the isomers. The presence of two positive charges in the molecules prevents further protonation at the carbonyl group in the ground states as well as in the excited states.

The photokinetic behavior of a photochromic compound, 2,2-spiro-adamantylidene-2H-naphtho[1,2-b]pyran (Py), has been investigated by using monochromatic irradiation in the UV and visible ranges. An unusually complex photochemistry occurs whereby two colored forms (o-quinone-allides) are produced, one of which is thermoreversible whereas the other is not thermoreversible but photoreversible. These are the result of ring opening of the pyran C-O bond to a short lived open-form intermediate which converts to the two different colored forms by twisting around different C-C bonds. Their spectra and molar absorption coefficients were obtained in acetonitrile solution. The kinetic parameters of the thermal bleaching (rate constant and activation energy) and photobleaching (quantum yield) were measured. A plausible reaction mechanism is proposed. Based on this mechanism, mathematical methods were devised which were capable of analyzing the system during its dynamic evolution and evaluating the quantum yields of the colorforming photoreactions.