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Why Some Pigments Fade

Organic Pigment Identification and Spectroscopy

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A field of interest that is prominent within the art and art history community is the determination of the photostability of different pigments. As paintings grow older, the original color deteriorates causing fading and in some cases a complete change in color from the original work. This can be seen in the Van Gogh painting above. Due to a lack of photostability in the red pigment in the walls, the current walls now appear to be blue. In the de Vries group, we measure the excited state lifetimes of a variety of organic pigments to provide insight as to why some colors fade faster than others. 

The Anthroquinone Dyes

Exposure to light can lead to the degradation of many molecules by the absorption of ultraviolet and visible wavelength photons placing them into an energetically excited state prone to chemical and physical changes. Whereby the canonical nucleobases have ultrafast relaxation as a protective mechanism to UV absorption, organic dyes must have similar mechanisms to maintain molecular structure, termed lightfastness, when repeatedly absorbing visible light. Here we study that potential for protective photomechanisms in a subset of naturally occurring organic dyes, red anthraquinone dyes, which were used prominently prior to the 20th century in daily life and in antiquity but due to structural variation degrade in unpredictable ways. We study the isolated chromophores in the gas phase by 2-color REMPI (R2PI) and time resolved pump-probe spectroscopy to understand how functional substitution affects excited state lifetimes.

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The color blue is hard to naturally find on Earth. Indigo is one natural blue compound, a pigment that is widely used in art as well as dyeing goods such as blue jeans and other textiles. In addition to its color, indigo also is very photo-stable. Because of this, paintings that use indigo maintain their blue color much longer than the other colors that were used. Although indigo is so widely used, there have been very little studies on the compound since it is not very soluble. As a result, most research involving indigo uses a similar compound, indigo carmine, since it is more soluble. In order to better understand the relaxation mechanisms that indigo undergoes, we performed many REMPI and pump-probe experiments on indigo in the gas phase. Indigo's major deactivation pathway is due to ESIPT and results in a short lifetime that prevents photodegradation. 

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