Abstract (click to view)
Naphthol reds are a group of widely used pigments with prominent historical, commercial, and cultural significance and are frequently characterized as part of a wider group called azo pigments. Azo pigments have two fundamental properties, they have the vibrant colors and low solubilities characteristic of pigments, and the presence of an azo group within their structure. The latter property, however, as other studies have found is oftentimes completely unfounded. Some of these pigments do not even possess azo groups at all, rather they possess a similar, but fundamentally different group known as a hydrazone group. In other cases these pigments can possess both the azo and hydrazone forms at the same time or undergo an enol/keto tautomeric shift under certain conditions. There are however still many common pigments which are still marketed and referred to as azo pigments with frequently little to no evidence. In this study by using the capabilities afforded to us by X-ray diffraction techniques as well as Rietveld refinement it was possible to determine not only the presence of the hydrazone group but also to elucidate the crystal structures of two pigments. Ultimately these results can fundamentally change both how we refer to these pigments from now on and how these pigments may be better used and applied in the future.
Matthew Heaney
Major: Chemistry; Minor: Geology
Faculty mentor: Tomče Runčevski
Matthew, very nice work and I liked the graphics. Interesting that the two dyes you studied only exist in the hydrazone form. Does that mean there is no equilibrium with the azo form? They exist exclusively in the hydrazone form? Is it possible to have crystals of both forms in the mixture?
As another note, I liked the pictures of your crystals. In the old days when I was in grad school (many years ago), those crystals would not be suitable for single-crystal X-ray analysis. The crystallographers always wanted block-shaped transparent crystals. Technology apparently has changed!
Dr. Son, to answer your first question, yes it does mean that there is no equilibrium with the azo form and that they exclusively exist in the hydrazone form. As for your second question, normally with single crystal that is a possibility as sometimes one crystal may be one form and another a different form. However, since we also used powder x-ray diffraction we were able to obtain an entirely random population. Single crystal analysis in both cases matched the powder pattern, which affirmed that there was only one form present in the crystals.
I would also like to note that a few weeks ago, we sent a sample of some microscopic crystals to Dr. Unruh, who did the single crystal measurements for this project. As it turns out they were even able to use those crystals. For mounting the crystals they can apparently use a nylon loop to pick them up individually and put them in the mounting medium, which is astonishing to me.
Thank you for your reply. Tom had indicated to me earlier that it was indeed possible to get good structures from powders, which is something I wish had back in the old days!