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  • Writer's pictureMichael Skalka

Pyrrole Red, The Allure of Rosso Corsa

Summary: Pyrrole Red is one of a number of pigments that were accidentally discovered. In this essay, take a trip from East Lansing, Michigan to Maranello, Italy to learn more about Pyrolle Red.

Most modern organic, synthetic pigments have little known or unknown inventors because they were created by employees of a corporation who were tasked with formulating a colorant to address specific needs and/or to add products that increase a company’s revenue stream.

The history of Pyrrole Red comes to light in 1974, when chemistry professor Donald G. Farnum who taught at Michigan State University, attempted to create a new chemical but his experiment failed. The "failure" that remained from his experiment became the basis for the pigment known as Pyrrole Red.

Like all synthetic organic pigments, the proper full name of Pyrrole Red is diketopyrrlopyrrole. It is a mouthful to pronounce and takes a bit of practice to say the chemical name. Sounded out it is di - keto – py -rr - lo - pyrrole. It is far easier to refer to it as DPP. DPPs are formally classified as organic dyes.

A cursory glimpse into what it takes to create DPP gives us a window into the complexity of chemical reactions and the knowledge it takes to perform an experiment. Unlike the act of mining minerals or processing a natural colorant to convert it into a usable pigment, organic chemistry requires a completely different set of skills.

If you are curious about what Farnum and his associates accomplished, making DPP involves heating benzonitrile, ethyl bromoacetate, and activated zinc-copper in toluene under reflux conditions. Farnum was originally seeking to create 2-azetinone derivatives. The red pigment is further reacted by boiling it in xylene to create DPP. (A minireview on diketopyrrolopyrrole chemistry: Historical perspective and recent developments, Published in: Results in Chemistry, Vol. 4, January, 2022)

An additional function of DPP is its physical properties that are useful in electronics. In 1974 Farnum and his colleagues only took the initial first step when accidentally making DPP. Their published paper did not go beyond the process that created DPP and the unintended color that resulted. Farnum did not take his discovery further and explore other potential uses of DPP. After the publication of Farnum's work, other scientists carried the work further, manipulating the DPP molecule and discovered additional uses for the DPP molecule. (Diketopyrrolopyrrole (DPP) - Based Materials and Its Applications: A Review. Frontiers in Chemistry, Vol. 8, 2020)

By modifying the DPP molecule, a host of properties related to electronics unfolded. DPP variants could be used in Organic Field-Effect Transistors, Organic Photovoltaics, and several other applications. (Frontiers in Chemistry, Vol. 8, 2020)

Research related to color indicates that DPP-based polymers showed promise in color-change windows due to their robust stability. DPP added to one-coat epoxy coatings demonstrated that it provides improved UV-light stability of the epoxy. (Frontiers in Chemistry, Vol. 8, 2020)

No matter how much you understand, want to understand, or don't understand the synthesis of DPP, it becomes clear that creating materials in the modern world is nothing like it was when pigments were created from rocks, roots, and insects. Further, while you might attempt to make a simple dye from plant extracts, you would never, ever casually try synthesizing DPP.

The history of Pyrrole Red takes an interesting financial turn. Farnum worked in academia and in this discipline, it is routine to document and publish the results of your work. Farnum did as his training and experience dictated and published the work on the failed experiment that yielded a red colorant.

The "unsuccessful" results of the experiment that occurred in 1974 were likely published sometime during or after 1974. However, the story of where DPP went did not end there. Between 1974 and 1983 Farnum’s paper was noticed and used as the basis for more experimentation. In 1983, Ciba Specialty Chemicals concluded a sufficient amount of research on DPP that enabled them to patent a manufacturing process for synthesizing Pyrrole Red. It was registered as a pigment in the Chemical Index and it received the designation PR 254.

Pyrrole Red displays high chroma, is lightfast, and very stable for use in applications that require a robust colorant. It becomes an ideal red colorant for automobile paint. Ciba did the work necessary to provide the marketplace with a product that performed admirably, especially when compared to other red predecessors.

Automobile manufacturers and car enthusiasts have documented significant deficiencies regarding red pigments for painting automobiles prior to the introduction of PR 254. The problem is two-fold. Prior to having PR 254 as a red color choice, the red pigments available had fading problems along with stability issues related to the paint binders. These caused red colors to turn pinkish and chalky. The color red also has inherent physical properties that make it susceptible to fading. Envision a spectral chart of visible light. Red is at the far right of the spectrum. Because of this, red absorbs a greater range (violet, blue, green, and yellow) of energetic short-wavelength light than colors that are closer to the blue side of the spectrum.

The "road" from Lansing, Michigan to Maranello, Italy was "paved" by the production of PR 254. When an average person is asked to name an iconic luxury sports car that is painted bright red, the company that comes to mind is the Italian car maker Ferrari. Ferrari’s decision to paint cars red was influenced by the international car racing community. Each country painted their cars with a color assigned to them. Italy's designated color for racing is red. Thus, the Italian term Rosso Corsa (Racing Red) is fused with Ferrari and was amplified to enhance the mystique of its unique brand.

Car enthusiasts report that PR 254 was held "captive" by Ciba's patent which allowed them to charge far greater for the pigment than the average cost of other colors used to paint automobiles. However, would any Ferrari owner ever have a concern that PR 254 in the 1980s cost $100 per kilo for the pigment as opposed to $20 or $30 per kilo for other colors? (Note: Not all Ferraris are red. Other colors have and continue to be offered and even the iconic bright red has changed over model years to include a range of dark red to orange-red hues.)

Since the 1980s the cost of Pyrrole Red has gone down in price. It is less expensive than a typical cadmium red. However, it does not behave and is not a mirror-like substitute for cadmium red if your goal is to avoid cadmium pigments on your palette.

Pyrrole Red has intense tinting strength. It will rise slightly in chroma when mixed with a small quantity of white. When mixed with a substantial amount of white, Pyrrole Red will display a hot pink hue while cadmium red will reveal a muted pink color.

Comparing Pyrrole Red with Cadmium Red is much like comparing the working properties of Ultramarine Blue versus Phalthocyanine Blue. Comparing inorganic colors to synthetic organics share many of the same characteristics related to tinting strength and color intensity. Organic colors are more intense and do not dull when mixed with white like inorganic colors display. It is likely why synthetic organics are chosen in many modern applications where robust, intense, lightfast color is a requirement.

The complete story of Pyrrole Red does not end with only one pigment color. A research paper written by Iqbal et al., in1988 provides us with a "Star Wars" like moment when Yoda gives Luke the indication that "there is another." The Iqbal paper reveals that DPP derivatives were created that had a color range from red to blue. Is Pyrrole Blue the "Princess Leia" of the DPP family of molecules? Indeed, it is true. Pyrrole Blue has a CAS number 1324-86-3. However, it appears to be used for scientific research purposes rather than as a colorant for paint or plastics. This is likely due to it having sensitivity to water and strong acid.

Without progressing too much further down the "rabbit hole," the pyrrole molecule ring has a wide range of uses. From pigments to electronics and even to pharmaceutical applications related to fighting cancer, viruses, and microbes. While science achieved an enormous stride with the creation of DPP, unfortunately, Farnum did not attain any financial gain from the discovery, most likely due to his position as an academic. (However, the same lack of achieving a monetary fortune by the discovery of DPP would have befallen Farnum if he worked as an employee of a chemical company.)

I have a goal to find a contest where a Ferrari is a grand prize if a participant can identify the color of the pigment used to paint the car. I think I have a shot at winning if the car is painted bright red. Oh, wait. Now you do as well...

The Syntax of Color

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