The Surprising Science Behind Yellow, the Brightest-Looking Color: What Are Mechanochromism and Organic Minerals? @ Science ZERO (Former Professor, 745th Day of Retirement)

Why is a yellow card the most conspicuous?

I have occasionally featured the "Color Series" from NHK's "Science ZERO." The characteristics of colors like green, white, black, and red have been very interesting, and just as I was thinking it had been a while, the theme this time was yellow. It is true that compared to the colors covered so far, there may not be many opportunities to be strongly conscious of yellow itself in our daily lives. However, it is an indispensable color for warning signs and restricted area markers, and it plays an important role in catching people's eyes in times of need. In modern society, its high visibility is also utilized as a tool for communication. For example, the soccer "yellow card" has become established worldwide as a visual language that conveys warnings across language barriers (I bought an official yellow card and used it half-jokingly on noisy or sleeping students during class (laughs)). I also feel that the IKEA logo and the Ferrari emblem are successful examples of branding that skillfully utilize color psychology and cultural identity. Personally, when I was making PowerPoint materials during my active career, I often used yellow as a background color for parts I wanted to highlight. Yellow stands out very well with both red and black text. Of course, I did not overuse it, limiting it to only a few particularly important points during my lectures. (Please also see the photo below.)

Temptation or warning? The duality of "yellow" used by the natural world. And the mystery of red + green = yellow

On the program, Professor Horiuchi of Chiba University, a familiar "master" of the color series, first spoke about how yellow is a color of "duality." First, there is "temptation" and "warning" in the natural world. Yellow flowers in plants function as a color of "temptation" to attract pollinators like bees. On the other hand, yellow jackets and venomous snakes use yellow as a warning color, emitting a survival strategy signal that says "stay away." The program also introduced an interesting experiment. When red light and green light from penlights are overlapped and shone, the overlapping part looks brilliantly yellow. I had seen similar things as experiments with the three primary colors of light, but this was the first time I saw yellow appear with just these two colors, and I was surprised by its vividness. However, when you think about the logic properly, it is quite mysterious why it is perceived as yellow just by mixing red and green light, which do not contain a "monochromatic yellow wavelength." This is a phenomenon called "additive color mixing," and it means that two types of yellow exist: yellow as a single wavelength and yellow perceived through a combination of red and green. I felt this point was also very interesting as it demonstrates the "duality" of yellow. (Please also see the photo below.)

A magical yellow powder that turns blue-green when pressed

The program also introduced a material developed by Professor Yutaka Matsuo and his colleagues at Nagoya University that exhibits "mechanochromism," where the color changes due to mechanical stimuli such as pressure. When a yellow powder or paper-like material is pressed with a glass rod, it changes to a blue-green color, and the phenomenon can be visualized by writing characters. What is interesting is that this compound was discovered by chance. Originally, it was obtained unintentionally as a yellow substance during the synthesis process of solar cell materials. It is said that what was originally expected was a dark green compound, but the synthesized product was yellow. However, it was discovered that when the substance was pressed (ground) with a rod in a mortar, it changed to a dark green color, and research progressed from there. This compound was called an FA compound (a compound in which fluorene and acridan are bonded).

"Bending" and "twisting" stereoisomerism creates future film technology

Even more interesting is the cause of its mechanochromism. Through X-ray structural analysis, it was found that this compound has two stereostructures: a bent type (non-planar structure) and a twisted type (a structure that has planarity but is twisted). The bent type absorbs blue light and reflects red and green, so it looks yellow. On the other hand, the twisted type absorbs red light and reflects blue and green, so it looks blue-green. In other words, this was a phenomenon where the color changes due to the control of stereoisomerism. Furthermore, interestingly, it was also found that the ratio of these two structures changes by applying external force, and as a result, a continuous color change occurs. In other words, the correlation between the magnitude of the applied force and the rate of color change can be handled quantitatively. For example, in the figure below, if the color change is 20%, the pressure is estimated to be about 200 MPa, and its application as a pressure-measuring film is also expected. Moreover, this material also has reversibility, returning to its original yellow color when alcohol is applied. Conventionally, microcapsule-type pressure-measuring films used in dental clinics and the like were irreversible, so it seems that development into more advanced and flexible applications will be possible. (Please also see the title photo and the photo below: Note 1)

A cliff that glows vividly under ultraviolet light! A new type of organic mineral, "Hokkaidoite"

As another unique function of yellow, the program also introduced "fluorescence." In recent years, it is said that various fluorescent rocks were observed under ultraviolet irradiation at a "glowing cliff" in Hokkaido, known for the formation of opal where there used to be hot springs. Among them, small crystals that emitted a strong yellow light were found. These crystals had the property of emitting light only when they absorbed ultraviolet rays. As a result of X-ray analysis, it was found to be an "organic mineral" composed mainly of organic matter. It is said to be a very rare existence, with organic minerals accounting for less than 1% of the approximately 6,200 registered minerals. Therefore, specialized methods of organic chemistry, not just ordinary mineralogy, were required for analysis, and joint research proceeded. First, a sample containing carpathite, a known organic mineral whose main component is coronene (seven benzene rings), was confirmed. When investigated in more detail, an unknown peak was detected, and a composition of 22 carbons and 12 hydrogens, or C22H12, was revealed. This is an organic mineral containing a component corresponding to benzoperylene (six benzene rings), and it was internationally approved as "Hokkaidoite" in 2023. On the program, the MC shone invisible 365 nm ultraviolet rays onto the rock, and the appearance of the mineral absorbing that light and emitting a yellow glow in granules on the surface was shown. The vivid yellow fluorescence emitted by this mineral was truly a mystery created by nature.

On the other hand, from a biological perspective, there was also a noteworthy topic regarding the acquisition of yellow. While Cambrian organisms only possessed melanophores, it is said that 'xanthophores' were first acquired by fish such as cartilaginous fish that appeared in the Devonian period. Furthermore, it was introduced that in modern medaka and other species, yellow acts as a courtship color to appeal to females. In this way, there are still many topics I would like to delve into, and I once again felt the depth of the world of yellow. I would like to write about the rest on another occasion. See you then.

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Note 1: From the NHK 'Science ZERO' special feature 'Color Series: Yellow'