ELi5: How do thermochromatic pigments work? Is there a reason darker colors tend to be cold activated and lighter colors tend to be heat activated?

Question

ELi5: How do thermochromatic pigments work? Is there a reason darker colors tend to be cold activated and lighter colors tend to be heat activated?

In: 2

Answer 1

Thermochromatic pigments most commonly work because of the expansion and contraction associated with heating and cooling respectively. For example, we would make body-heat reactive cards in my chemistry department at my school out of a blend of sterols. Cholesterol, and it’s many derivities, are very flat planar molecules. However, because of their tail chains, when they stack on top of each other the offset a bit and form a rotating stack somewhat similar to the way a DNA helix twists back on itself. When you apply your body heat to such a stack of sterols, the space between each molecule expands and this spacing changes. It just so happens that those expansions move through the wavelengths of visible light, meaning that some wavelengths will be able to pass through the stack without interference while most other wavelengths get refracted.

Tl/DR: What this means is changes in the physical shape of the molecules, cause by expansion and contraction of heating and cooling, change the way they interact with certain wavelengths of light. This changes the wavelengths refracted, which changes what color they appear!