The Chemistry behind Highlighter Colors

I'm sure many students have already gone to school or their schools are staring soon. So, as a student you have probably used a highlighter before to annotate an article or just decorate your notes. I am here to tell you about the chemistry behind those vibrant colors that highlighters are known for.

Each color of a highlighter uses a different dye to create the bright colors. Yellow highlighters usually use pyrene based dye(pyranine) or fluorescein. The blue colors use dies known as trphenylmethan that can be mized with pyrene based dyes to create green or mized with rhodamine dyes that are used to make pink or purple. Combining coumarin dye with xanthene dye can create orange ink. Now moving on from that, you are probably wondering why those different dyes mix together to create such vibrant colors.

First I will start with the interaction of light and chemical structures. Chemicals are colored because they absorb some wavelengths of light and not other wavelengths. Highly conjugated molecules(molecules with large number of alternating single/double bonds) are able to absorb wavelengths that are visible to us, so they have different colors dpending on the wavelength it absorbed. The dyes have color because they have highly conjugated molecules. The vibrant color of a highlighter can be explained through the ultraviolet spectrum. When the electrons absorb light, they are at an "excited state." The electrons go back to their original state which releases the energy in the form of light. The fluorescent pigments emit visible light as a result of absorbing UV light.

Fluorescense is when a material takes in light of a certain color and emits the light of a different color with a long wavelength. As mentioned before highlighter's fluorescense involves the in take of ultraviolet rays and the emission of light in the visible spectrum. Since humans cannot see ultraviolet light, a fluorescent object will look like its glowing when illuminated by ultraviolet light. Since highlighters are made with fluorescent materials, it will seem to glow.

However, highlighter ink is not mainly made of this dye. Most of the dye is made from the combination of glycol solvent and water.

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