The more UV a colored surface absorbs, the more chances UV radiation has to affect the pigmentation. Some pigments are more durable than others, while some pigment breaks down quite easily. This is why when a multi-color print, sign or textile experiences sun fading, some colors may seem to have evaporated while others appear hardly changed.
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If you leave something outside, its colors seem to inevitably fade or bleach due to exposure. Is this due to UV absorption? What sort of mechanism causes this - is it that man-made dyes deform on a molecular level? Are there notable materials that are exceptions? I have a full undergraduate background in physics but not so much training in chemistry.
A technical explanation would be cool, though. Dyes and pigments work by absorbing certain wavelengths of light and reflecting or transmitting the rest.
When a dye molecule absorbs a photon, an electron is excited to a higher energy state. Most of the time neglecting fluorescence , the molecule de-excites by giving off heat and returns to the ground state intact.
However, because the excited state is a high energy state, it has the potential to undergo a chemical reaction, breaking a covalent bond or otherwise irreversibly reacting with another molecule.
This changes the electronic structure of the molecule which changes its absorption properties: e. How likely this kind of destructive chemistry is depends on the nature of the dye. To look at things a bit more closely, the color of the dyes used in clothing, colored paper, etc. For example, take a look at the structure of this common dye called Orange II. Notice how there are two rings with alternating double bonds we call these aromatic rings , linked by a nitrogen-nitrogen double bond.
This extended ring structure is what allows this compound to strongly absorb blue and green light and to reflect the yellow-red part of the spectrum. Now as you might expect this compound will initially have a vivid orange color.
However, if you expose this dye it to light, over time this color will go away and you will be left with a product that is mostly clear, as shown here. So what exactly happened that made the color go away?
Well, in one word photochemistry happened, namely light promoted chemical reactions.
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