Chromophores

The colorfulness of pigments (see last chapter) is based on different structural conditions than the colorfulness observed with soluble dyes.

Colorants are substances that have all the properties needed to prepare colors,
or that are suitable for coloring.
A distinction is made between dyes and (color) pigments :

colorants

The structure of dye molecules can be easily demonstrated using polyenes. Polyene are hydrocarbons with several Double binds (Dien, Trien . Polyen). In the above examples "n" means the number of double bonds.

It is noticeable that polyenes which have more than nine conjugated double bonds appear colored to us.

Polyene

Let’s take a look at what it means with these conjugated double bonds has on itself.

Please be sure to read the chapter Bonding Relationships in Butadiene (lesson content from the 1. half-year) and then return here! There the term mesomerism (resonance) is explained and it is described how to establish mesomeric boundary formulas. You need this for this topic here!

Conclusion from reading: In a double bond, two electrons are relatively easy to shift resp. to distribute when a conjugated other double bond (or/and a non-bonding electron pair) is nearby. In conjugated double bond systems, these so-called π-electrons are distributed throughout the system. This idea is plausible even without the knowledge of orbital theory. The classic example of this is benzene:

red = delocalized π-electron system!

But what is the relationship between the color and the length of a conjugated double bond system??

Imagine a short molecule with delocalized π-electrons in which all carbon-carbon bonds are equivalent. And now imagine that you could strike the molecule like a guitar string and thus make it vibrate. It takes a relatively large amount of force to make a short string vibrate. Less force is needed to make a long string vibrate. Or: imagine a short and a long skipping rope, which are attached to a wall at one end. How much energy do you need in the different ropes to generate a (standing) wave?. The required energy decreases with increasing length. Analogously, we can say:

Molecules with a long delocalized π-electron system can absorb longer-wavelength, i.e. lower-energy light in the visible range, short-chain molecules with a short delocalized π-electron system, on the other hand, can absorb short-wave, i.e. higher-energy light, i.e. ultraviolet, invisible light.

The color of an object perceived by us humans results from the mixture of the light portions of the white, visible light not absorbed by the object.

What happens to the absorbed longwave light?

According to the so-called Energy level model leads those of the molecules absorbed light energy to the fact that the delocalized π-electrons make a quantum jump from their energetic ground state (highest energy level just occupied by electrons, HOMO) to the following, lowest, previously unoccupied energy level (LUMO). However, the electrons cannot remain in this excited state. Thus, they jump back to the ground state, giving off Energy in the form of heat from.

This is the essential difference to Light emission (Fluorescence, phosphorescence, chemiluminescence, flame sample of alkali metal salts), in which non-visible electromagnetic radiation is responsible for exciting the electrons and light in the visible range is emitted when the electrons bounce back.

Examples of dyes in naturetomate-small

Example 1

The following dye is red according to its long delocalized π-electron system. This dye is called Lycopene (lycopene). You find it in Tomatoes and rose hips.

example 2

lobsterAstaxanthin is responsible for the intense coloration of salmon flesh and cooked lobster. A living lobster has a bluish color!
In living lobsters, these dye molecules are bound to a protein, always crossing two in an X-shape, thus expanding the delocalized π-electron system. Yellow light is absorbed, we see the complementary color blue. When boiled, the dye molecules are released, and there is an intense color change from blue (absorption: λmax = 632 nm) to red (absorption: λmax

color-shift-hummer

(Upper bar: Absorbed light,
lower bar: complementary color)

Example 3carrot-small

β-carotene (Absorption: λmax = 460 nm)

beta-carotene-a

To see the more detailed structural formula, you need to point the mouse on the image.

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