Why do Molecules Have Color?(분자구조에 따른 색)

When a light  in the visible spectrum reaches our eyes, we percieve it as a 'color'. Objects absorb light of a spectrum, and reflect the 'opposite' spectrum. For light in the visible spectrum, red is opposite to cyan, yello to blue, and green to magenta. For example, when an object absorbs red, it reflects cyan, and therefore we see it as having a cyan color. Objects that do not absorb light in the visible spectrum appear white or cololess.

https://chem.libretexts.org/

Then how do molecules appear in a specific color? The reason is in the unique structures of some molecules. Molecules such as chlorophyll, red blood cells, and vitamin A all have the same thing in common: parts of their bonds are conjugated, or has a conjugated system. A conjugated system is when a single bond-double bond, or a double bond-triple bond, or a single bond-triple bond pattern continues. Molecules having conjugated bonds appear to have color. The more atoms a conjugatd system is made up of, the less energy it takes to make an electron to an excited state(see picture below). This also means that molecules with longer conjugated systems absorb less energy. For instance, since the energy of light is inverse to its wavelength, so molecules of longer conjugated systems will absorb light of longer wavelengths and less energy. 

https://chem.libretexts.org/

Let's look at a few examples. Heptatriene, whose conjugated system is made up only of 7 atoms, doesn't absorb light in the visible spectrum, and can be seen as being colorless.

http://www.chemspider.com/Chemical-Structure.4518902.html

However, Retinol(or vitamin A), which has a longer conjugated system consisted of 11 atoms, absorbs purple light and reflects yellow.

https://commons.wikimedia.org/wiki/File:Retinol.png

One more example. Beta Carotine, which has a conjugated system of 23 atoms, absorbs blueish light, which has a longer wavelength than purple, and reflects orange.

http://www.chm.bris.ac.uk

In conclusion, the more atoms the conjugated system in a molecule is made up of, the less energy it absorbs, and the reflected color is the opposite to the absorbed color.

 

Energy of Colors: red(weakest), range, yellow, green, blue, magenta(strongest)