Fluorescence of chlorophyll with UV light

Summary

Watch a solution of ethanol and chlorophyll change color. Regular light shows a green solution while a UV light shows a red solution.

Materials

• Spinach leaves
• Ethanol (95% or 200 proof)
• Coffee filter
• Funnel
• Cup
• UV light

Procedure

1. Tear spinach leaves into small bits and place in a mortar and pestle
2. Crush the leaves into a mushy paste
3. Place coffee filter in funnel over a clear cup
4. Scrape leaves and paste into a coffee filter
5. Pour ethanol over the leaves
6. Shine a UV light on the extract

Discussion

Molecules have varying “bands” within discrete energy levels due to the translational, rotational, and vibrational variation in energy that can occur due to the electron bonds. When energy is absorbed by a molecule like chlorophyll from sources such as the UV light photons, electrons will be promoted from the ground state (S0) to an excited band (S₁ or S₂). As the molecule gives off the energy absorbed, the electron will return to the ground energy state. This phenomenon is termed fluorescence. In the case of the spinach used in this experiment, crushing the leaves releases the pigment chlorophyll a, which naturally absorbs blue and red wavelengths of light, thus emitting a green color under natural light. When the UV light is placed next to the chlorophyll solution, electrons are promoted from the S₀ state to the S₂ state. The pigment then absorbs only the blue-violet range of wavelengths. The electrons will fluoresce, and the red color is observed.