Fluorescence of Chlorophyll

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Chlorophyll

In reality, chlorophyll is not a single substance; rather, it is an entire class of chemical compounds with similar structures and properties. Chlorophylls are a group of organic chemical compounds found, among others, in plants, algae, and photosynthesizing bacteria. They give plants their characteristic green color.

The role of chlorophylls in photosynthetic organisms is to capture photons and transfer the energy later used in subsequent stages of photosynthesis. The green color of chlorophyll arises from its high absorption in the red and blue parts of the light spectrum, and its low absorption in the green part.

There are many types of chlorophylls. The most widespread in nature are chlorophyll a and chlorophyll b, found in all plants that carry out photosynthesis. Chlorophylls c and d occur only in certain algae.

As an example, here is the structural formula of chlorophyll b:

źródło: http://upload.wikimedia.org/wikipedia/commons/0/05/Chlorophyll_b.svg, dostęp: 10.06.2012

Its molecular formula is C₅₅H₇₀O₆N₄Mg, which makes it a relatively complex molecule.

Today, we will use a simple method to isolate chlorophyll from plant tissues and then see for ourselves that it fluoresces under ultraviolet light.

Experiment

We need a source of chlorophyll. Any green parts of plants will do; fleshy leaves work best. In winter, when fresh green plants are scarce, you can use spinach (even frozen) for this purpose.

The leaves should first be cut into smaller pieces. Since chlorophylls are insoluble in water, we must use another solvent; acetone (C₃H₆O) is suitable.

Warning: Acetone is not highly toxic, but it is classified as an irritant or a substance that may cause temporary health issues. It is a very volatile liquid; avoid inhaling its vapors. The author assumes no responsibility for any potential damage. You proceed at your own risk!

Grind the leaf fragments in a mortar with a small amount of acetone. During this process, you’ll notice the solvent extracting the chlorophyll, turning a characteristic green color. The solution should then be filtered to remove the fragments of plant tissue.

The resulting acetone solution of chlorophylls has a characteristic, grass-green hue. This is visible in the photo below.

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Is the solution truly green? Under visible light, it undoubtedly is, but let’s try illuminating our sample with ultraviolet light. The result looks quite different:

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Under UV light, the solution begins to glow a strong, carmine-red color. You can also see this effect by illuminating the solution with a semiconductor laser that emits light at a wavelength of λ=405 nm:

na świetle

w ciemności

In this simple way, we have verified the fluorescent properties of a substance as commonly found as chlorophyll.

Explanation

Each chlorophyll molecule is built from a pheoporphyrin structure, which is derived from porphyrin. Four of the rings are pyrrole rings, while the fifth ring is composed of carbon atoms alone. The bonds between the atoms forming the rings are alternating single and double bonds, creating a conjugated bonding system.

The central position in the porphyrin system is occupied by a magnesium atom, which forms bonds with the nitrogen atoms of each ring. A porphyrin complexed with magnesium is capable of absorbing electromagnetic radiation in the visible range.

Thanks to this structure, a chlorophyll molecule can also be excited by ultraviolet light. An excited molecule has higher energy than in its ground state. This excited state is unstable, and after a very short time, the molecule spontaneously returns to the ground state. According to the conservation of energy, the energy difference between these two states is released into the environment as radiant energy—this is the red light we observe.

Enjoy! :)

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