Well, the answer to that is pretty simple. There are molecules of a specific compound that fluoresces when exposed to ultraviolet light. Contrary to popular belief, it isn’t actually the gin that is glowing. Instead, it is the tonic. Tonic water contains a molecule called quinine, which was actually a compound developed to treat malaria. Quinine has a molecular structure that allows for the absorption and reemission of ultraviolet light, also known as fluorescence.
An Introduction to Electromagnetic Radiation
Electromagnetic radiation (electromagnetic waves) is a form of energy that presents as a wave. The variations of electromagnetic waves exist on a spectrum, organized by a characteristic called wavelength. Wavelength is the distance from one peak to another, and ranges from 10-16 to 108meters. Wavelength is inversly related to the energy associated with the wave. The larger or longer the wavelength, the less energy, and vice versa. Along this spectrum, we find visible light in a small section from roughly 10-7 to 10-6 meters.
Light is electromagnetic radiation, and it’s how we can perceive our surroundings; the grass is green, the sky is such a beautiful blue, that IPA I had last night was such a sweet honey gold color. All of those colors fall on the electromagnetic spectrum, at wavelength that are detected by our eyes.
What Is Fluorescence?
Fluorescence is a light phenomenon, a process in which a chemical absorbs electromagnetic radiation at a certain wavelength and reemits that energy in the same form, but at a different wavelength and thus, a different energy. Fluorescence is a subclass of luminescence, which is an all-encompassing term for all methods of glowing. Fluorescence is merely just one process in which things glow.
Excitation and Emission
Atoms and Electrons
Atoms make up the things we observe and interact with. They’re made of protons, neutrons and electrons. Electrons occupy discrete amounts of volume (orbitals) around the nuclei, which consist of protons and neutrons. Electrons are exposed to the surroundings of the atom, so they govern chemical interactions. Electrons have certain properties and characteristics, and the exploitation of these properties are exactly what is being manipulated in fluorescence.
When atoms are brought together, they form bonds, sometimes called covalent bonds. Two or more atoms bound together create a molecule. The volume that electrons occupy, as mentioned previously, overlap and create these new weirdly shaped volumes. Electrons from both atoms that now reside in those bonds, are the ones that are able to undergo excitation and emission.
Electron Energy States
Electrons exist in discrete energy levels. Think of a stair case, which are discrete levels that you put your feet on, from one to the next to go up or down. Electrons at the bottom of the staircase are in their ground state, and it takes a specific amount of energy to go up a step; no more, no less. When they receive that specific amount of energy, they go up to the next step, called the excited state. Because light (electromagnetic radiation) is a form of energy, it can be absorbed by the molecule to increase or step up the energy level of the associated electron. This is known as excitation.
Upon falling down to the original step, the electron loses some of that energy, which is again emitted as electromagnetic radiation. This time, that electromagnetic radiation has less energy, and thus, a larger wavelength, a process called emission. In the Gin and Tonic example, UV light is absorbed by the quinine and Blue light is emitted as a result. UV light has a greater associated energy and a smaller wavelength. Blue light has a larger wavelength, and a smaller level of energy.
The light emitted is at a lower energy because the electron loses energy in the first excited state in the form of heat. Heat is just another form of energy. When this happens to electrons, it is called non-radiative relaxation. So upon emission, some of the original energy is release as an electromagnetic wave and some gets lost to heat, non-radiative form of energy.
Electrons that have a lot of double bonds are very good at fluorescing because of that abundance of electrons. Rhodamine 6G is a chemical that is used as a fluorescent dye. It is very fluorescent because it has a ton of double bonds.
There are a lot of sites for absorption and emission. The two different modes of excitation are: pi to pi* (pronounced “pie star”) excitation – where an electron in a double bond gets excited and nonbonding electron to pi* – which is just a lone pair electron committing to the process.
- McQuarrie, D. A., & Simon, J. D. (1997). Physical chemistry: A molecular approach. Sausalito (CA): University Science Books
- Physical and Theoretical Chemistry: https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Electronic_Spectroscopy/Radiative_Decay/Fluorescence