Fluorescence ✖ Ralph Williams

Background reading music curated for this article is available at the bottom. Click to enjoy

01 Fluorescence in Real Life

Many by-sea tourist attractions advertise their seas to be fluorescent. Indeed, at night, the wavefronts show breath-taking glows of blue fluorescence. 

glowing beach lines

This phenomenon is named biofluorescence, driving me to discover the mechanism of glowing particles. In fluorescence, there is absorption of highly energetic light waves by the material under consideration, such as ultraviolet light. This results in excitation of the electrons to an energy level higher than their ground state. The process takes place within a few nanoseconds after which the electrons dissipate some of the energy in the form of molecular vibration and then revert back to the ground state giving out light energy. 

light emission

Since there is some loss of energy in the process as heat, the wavelength of the light emitted is longer than that of the light absorbed, a process called the Stokes shift. Fluorescence is mainly divided into two categories, organic fluorescence and inorganic fluorescence.

02 Organic Fluorescence

Organic fluorescence results from the presence of molecules that contain extended π-electron conjugation system such as fluorescein, rhodamine, and quinine. Upon interaction with ultraviolet or visible light, the electrons in these molecules become excited, going from the ground electronic state to an excited singlet state. The excited electrons quickly dissipate their energy through non-radiative de-excitation processes mainly through vibrations and collision of the molecules in their environment. Thereafter, they relax back to the ground electronic state and emit their energy in the form of a photon of visible light. Due to the fact that the electrons have already dissipated some energy, the emitted photon will have lower energy and a longer wavelength compared to the incident light, and this difference in energy is referred to as the Stokes shift. 

The whole process takes only a few nanoseconds, hence appears as almost instantaneous appearance and disappearance as soon as the source of excitation stops. The effectiveness of organic fluorescence largely depends on the molecular rigidity and π-electron conjugation, which limit the loss of energy through molecular movement. The color and intensity of the fluorescence can be controlled through modification of the molecular structure by chemists.

03 Inorganic Fluorescence

The source of inorganic fluorescence is a crystalline material called phosphor, consisting of the host crystal doped with ions capable of emitting luminescence, such as europium, terbium or cerium ions. 

phosphor glowing green light

When phosphor is excited with UV or blue light, its luminescent dopant ions absorb energy, exciting the electrons to a higher energy state. The stiff crystal lattice prevents excited electrons from releasing their extra energy through vibration; thus, all the energy acquired in the excitation process is transformed into visible light emission. Emission of photons in a process of electrons' return to their normal state creates bright colours. Colour emission depends on electronic transitions within the dopant ions and the structure of the crystal. In contrast to organic fluorophores, inorganic phosphors are highly stable to temperature, chemicals and photodecomposition. They preserve their fluorescent properties when excited with strong light for a long time, which makes them suitable for use in white LEDs, fluorescent lamps, displays backlights, X-ray scintillators and security marks. 

fluorescent lamps of all scales

Using different types of host crystals and dopant ions, scientists are able to obtain phosphors that emit nearly all kinds of visible colours with high luminosity and lifetime.

In conclusion, fluorescence transforms invisible energy into visible light through precise electronic transitions. Whether in organic molecules or inorganic phosphors, this phenomenon underlies both nature's glowing coastlines and countless modern technologies. By understanding its mechanism, we uncover how chemistry connects breathtaking natural beauty with practical innovations that shape everyday life.

"The Lark Ascending" by Ralph Vaughan Williams fits well with the concept of fluorescence.

The Lark Ascending – Ralph Vaughan Williams

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It perfectly depicts the quality of fluorescence because of its brilliance that is fleeting and radiant. Melodies played with the violin ascend easily amidst the calm setting of an orchestra and reflect the excitation of electrons to higher energy levels where they emit energy in the form of light.