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What is Fluorescence & How to observe it
What is Fluorescence & How to observe it
Fluorescence is a commonly encountered yet often misunderstood term referring to specific light-emitting processes. Scientifically, emission of light qualifies as fluorescence only when two essential conditions are met: first, the energy required to excite the material is provided by light; and second, the emitted radiation originates from an excited singlet state. The term "fluorescence" originates historically from the mineral Fluorite, where this phenomenon was first observed and described. Interestingly, the luminescence seen in fluorite is now correctly identified as phosphorescence rather than fluorescence.
If you are closer interested in the photoluminescence of minerals you can find more information on this topic on the dedicated page:
To observe the fluorescence of a substance, it is essential to use an appropriate excitation source. UV lamps are particularly suitable because many substances exhibit absorption bands within the UV-A range (315–380 nm) and UV-B range (200–315 nm). UV LED lamps are now easily available through online shops. When purchasing a UV light one should look for 365 nm LEDs for UV-A and 254 nm lamps for UV-B excitation. The commonly found 405 nm UV-LEDs are not recomandable since they produce to much light in the blue region of the visible spectra weakening the optically visible fluorescence effect. Since UV light below ~380 nm is invisible to the human eye, it conveniently does not interfere with the fluorescence emission, which generally occurs in the visible spectrum.
Photophysical Explanation of Fluorescence
Photophysical Explanation of Fluorescence
The fluorescence process is best understood by examining the Jablonski diagram, a graphical representation of electronic states in molecules and their transitions. Initially, absorption of photons from an external light source (often UV) promotes electrons from the ground electronic state (S0) to higher-energy excited singlet states (S1, S2, etc.). These electronic transitions are rapid, occurring within femtoseconds to picoseconds.
The fluorescence process is best understood by examining the Jablonski diagram, a graphical representation of electronic states in molecules and their transitions. Initially, absorption of photons from an external light source (often UV) promotes electrons from the ground electronic state (S0) to higher-energy excited singlet states (S1, S2, etc.). These electronic transitions are rapid, occurring within femtoseconds to picoseconds.
Following excitation, the molecule undergoes internal conversion, a non-radiative relaxation process, to the lowest vibrational level of the first excited singlet state (S1). From this state, electrons can return to various vibrational levels of the ground state (S0), emitting photons in the process. This emission is called fluorescence and typically occurs within nanoseconds. Due to the time scale of the involved processes fluorescence typically arises from the first excited singlet state (S1). This observation is known as Kasha’s rule.
The emitted fluorescence photons have lower energy (and hence longer wavelengths) compared to the absorbed photons due to energy losses during internal conversion and vibrational relaxation. This shift to lower energy is known as the Stokes shift and is characteristic of fluorescence processes. The rapid and efficient emission of photons makes fluorescence a widely utilized phenomenon across various scientific and technological applications, including bioimaging, diagnostics, and material sciences.
What Makes an Organic Molecule Fluorescent?
What Makes an Organic Molecule Fluorescent?
Organic molecules become fluorescent primarily due to their electronic structure, specifically the presence of conjugated π-systems and charge-transfer states. Conjugated π-systems, characterized by alternating single and double bonds, enable electrons to delocalize over extended regions of the molecule. When such molecules absorb photons, electrons transition to higher-energy excited states. Upon returning to the ground state, these electrons release energy as fluorescence. The extent of π-conjugation directly influences the energy of emitted photons, with longer conjugation typically resulting in fluorescence emission at longer wavelengths.
Among many other mechanism, fluorescence can also be significantly enhanced in molecules featuring charge-transfer states, particularly in donor-acceptor dyes. These molecules consist of electron-rich donor groups and electron-deficient acceptor groups. Upon excitation, intramolecular charge transfer from donor to acceptor states occurs, stabilizing the excited state and often leading to highly efficient fluorescence emission with tunable colors. This phenomenon makes donor-acceptor dyes particularly valuable in optoelectronics, sensing, and bioimaging applications.
Applications of Fluorescence
Applications of Fluorescence
Today, fluorescence is an everyday phenomenon, widely encountered in numerous applications. Optical brighteners, signal colors, and various plastic consumer goods frequently incorporate fluorescent dyes. Fluorescence has gained particular importance in biochemistry and medical diagnostics, where large quantities of fluorescent dyes are employed to label otherwise colorless biomolecules for easier detection and analysis.
The literature on fluorescent materials and their applications is extensive and continues to expand rapidly. The production and processing of fluorescent dyes have grown into a significant industry, and these materials have become ubiquitous in everyday life. For instance, UV lamps can vividly demonstrate the prevalence of optical brighteners by revealing fluorescent contamination in household dust or biofilms on bathroom surfaces. Fluorescent products available commercially include glow-in-the-dark pigments, body paints, various fluorescent plastic items, adhesive tapes, and more, making fluorescence readily accessible and practically experienced by everyone.
Our Contributions to the Field of Fluorescent Molecules & Materials
Our Contributions to the Field of Fluorescent Molecules & Materials
Our research has significantly contributed to advancing fluorescent materials through the development of new, highly efficient, and tunable fluorophores, particularly focusing on the classes of 4-Hydroxy/4-Alkoxy-thiazoles and Azaacenes. These molecules are distinguished by their unique structural properties that allow precise tuning of their photophysical characteristics, enabling high quantum yields, tunable emission spectra, and improved photostability.
Inspired by Fireflies, 4-Hydroxy/4-Alkoxy-thiazoles are valued for their compact molecular structure, leading to intense luminescence and excellent stability, making them ideal candidates for efficient optoelectronic applications. Azaacenes, characterized by their nitrogen-incorporated polycyclic frameworks, provide an exceptional degree of tunability and enhanced environmental stability, making them promising for use in organic light-emitting diodes (OLEDs).
Our recent research emphasizes enhancing photoluminescence efficiency and material stability through molecular engineering of extremely small fluorophores, as well as developing bio-inspired, biodegradable luminescent materials (BiOLEMs). These innovative materials hold potential for sustainable and environmentally friendly optoelectronic technologies.
More detailed information on 4-Hydroxy/4-Alkoxy-thiazoles and Azaacenes can be found via the links provided below.
Fluorescent Natural Products & Fluorescent Dyes in Nature
Fluorescent Natural Products & Fluorescent Dyes in Nature
Dyes are widespread in nature, yet true fluorescent dyes remain relatively rare. This scarcity is likely linked to the limited natural occurrence of UV light, which primarily stems from sunlight. In bright daylight, fluorescence is barely visible to the human eye and therefore rarely holds ecological relevance—at least at first glance.
Nonetheless, nature does produce a fascinating spectrum of fluorescent compounds. A striking example is aesculin, a 6,7-dihydroxycoumarin glycoside found in horse chestnut (Aesculus hippocastanum). When a freshly cut twig is placed in water and irradiated with 366 nm UV light, a vivid blue fluorescence becomes visible as the dye diffuses into the water. Related coumarin derivatives, such as fraxin in ash trees and daphnin in daphne species, show similar behavior. These water-soluble dyes can often be extracted with simple means and exhibit characteristic blue fluorescence.
The early discovery of coumarin fluorescence in 1929 inspired the development of optical brighteners for textiles—a compelling example of how natural observations inform technological applications.
Another class of naturally occurring fluorescent compounds are the protoberberine alkaloids, with berberine being the most well-known. Found in Berberis, Mahonia, and the common celandine (Chelidonium majus), berberine exhibits bright yellow fluorescence under UV light. Beyond its optical properties, it has long been used in traditional medicine and continues to be of pharmacological interest due to its antimicrobial and antispasmodic effects.
Historical records also point to lignum nephriticum, a wood used since medieval times to treat kidney disorders. Immersing shavings of this wood in water produces an unusually strong blue fluorescence—even observable in daylight. Though the botanical source was long debated, it is now understood that contact with air and water triggers a multistep reaction cascade forming the actual fluorescent compound matlaline. This delayed formation is a rare case of fluorescence generated in situ.
Even modern studies continue to uncover new examples. In 2008, the group of Kräutler reported in Angewandte Chemie a surprising observation: ripe banana peels exhibit distinct blue fluorescence under UV light, particularly around small brown spots during ripening. This fluorescence likely arises from breakdown products of chlorophyll and is visible only at certain maturation stages. Similar chlorophyll-derived porphyrin breakdown products, such as ooporphyrins, are found in brown eggshells. Their red fluorescence becomes evident only after acid treatment, which releases the dye from the calcified shell matrix.
Observing the natural fluorescence of chlorophyll in living organisms is often challenging. In intact photosynthetic systems, most of the absorbed excitation energy is rapidly funneled into the photosystems for photochemical reactions, leaving little for radiative emission. However, one clever workaround involves the common yellow lichen Xanthoria parietina. When kept moist and under bright—but not direct—light, this lichen begins to proliferate its symbiotic algae and bring them closer to the surface. Under UV illumination (366 nm), this setup reveals a faint yet distinct red fluorescence characteristic of chlorophyll.
Remarkably, Xanthoria parietina also produces its own fluorescent dye: parietin, a rare naturally occurring anthraquinone derivative. Parietin can be extracted with solvents such as ethanol or even sublimed directly from the lichen under mild heating. In solution, parietin appears yellow and exhibits a yellow fluorescence. This behavior is pH-sensitive—addition of a base like aqueous KOH shifts the solution color to red and quenches the fluorescence. Upon acidification (e.g., with HCl), both the original yellow color and fluorescence reappear, reflecting a reversible change in the molecular structure and its photophysical properties.
Fungi, too, exhibit remarkable fluorescent diversity. Among the best studied are species of the Russula genus, which produce fluorescent pigments such as russupteridines and russulumazines. Multiple emission colors can often be found within a single fruiting body—bright blue beneath the cap, greenish lamellae, and sometimes red fluorescence in the stem. These pigments remain stable upon drying, offering excellent material for scientific collections and fluorescence surveys.
The genus Cortinarius includes some of the most colorful fungi in nature, with many species containing enough dye to color textiles. Cortinarius venetus, for example, produces dermoxanthone, a compound responsible for its characteristic yellow-green fluorescence—visible both in the mushroom and in dyed fabrics under UV light. When extracted with ethanol, the dye fluoresces yellow-green. Upon adding a base, the fluorescence shifts to blue-green, reflecting pH-dependent structural changes.
Another well-known natural fluorophore is riboflavin (vitamin B2), a yellow-green fluorescent compound found in milk, eggs, vegetables, and fungi. It is widely used as a food colorant (E101) and produced at scale via microbial fermentation. It can even be wound in fermented cucumbers as seen in image on the left. Riboflavin’s role in energy metabolism and its photophysical properties make it a molecule of dual biological and technological significance.
In contrast, the fluorescence of insects remains a largely unexplored field. Historical limitations in UV light sources have contributed to this knowledge gap. Yet a large-scale museum survey of over 10,000 butterflies and moths revealed fluorescence in nearly one third of all specimens—an open invitation for further discovery.
In a collaborative study published 2019 in Scientific Reports, we contributed to the discovery of intense bone fluorescence in two species of pumpkin toadlets (Brachycephalus ephippium and B. pitanga). This fluorescence originates from dermal bones located in the head and back, which are visible through the toadlets' exceptionally thin skin. Our research demonstrated that these bones exhibit significantly higher fluorescence intensity compared to closely related species, such as Ischnocnema parva. This finding suggests a unique adaptation in these toadlets, potentially linked to their visual communication or aposematic signaling.
For a detailed exploration of our findings, please refer to the full study:
S. Goutte, M. J. Mason, M. M. Antoniazzi, C. Jared, D. Merle, L. Cazes, L. F. Toledo, H. el-Hafci, S. Pallu, H. Portier, S. Schramm, P. Gueriau, M. Thoury, Scientific Reports 2019, 9, 5388. Link
Natural fluorescence may be rare, but where it does occur, it reveals nature's capacity to produce structurally complex, photophysically active molecules with a spectrum of ecological, aesthetic, and functional significance. Continuing to explore these phenomena promises not only scientific insights but also novel molecular tools for a range of disciplines.
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