Glowing Candy - Phosphorescent Organic Materials
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The following article was originally published in the journal for educators Chemia w Szkole (eng. Chemistry in School) (6/2019):
Light-related phenomena, or more precisely any effect that makes matter visibly glow, hold a special appeal for both students and teachers. Witnessing them firsthand opens a window onto the molecular machinery that governs our world. A vivid demonstration captures the imagination far more quickly than a purely theoretical discussion, and the spectacle itself ignites curiosity and the urge to understand, which is the very essence of science. In this article, I explore one such captivating phenomenon.
Phosphorescence is a branch of luminescence, a family of processes that generate visible electromagnetic radiation. In a phosphorescent material, exposure to photons of an appropriate wavelength (and energy) stores energy that later re-emerges as light. The resulting afterglow can last from a few seconds to many hours. This unusually long lifetime sets phosphorescence apart from fluorescence, whose emission fades almost instantaneously (about 10-8 s) once the excitation source is removed.
Numerous substances exhibit phosphorescence, including zinc and alkaline-earth sulfides doped with heavy-metal salts, strontium aluminate SrAl2O4, and aluminum and boron nitrides [1]. Preparing these inorganic phosphors is often labor-intensive, and they are not always easy to obtain. Fortunately, a straightforward method for making phosphorescent material is simple enough for the home or classroom laboratory.
Experiment
Warning: The experiments described here do not involve highly toxic reagents, but standard precautions are essential. Large doses of ethacridine lactate or fluorescein can be harmful. Both compounds are potent dyes and readily stain skin or clothing. Heating molten sugar is particularly hazardous, because burns from this viscous liquid are difficult to treat. Always wear appropriate personal protective equipment!
Only readily available chemicals are required:
- sugar (a glucose–fructose mixture)
- ethacridine lactate C18H21N3O4 [2]
A clear hard candy works best; aside from trace flavorings and colorants, its main constituents are the carbohydrates listed above. The corn-syrup routinely added to candy suppresses crystallization, which would otherwise interfere with the demonstration.
Ethacridine lactate is an over-the-counter antiseptic sold under the trade name Rivanol. The structural formula of this highly efficient fluorescent dye appears in Fig. 1 [3].
Crush one candy (Photo 1). Its color is unimportant, although clear or lightly tinted pieces are more convenient.
Blend the sugar thoroughly with a small pinch (just a few milligrams) of ethacridine lactate, a yellow powder (Photo 2).
Gently heat the mixture until it liquefies, using an alcohol/gas burner, or electric hot plate. Keep the temperature moderate and stir constantly to prevent excessive caramelization. As the mixture warms, it becomes progressively less viscous. After one or two minutes of stirring, pour the melt onto a cold surface, ideally lined with household aluminum foil.
Once cooled, the material forms a glassy solid (Photo 3) tinted green by Rivanol. Needless to say, this “candy” is no longer edible.
Under ordinary light the material looks unremarkable. Briefly irradiate it with a light bulb, then switch the light off: an unexpectedly bright green afterglow lingers for several to more than ten seconds (Photo 4).
For stronger excitation, use higher-energy light. Monochromatic red light has no effect, whereas violet–blue light (λ = 405 nm) from a semiconductor laser produces a much brighter afterglow and prolongs its duration (Photo 5).
The sample can be recharged repeatedly; each exposure is followed by phosphorescence. Because sugar is hygroscopic, store it in a dry place.
You can also prepare a second organic phosphor. Gather:
- poly(vinyl alcohol) [CH2CH(OH)]n
- fluorescein C20H12O5
Poly(vinyl alcohol) (PVA) is a synthetic polymer made of vinyl-alcohol units C2H4O. Curiously, it is usually produced by hydrolyzing poly(vinyl acetate) (C4H6O2)n. PVA is widely used in the chemical and pharmaceutical industries. The sample used here (average molar mass 80 000 g/mol ≈ 1.76 × 102 lb/mol-1) appears as fine white granules (Photo 6).
Fluorescein is a xanthene derivative C13H10O (Fig. 2). In alkaline solution it glows bright green-yellow. The dye is remarkably efficient: under ultraviolet light, fluorescence remains visible even after a dilution of one part in tens of millions.
Fluorescein is an orange-red crystalline solid (Photo 7); its disodium salt is commercially known as uranine.
Dissolve PVA in a few to several dozen cm3 (≈ 0.5–2 fl oz) of distilled water. Because PVA dissolves slowly, heat the mixture and stir vigorously. Add enough polymer to obtain a viscous solution, then stir in a pinch of fluorescein. Remove any insoluble residue by vacuum filtration. The solution appears orange in visible light but fluoresces brilliant green under UV (Photo 8).
Pour a thin layer, just a few cm3 (≈ 0.2 fl oz), into a broad, shallow glass dish or onto a glass plate (Photo 9).
Allow the layer to dry at room temperature (≈ 68 °F) or slightly warmer. When the water has evaporated, a thin yellowish film remains, composed of PVA doped with fluorescein (Photo 10).
The film fluoresces under UV and, like the sugar doped with Rivanol, phosphoresces after excitation (Photo 11).
An engaging effect can also be achieved by soaking paper or filter paper in the PVA solution and letting it dry. In darkness, a blue or violet laser pointer allows you to “draw” on the paper; the beam leaves luminous tracks that fade only after some time.
Explanation
How can these observations be explained? Most luminescent phenomena, that is, those that absorb radiant energy and then re-emit it, are extremely brief, lasting up to about 10 nanoseconds. In fluorescence, an electron relaxes directly from the excited state to the ground state within that fleeting interval.
In phosphorescence, the molecule is likewise excited to a singlet state, but within picoseconds it undergoes vibrational-rotational (VR) relaxation, dissipating some energy as heat. The molecule then drops to a lower singlet excited state. In phosphorescent materials, intersystem crossing (ISC) frequently occurs, moving the molecule to a triplet state. Quantum selection rules make transitions between states of different multiplicity unlikely, so the return to the ground state is hindered and can be delayed long after the excitation light is gone. Eventually, after further VR relaxation, the molecule returns to the electronic ground state by emitting a photon, an event we observe as the afterglow. The remaining energy exits the system as visible light.
Because of the energy lost during VR relaxation, the wavelength of phosphorescent emission is always longer than that of the excitation light and often longer than that of the corresponding fluorescence; this difference is known as the Stokes shift [4].
Both the intensity and duration of phosphorescence depend strongly on temperature. At higher temperatures, thermal vibrations enhance non-radiative relaxation, reducing, and in some cases even quenching, the glow.
Many substances exhibit phosphorescence. The first synthetic examples were the so-called Bologna stones, one of seventeenth-century alchemy’s more intriguing achievements. Chemically, they were metal sulfides, chiefly of barium (Ba), calcium (Ca), and zinc (Zn), whose crystal lattices had been doped with trace amounts of other metal ions such as copper (Cu) or silver (Ag). I encourage readers to experiment both with the organic phosphors described here and with the inorganic compounds analogous to Bologna stones discussed in an earlier issue of “Chemia w Szkole” [5].
References:
- [1] Katsumata T., Sasajima K., Nabae T., Komuro S., Morikawa T., Characteristics of Strontium Aluminate Crystals Used for Long-Duration Phosphors, Journal of the American Ceramic Society, 2005, 81 (2), pp. 413–416 back
- [2] Pluciński T., Doświadczenia chemiczne, Wydawnictwo Adamantan, Warszawa, 1997 back
- [3] Ples M., Więcej światła! O fluorescencji rywanolu (eng. More Light! On the Fluorescence of ethacridine lactate), Chemia w Szkole (eng. Chemistry in school), 6 (2015), Agencja AS Józef Szewczyk, pp. 16-18 back
- [4] Jabłoński A., Efficiency of Anti-Stokes Fluorescence in Dyes, Nature, vol. 131, 1933 back
- [5] Ples M., Jak uwięzić światło? O skutkach domieszkowania siarczku cynku (ang. Trapping Light: Exploring the Effects of Zinc Sulfide Doping), Chemia w Szkole (eng. Chemistry in school), Agencja AS Józef Szewczyk, pp. 12-18 back
All photographs and illustrations were created by the author.
Addendum
The phosphorescence effect can be seen in the video below:
Marek Ples