Fluorescence

• 1. The Word That Was Coined

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  In his 1852 paper on the "Refrangibility" of light, George Gabriel Stokes wrote a sentence that named a phenomenon humans had been staring at for centuries. "I am almost inclined to coin a word, and call the appearance fluorescence, from fluor-spar," he said, borrowing from the mineral fluorite the way opalescence borrows from opal. He was describing how fluorspar, uranium glass, and many other substances could take invisible light from beyond the violet end of the spectrum and turn it into light the eye can see. A substance soaks up one kind of radiation and gives back another, usually at a longer wavelength and lower energy. Expose it to ultraviolet light and it can glow with colored visible light. The color depends on the chemical makeup of the substance itself. But why does a rock seem to hold light it was never given? Why do some glowing things go dark the instant the lamp clicks off, while others keep shining for hours? And why has this single property turned up in laundry detergent, in deep-sea fish, in road signs, and in the lasers that read our data? The answers begin with the Aztecs and a piece of wood, and they run all the way to a jellyfish off the coast of North America.

• 2. A Long Misunderstanding

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  The Aztecs knew an infusion called lignum nephriticum, Latin for "kidney wood," and it was described in 1560 by Bernardino de Sahagun and in 1565 by Nicolas Monardes. The wood came from two tree species, Pterocarpus indicus and Eysenhardtia polystachya, and the compound responsible for its glow is matlaline, an oxidation product of one of the wood's flavonoids. For centuries afterward, observers saw the effect but reached for the wrong explanation. In 1819 E.D. Clarke and in 1822 Rene Just Hauy described fluorites that looked one color by reflected light and another by transmitted light. Hauy wrongly called it light scattering, like opalescence. In 1833 Sir David Brewster saw a similar effect in chlorophyll and also thought it a form of opalescence. Sir John Herschel studied quinine in 1845 and reached yet another wrong conclusion. The first person to grasp a central fact was A.E. Becquerel, who in 1842 watched calcium sulfide emit light after exposure to solar ultraviolet and stated that the emitted light has a longer wavelength than the incident light. Ironically, the effect Becquerel described is now called phosphorescence, not fluorescence at all.

• 3. Glow That Stays, Glow That Goes

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  Fluorescent materials generally stop glowing almost the instant the radiation source is switched off, and this single behavior separates them from their cousin, phosphorescence, which keeps emitting light for some time afterward. The difference comes down to quantum spin. When a molecule absorbs a photon and returns to a lower energy state with the same electronic spin multiplicity, the process is fluorescence. When the initial and final states have different spin, it is phosphorescence. A photoexcited molecule can cross over from an excited singlet state to a triplet state through intersystem crossing, and decay from that triplet back to the ground state is slower and dimmer. Advances in spectroscopy and quantum electronics between the 1950s and 1970s let scientists finally separate these mechanisms by their timescales. Lasers needed the fastest decay, in the nanosecond range, and that singlet emission was called fluorescence, common in laser mediums such as ruby. Triplet phosphorescence decays over a few microseconds to a second, still fast enough that people loosely call materials like fluorspar fluorescent. Slowest of all is persistent phosphorescence, the glow-in-the-dark range that runs from one second to many hours.

• 4. Inside The Molecule

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  When a molecule sits in its ground state, labeled S0, a photon can lift it into one of several excited electronic states, S1, S2, S3 and beyond, each with its own vibrational levels populated according to the Franck-Condon principle. States above the first excited level relax quickly, transferring energy to surrounding solvent molecules as heat through internal conversion and vibrational relaxation. Because that energy is shed before light comes back out, the emitted fluorescence carries less energy than the photon that started the process. This is why emitted light usually has a longer wavelength than absorbed light, a gap known as the Stokes shift. The excited state also faces competition. Non-radiative escape routes such as internal conversion, intersystem crossing, and energy transfer to another molecule all drain efficiency away from light emission. Molecular oxygen is an unusually efficient quencher of fluorescence because of its triplet ground state. The fluorescence quantum yield measures the success of the process, the ratio of photons emitted to photons absorbed, with a maximum of 1.0. Compounds with a yield of just 0.10 are still considered quite fluorescent. For common fluorescent compounds, the excited state lasts somewhere between 0.5 and 20 nanoseconds before a photon escapes.

• 5. Life That Lights Up

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  Some deep-sea animals, such as the greeneye, carry fluorescent structures, and the phenomenon turns out to be widespread across all kingdoms of life. The incidence of fluorescence has been studied most extensively in cnidarians and fish, and it appears to have evolved many separate times, in eels, in gobies and cardinalfishes, in triggerfishes, and beyond. Underwater, the physics shapes the function. Water absorbs long wavelengths, so warm colors fade with depth while cooler colors dominate the photic zone. Light intensity falls tenfold for every 75 meters, so at 75 meters only ten percent of surface light remains, and at 150 meters just one percent. Against that blue field, fluorescent reds, oranges, yellows, and greens stand out, with green the most common marine color and red the rarest. Many fluorescent fish, including sharks, lizardfish, scorpionfish, wrasses, and flatfishes, carry yellow intraocular filters that act as long-pass filters, letting them see signals invisible to predators. The fairy wrasse uses red fluorescence for private communication that other reef fish cannot detect. The polka-dot tree frog, Hypsiboas punctatus of South America, was the first fluorescent amphibian discovered, in 2017, its blue-green glow traced to a compound called Hyloin-L1.

• 6. Unexpected Glowers

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  A flying squirrel fluoresced pink under ultraviolet light, noticed by chance in 2019, and biologists at Northland College in Northern Wisconsin then confirmed it for all three species of North American flying squirrels, while non-flying squirrels showed nothing. The scientists suggested this fluorescence may be an accident of evolution serving no biological purpose at all. Fluorescence was also reported for several platypus specimens in 2020. Among invertebrates, scorpions glow because of beta-carboline in their cuticles, and spiders carry a huge diversity of fluorophores, the only known group in which fluorescence is, as one set of researchers put it, "taxonomically widespread, variably expressed, evolutionarily labile." Swallowtail butterflies of the genus Papilio grow pigment-infused crystals in their wings that produce directed fluorescent light, working best when they absorb sky-blue light around 420 nanometers. Budgerigars showed something stranger still. In mate-choice experiments, both males and females preferred birds bearing a fluorescent stimulus, suggesting the glowing plumage of parrots is an honest signal of quality rather than a byproduct of pigment. The most scientifically significant glower of all is the jellyfish Aequorea victoria, identified by Osamu Shimomura as the carrier of green fluorescent protein, whose gene is now widely used to mark the expression of other genes.

• 7. Glowing Stones And Oil

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  Divalent manganese, present at concentrations of up to several percent, drives the red or orange fluorescence of calcite, the green of willemite, the yellow of esperite, and the orange of wollastonite and clinohedrite. The chemistry of mineral fluorescence is a catalog of activators, each tuned to a color. Hexavalent uranium as the uranyl cation fluoresces yellow-green at any concentration, lighting up autunite and andersonite. Trivalent chromium at low concentration produces the red glow of ruby, while divalent europium gives fluorite its blue. Rubies, emeralds, and diamonds glow red under long-wave ultraviolet, and diamonds even emit light under X-rays. The mineral must stay free of impurities such as iron or copper, which quench the emission. The effect reaches into industry as well. Crude oil fluoresces across a range of colors, from dull-brown for heavy oils and tars to bright-yellowish and bluish-white for the very lightest oils and condensates. Oil exploration drilling exploits this to spot tiny amounts of oil in drill cuttings and core samples. Even groundwater tells on itself, since humic and fulvic acids from decaying organic matter fluoresce thanks to the aromatic rings in their tangled molecular structures.

• 8. Light Put To Work

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  Fluorescent lights were first available to the public at the 1939 New York World's Fair, and their working principle is a small drama inside a glass tube. A partial vacuum holds a little mercury, an electric discharge makes the mercury atoms emit mostly ultraviolet light, and a phosphor coating absorbs that ultraviolet and re-emits visible light. The mercury vapor's spectrum is dominated by a short-wave ultraviolet line at 254 nanometers, with visible lines at 436, 546, and 579 nanometers. White light-emitting diodes arrived in the mid-1990s, using blue light from a semiconductor to strike phosphors that fill in the green-to-red range. The same trick of catching faint signals powers the laboratory. A fluorometer can measure fluorescent molecule concentrations as low as one part per trillion, and lasers use the fluorescence of materials such as ruby and titanium sapphire as their active media. In the life sciences, a protein can be labeled with an extrinsic fluorophore and tracked without harm, because few cellular components glow on their own. Ethidium bromide barely fluoresces in water but lights up sharply once it slips between the base pairs of DNA, revealing where fragments sit in a gel. The same property that puzzled Hauy and Brewster now whitens paper through optical brighteners, marks high-risk plaque inside coronary arteries, and makes fluorescent orange road signs visible from far down the highway.