A Brief History of Fluorescence and Phosphorescence before the Emergence of Quantum Theory

Abstract

Fluorescence and phosphorescence are two forms of photoluminescence used in modern research and in practical applications. The early observations of these phenomena, before the emergence of quantum theory, highlight the investigation into the mechanism of light emission. In contrast to incandescence, photoluminescence does not require high temperatures and does not usually produce noticeable heat. Such a “cold light” was the object of an interesting controversy in the 19th century: does it fit into thermodynamics? The early applications, such as the fluorescent tube, fluorescence analysis, and fluorescent tracers, are described.Keywords: General Public; Upper-Division Undergraduate; Analytical Chemistry; Physical Chemistry; History/Philosophy; Textbooks/Reference Books; Fluorescence Spectroscopy; Quantitative Analysis

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Published: March 18, 2011
Copyright r2011 American Chemical Society and
Division of Chemical Education, Inc. 731 dx.doi.org/10.1021/ed100182h |J. Chem. Educ. 2011, 88, 731–738
ARTICLE
pubs.acs.org/jchemeduc
A Brief History of Fluorescence and Phosphorescence before the
Emergence of Quantum Theory
Bernard Valeur*
,†
and Mario N. Berberan-Santos*
,‡
†
Conservatoire National des Arts et Metiers, 292 rue Saint-Martin, F-75003 Paris, and Institut d’Alembert, Laboratoire PPSM,
ENS-Cachan, 61 Avenue du President Wilson, F-94235 Cachan Cedex, France
‡
Centro de Química-Física Molecular and Institute of Nanoscience and Nanotechnology, Instituto Superior Tecnico, 1049-001 Lisboa,
Portugal
Photoluminescence, the emission of light arising from
excited electronic states following absorption of light, is
important in many scientific and technological fields, namely,
physics, chemistry, materials science, biology, and medicine.
1
Many important applications based on photoluminescence
have been developed, such as fluorescence microscopy,
fluorescent tubes and lamps, optical brighteners, plasma
screens, forensics, tracers in hydrogeology, fluorescent and
phosphorescent paints, phosphorescent labels, safety signs,
and counterfeit detection (security documents, bank notes,
art works).
2
The aim of this paper is to briefly describe some of the early
milestones in the study of photoluminescence. The explana-
tion of how light can be emitted by atomic or molecular
excited states following absorption of light can include a
historical introduction or be intermingled with historical
remarks. Generally speaking, luminescence can be distin-
guished from incandescence, which is light emitted by bodies
heated at high temperatures. The question as to whether
photoluminescence, often considered as cold light, fits into
thermodynamics was the object of a controversy in the 19th
century. This could lead to an interesting discussion in a
thermodynamics course. Information provided here might
also be useful for instrumental analysis and quantum me-
chanics courses.
’WHAT IS PHOTOLUMINESCENCE
The term luminescence comes from a Latin root (lumen =
light). It was first introduced as luminescenz by the German
physicist and science historian Eilhard Wiedemann in 1888
for all phenomena of light that are not solely conditioned
bytheriseintemperature,thatis,incandescence.Before
considering the historical evolution of the understanding
of luminescence, it should be noted that the present defini-
tion of luminescence is “a spontaneous emission of radia-
tion from an electronically excited species (or from a vibra-
tionally excited species) not in thermal equilibrium with its
environment”.
3
The various types of luminescence are classified according
to the mode of excitation. In particular, photoluminescence is
the emission of light arising “from direct photoexcitation of
the emitting species”.
3
Fluorescence, phosphorescence, and
delayed fluorescence are well-known forms of photolumines-
cence. There are other types of luminescence that differ by
the mode of excitation (chemiluminescence, bioluminescence,
electroluminescence, cathodoluminescence, radioluminescence,
sonoluminescence, thermoluminescence, triboluminescence).
ABSTRACT: Fluorescence and phosphorescence are two forms of photoluminescence used in
modern research and in practical applications. The early observations of these phenomena, before
the emergence of quantum theory, highlight the investigation into the mechanism of light
emission. In contrast to incandescence, photoluminescence does not require high temperatures
and does not usually produce noticeable heat. Such a “cold light”was the object of an interesting
controversy in the 19th century: does it fit into thermodynamics? The early applications, such as
the fluorescent tube, fluorescence analysis, and fluorescent tracers, are described.
KEYWORDS: General Public, Upper-Division Undergraduate, Analytical Chemistry,
Physical Chemistry, History/Philosophy,Textbooks/Reference Books, Fluorescence Spectroscopy,
Quantitative Analysis

732 dx.doi.org/10.1021/ed100182h |J. Chem. Educ. 2011, 88, 731–738
Journal of Chemical Education ARTICLE
For a long time after the introduction of the term fluorescence
by G. G. Stokes
4
in the middle of the 19th century, the distinction
between fluorescence and phosphorescence was based on the
duration of emission after the end of excitation: fluorescence was
considered as an emission of light that disappears simultaneously
with the end of excitation, whereas in phosphorescence, the
emitted light persists after the end of excitation. But such a
criterion is insufficient because there are long-lived fluores-
cences (e.g., divalent europium salts) and short-lived phos-
phorescences (e.g., violet luminescence of zinc sulfide) whose
durations are comparable (several hundreds of nanoseconds).
The usual condition for observing phosphorescence is that the
excited species passes through an intermediate state before
emission, as stated for the first time by Francis Perrin in 1929.
5
More precisely, in the frame of molecular photochemistry, we
now say that the spin multiplicity
a
is retained in the case of
fluorescence, whereas phosphorescence involves a change in
spin multiplicity, typically from triplet to singlet or vice versa
(Figure 1).
3
’EARLY OBSERVATIONS OF
PHOTOLUMINESCENCE
68
Phosphorus was the ancient Greek name given to planet Venus
when appearing as the morning star and thus announcing the
imminent sunrise. The term means the light bearer:φως = light;
φεFειν = to bear.
b
The term phosphor has been used since the
Middle Ages to designate materials that glow in the dark after
exposure to light. There are many ancient reports of glow-in-
the-dark minerals, and the most famous of them was the
Bolognian phosphor (impure barium sulfide) discovered by a
cobbler from Bologna in 1602, Vincenzo Cascariolo. Later,
thesamenamephosphor was assigned to the element phos-
phorus isolated by Brandt in 1677 because, when exposed to
air, it burns and releases glowing vapors. But emission of light
is in this case chemiluminescence, not photoluminescence; the
species that emit light are excited by the energy provided by
the combustion reaction and not by the absorption of a
photon.
In 1565, a Spanish physician and botanist, Nicolas Monardes
(Figure 2), reported the peculiar blue color (under certain
conditions of observation) from an infusion of a wood from
Mexico and used to treat kidney and urinary diseases
(Figure 3).
912
This wood (later called Lignum nephriticum),
whose peculiar color effect and diuretic properties were already
known to the Aztecs, was a scarce and expensive medicine.
Therefore, it was of interest to detect counterfeited wood.
Monardes wrote on this respect,
12
Make sure that the wood renders water bluish, otherwise it is a
falsification. Indeed, they now bring another kind of wood that
renders the water yellow, but it is not good, only the kind that
renders the water bluish is genuine. (in Spanish in the original).
This method for the detection of a counterfeited object can be
considered as the first application of the phenomenon that would
be later called fluorescence. Extracts of the wood were further
investigated by Boyle, Newton, and others,
6
but the phenomen-
on was not understood at the time.
The chemical species responsible for the intense blue fluor-
escence was recently identified in an infusion of L. nephriticum
(E. polystachya): it is called matlaline (from Matlali, the Aztec
word for blue) (Figure 4).
12c
This compound is not present in
the plant but results from an unusual spontaneous oxidation of at
least one of the tree’sflavonoids.
In 1819, a peculiar property of some crystals of fluorite
(calcium fluoride, then called fluated lime, spath fluor, or fluor
spar) from Weardale, Durham, England, was reported by
Edward D. Clarke, Professor of Mineralogy at the University of
Cambridge.
13
These crystals of the “Durham Fluor”surpassed in
magnificence and beauty any other mineral substance he had ever
seen. The finer crystals, perfectly transparent, had a dichroic
nature: the color by reflected light was a deep sapphire blue,
whereas the color by transmitted light was an intense emerald
green (Figure 5). Clarke offered no explanation for the observa-
tions reported.
In the second edition of his famous treatise on mineralogy,
published in 1822,
16
the French mineralogist Rene-Just Ha€uy
mentions the double color of some crystals of fluorite (reported
to be from Derbyshire County, England) The color by reflected
light was described as violet, whereas the color by transmitted
light was again green. Ha€uy explained the phenomenon as a type
of opalescence (which is observed with opal, a naturally occurring
hydrated silica glass, and results from light scattering): the two
colors were complementary, violet being the dominant hue of
the scattered light and green the dominant hue of transmitted
(i.e., unscattered) light. Although the explanation was incorrect
Figure 1. Simplified PerrinJablonski diagram showing the difference
between fluorescence and phosphorescence. Fluorescence occurs when
radiation is emitted from the first excited singlet state S
1
that is reached
by previous absorption of a photon.
a
Phosphorescence occurs when
radiation is emitted from the triplet state T
1
after intersystem crossing
from S
1
.
Figure 2. Portrait of Nicolas Monardes. From the front page of the
book Dos Libros, el Vno Qve Trata de Todas las Cosas que traen de nuestras
Indias Occidentales, que siruen al vso de la Medicina, y el otro qve trata de la
Piedra Bezaar, y de la Yerva Escuerc-onera, Seville, Spain, 1569.

733 dx.doi.org/10.1021/ed100182h |J. Chem. Educ. 2011, 88, 731–738
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(see the caption of Figure 5) and a correct one was still a long way
off, Hauy’s view and the two minerals mentioned, fluorite and
opal, were going to play a central role in the understanding and
naming of fluorescence.
In 1833, Sir David Brewster, the well-known Scottish physi-
cist, described the beautiful red fluorescence of chlorophyll in the
article “On the Colour of Natural Bodies”,
17
where it is reported
that a beam of sunlight passing through a green alcoholic extract
of leaves (mainly a chlorophyll solution) appears to be red when
observed from the side. He pointed out the similarity with the
blue light coming from a light beam when entering some fluorite
crystals. Again, similar to Ha€uy, Brewster interpreted these
phenomena as manifestations of opalescence (light scattering).
In 1845, the polymath Sir John Herschel, son of the famous
astronomer and the originator of the word photography (where
light is again involved), prepared an acid solution of quinine
sulfate and stated,
18
Though perfectly transparent and colorless when held between
the eye and the light, it yet exhibits in certain aspects,and under
certain incidences of the light, an extremely vivid and beautiful
celestial blue color.
As the color was always superficial, he believed it to be a hitherto
unidentified phenomenon, “a case of superficial colour presented
by a homogeneous liquid, internally colourless”.
18
Herschel
called this phenomenon epipolic dispersion, from the Greek:
επιπολη = surface. In fact, the solutions observed by Herschel
were very concentrated so that the majority of the incident light
was absorbed near the surface and all the blue fluorescence
originated from there. Herschel used a prism to show that the
epipolic dispersion could be observed only upon illumination by
the blue end of the spectrum and not the red end. The crude
spectral analysis of the emitted light with the prism revealed blue,
green, and a small quantity of yellow light, but Herschel did not
realize that the superficial light was of longer wavelength than the
incident light. Herschel also discussed the superficial blue color
of some green fluorite crystals, noting the similarity with the
observations made with quinine solutions, and considered that it
was also a case of epipolic dispersion. On the other hand, he
attributed the blue color of L. nephriticum extracts to ordinary
dispersion by tiny particles in suspension.
When the epipolic theory, which contradicted Brewster’s
earlier views, became known to Brewster, he carried out further
experiments showing that the phenomenon was not superficial,
as assumed by Herschel. On the basis of polarization measure-
ments, Brewster proposed instead
19
that “...unless this [...] is a
new property of light, produced by a peculiar action of certain
solid and fluid bodies...”all the media studied contain minute
crystals randomly oriented, with the consequence that unpolar-
ized light is dispersed in all directions. He coined the term
internal dispersion for this explanation. However, Brewster had
made the wrong choice.
’INVENTION OF THE TERM FLUORESCENCE
4,6,8
A major event in the history of photoluminescence was the
publication by Sir George Gabriel Stokes (Figure 6), physicist
and professor of mathematics at Cambridge, of his famous paper
entitled “On the Refrangibility of Light”in 1852.
4
In it, and in
detailed experimental studies on several samples, both organic
(including quinine) and inorganic (including a fluorite crystal
similar to that shown in Figure 4, reported to be from Alston
Moor, England
c
), he clearly identified a common phenomenon
he called dispersive reflection: the wavelengths of the dispersed
light are always longer than the wavelength of the original light.
One of Stokes’s experiments that is spectacular and remarkable
by its simplicity deserves attention. Stokes formed the solar
spectrum by means of a prism. When he moved a test tube
filled with a solution of quinine through the visible part
of the spectrum, nothing happened: the solution remained
transparent.
20
But beyond the violet portion of the spectrum,
that is, in the invisible zone corresponding to ultraviolet
radiation, the solution glowed with a blue light (Figure 7).
Stokes wrote,
4
It was certainly a curious sight to see the tube instantaneously
light up when plunged into the invisible rays: it was literally
Figure 3. Absorption and fluorescence colors of infusions of L. ne-
phriticum in daylight: (left) image taken from ref 9 and (right) mildly
alkaline aqueous solution to which chips of Eysenhardtia polystachia
(kindly provided by Dr. A. U. Acu~na) were added.
Figure 4. Structure of tetrahydromethanobenzofuro[2,3-d]oxacine,
which is responsible for the fluorescence of L. nephriticum.
Figure 5. Twinned crystals of green fluorite (from Rogerley, Weardale,
Durham County, England) illuminated with sunlight (left) and a UV
lamp (right). The double color is apparent. Pure fluorite is colorless and
nonfluorescent; however, natural fluorites usually contain many ele-
ments from the rare-earth family. The green color is due to Sm
2þ
absorption (in the blue and in the red),
14
whereas the deep blue color is
due to Eu
2þ
fluorescence. The states involved in the emission have 7
unpaired electrons; hence, their spin multiplicity
a
is 8.
15
Both elements
are present as substitutional impurities in the range 10100 ppm.

734 dx.doi.org/10.1021/ed100182h |J. Chem. Educ. 2011, 88, 731–738
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darkness visible. Altogether the phenomenon had something of
an unearthly appearance.
From his experiments with a wide range of substances, Stokes
concluded that the dispersed light was always of longer wave-
lengths than the incident light. Later this statement became the
Stokes law.
Stokes also noted that the dispersion of light took place in all
directions, hence, the fluid behaved as if it were self-luminous. In
his paper, Stokes called the observed phenomenon true internal
dispersion or dispersive reflection but in a footnote,
4
he wrote,
I confess I do not like this term. I am almost inclined to coin a
word, and call the appearance fluorescence, from fluorspar, as
the analogous term opalescence is derived from the name of a
mineral.
In his second paper,
21
Stokes definitely resolved to use the word
fluorescence.
It is often ignored that, 10 years before Stokes’sfirst paper, the
French physicist Edmond Becquerel (Figure 8) (discoverer of
the photovoltaic effect and father of Henri Becquerel, the
discoverer of radioactivity) published an original paper
22
in
which he described the emission of light by calcium sulfide
deposited on paper when exposed to solar light beyond the violet
part of the spectrum. Therefore, he was the first to state that the
emitted light is of longer wavelength than the incident light.
Stokes’s paper led Becquerel to a priority claim for this result.
23
The difference between the Stokes and Becquerel experiments is
that, according to the definitions given in the preceding section,
quinine is fluorescent whereas calcium sulfide is phosphorescent,
but both species are relevant to photoluminescence.
All modern accounts of Stokes’s contributions to the under-
standing of fluorescence fail to mention that he viewed it as an
instantaneous scattering process that ceases immediately after
the exciting light is cut off. In his own words,
4
...in the phenomenon of internal dispersion, the sensitive body, so
long as it is under the influence of the active light, behaves as if it
were self-luminous. Nothing then seems more natural to suppose
that the incident vibrations of the luminiferous ether produce
vibratory movements among the ultimate molecules of sensitive
substances, and that the molecules in turn, swinging on their own
account, produce vibrations in the luminiferous ether, and thus
cause the sensation of light.
The change of refrangibility (i.e., the wavelength or the Stokes
shift) could be explained in the same way: “The periodic times
of these vibrations depend upon the periods in which the
molecules are disposed to swing, not upon the periodic time of
the incident vibrations”.
4
Thus, internal dispersion would corre-
spond in this respect to what is now known as inelastic scattering,
for example, vibrational Raman scattering, and not to the
postquantum description of fluorescence as a two-step process
with a finite waiting time between absorption and emission.
Interestingly, such a connection can be found in the well-
known terminology of Raman lines as either Stokes or anti-
Stokes. Nevertheless, in vibrational Raman scattering, a charac-
teristic and fixed emission spectrum does not exist, and it is only
the shift in energy that is constant and specific of the molecular
vibrations.
Becquerel, on the other hand, considered that phosphores-
cence and Stokes’sfluorescence were one and the same emission
phenomenon, always with a finite duration that was simply
shorter in the case of fluorescence and longer in the case of
phosphorescence. He even advocated the term fluorescence to
be abandoned, which according to his view was a short-lived
phosphorescence (ref 24, p 321).
From our present vantage point, it can be seen that both
Stokes’s and Becquerel’s views of fluorescence and phosphores-
cence contain important elements of truth derived from carefully
designed experiments, but none is completely correct in its
theoretical aspects. As is often the case, theories are subject of
gradual growth and progressive improvement.
Figure 6. Portrait of Sir George Gabriel Stokes (permission obtained
from AIP Emilio Segre Visual Archives, E. Scott Barr Collection).
Figure 7. Principle of Stokes’s experiment showing that a solution of
quinine irradiated with UV emits blue light, whereas no effect is observed
when it is placed in the visible part of the solar spectrum.
20
.
Figure 8. Portrait of Edmond Becquerel (author’s collection).

735 dx.doi.org/10.1021/ed100182h |J. Chem. Educ. 2011, 88, 731–738
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’COLD LIGHT VERSUS INCANDESCENCE
25
In contrast to incandescence, fluorescence and phosphore-
scence do not require high temperatures and do not usually
produce noticeable heat. Such emissions were named cold light
for this reason.
In the late 19th century, the laws of radiation were thoroughly
studied. In particular, Kirchhoff’s law of radiation stated that the
ratio of the absorptive and emissive powers of any material was a
universal function of temperature and wavelength. Fluorescence
and phosphorescence were in apparent contradiction with this
law because they were highly dependent on the chemical nature
of the substance and did not show a strong temperature
dependence.
The mechanism of cold-light emission invoked by Wiede-
mann was based on the kinetic theory stating that matter is
composed of molecules in motion and postulating that each
molecule (or atom) is surrounded by an ether shell. At that time,
ether was assumed to be necessary for explaining the propagation
of light that should require, similar to sound, a supporting
medium. Collisions cause vibrations that are transmitted to the
ether shells, which produce light whose intensity depends on the
strength of vibrations. These considerations led Wiedemann to
reject the term cold light and to propose instead the term
luminescence to designate any emission of light more intense
than expected from the source’s temperature.
26
’DOES COLD LIGHT FIT INTO
THERMODYNAMICS?
25,27
In the late 19th century, the question arose whether cold light
violates the second law of thermodynamics, in that heat cannot
flow from a colder body to a warmer body. In 1889, Wiedemann
envisioned a case where the second law seems to be violated: a
luminescent material could transfer radiant energy to an object
having a higher temperature if this object absorbed the lumines-
cence. To rescue the second law, Wiedemann introduced the
concept of luminescence temperature, that is, the temperature
required for the incandescent emission from a body to match the
intensity of the body’s luminescence. But this concept was found
to be unnecessary because a fundamental distinction should be
made between energy transferred from a body with a well-
defined temperature (i.e., in internal thermal equilibrium) and
energy transferred from a body not in internal thermal equilib-
rium, in the same way as Kirchoff’s law applies to thermal
radiation but not to nonthermal radiation (cold light).
At the end of the 19th century, the Berlin physicist Wilhelm
Wien
25
considered that the Stokes law was simply a special case
of the second law. But several cases of violation of the Stokes law
were reported. The first of them is due to Eugen Lommel in
1871: upon excitation of a solution of a dye (naphthalene red)
with the yellow lines from a sodium flame, he was able to detect a
weak green fluorescence, that is, of shorter wavelength.
28
The
contamination of the light source was suspected by other
researchers. In 1886, after checking carefully that no extraneous
light contaminated his experiments, Franz Stenger studied not
only naphthalene red, but also fluorescein and eosin: he found
that all samples showed fluorescence at shorter wavelengths than
excitation.
29
Wien and also Karl von Wesendonck
30
considered
that in the cases where the Stokes law failed, there must be an
increased absorption of energy by the fluorescent species.
Additional evidence for the Stokes law violation was provided
in 1904 by Edward Nichols and Ernest Merritt, physicists at
Cornell University, who were able to record the fluorescence
spectra of naphthalene red, fluorescein, and eosin.
31
The spectra
extended beyond the short-wave limits of the exciting light
(Figure 9). The Stokes law violation happens only in the region
where the absorption and fluorescence curves overlap, as first
noted by Lommel.
32
A major event in the turn of the 19th century was the Planck
theory of quanta that Albert Einstein applied to the photoelectric
effect and also to luminescence. Considering that the energy of
the absorbed and emitted light quanta (later on called photons)
should be proportional to their respective frequencies, the Stokes
law simply obeys the first law of thermodynamics (conservation
of energy). But how can the exceptions to the Stokes law be
explained? The bell-shaped intensity curves for emission suggest
a statistical process. Einstein proposed that molecular motion
provides the additional energy required for the violation of the
Stokes law. If this assumption is correct, then the departure from
the Stokes law should be larger at higher temperatures. A
discussion between Einstein and Joseph von Kowalski on this
topic led the latter to study the effect of temperature on the
emission of rhodamine. The results showed agreement (within
an order of magnitude) with calculations based on Einstein’s
assumption.
33
As vibrational energy is converted into radiation,
cooling of the medium can occur upon anti-Stokes emission. An
interesting consequence is laser cooling of solids, a subject where
significant developments occurred over the past decade.
34
’BECQUEREL’S PIONEERING WORK ON TIME-RE-
SOLVED PHOTOLUMINESCENCE
Edmond Becquerel did not participate in the controversy on
the Stokes law. He showed great interest for various aspects of
light,
24
but his most important contribution concerns phosphor-
escence. In fact, Kayser and Konen
35
considered that Becquerel
introduced a revolution in this field. In particular, Becquerel
Figure 9. Fluorescence spectra of fluorescein upon excitation by various
sources (A, B, C) whose wavelength ranges are represented on the
horizontal axis (values in nm). L represents the longest excitation
wavelength providing fluorescence. D is the transmittance of the sample.
(Adapted from ref 31).

736 dx.doi.org/10.1021/ed100182h |J. Chem. Educ. 2011, 88, 731–738
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measured the decay times of the phosphorescence of various
compounds by means of his outstanding phosphoroscope built in
1858: this was the very first time-resolved photoluminescence
experiment.
Becquerel designed his phosphoroscope at the Conservatoire
Imperial des Arts et Metiers, where he was appointed to the chair
of physics from 1852 to 1891. The instrument consists of two
disks rotating together at variable speeds up to 3000 revolutions
per second (rev/s). The sample is placed between the two disks.
Each disk possesses four windows in such a way that the incident
light cannot go through the second disk (Figure 10), and
therefore, there is a time lag between excitation and observation
of emission that depends on the speed of rotation. By changing
the latter, the intensity of emission can be measured as a function
of time. Phosphorescence lifetimes shorter than 0.1 ms could be
determined in this way. The first results were published in
1861.
24
For the description of the experimental phosphorescence
decays, Becquerel used an exponential of time and also a sum
of two exponentials. However, in the case of inorganic solids, he
obtained a better fit with the following equation:
i2ðtþcÞ¼cð1Þ
where iis the normalized intensity so that i(0) = 1, tis time, and c
is a time constant. Later, he proposed a more general equation in
the form:
36
imðtþcÞ¼cð2Þ
where 1 gmg
1
/
2
. He obtained good fits for alkaline-earth
sulfides. Equation 2 can be rewritten as
iðtÞ¼ 1
ð1þt=cÞ1=mð3Þ
Because mis less than 1, this function decays faster than a
hyperbola (for which m= 1) and can thus be called a compressed
or squeezed hyperbola. But owing to Becquerel’s pioneering
studies, and reviving a now almost forgotten terminology,
37
we
suggest calling this function the Becquerel decay law.
38
We have
recently shown that this function is of great interest in the
analysis of complex luminescence decays with underlying dis-
tributions of decay times.
3941
’EARLY APPLICATIONS OF
PHOTOLUMINESCENCE
2,6,4244
The fluorescent tube was one of the oldest applications of
fluorescence. Edmond Becquerel in 1857, and probably German
scientists at the same time, conceived the idea of coating the
inner surface of an electric discharge tube with a luminescent
material (Figure 11). Such tubes are similar to the fluorescent
tubes that are made today. In fact, the inner coating is nowadays
made of Eu
II
,Eu
III
, and Tb
III
, so that addition of blue, red, and
green lights yields white light.
In the field of chemistry, fluorescence has long been used as an
analytical tool for the determination of the concentrations of
various neutral or ionic species.
6,45
Stokes had this idea in mind
since his 1852 paper, where one of conclusions states
4
[The phenomenon] furnishes a new chemical test, of a remark-
able searching character, which seems likely to prove of great
value in the separation oforganic compounds. The test (...) leads
to the independent recognition of one or more sensitive sub-
stances in a mixture (...).
He lectured “on the application of the optical properties to
detection and discrimination of organic substances”before the
Chemical Society and the Royal Institution in 1864.
Victor Pierre
46
who was a professor in Prague and later in
Vienna, published papers in 1862 where he studied solutions of
single fluorescent compounds and mixtures. He noticed that
bands of fluorescent spectra were characteristic of a particular
substance. He noted also the effect of solvent and acidity or
alkalinity. A well-known application of fluorescence to analysis
was reported by G€oppelsr€oder
d
in 1868:
47
the complexation of
morin (a hydroxyflavone derivative) with aluminum produces a
drastic enhancement of fluorescence intensity (Figure 12), offer-
ing a straightforward way to detect this metal. It was the first time
that the term fluorescence analysis was employed.
Figure 11. Photograph of an early fluorescent tube made by W. S.
Andrews in 1912 (reproduced with permission from ref 43; Copyright
1941 Chemical Publishing Co.).
Figure 10. Two of Edmond Becquerel’s phosphoroscopes: (left)
drawing from ref 24 and (right) photo of the phosphoroscope at the
Musee des Arts et Metiers in Paris. The speed of rotation of both disks
bearing four windows can reach 3000 rev/s, which allowed Becquerel to
analyze phosphorescence decays whose time constant is shorter than 0.1
ms. The authors of the present article had the privilege to operate it very
carefully, with special gloves! It still works very well.
Figure 12. Complexation of morin with aluminum ion leads to a
fluorogenic effect (enhancement of fluorescence intensity).

737 dx.doi.org/10.1021/ed100182h |J. Chem. Educ. 2011, 88, 731–738
Journal of Chemical Education ARTICLE
Among the old applications of fluorescence, it is worth
mentioning that uranin (the disodium salt of fluorescein) was
used for the first time in 1877 as a tracer for monitoring the flow
of the Danube river.
48
On all maps, it is shown that the Danube
springs in the Black Forest and, after many hundreds of kilo-
meters, flows into the Black Sea (Figure 13). But there are several
sinks (swallow holes) in the bed of Danube. The biggest one is
near Immendingen (red point in Figure 13). Ten liters of a
concentrated solution of uranin were poured by Knop into the
bed of the upper current of the Danube, and 50 h later, the
fluorescence could be observed in the water of the river Aache 12
km to the south. This river flows into the lake Constanz that feeds
the Rhine. Therefore, only a small part of the water from the
Danube spring arrives at the Black Sea. Most of it flows into the
North Sea! Nowadays, fluorescence tracing is currently used in
hydrogeology, especially to simulate and trace the discharge of
pollutants.
48
’CONCLUSION
Photoluminescence is an attractive topic that deserves various
discussions in the classroom related to (i) the excited states of
atoms and molecules, (ii) the various types of emission of light
by matter, (iii) the distinction between luminescence and
incandescence, (iv) the thermodynamic aspects of photolu-
minescence in the frame of a historical controversy, (v) the
Stokes law and its violation, (vi) the temporal characteristics
of photoluminescence exemplified by the early time-resolved
measurements, and (vii) the early applications of fluorescence
that are still used today.
’AUTHOR INFORMATION
Corresponding Author
*E-mail: (B.V.) bernard.valeur@cnam.fr; (M.N.B.-S.) berberan@
ist.utl.pt.
’ADDITIONAL NOTE
a
The spin multiplicity is the number of possible orientations,
calculated as 2Sþ1, of the spin angular momentum correspond-
ing to a given total spin quantum number (S), for the same
spatial electronic wavefunction. A state of singlet multiplicity
has S= 0 and 2Sþ1 = 1. A state of triplet multiplicity has S=1
and 2Sþ1=3.
b
We find the same root in the common name of COCl
2
,
phosgene (generated by light), the infamous poisonous gas first
synthesized by Davy in 1812 by the photodissociation of chlorine
in the presence of carbon monoxide.
c
According to a contemporary writer, most mineral shops were
located at Alston, in the Alston Moor district, under whose name
the fluor spar from the area (including the green variety
characteristic of Weardale) was known, see Sopwith, T. An
Account of the Mining Districts of Alston Moor, Weardale and
Teesdale in Cumberland and Durham; Davison: Alnwick,
1833; p 110.
d
The first paper of the series written by F. G€oppelsr€oder (J. Prakt.
Chem. 1867,101, 408) is often erroneously cited as the first
reported application of fluorescence to analysis. In fact, the
application to aluminum detection was not described in
this paper.
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The red point indicates the location of Immendingen.

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