Have you ever wondered why glow sticks (see Fig. 1)
are glowing after you have bent them? Or why those plastic stars are
still glowing at night after the lights have been switched off? You may
not have.
However, whether you have asked yourself those questions or
not, the following text will hopefully provide you with answers and
explanations of the phenomena of glowing bracelets, shining jellyfish,
black light, and many more.
|
 |
| Figure 1: Glowstick |
|
Luminescence:
In general, the glow that occurs in all the above mentioned phenomena
is called luminescence.
Luminescence is energy released by a substance
in the form of light. Several types of luminescence can be
differentiated:
- One example is Chemiluminescence. During some chemical
reactions, energy is released as light. This occurs after bending a glow
stick. It is also the reason for the glowing of animals like jellyfish
or some microorganisms. In this case, it is called bioluminescence.
- Another kind is Triboluminescence. This can be observed when a
self-adhesive envelope is opened in complete darkness or when adhesive
tape is unrolled in a dark room. In doing so, mechanical energy is put
into the system and serves as an activator for the glow.
- Probably the most familiar type of luminescence is Photoluminescence. Here, energy is provided by electromagnetic
radiation, for instance through sunlight or an ultraviolet lamp, as in
some discotheques. This causes phenomena like the ongoing glow of
plastic stars or the extreme brightness of white clothes under black
light. One can differentiate fluorescence and phosphorescence, which
will be explained below.
What are Electronically Excited States?
 |
| Fluorescein |
Generally, all kinds of luminescence are based on so-called
photo-physical processes. Usually, molecules themselves are described
as fluorescent. This is the case with fluorescent dyes like fluorescein
or curcumin. However, to explain photo-physical processes, one
has to take a closer look at an even smaller level than the molecular
one.
 |
| Flourescein Chemical Structure |
Atoms of different elements have a different number of electrons
distributed into several shells and orbitals. Electrons are a type of
elementary particle. Electronic transitions are responsible for
luminescence. When the system absorbs energy, electrons are
excited and are lifted into a higher energetic state. Before excitation,
in the ground state, some of the electrons are in the so-called HOMO
(Highest Occupied Molecular Orbital). After they reach an excited state,
they are in the LUMO (Lowest Unoccupied Molecular Orbital) (see
Fig. 2). How this works exactly will be explained using Photoluminescence as a specific example.
 |
| Figure 2. Electronic excitation. |
|
|
Different energetic states of an atom or molecule are known as
"energy levels". Depending on the molecule and atom, the electrons can
only occupy discrete energy levels since the energy is quantized, which
means, energy can only be absorbed and emitted in certain amounts.
The difference between two levels can be calculated with
equation 1.
ΔE = E2–E1 ...................(1)
(where E2 is the higher energy level and E1 the lower one).
Photons, particles of which electromagnetic radiation or light consists,
have to have a certain energy value to be able to excite electrons. The
energy of a photon can be calculated with
equation 2,
Ephoton = hν .......(2)
(where h is the
Planck constant and ν is the frequency of the light.)
The necessary excitation energy for the electrons equals the difference
between the energy levels. Only light with a certain energy, and
accordingly with a certain frequency and wavelength, is capable of
exciting electrons. By equalizing equations 1 and 2, and with the
help of
equation 3 (where c stands for the speed of light), the
necessary frequency and wavelength can be calculated
(see eq. 4). In
many cases, UV-radiation is used for excitation.
λ = c/ν ..............(3)
ΔE = Ephoton ⇔ E2 – E1 = hν ..............(4)
ν = (E2 – E1)/h
λ = hc/(E2 – E1)
Deactivation of Electronically Excited States
Such electronically excited states are unstable. Electrons drop back
to their ground states. At the same time, the excitation energy is
released again. One distinguishes between radiative and non-radiative
decay processes. Most of the time, the decay is non-radiative, for
example through vibrational relaxation, quenching with surrounding
molecules, or
Internal Conversion (IC). These processes will be
explained in detail later.
Sometimes, a radiative decay can occur in form of fluorescence and
phosphorescence. The energy is emitted as electromagnetic radiation or
photons. The emitted light has a longer wavelength and a lower energy
than the absorbed light because a part of the energy has already been
released in a non-radiative decay process . This is the reason that
an emission in the visible spectrum can be achieved by excitation with
non-visible UV-radiation. This shift towards a longer wavelength is
called Stokes shift.
Comparing: Fluorescence and Phosphorescence:
Both fluorescence and phosphorescence are spontaneous emissions of
electromagnetic radiation. The difference is that the glow of
fluorescence stops right after the source of excitatory radiation is
switched off, whereas for phosphorescence, an afterglow with durations
of fractions of a second up to hours can occur.
To compare the photo-physical processes behind both phenomena, there are
some facts about electrons that are helpful for understanding:
Electrons are particles that have a so-called spin and a spin quantum
number. This can have two different values, namely either +1/2 or –1/2. This number is a property that we actually cannot imagine or
describe easily. It is often compared with a spinning top, either
spinning in a clockwise or anti-clockwise direction. However, this
description is neither mathematically nor physically quite correct. Two
electrons in a single orbital of an atom have antiparallel spin, which
is noted as
(↑↓).
Fluorescence:
In the
Jablonski diagram for fluorescence (see Fig. 3), the singlet spin state
S0 is the ground state of the electrons, and
S1 and
S2
are singlet excited states (the states are only used as an example in
this text and do not necessarily apply to certain atoms, molecules,
etc.). Within those states, there are several energy levels. The higher
the level is, the more energy an electron possesses when being in that
level. In the case of singlet states, the electrons have antiparallel
spins.
 |
| Figure 3. Jablonski diagram for fluorescence. |
|
|
The electrons are lifted from the ground state
S0, for example, to an energy level of the second excited state
S2,
when excited by electromagnetic radiation. After excitation stops, the
electrons only stay in that excited state for a short period of time
(ca. 10
–15 s) and then immediately start falling back down into the
ground state. In doing so, energy initially can be released to the
surroundings by vibrational relaxation. That means thermal energy is
released by the motion of the atom or molecule until the lowest level
of the second excited state is reached.
The bigger gap between the second and first excited state is overcome by
internal conversion. That describes an electronic transition between
two states while the spin of electrons is maintained. Now, the electrons
can relax further due to more vibrational relaxation until they reach
the lowest energy level of the
S1 state.
Theoretically, the electrons could relax even further in a non-radiative
way until they eventually reach the ground state again. However, it can
be the case that the last amount of energy is too large to be released
to the surroundings because the surrounding molecules cannot absorb this
much energy. Then, fluorescence occurs, which leads to an emission of
photons possessing a certain wavelength. The emission lasts only until
the electrons are back in the ground state. Since during all those
transitions the electron spin is kept the same, they are described as
spin-allowed.
Phosphorescence:
For phosphorescence, things are a bit different (see Fig. 4). There are again an
S0 ground state and the two excited states,
S1 and
S2. Additionally, there is an excited triplet
T1 state which lies energetically between the
S0 and
S1 state. The electrons again have antiparallel spins in the ground state.
 |
| Figure 4. Jablonski diagram for phosphorescence. |
|
|
Excitation happens in the same way as in fluorescence, namely through
electromagnetic radiation. The release of energy through vibrational
relaxation and internal conversion while maintaining the same spin is
the same here, as well, but only until the
S1 state is reached.
Alongside the singlet states, a triplet state exists and so-called intersystem crossing (ISC) can occur since the
T1 state is energetically more favorable than the
S1
state. This crossing, like internal conversion, is an electronic
transition between two excited states. But contrary to internal
conversion, ISC is associated with a spin reversal from singlet to
triplet. Electrons in the triplet state have parallel spins, which is
noted as
(↑↑). This ISC process is described as "spin-forbidden". It
is not completely impossible – due to a phenomenon called "spin-orbit
coupling" – however, it is rather unlikely.
In the
T1 state, non-radiative decay is possible as
well. However, a transition between the lowest energy level of the
triplet state and the
S0 state is not readily
possible, because that transition is spin-forbidden, too. Still, it can
happen anyway with a small possibility. It causes a rather weak emission
of photons because the electron spin has to be reversed again. The
energy is trapped in this state for a while and can only be released
slowly. After all energy has been released, the electrons are back
in the ground state.
Summary:
The spin-allowed and -forbidden processes serve as explanations for
an immediately ceasing glow of fluorescence and for the afterglow of
phosphorescence. Phosphorescence usually occurs only with "heavier"
molecules since the spin has to be reversed with the help of
spin-orbit-coupling. Whether electromagnetic radiation is emitted at
all, and with which wavelength, depends on how much energy can be
released beforehand by non-radiative decay. It also depends on the
properties of so-called quenchers that are surrounding molecules and
are able to take up larger amounts of energy.
All processes that can lead to an inhibition of radiative decays can
cause fluorescence quenching. Examples are non-radiative decay
processes, but also the destruction of the fluorescent molecule.
The quantum efficiency describes the efficiency of the process and is
defined as the ratio of absorbed and emitted photons. This property
is different for each substance.
Even though this text focuses on photoluminescence, the photo-physical processes are the same for all types of luminescence.
Liked this post? Get Connected with us and Stay Updated with more upcoming Posts
.png)