Download Fluorescence

Chapter 3
Photophysics
Emission and loss processes:
• Several of pathways for loss of electronic
excitation shown in Fig. as a emission of light
as luminescence
• The individual luminescence phenomena are
named according to the mode of excitation of
the energy-rich species. Emission from species
excited by absorption of radiation is referred
to fluorescence or phosphorescence Emission
following excitation by chemical reaction (of
natural or changed species) is
chemiluminescence.
• If emission of radiation continued after the
exciting radiation was shut off, the emitting
species was said to phosphorescent, while if
emission appeared to cease immediately, then
the phenomenon was one of fluorescence.
Bioluminescence
Fireflies
Artistic rendering of bioluminescent Antarctic krill
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Oxyluciferin
Firefly luciferin
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Image of bioluminescent red tide event of 2005 at a beach in Carlsbad California showing brilliantly glowing crashing waves
containing billions of Lingulodinium polyedrum dinoflagellates
• In 1935, Jablonski phosphorescence was
emission from some long-lived metastable
electronic state lying lower in energy than the
state populated by absorption of radiation
(see section 1.9). This long-lived metastable
state is now known to be the first excited
triplet in many cases where the ground state
of a molecule is a singlet. The long lifetime of
the emission is a direct consequence of the
forbidden.
Absorption and emission pathways
and
Dr.McGarvey
Suzan A. Khayyat
Gaillard, Basic Photochemistry at 9
http://classes.kumc.edu/grants/dpc/instruct/index2.htm
Quenching or collisional deactivation
• In addition to loss by emission and
intermolecular energy transfer (IC and ISC ),
excited states often suffer losses in quenching
processes. Quenching is especially important
in the liquid phase where collisions are very
frequent.
Fluorescence, Phosphorescence and
Chemiluminescence
• There are three main types of “cold light”
reactions in chemistry which are often
confused but are in fact very different. It is
important to make sure that you know the
difference.
• Fluorescence is the emission of light by a substance that has
absorbed light or other electromagnetic radiation of a
different wavelength It is a form of luminescence. In most
cases, emitted light has a longer wavelength, and therefore
lower energy, than the absorbed radiation. However, when
the absorbed electromagnetic radiation is intense, it is
possible for one electron to absorb two photons; this twophoton absorption can lead to emission of radiation having a
shorter wavelength than the absorbed radiation.
• The most striking examples of fluorescence occur when the
absorbed radiation is in the ultraviolet region of the
spectrum, and thus invisible to the human eye, and the
emitted light is in the visible region.
• Fluorescence has many practical applications, including
chemical sensors (fluorescence spectroscopy), fluorescent
labelling, dyes, biological detectors, and, most commonly,
fluorescent lamps.
• Fluorescence occurs when an orbital electron of a
molecule, atom or nanostructure relaxes to its ground
state by emitting a photon of light after being excited to
a higher quantum state by some type of energy:
• Excitation: :
• Fluorescence (emission):
•
• hν is a generic term for photon energy with h = Planck's
constant and ν = frequency of light. (The specific
frequencies of exciting and emitted light are dependent
on the particular system.)
• State S0 is called the ground state of the fluorophore
(fluorescent molecule) and S1 is its first (electronically)
excited state.
Fluorescence
• 1- First, absorption must occur at a wavelength long enough to
ensure that chemical dissociation does not take place.
• a- Absorption to unstable state is clearly very unlikely to result in
fluorescence.
• b- Further, in many molecules in which the absorption
corresponds to energy greater than the cleavage energy of the least
stable bond, no fluorescence is observed.
• 2-Secondely, intermolecular energy transfer must be relatively slow
compared to the rate of radiation. This appears to mean the ISC
from S T must be slow. Geometrical factors such as rigidity and
planarity can also affect the efficiency of fluorescence. There are
two different types of spectrum discussed. First, obtained by
measuring the intensity.
Fluorescence
• higher energy radiation is absorbed by a species
and stored for long enough that the molecules
have collided and lost vibrational energy in
collisions. When the excited molecule re-emits,
the photon is of less energy depending on the
species and is therefore of a lower frequency.
• . Note that Fluorescence is normally emitting
from a singlet state whereas phosphorescence
(below) occurs from triplet states.
Phosphorescence
• light is absorbed by a molecule which becomes excited
to a higher singlet state. The light can either be reemitted straight away or (very-rarely), the electron can
switch to a triplet state (this is technically forbidden
but can happen often enough through a process known
as intersystem crossing). Transitions between different
multiplicities are forbidden by quantum selection rules
and as the ground state of the molecule will often be a
singlet state (recall all closed shells are singlets), the
molecule is not allowed to emit a photon that will get
it directly to the ground state. The molecule must wait
a long time (in quantum chemistry) until it can make
the forbidden transition.
What is Chemiluminescence?
• Chemiluminescence is the production of light
from a chemical reaction in excess of the black
body radiation expected from that body. As
such, it is often referred to as “cold light”.
• Normally, chemiluminescence involves the
production of an electronically excited species
from a number of reactants which goes on to
release visible light in order to revert to its
ground state energy.
• The mechanism for this reaction is outlined
extensively throughout this site but the key
difference to note is that no radiation is
absorbed – the energy required to emit light
comes from the energetics of the chemical
reaction. The emission could proceed either
from a singlet (fluorescence) or triplet
(phosphorescence) state. "Light Sticks" are a
popular example of Chemiluminescence
• Emission of electromagnetic radiation, usually visible light, caused
by excitation of atoms in a material, which then reemit almost
immediately (within about 10 8 seconds). The initial excitation is
usually caused by absorption of energy from incident radiation or
particles, such as X-rays or electrons. Because reemission occurs so
quickly, the fluorescence ceases as soon as the exciting source is
removed, unlike phosphorescence, which persists as an afterglow. A
fluorescent lightbulb is coated on the inside with a powder and
contains a gas; electricity causes the gas to emit ultraviolet
radiation, which then stimulates the tube coating to emit light. The
pixels of a television or computer screen fluoresce when electrons
from an electron gun strike them. Fluorescence is often used to
analyze molecules, and the addition of a fluorescing agent with
emissions in the blue region of the spectrum to detergents causes
fabrics to appear whiter in sunlight. X-ray fluorescence is used to
analyze minerals.
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sensitized and delayed fluorescence.
Chemiluminscence
Photochemistry in nature
Applied Photochemistry