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Molecular Luminescence
Spectroscopy
Lecture 29
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Molecular Luminescence
Spectroscopy
Luminescence spectroscopy is a
technique which studies the
fluorescence, phosphorescence, and
chemiluminescence of chemical
systems. The analyte or its reaction
product needs to be luminescent. The
relative luminescence intensity is
related to analyte concentration as will
be seen shortly.
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Singlet and Triplet States
Electrons in molecular orbitals are paired,
according to Pauli exclusion principle. When
an electron absorbs enough energy it will be
excited to a higher energy state; but will keep
the orientation of its spin. The molecular
electronic state in which electrons are paired
is called a singlet transition. On the other
hand, the molecular electronic state in which
the two electrons are unpaired is called a
triplet state. The triplet state is achieved
when an electron is transferred from a
singlet energy level into a triplet energy level,
by a process called intersystem crossing;
accompanied
by a flip in spin
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In a singlet state, the spins of the two electrons are
paired and thus exhibit no magnetic field and called
diamagnetic. Diamagnetic molecules, containing
paired electron, are neither attracted nor repelled by
a magnetic field. On the other hand, molecules in the
triplet state have unpaired electrons and are thus
paramagnetic which means that they are either
repelled or attracted to magnetic fields. The terms
singlet and triplet stems from the definition of
multiplicity where:
Multiplicity = 2S + 1
Where, S is the total spin. The total spin for a singlet
state is zero since electrons are paired which gives a
multiplicity of one (the term singlet state).
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Multiplicity = (2 * 0) + 1 =1
In a triplet state, the total spin is one (the two
electrons are unpaired) and the multiplicity is
three:
Multiplicity = (2 * 1) + 1 = 3
It should also be indicated that the probability
of a singlet to triplet transition is much lower
than a singlet to singlet transition. Therefore,
the intensity of the emission from a triplet
state to a singlet state is much lower than
emission intensities from a singlet to a
singlet state.
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Energy Level Diagram for
Photoluminescent Molecules
The following diagram represents the
main processes taking place in a
photoluminescent molecule when it
absorbs and emits energy.
The different processes will be discussed
below:
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Absorption
The absorption of UV-Vis radiation is necessary to
excite molecules from the ground state to one of the
excited states. Absorption of radiation promotes
electrons in chemical bonds to be excited. However,
we have seen earlier that not all transitions have the
same probability and while certain transitions are
practically very important, others are seldom used
and are of either no or marginal importance.
Therefore, from information we have discussed in
Chapter 14 we concluded that there are four different
types of electronic transitions which can take place
in molecules when they absorb UV-Vis radiation. A
s-s* and a n-s* are not useful while the n-p*
transition requires low energy but the molar
absorptivity for this transition is low and transition
energy will increase in presence of polar solvents.
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The most frequently used transition is the p-p*
transition for the following reasons:
a.
The molar absorptivity for the p-p* transition is
high allowing sensitive determinations.
b.
The energy required is moderate, far less than
dissociation energy.
c.
In presence of the most convenient solvent
(water), the energy required for a p-p* transition is
usually smaller.
Therefore, best molecules that may show absorption
are those with p bonds or preferably aromatic nature
as discussed earlier. Absorption to higher excited
singlet states requires a very short time (in the range
of 10-14s).
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Vibrational Relaxation
Absorption of radiation will excite molecules to
different vibrational levels of the excited
state. This process is usually followed by
successive vibrational relaxations (VR) as
well as internal conversion to lower excited
states. In cases where transitions occur to
the first excited state, vibrational relaxation
to the main excited electronic level will take
place and/or an intersystem crossing (ISC) to
the triplet state can occur.
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Fluorescence
After vibrational relaxation to first excited electronic
level takes place, a molecule can return to the
ground state by emission of a photon, called
fluorescence (FL). The fluorescence lifetime is much
greater than the absorption time and occurs in the
range from 10-7 to 10-9s. As the lifetime in the excited
state is increased, the probability of fluorescence
will be decreased since radiationless deactivation
processes may take place. However, not all excited
molecules can show fluorescence by returning to
ground state and most return to ground state by
losing excitation energy as heat or through
collisions with other molecules or solvent.
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Internal and External Conversion
Internal conversion (IC) is a radiationless
deactivation process whereby excited
molecules return to the ground state without
emission of a photon. This process lacks
rigid understanding but seems to be the
most efficient deactivation process in
luminescence spectroscopy, since most
molecules do not show fluorescence.
However, molecules with close electronic
energy levels, to the extent that their
vibrational energy levels of ground and
excited states are overlapped, are believed to
cause efficient internal conversion.
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Dissociation and predissociation
Internal conversion can result in a phenomenon called
predissociation (PD) where an electron relaxes from
a higher electronic state to an upper vibrational
energy of a lower electronic state. When the
vibrational energy is large enough and is greater
than the bond senergy, bond rupture occurs in a
process called predissociation. Dissociation should
be differentiated from predissociation where
dissociation involves absorption of high energy so
that the molecule is directly promoted to a high
energy vibrational level where bond rupture directly
occurs.
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External conversion (EC) is a process
whereby excited molecules lose their
energy due to collisions with other
molecules or by transfer of their energy
to solvent or other unexcited
molecules. Therefore, external
conversion is influenced by
temperature, solvent viscosity, as well
as solvent composition.
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Intersystem Crossing
Electrons present at the first excited electronic
level can follow one of three choices
including emission of a photon to give
fluorescence, radiationless deactivation to
ground state, or intersystem crossing (ISC).
The process of intersystem crossing
involves transfer of the electron from an
excited singlet to a triplet state. This process
can actually take place since the vibrational
levels in the singlet and triplet states overlap.
However, crossing of the singlet state to the
triplet state involves a flip in electron spin in
order to satisfy the triplet state.
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Intersystem crossing is facilitated by presence
of nonbonding electrons as well as heavy
atoms. The presence of paramagnetic atoms
or species also enhances intersystem
crossing.
An electron in the triplet state can also cross
back to the singlet state and can result in a
photon as fluorescence but at a much longer
time than regular fluorescence. This process
is termed delayed fluorescence and has the
same characteristics as direct fluorescence
except for the large increase in lifetime.
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Phosphorescence
Electrons crossing the singlet state to the triplet state
with a flipped spin can also follow one of three
choices including returning to the singlet state
(including a flip in spin), relax to ground state by
internal or/and external conversion, or lose their
energy as a photon (phosphorescence, Ph) and relax
to ground state with a second flip in spin to satisfy
the singlet ground state. As can be rationalized from
the processes involved in collecting
phosphorescence photons, this involves an
intersystem crossing and two flips in spin. This, in
fact, requires a much longer time than fluorescence
(10-4s to up to few s). Therefore, the probability of
phosphorescence, and hence the intensity of the
phosphorescence spectrum, is very low due to high
possibility of radiationless deactivation.
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