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Molecular Luminescence
Spectroscopy
Lecture 30
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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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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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Internal 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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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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Quantum Yield and Efficiency
The quantum yield or efficiency of
fluorescence is the ratio of the number of
fluorescing molecules to the total number of
excited molecules. For highly fluorescent
molecules, a quantum efficiency
approaching one can be obtained. The
quantum efficiency can be represented by
the relation:
F = kFL/(kFL + kISC + kIC + kEC + kPD + kdiss)
Phosphorescence quantum efficiency is
defined in the same manner
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The transitions most important in
luminescence spectroscopy are n-p* and pp*. However, fluorescence is encountered
more often in molecules having p-p*
transitions since this transition has a higher
quantum efficiency in terms of a higher
molar absorptivity and a shorter lifetime (10-7
– 10-9s) than the n-p* transition which has a
longer lifetime (10-5 – 10-7s). Once again, a
longer lifetime means a lower luminescent
probability due to increased possibility of
radiationless deactivation.
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Variables That Affect Fluorescence and
Phosphorescence
Factors affecting fluorescence and phosphorescence
include both environmental and structural factors.
Some of the important factors are discussed below:
Fluorescence and Structure
As indicated earlier, best luminescence is observed for
molecules with p bonds and preferably those having
aromatic rings due to presence of low energy p-p*.
However, some heterocyclic aromatic rings do not
show fluorescence. These include pyridine, furan,
pyrrole, and thiophene
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The lack of fluorescence in such molecules
is largely believed to be due to existence of a
low lying n-p* transition that rapidly converts
the excited molecule to the triplet state and
prevent fluorescence. However, fusion of a
phenyl ring to any of the above molecules
increase the possibility of the p-p*
transitions
and
thus
increase
the
fluorescence quantum efficiency.
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Substitution of halogens to the aromatic ring
has important influence on the fluorescent
signal where a decrease in fluorescence is
observed with an increase in the atomic
weight of the halogen and a subsequent
increase in phosphorescence. This is
referred to as the heavy atom effect where
promotion of intersystem crossing takes
place. In addition, substitution of a carbonyl
or carboxylic acid groups decreased
fluorescence due to enhancement of
intersystem crossing.
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Effect of Structural Rigidity
The nature of the chemical structure of a
molecule in terms of flexibility and rigidity is
of major influence on the fluorescence and
phosphorescence signal. Molecules that
have high degree of flexibility will tend to
decrease fluorescence due to higher
collisional probability. However, more rigid
structures have lower probability of
collisions and thus have more fluorescence
potential. Biphenyl has very low
fluorescence quantum efficiency (~ 0.2) while
fluorine has a quantum efficiency close
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In addition, some metal complexes have
higher fluorescence efficiency than do
the ligands; also as a result of
increased structural rigidity. For
example, the fluorescence intensity of
8-hydroxyquinoline is increased to a
large extent in presence of zinc ions,
due to more rigid complex formation.
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Effect of Solvent Nature
Solvents characteristics have important effects on
luminescent behavior of molecules. Three main
effects can be recognized:
a. Solvent Polarity
A polar solvent is preferred as the energy required for
the p-p* is lowered.
b. Solvent Viscosity
More viscous solvents are preferred since collisional
deactivation will be lowered at higher viscosities.
c. Heavy Atoms Effect
If solvents contain heavy atoms, fluorescence quantum
efficiency will decrease and phosphorescence will
increase.
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Effect of Temperature
Higher temperatures result in larger collisional
deactivation due to increased movement and
velocity of molecules. Therefore, lower temperatures
are preferred.
Effect of pH
The pH of the solution is a very important factor that
influences luminescence. For example, aniline
shows fluorescence while aniline in acid solution
(anilinium ion) does not. Most compounds luminesce
in basic or slightly basic solutions while some show
fluorescence in acidic medium. It is therefore
important to adjust the pH so that maximum
luminescence intensity is obtained. The pH also
affects the emission wavelength where usually a
longer emission wavelength is observed at higher
pH.
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Effect of Dissolved Oxygen
Dissolved oxygen largely limits fluorescence since it
promotes intersystem crossing because it is
paramagnetic. However, dissolved oxygen affects
phosphorescence more than it does to fluorescence.
Although one would think that as far as intersystem
crossing is increased in presence of oxygen,
phosphorescence is expected to increase. On the
contrary, phosphorescence is completely eliminated and
quenched in presence of dissolved oxygen. This may be
explained on the basis that the ground state of oxygen is
the triplet state and it is easier for an electron in the triplet
state to transfer its energy to triplet oxygen rather than
performing a flip in spin and relax to singlet state.
Therefore, oxygen will be excited and what we really
observe is oxygen emission rather than phosphorescence.
It is for this reason that oxygen should be totally excluded
to be able to detect phosphorescence.
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Effect of Concentration on
Fluorescence
It can be undoubtedly said that the
fluorescence is directly proportional to the
amount of absorbed radiation where:
F = k(Po-P)
P = Po 10-A
Substitution gives:
F = kP0 (1-10-A)
This relation can be expanded by Mc Lauren
series giving:
F = kP0 (2.303 A – (2.303 A)2/2! + (2.303 A)3/3! (2.303 A)4/4! + ….)
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Only the first term is important at the
very low concentrations used. The
relation simplifies to:
F = kPo (2.303 A)
F = KPoebc
Where K = 2.303k
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