De-excitation of electronically excited molecules
When a molecule has been photochemically promoted to an excited
state. It does remain there for a long time.
Most of the promotions are from So to S1 state. Promotion to S2
and higher singlet state may take place. The energy lost when an S2
or S3 molecule drops to S1 is given up in small increments to the
environment by collisions with neighboring molecules. Such a
process is called energy cascade. In a similar manner, the initial
excitation and the decay from higher singlet states initially populate
many of viberational levels of S1,but these also cascade down to the
lowest vibrational level of the S1.
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Therefore, in most cases, the lowest vibrational level of the S1 state is
the only important excited singlet state. This state can undergo
various physical and chemical of different possible de-excitation
pathways, which are often characterized by very rapid rates.
We may classify de-excitation processes to two broad categories:
1.Phothophysical de-activation process e.g. photoluminescence,
vibration transition, rotional transition, etc……)
2.Photochemical de activation process e.g. (photodecomposition,
photoaddition, etc…..)
These two de-activation processes can be illustrated as follow.
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Radiationless Transitions (Heat)
✛ Vibrational / rotational deactivation
✛ Energy transfer during molecular collisions
✛ Internal conversion
✛ Intersystem crossing
✛ Photochemical reactions
Radiant Transitions (Photoluminescence)
✛ Fluorescence
✛ Phosphorescence
✛
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A classification of deactivation processes for excited state molecules.
Self quenching
Bimolecular de-activation (quenching
Impurity quenching
Vibration relaxation(V.R)
Photophysical
De-activation
Non-radiative
Unimolecular de-activation
Internal conversion (I.C)
Intersystem crossing (I.S.CC)
Fluorescence hν f
Photodiamerization(AB)2
R
Phosphorescence hν ph
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AB*
Biomolecular
deactivation
Photochemical (quenching)
De-activation
Unimolecular
Deactivation
Photodiamerization(AB)2
Intermolecular electron transfer reactions
Photosubstitution
Photoaddition(intermolecular hydrogen atom
abstraction) P + Q ----- PQ
Radical formation A+B
Intermolecular decomposition C+D
Rearrangement BA
Photoisomerization AB`
Intermolecular H atom abstraction
Photoionization AB+ +eElectron transfer reactions AB+ +e-, AB- + e+
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Basic Concepts in Fluorescence
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Multiplicity States
Singlet State
Triplet State
S2
T2
S1
T1
S0
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FIRST PHOTOPHYSICAL DE-ACTIVATION PROCESS:
1- Photoluminescence.
When the external energy is supplied by means of the absorption of
infrared, visible or ultraviolet light , the emitted light is called
photoluminescence.
Photoluminescence can be subdivided into:
a- Fluorescence, when the molecule in the excited state s1 can drop
to some lower vibrational level of so state all at once by giving of the
energy in the form of light. This process, generally happens within
10-12 to 10-9 sec.
b- Phosphorescence, when the molecule in the s1 state can undergo
an intersystem crossing (ISC) to the lowest triplet state T1 , and emit
the energy in the form of light within 10-3 to 10 sec, and in all cases
the molecule cascade down to the ground state. These pathways are
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shown in Jablonsky diagram as illustrated in the upper figure.
In short briefly the commonly encountered photophysical radiative
and nonradiative processes are:
FIRST FOR RADIATIVE PROCESSES
- “Allowed” or singlet- singlet absorption (So + hn- ----- S1 ).
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- “Allowed”or singlet- singlet emission,called fluorescence S1
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- - So+hn.
- “Forbidden” or singlet- triplet absorption (So + hn- --- T1)
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- “Forbidden”or triplet- singlet emission phosphorescence(T1
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- - So+hn).
SCOND FOR NONRADIATIVE PROCESSES
- “Allowed” transitions between states of the same spin, called internal
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conversion (Sn
- - -- S1+ heat).
- “Allowed” transitions between states of the same spin, called external
2
(by collision, to other molecules especially solvent) and internal
conversion (S1
- - -- So+ heat).
- “Forbidden” transitions between excited states of different spin,
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called intersystem crossing (S1
- - -- T1+ heat).
- “Forbidden” transitions between triplet states and the ground states
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also called intersystem crossing (T1---- So + heat).
Vibronic Transitions – The Franck-Condon Principle
The bold horizontal lines of the Jablanski diagram corresponds to
the minimum of the potential energy diagram of the molecule. As
illustrated in this Figure
Energy
Excited electronic
level
Ground level
Rtational
energy
level
Vibrational energy
levels
Interatomic distance
The potential energy diagram plots the electronic and vibrational
energies of the molecules as a function of the the nuclear separation
r, and the wavefunctions of the viberational modes approximate
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those of a harmonic oscillator.
The Franck-Condon Principle State That
The Frank-Condon principle state that, electronic transitions are
so fast (10-15 sec.) in comparison to the nuclear motion(10-12 sec.)
that immediately after the transition, the nuclei have nearly the
same relative position and velocities as they did just before the
transition.
Excited electronic
level
Ground level
Rtational
energy
level
Vibrational energy
levels
Interatomic distance
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Mirror Symmetry
According to the Frank-Condon principle, all electronic transitions
are vertical, that is , they occur without change in the position of
nuclei. When we record the absorption spectrum of an organic
compound in a nonpolar solvent, we may observe that the spectrum
is broad, with features that we can assign to different Frank-condon
vibronic transitions. The most intense of these will be determined by
the vibrational overlap integral, or the Frank-condon factor for the
transition. If there is not a great difference in the nuclear
configuration of the molecule in the excited state compared to the
ground state, the operation of the Frank-condon principle will mean
that the o-o transition (ν
ν =o in the ground state to ν = o in the excited
state) will be the most intense fig ( ),
The mirror image rule: emission spectra are mirror images of the
lowest energy absorption band.
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the Mirror Image Rule
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although the possibility of the excitation to ν= 1,2,3,…. Etc. will still
exist. In solution, any vibrational excitation is rapidly lost through
collisions with solvent molecules and emission therefore occurs from
the ν=0 level of the excited electronic state.
The operation of the Frank-condon principle for the reverse
emission process means that the emission intensity is great for the 00 transition. Fig. ( ), but there will also be the possibility of emission
from ν = 0 down to ν = 1,2,3,…etc. Whereas excitation to the ν=
1,2,3,…etc. levels in absorption costs more energy( requiring shorter
wavelength radiation) than excitation to ν = 0, de-excitation to ν=
1,2,3,…etc. is accompanied by the emission of radiation to lower
energy (longer wavelength) than that of the 0-0 emissive transition.
If the vibrational level spacing are similar in both states. The
emission spectrum will appear as a mirror image of the absorption
spectrum, as shown in fig. ( ). The reason for the small difference in
the wavelengths of the 0-0 absorption and emission transitions is
related to the properties of the solvent molecules and their slightly
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different interactions with the two electronic states involved.
Some definitions
The categories of molecules capable of undergoing electronic
transitions that ultimately result in fluorescence are known as
fluorescent probes, fluorochromes, or simply dyes.
Fluorochromes that are conjugated to a larger macromolecule
(such as a nucleic acid, lipid, enzyme, or protein) through
adsorption or covalent bonds are termed fluorophores.
fluorophores are divided into two broad classes:
Intrinsic fluorophores are those that occur naturally such as
aromatic amino acids, neurotransmitters, porphyrins, and green
fluorescent protein.
Extrinsic fluorophores are synthetic dyes or modified biochemicals
that are added to a specimen to produce fluorescence with
specific spectral properties.
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