Factors Affecting on luminescence

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3. Factors Affecting Fluorescence
And Phosphorescence
Name : Ramsha Afzal
Roll No. 35322
Bs chemistry: 8th semester
Course Code: CHM-606
Submitted To: Mrs.Mahwish
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5. List Of Contents
• Introduction Of Fluorescence and Phosphorescence
• Possible de-excitation pathways of excited molecules
• Jablonski energy-level Diagrams
• Comparison between Fluorescence and Phosphorescence
• Variables that Affect Fluorescence
• FactorsThat Affect Photoluminescence
• References
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Introduction Of Fluorescence and
Phosphorescence

7. Fluorescence and Phosphorescence
• Fluorescence and phosphorescence are types of molecular
luminescence methods.
• A molecule of analyte absorbs a photon and excites a species.
• The emission spectrum can provide qualitative and quantitative
analysis.
• The term fluorescence and phosphorescence are usually referred as
photoluminescence because both are alike in excitation brought by
absorption of a photon, only differ in relaxation process.
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8. Possible De-excitation Pathways Of Excited
Molecules
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9. Jablonski Energy-Level Diagrams
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10. Fluorescence Phosphorescence
• short-lived
• Need source for excitation
• No change in electron spin
• Endure for several seconds
• Need source for excitation
• Change in electron spin
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11. Various Parameters Influence The Emission Of
Fluorescence
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12. Variables that Affect Fluorescence
• Structure and structural Rigidity
• Temperature – increased temperature, decreased quantum yield
• SolventViscosity – lower viscosity, lower quantum yield
• Fluorescence usually pH-dependent
• Dissolved oxygen reduces emission intensity
• Concentration: Self-quenching due to collisions of excited molecules.
Self-absorbance when fluorescence emission and absorbance
wavelengths overlap.
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13. Fluorescence And Structure
• As indicated earlier, best luminescence is observed for molecules with π bonds
and preferably those having aromatic rings due to presence of low energy 𝛑 − 𝝅*.
• Compounds containing aliphatic and alicyclic carbonyl structures or highly
conjugated double-bond structures may also exhibit fluorescence.
• Electron withdrawing groups like –COOH ,-N=N-, NO2 and halides decrease
fluorescence.
• Electron donating groups like –OH, and –NH2 are strongly fluorescence.
• Most unsubstituted aromatic hydrocarbons fluoresce in solution; the quantum
efficiency usually increases with the number of rings and their degree of
condensation.
• However, some heterocyclic aromatic rings do not show fluorescence.
• These include pyridine, furan, pyrrole, and thiophene 13

14. Cont.…
The lack of fluorescence in such molecules is largely believed to be due to:
• With nitrogen heterocyclic, the lowest-energy electronic transition is believed to involve n to 𝜋*
system that rapidly converts to the triplet state and prevents fluorescence.
• However, fusion of a phenyl ring to any of the molecules increase the possibility of the 𝝅 − 𝝅 ∗
transitions and thus increase the fluorescence quantum efficiency.
• Fusion of benzene rings to a heterocyclic nucleus, however, results in an increase in the molar
absorptivity of the absorption peak. The lifetime of an excited state is shorter in such structures;
fluorescence is thus observed for compounds such as quinoline, isoquinoline, and indole.
• Substitution of a carboxylic acid or carbonyl group on an aromatic ring generally inhibits
fluorescence.
• In these compounds, the energy of the n to 𝜋* transition is less than that of the 𝜋 to 𝜋* transition;
as pointed out earlier, the fluorescence yield from the former type of system is ordinarily low
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15. Heavy Atom Effect
• Halogens constituents cause a decrease in fluorescence and the decrease
increases with atomic number of halogens.
• The decrease in fluorescence with increasing atomic number of the halogen is
thought to be due in part to the heavy atom effect, which increases the
probability for intersystem crossing to the triplet state.
• Spin/orbital interactions become large in the presence of heavy atoms and a
change in spin is thus more favorable.
• Predissociation is thought to play an important role in iodobenzene (for example)
that has easily ruptured bonds that can absorb the excitation energy following
internal conversion.
• Substitution of a carboxylic acid or carbonyl group on an aromatic ring generally
inhibits fluorescence. In these compounds, the energy of the n-𝝅* transition is
less than that of the 𝝅 − 𝝅 * transition.
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16. Cont.…
• The electromagnetic fields that are associated with relatively heavy atoms affect electron
spins within a molecule more than the fields associated with lighter atoms.
• The addition of a relatively heavy atom to a molecule causes excited singlet and triplet
electrons to become more energetically similar. That reduces the energetic difference
between the singlet and triplet states and increases the probability of intersystem
crossing and of phosphorescence. The probability of fluorescence is simultaneously
reduced.
• The increased phosphorescence and decreased fluorescence with the addition of a heavy
atom is the heavy-atom effect.
• If the heavy atom is a substituent on the luminescent molecule, it is the internal heavy-
atom effect. The external heavy-atom effect occurs when the heavy atom is part of the
solution (usually the solvent) in which the luminescent compound is dissolved rather than
directly attached to the luminescent molecule.
• The effect that the halides have upon a luminescent molecule is an example of the
internal heavy-atom effect.
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17. Factors That Affect
Photoluminescence
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18. Photoluminescence is favored when the absorption is efficient (high absorptivities).
• Fluorescence is favored when
1. The energetic difference between the excited singlet and triplet states is relatively
large
2. The energetic difference between the first excited singlet state and the ground state
is sufficiently large to prevent appreciable relaxation to the ground state by
radiationless processes.
• Phosphorescence is favored when
1. The energetic difference between the first excited singlet state and the first excited
triplet state is relatively small
2. The probability of a radiationless transition from the triplet state to the ground state
is low.
• Any physical or chemical factor that can affect any of the transitions can affect the
photoluminescence.
• These factors include: structural rigidity, temp., solvent, pH, dissolved oxygen.
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19. Effects of Structural Rigidity
• The nature of the chemical structure of a molecule in terms of flexibility and
rigidity is of major influence on the Photoluminescence signal.
• High degree of flexibility will trend to decrease the fluorescence(due to collision)
• More rigid structure have lower collision, thus have more fluorescence potential.
• Photoluminescent compounds are those compounds in which the energetic levels
within the compounds favor de-excitation by emission of UV-visible radiation
rather than by loss of rotational or vibrational energy
• Fluorescing and phosphorescing compounds usually have a rigid planar structure
• The quantum efficiencies for fluorene and biphenyl are nearly 1.0 and 0.2,
respectively, under similar conditions CH2 causes more rigidity
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20. Cont.…
• The rigidity of the molecule prevents loss of energy through rotational and vibrational
energetic level changes.
• Any subsistent on a luminescent molecule that can cause increased vibration or rotation
can quench the fluorescence.
• The planar structure of fluorescent compounds allows delocalization of the 𝜋-electrons
in the molecule. That in turn increases the chance that luminescence can occur because
the electrons can move to the proper location to relax into a lower energy localized
orbital.
• Organic compounds that contain only single bonds between the carbons do not
luminesce owing to lack of absorption in the appropriate region and lack of a planar and
rigid structure.
• Organic compounds that do luminesce generally consist of rings with alternative single
and double bonds between the atoms (conjugated double bonds) in the rings.
• The sp2 bonds between the carbons in the rings cause the desired planar structure, and
the alternating double bonds give rigidity and provide the 𝜋-electrons necessary for
luminescence. 20

21. Effect Of Temperature
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• The quantum efficiency of fluorescence in most molecules decreases with
increasing temperature
• Higher temperature result in larger collisional deactivation due to
increased movement and velocity of molecules.
• Therefore, lower temperature are preferred.

22. Effect Of PH
• The pH of the solution is a very important factor
• Fluorescence of an aromatic compound with acidic ring substituents is usually
pH-dependent.
• Both 𝛌 and the emission intensity are likely to be different for the ionized and
nonionized forms of the compound
For Example:
Aniline shows fluorescence, while Aniline in acid solution(anilinium ion) does
not
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23. Cont.…
• Most compound luminesce in basic or slightly basic solution
• While some show fluorescence in acidic medium
• So, It is important to adjust pH to obtained maximum luminescence intensity
• pH also affect the emission wavelength, longer emission wavelength is
observed at higher pH
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24. Effect Of Solvent Nature
Solvents characteristics have important effect on luminescent behavior of molecules.
• Three main effect can be recognized:
Solvent Polarity:
A polar solvent is preferred as the energy required for the π-π* is lowered.
SolventViscosity:
More viscous solvents are preferred since collisional deactivation will be lowered at higher
viscosities.
Heavy Atoms Effect
Fluorescence quantum efficiency will decrease, Phosphorescence will increase.
• Other solutes with such atoms in their structure; carbon tetrabromide and ethyl iodide are
examples.
• The effect is similar to what occurs when heavy atoms are substituted into fluorescing
compounds; orbital spin interactions result in an increase in the rate of triplet formation and a
corresponding decrease in fluorescence. 24

25. Effect Of Dissolved Oxygen
• Dissolved Oxygen largely limits fluorescence , since it promotes intersystem
crossing because it is paramagnetic.
• Dissolved Oxygen affects phosphorescence more than fluorescence
• As far as intersystem crossing is increased in the 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 is easier for an electron in the triplet state to transfer its energy to
triplet oxygen rather than performing flip in spin and relax to single.
• Therefore, oxygen will be excited and we really observe oxygen emission rather
than phosphorescence.
• For this reason oxygen should be totally excluded to detect phosphorescence
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26. Effects Of Inner-Filter
• Fluorescence intensity will be reduced by the presence of any compound
which is capable of absorbing a portion of either the excitation or emission
energy.
• At high concentrations this can be caused by absorption due to the
fluorophore itself.
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27. Quenching
• Interaction of the excited state of the fluorophore with its surroundings is
known as quenching
• Decreasing fluorescence intensity
• Relatively rare
• Quenching is not random
• Quinine fluorescence is quenched by the presence of halide ion despite the
fact that the absorption spectrum and extinction coefficient of quinine is
identical in 0.5M H2SO4 and 0.5M HCl.
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28. References
• https://www.philadelphia.edu.jo/academics/ajaber/uploads/CHEM%20540-061-
Fluorescence%20and%20phoephorescence-All.pd
• https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Tex
tbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spec
troscopy/Electronic_Spectroscopy/Fluorescence_and_Phosphorescence
• http://www.phys.ubbcluj.ro/~dana.maniu/OS/BS_7.pdf
• https://webcache.googleusercontent.com/search?q=cache:u7t8IgKKVkQJ:https://
www.chem.uci.edu/~dmitryf/manuals/Fundamentals/Fluorescence%2520Spectros
copy.pdf+&cd=2&hl=en&ct=clnk&gl=pk
• https://www.youtube.com/watch?v=His36eKQ8HQ
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