FLUORESCENCE

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FLUORESCENCE

• Joanna Piechowska

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• 1. Colors of the visible spectrum
• 2. Absorption of UV-VIS light
• Types of electronic transitions in polyatomic
  molecules
• The Franck-Condon principle
• Terminology for Absorption Shifts
• 3. Fluorescence emission
• Perrin-Jablonski diagram
• Radiative and non-radiative transitions between
  electronic states
• Internal conversion
• Fluorescence
• Intersystem crossing
• Phosphorescence
• Delayed fluorescence
• Stokes shift
• 4. Fluorescent compounds and types of fluorofores
• 5. Intramolecular Charge Transfer (ICT)
• 6. Photoinduced Electron Transfer (PET)

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1. Colors of the visible spectrum
Violet   400 - 420 nm Indigo   420 - 440 nm
Blue   440 - 490 nm Green   490 - 570 nm
Yellow   570 - 585 nm Orange   585 - 620 nm
Red   620 - 780 nm

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2. Absorption of UV-VIS light
Types of electronic transitions in polyatomic
molecules
Rys. Energy levels of molecular orbitals in
formaldehyde
The energy of these electronic transitions is
generally in the following order
n -gt p lt p -gt p lt n -gt s lt s -gt p lt s -gt s
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2. Absorption of UV-VIS light
Distinction between singlet and triplet states
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2. Absorption of UV-VIS light
The Franck-Condon principle an electronic
transition is most likely to occur without
changes in the positions of the nuclei in the
molecular entity
Top Potential energy diagrams with vertical
transitions (Franck-Condon principie). Bottom
shape of the absorption bands
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2. Absorption of UV-VIS light
Terminology for Absorption Shifts
Nature of Shift Descriptive Term
To Longer Wavelength Bathochromic
To Shorter Wavelength Hypsochromic
To Greater Absorbance Hyperchromic
To Lower Absorbance Hypochromic
Rule if the extent of the p electron system is
greater, then the energy of the low-lying p -gt p
transition is lower, and consequently, the
wavelength of the corresponding absorption band
is larger
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3. Fluorescence emission
Perrin-Jablonski diagram
ilustration of the relative positions of
absorption, florescence and phosphorescence
spectra
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Stockes shift
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4. Fluorescent compounds and types of fluorofores
aromatic compounds
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1.Internal heavy atom effect
Z4
2. Electron-donating substituents -OH, -OR,
-NH2, -NHR, -NR2
3. Electron-withdrawing substituents carbonyl
and nitrocompounds
4. Heterocycle compounds
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5. Intramolecular Charge Transfer (ICT)
The reaction coordinate contains both solvent
relaxation and rotation of the dimethylamino group
. Room temperature fluorescence spectrum
in hexane and tetrahydrofurane
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6. PET Photoinduced Electron Transfer
(reductive and oxidative)
LUMO
HOMO