Download - FLUORESCENCE
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FLUORESCENCE
Joanna Piechowska
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1. Colors of the visible spectrum2. 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 fluorofores5. Intramolecular Charge Transfer (ICT)6. Photoinduced Electron Transfer (PET)
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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
1. Colors of the visible spectrum
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Rys. Energy levels of molecular orbitals in formaldehyde
Types of electronic transitions in polyatomic molecules
2. Absorption of UV-VIS light
The energy of these electronic transitions is generally in the following order:
n * < * < n * < * < *
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Distinction between singlet and triplet states
2. Absorption of UV-VIS light
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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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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 electron system is greater, then the energy of the low-lying* transition is lower, and consequently, the wavelength of the correspondingabsorption band is larger
2. Absorption of UV-VIS light
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3. Fluorescence emission
ilustration of the relative positions of absorption, florescence and phosphorescence spectra
Perrin-Jablonski diagram
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Stockes shift
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aromatic compounds
4. Fluorescent compounds and types of fluorofores
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1.Internal heavy atom effect
4. Heterocycle compounds
2. Electron-donating substituents : -OH, -OR, -NH2, -NHR, -NR2
3. Electron-withdrawing substituents: carbonyl and nitrocompounds
Z4
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5. Intramolecular Charge Transfer (ICT)
The reaction coordinate containsboth solvent relaxation androtation of the dimethylaminogroup. Room temperaturefluorescence spectrum inhexane and tetrahydrofurane
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6. PET – Photoinduced Electron Transfer (reductive and oxidative)
LUMO
HOMO