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FLUORESCENCE

FLUORESCENCE. Joanna Piechowska. 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

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FLUORESCENCE

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  1. FLUORESCENCE Joanna Piechowska

  2. 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)

  3. 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

  4. 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 ->p* < p->p* < n ->s* < s->p* < s ->s*

  5. 2. Absorption of UV-VIS light Distinction between singlet and triplet states

  6. 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;

  7. 2. Absorption of UV-VIS light Terminology for Absorption Shifts Rule: if the extent ofthe p electron system is greater, then the energy of the low-lying p->p*transition is lower, and consequently, the wavelength of the corresponding absorption band is larger

  8. 3. Fluorescence emission Perrin-Jablonski diagram ilustration of the relative positions of absorption, florescence and phosphorescence spectra

  9. Stockes shift

  10. 4. Fluorescent compounds and types of fluorofores aromatic compounds

  11. 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

  12. 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

  13. 6. PET – Photoinduced Electron Transfer (reductive and oxidative) LUMO HOMO

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