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 –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)

Violet: nm Indigo: nm Blue: nm Green: nm Yellow: nm Orange: nm Red: nm 1. Colors of the visible spectrum

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   * <    * <    *

Distinction between singlet and triplet states 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;

Terminology for Absorption Shifts Nature of ShiftDescriptive Term To Longer WavelengthBathochromic To Shorter WavelengthHypsochromic To Greater AbsorbanceHyperchromic To Lower AbsorbanceHypochromic 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 corresponding absorption band is larger 2. Absorption of UV-VIS light

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

Stockes shift

aromatic compounds 4. Fluorescent compounds and types of fluorofores

1.Internal heavy atom effect 4. Heterocycle compounds 2. Electron-donating substituents : -OH, -OR, -NH 2, -NHR, -NR 2 3. Electron-withdrawing substituents: carbonyl and nitrocompounds Z4Z4

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

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