FLUORESCENCE

Joanna Piechowska

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)

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

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

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 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

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, -NH2, -NHR, -NR2

3. Electron-withdrawing substituents: carbonyl and nitrocompounds

Z4

5. Intramolecular Charge Transfer (ICT)

The reaction coordinate containsboth solvent relaxation androtation of the dimethylaminogroup. Room temperaturefluorescence spectrum inhexane and tetrahydrofurane

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

LUMO

HOMO