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)
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
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*
2. Absorption of UV-VIS light
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;
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 -> p* transition is lower, and consequently, the wavelength of the corresponding
absorption band is larger
3. Fluorescence emission
Perrin-Jablonski diagram
ilustration of the relative positions of
absorption, florescence and phosphorescence spectra
Stockes shift
4. Fluorescent compounds and types of fluorofores
aromatic compounds
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
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