An excited molecule can return to its ground state by combination of several steps. Fluoroscenece and phoshoresence involve emission of a photon of radiation. Other deactivation steps,indicated by wavy arrow are radiationless processes.

1. M R . P . R . J A D H A V
A S S T . P R O F E S S O R ,
M O D E R N C O L L E G E O F P H A R M A C Y , M O S H I , P U N E .
1
Deactivation processes in
Molecular Luminescence

2. Deactivation Processes
2
 An excited molecule can return to its ground state by
combination of several steps.
 Fluoroscenece and phoshoresence involve emission
of a photon of radiation.
 Other deactivation steps,indicated by wavy arrow are
radiationless processes.

3. 3
Figure 15-1

4. Processes that compete with fluorescence
4
1) vibrational relaxation
2) internal conversion
3) predissociation
4) dissociation
5) external conversion
6) intersystem crossing
7) phosphorescence

5. Vibrational relaxation
5
A molecule may promoted to any of several vibrational
level during electronic excitation process
much faster than fluorescence
fluorescence occurs at wavelengths longer than absorbed
radiation, except were no vibrational relaxation can take
place
Seen as l'r and lr
resonance fluorescence − labsorbed = lemitted

6. Internal conversion
6
processes that allow molecule to drop to lower electronic E state
without emission of photons
occurs by overlap of lowest vibrational level in a higher
vibrational level with an upper vibrational level in a lower E
electronic state which then leads to vibrational relaxation
can occur between:
2 or more upper electronic states (S1 and S2 in Fig. 15-1)
lowest excited electronic state and electronic ground state (S1 and
S0 in Fig. 15-1).
rate constant = kic

7. Predissociation
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electron movesfrom higher electronic state to upper
vibrational level of lower electronic state,in which
vibrational energy is great enough to cause rupture of
bond.
rate constant = kpd

8. Dissociation
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in this,absorbed radiation directly excites electron of
chromophore to a sufficently high vibrational level to
cause rupture of chromophoric bond.
rate constant = kd

9. External Conversion
9
solvent effect
radiationless transition
E transfer between excited molecule and solvent or other
solutes
rate constant = kec

10. Intersystem Crossing
10
process in which there is a crossover between electronic
states of different multiplicity.
spin of electron is reversed (singlet to triplet)
most common in moleculed containing heavy atoms.
molecules containing Cl, I,or Br (heavy atom effect) or
in solutions containing these atoms (or ions)
dissolved O2 , or other paramagnetic species
rate constant = ki

11. 11
 Phosphorescence
-intersystem crossing to triplet state.
-Low temp. required.
-External and internal conversions compete with
Phosphorescence