Molecular Luminescence

1
Molecular Luminescence

• Emission of a photon as an excited state molecule
  returns to a lower state
• Chemoluminescence
• Bioluminescence
• Crystalloluminescence
• Electroluminescence
• Photoluminescence
• Radioluminescence
• Sonoluminescence
• Thermoluminescence
• Triboluminescence

http//www.shef.ac.uk/content/1/c6/01/89/68/lumine
scence.jpg
2
Jablonski Diagram
Skoog, Hollar, Nieman, Principles of Instrumental
Analysis, Saunders College Publishing,
Philadelphia, 1998.
3
Absorption
Selection Rules
DJ ?1 Dv ?1, ?2, ?3, DS 0 (i.e. S ? S, T
? T)
Very Fast ? 10-14 10-15 sec.
Skoog, Hollar, Nieman, Principles of Instrumental
Analysis, Saunders College Publishing,
Philadelphia, 1998.
4
Vibrational Relaxation

• Excited molecule rapidly transfers excess
  vibrational energy to the solvent / medium
  through collisions.
• Molecule quickly relaxes into the ground
  vibrational level in the excited electronic
  level.
• Non-radiative process
• 10-11 10-10 sec.

Skoog, Hollar, Nieman, Principles of Instrumental
Analysis, Saunders College Publishing,
Philadelphia, 1998.
5
Internal Conversion

• Transfers into a lower energy electronic state
  of the same multiplicity without emission of a
  photon.
• Favored when there is an overlap of the
  electronic states potential energy curves.
• Non-radiative process (minimal energy change)
• 10-12 s between excited electronic states.

Skoog, Hollar, Nieman, Principles of Instrumental
Analysis, Saunders College Publishing,
Philadelphia, 1998.
6
Fluorescence

• Radiative transition between electronic states
  with the same multiplicity.
• Almost always a progression from the ground
  vibrational level of the 1st excited electronic
  state.
• 10-10 10-6 sec.
• Occurs at a lower energy than excitation.

Skoog, Hollar, Nieman, Principles of Instrumental
Analysis, Saunders College Publishing,
Philadelphia, 1998.
7
Stokes Shift
Skoog, Hollar, Nieman, Principles of Instrumental
Analysis, Saunders College Publishing,
Philadelphia, 1998.
8
Relationship between the shape of the excitation
and fluorescence bands.
Franck-Condon Factor
shift
Ingle and Crouch, Spectrochemical Analysis
P.R. Callis et. al., Chem. Phys. Lett, 244
(1995), 53-58.
9
External Conversion

• Non-radiative transition between electronic
  states involving transfer of energy to other
  species (solvent, solutes).
• Also referred to as quenching.
• Modifying conditions to reduce collisions
  reduces the rate of external conversion.
• Occurs on a comparable time scale as
  fluorescence.

Skoog, Hollar, Nieman, Principles of Instrumental
Analysis, Saunders College Publishing,
Philadelphia, 1998.
10
Intersystem Crossing

• Similar to internal conversion except that it
  occurs between electronic states with different
  multiplicities.
• Slower than internal conversion.
• More likely in molecules containing heavy
  nuclei.
• More likely in the presence of paramagnetic
  compounds.

Skoog, Hollar, Nieman, Principles of Instrumental
Analysis, Saunders College Publishing,
Philadelphia, 1998.
11
Luminol Chemoluminescence
www.wikipedia.org
12
Phosphorescence

• Radiative transition between electronic states
  of different multiplicities.
• Much slower than fluorescence (10-4 104 s).
• Even lower energy than fluorescence.

www.wikipedia.org
13
Dissociation
Ingle and Crouch, Spectrochemical Analysis
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Predissociation

• Occurs if the molecule enters a vibrational
  level above the dissociation limit during an
  internal conversion.
• Dissociation and predissociation are more
  likely in molecules that absorb at low l.

Skoog, Hollar, Nieman, Principles of Instrumental
Analysis, Saunders College Publishing,
Philadelphia, 1998.
15
Quantum Yield
Fraction of absorbed photons that are converted
to luminescence, fluorescence or phosphorescence
photons. May approach unity in favorable
cases.
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Fluorescence Quantum Yield
All activation and deactivation processes
discussed so far can be described using first
order rate constants.
nS1, nS0 population densities of S1 and S0. kA
rate of absorption kF rate of
fluorescence knr rate of non-radiative
deactivation processes.
17
A continuously illuminated sample volume (V) will
reach steady-state.
18
unitless but describes photons/molecule
Fluorescence Quantum Efficiency of a Molecule
kec external conversion (S1 ? S0) kic
internal conversion (S1 ? S0) kisc intersystem
crossing (S1 ? T1) kpd predissociation kd
dissociation
typically 105-107 s-1
typically 106-109 s-1
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Time Scales of Processes
http//micro.magnet.fsu.edu/primer/techniques/fluo
rescence/fluorescenceintro.html
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Are you getting the concept?
For a given fluorophore under steady state
conditions, excitation of a 1 cm3 sample volume
yields the following first-order rate constants
kf 5 x 107 s-1, knr 9 x 105 s-1, and ka 1 x
1014 s-1 and an overall rate of fluorescence
photon emission of 9.8 x 1019 photons/second.
What is the molecule number density in the ground
electronic state?
21
Phosphorescence Quantum Yield
Skoog, Hollar, Nieman, Principles of Instrumental
Analysis, Saunders College Publishing,
Philadelphia, 1998.
22
Phosphorescence Quantum Yield

• Product of two factors
• fraction of absorbed photons that undergo
  intersystem crossing.
• fraction of molecules in T1 that phosphoresce.

knr non-radiative deactivation of S1. knr
non-radiative deactivation of T1.
Is phosphorescence possible if kP lt kF?
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Conditions for Phosphorescence
kisc gt kF kec kic kpd kd kP gt knr
Skoog, Hollar, Nieman, Principles of Instrumental
Analysis, Saunders College Publishing,
Philadelphia, 1998.
24
Luminescence Lifetimes
Emitted Luminescence will decay with time
according to
luminescence radiant power at time t luminescence
radiant power at time 0 luminescence lifetime
Skoog, Hollar, Nieman, Principles of Instrumental
Analysis, Saunders College Publishing,
Philadelphia, 1998.
25
Quenching
Static Quenching Lumophore in ground state and
quencher form dark complex. Luminescence is only
observed from unbound lumophore. Luminescence
lifetime not affected by static
quenching. Dynamic Quenching/Collisional
Quenching Requires contact between quencher and
excited lumophore during collision (temperature
and viscosity dependent). Luminescence lifetime
drops with increasing quencher concentration. Long
-Range Quenching/Förster Quenching Result of
dipole-dipole coupling between donor (lumophore)
and acceptor (quencher). Rate of energy transfer
drops with R-6. Used to assess distances in
proteins.
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Fluorescence Resonance Energy Transfer (FRET)
http//www.olympusfluoview.com/applications/fretin
tro.html
27
Are you getting the concept?
Determine the type of quenching being
demonstrated in the figures below.
S. Amemiya et al., Chem. Commun.,1997, 1027.
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Fluorescence or Phosphorescence?
p p transitions are most favorable for
fluorescence.

• e is high (100 1000 times greater than n
  p)
• kF is also high (absorption and spontaneous
  emission are related).
• Fluorescence lifetime is short (10-7 10-9 s
  for p p vs. 10-5 10-7 s for n p).