Download Energy and Electron Transfer

Energy and Electron Transfer
Chapter 7
7.1 Mechanisms for Energy and
Electron Transfer
By exchange mech.
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Processes that Compete with Energy Transfer
Radiative or radiationless
processes
Energy transfer (ET)
Energy wasted
Chemical reaction
Modes of deactivation of
D* by A
Efficiency of energy transfer
Quantum yield of energy transfer
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7.2 The Trivial Mechanisms for
Energy Transfer
• There is no interaction between D* and A
that triggers the transfer
• No encounter necessary
• D* is an excitation donor and A an
excitation acceptor
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Rate or Probability of Trivial
Energy Transfer
The rate or probability per unit time of energy transfer
from D* to produce A* will depend on:
(a) The quantum yield (e D ) of emission by D*.
(b) The number of A molecules (concentration) in the path of photons
emitted by D*.
(c) The light absorbing ability of A.
(d) The overlap of the emission spectrum of D* and the absorption
spectrum of A, with consideration given to the extinction coefficient
of A at the wavelength of overlap.
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7.2 Trivial Electron Transfer
Mechanism
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7.3 Energy and Electron Transfer by NonEmissive Mechanisms.
1. Coulombic Energy Transfer
2. Electron Exchange Mechanism
1. No analogy with electron
transfer since no electrons are
transferred. Electrons do not
change molecules
2. Electrons are transferred
As seen fig 1 energy transfer
is sum of electron and hole
transfer
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7.4 Transmitter-Antenna Mechanism for
Energy transfer by Coulombic Interactions
Induction of a dipole oscillation in A by D*
µ = µ0 cos (2πt)
Dipole-dipole coupling= Förster mech.
For light absorption
For energy transfer
If they don’t match : energy conservation is maintained by
the vibrational and rotational modes of D and A being
recipients of the excess energy
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Coulombic Energy Transfer
Förster Theory
(Interactin energy) 2
 varies with conc. And solvent
2 depends on orientation of dipoles
k°D radiative rate constant
J overlap integral
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Efficiency of Energy Transfer by
Dipole-Dipole Mechanism
R0 is distance at which
ET is 50% efficient
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7.5 Electron Exchange Process
• Processes that can occur by electron transfer
1. Energy transfer
2. Triplet-triplet annihilation
3. Charge transfer
4. Charge translocation
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1.Energy Transfer by Electron
Exchange
• Energy transfer can be dipole-induced (Förster or
Coulombic) or exchange-induced (Dexter)
K related to orbital interactions
J normalized spectral overlap
(no dependence on A)
rDA D_A separation relative to
Van der Waals radii
L
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2. Triplet-Triplet Annihilation
by Electron Exchange
1/9 singlet encounters
3/9 triplet encounters
5/9 quintet encounters
Since quintet encounters
are dissociative, max rate
is 4/9 of diffusion control
Long lived fluorescence (magnitude of the triplet lifetime depending on
other forms of decay of the triplet)
P-typed delayed fluorescence
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Energy Transfer Mechanism
Comparison
• Förster (Coulombic)
a) KETR-6
• Dexter(e- exchange)
a) KETexp(-2r/L)
b) depends on the
oscillator strengths of
D* to D and A to A*
transitions
c) Efficiency related to
oscillator strength of Ato
A* and of KD
b) independent of
oscillator strength
c) ET not related to an
experimental quantity
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7.6 Types and Energetics of
Electron Transfer
• Full electron transfer
3. Charge transfer 4. Charge translocation
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Oxidation and Reduction
Excited states of diamagnetic molecules with closed shell ground states
are better oxidizing and reducing agents than their corresponding g.s.
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Calculating G
Get from cyclic voltammetry
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Approximations and Example
• Approximations
coulombic energy gain ignored -e2/r
E*D is an enthalpy not a Gibbs energy
 is solvent dielectric constant
Forward e- transfer favored in the
excited state and the reverse for g.s.
Coulombic term
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Summary
• Energy Transfer
1) Trivial(radiative)
2) Coulombic ( Förster theory)
3) Electron Exchange (Dexter )
(sum of electron and hole
exchange)
• Electron Transfer
1) Trivial
(e- ejection-e- capture)
2) Marcus Theory
• Processes that occur by
e- exchange
1) Energy Transfer
2) TTA
3)Charge Transfer
4) Charge Translocation
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7.7 Marcus Theory of ElectronTransfer
• Solvent sphere needs to reorganize
• Follow isotopically
• Molecular or Solvent Reorganisation
Libby
Marcus
Following electron transfer
Libby violates energy conservation
so rearragements during
e- transfer
inner sphere (bond lengths and angles)
outer sphere (rearrangement of solvent)
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Marcus Theory of electron Transfer
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Marcus Theory of electron Transfer
 is the transmission coefficient
N is the electronic factor
 is the reorganisational energy
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Marcus Theory of electron Transfer
Reference: www.chem.unc.edu/undergrads/2002fall/chem145_murray/classnotes/ETtheory.pdf
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Marcus Theory of electron Transfer
Reference: www.chem.unc.edu/undergrads/2002fall/chem145_murray/classnotes/ETtheory.pdf
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Inverted Region
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Chemical Spectroscopy
• Determine ket from product ratios
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7.8 Contact and Solvent Separated Radical
Ion Pairs
• CRIP
• No solvent molecules
between D+ and A-
•SSRIP
•Shielding effect high in
polar solvents
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7.8 Contact and Solvent Separated Radical
Ion Pairs : Example
• Y=H
• CRIP is more Stable
than SSRIP
• k2 values vary with
structure
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CRIP Fluorescence
Gould & Farid
• C RIP is equivalent to an exciplex or an excited CT
complex in which charge transfer from D toA is complete
• Radiative and non-radiative return electron transfer where
the energy is dissipated into nuclear motions of A & D and
the solvent or is emitted as light
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