Energy and Electron Transfer

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Energy and Electron Transfer

• Chapter 7

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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 Non-Emissive
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 (2p?t)
• Dipole-dipole coupling Förster mech.
• For light absorption
• For energy transfer
• If they dont 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 kD 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
• 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

• Dexter(e- exchange)
• a) KET?exp(-2r/L)
• 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 ?
  is solvent dielectric constant
• ED is an enthalpy not a Gibbs energy

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/c
hem145_murray/classnotes/ETtheory.pdf
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Marcus Theory of electron Transfer
Reference www.chem.unc.edu/undergrads/2002fall/c
hem145_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

• SSRIP
• Shielding effect high in
• polar solvents

• CRIP
• No solvent molecules between D and A-

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7.8 Contact and Solvent Separated Radical Ion
Pairs Example

• YH
• 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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