Energy and Electron Transfer Chapter 7. 2 7.1 Mechanisms for Energy and Electron Transfer By...
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Transcript of Energy and Electron Transfer Chapter 7. 2 7.1 Mechanisms for Energy and Electron Transfer By...
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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 ofD* 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 electrontransfer since no electrons are transferred. Electrons do not change molecules
2. Electrons are transferredAs 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 (2πt)
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 solvent2 depends on orientation of dipolesk°D radiative rate constantJ overlap integral
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Efficiency of Energy Transfer by Dipole-Dipole Mechanism
R0 is distance at whichET 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 interactionsJ normalized spectral overlap(no dependence on A)rDA D_A separation relative to Van der Waals radiiL
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2. Triplet-Triplet Annihilation by Electron Exchange
1/9 singlet encounters3/9 triplet encounters5/9 quintet encounters
Since quintet encountersare dissociative, max rateis 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) KETR-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) KETexp(-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 ReductionExcited 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
E*D 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 MarcusFollowing 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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