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 transferfrom 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 photonsemitted by D*.(c) The light absorbing ability of A.(d) The overlap of the emission spectrum of D* and the absorptionspectrum of A, with consideration given to the extinction coefficientof 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 transfer2. Triplet-triplet annihilation3. Charge transfer4. 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 constantE*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 Transfer1) Trivial(radiative)2) Coulombic ( Förster theory)3) Electron Exchange (Dexter )(sum of electron and hole

exchange)

• Electron Transfer1) Trivial (e- ejection-e- capture)2) Marcus Theory • Processes that occur by e-

exchange1) Energy Transfer2) TTA3)Charge Transfer4) 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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