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Energy and Electron Transfer

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Presentation on theme: "Energy and Electron Transfer"— Presentation transcript:

1 Energy and Electron Transfer
Chapter 7

2 7.1 Mechanisms for Energy and Electron Transfer
By exchange mech.

3 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

4 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

5 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.

6 7.2 Trivial Electron Transfer Mechanism

7 7. 3 Energy and Electron Transfer by Non-Emissive Mechanisms. 1
7.3 Energy and Electron Transfer by Non-Emissive Mechanisms 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

8 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

9 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

10 Efficiency of Energy Transfer by Dipole-Dipole Mechanism
R0 is distance at which ET is 50% efficient

11 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

12 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

13 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

14 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

15 7.6 Types and Energetics of Electron Transfer
Full electron transfer 3. Charge transfer 4. Charge translocation

16 Oxidation and Reduction
Excited states of diamagnetic molecules with closed shell ground states are better oxidizing and reducing agents than their corresponding g.s.

17 Calculating G Get from cyclic voltammetry

18 Approximations and Example
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

19 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

20 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)

21 Marcus Theory of electron Transfer

22 Marcus Theory of electron Transfer
 is the transmission coefficient N is the electronic factor  is the reorganisational energy

23 Marcus Theory of electron Transfer

24 Marcus Theory of electron Transfer

25 Inverted Region

26 Chemical Spectroscopy
Determine ket from product ratios

27 7.8 Contact and Solvent Separated Radical Ion Pairs
SSRIP Shielding effect high in polar solvents CRIP No solvent molecules between D+ and A-

28 7.8 Contact and Solvent Separated Radical Ion Pairs : Example
Y=H CRIP is more Stable than SSRIP k2 values vary with structure

29 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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