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Energy and Electron Transfer Chapter 7. 2 7.1 Mechanisms for Energy and Electron Transfer By exchange mech.

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Presentation on theme: "Energy and Electron Transfer Chapter 7. 2 7.1 Mechanisms for Energy and Electron Transfer By exchange mech."— Presentation transcript:

1 Energy and Electron Transfer Chapter 7

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

3 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 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 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 6 7.2 Trivial Electron Transfer Mechanism

7 7 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

8 8 7.4 Transmitter-Antenna Mechanism for Energy transfer by Coulombic Interactions Induction of a dipole oscillation in A by D* µ = µ 0 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 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 10 Efficiency of Energy Transfer by Dipole-Dipole Mechanism R 0 is distance at which ET is 50% efficient

11 Electron Exchange Process Processes that can occur by electron transfer 1. Energy transfer 2. Triplet-triplet annihilation 3. Charge transfer 4. Charge translocation

12 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) r DA D_A separation relative to Van der Waals radii L

13 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 14 Energy Transfer Mechanism Comparison Förster (Coulombic) a) K ET  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 K D Dexter(e - exchange) a) K ET  exp(-2r/L) b) independent of oscillator strength c)  ET not related to an experimental quantity

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

16 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 17 Calculating  G Get from cyclic voltammetry

18 18 Approximations and Example Approximations coulombic energy gain ignored -e 2 /  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 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 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 21 Marcus Theory of electron Transfer

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

23 23 Marcus Theory of electron Transfer Reference:

24 24 Marcus Theory of electron Transfer Reference:

25 25 Inverted Region

26 26 Chemical Spectroscopy Determine k et from product ratios

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

28 Contact and Solvent Separated Radical Ion Pairs : Example Y=H CRIP is more Stable than SSRIP k 2 values vary with structure

29 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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