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Energy and Electron Transfer Chapter 7 7.1 Mechanisms for Energy and Electron Transfer By exchange mech. 2 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 3 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 4 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. 5 7.2 Trivial Electron Transfer Mechanism 6 7.3 Energy and Electron Transfer by NonEmissive 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 7 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 8 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 9 Efficiency of Energy Transfer by Dipole-Dipole Mechanism R0 is distance at which ET is 50% efficient 10 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 11 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 12 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 13 Energy Transfer Mechanism Comparison • Förster (Coulombic) a) KETR-6 • Dexter(e- exchange) a) KETexp(-2r/L) 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 b) independent of oscillator strength c) ET not related to an experimental quantity 14 7.6 Types and Energetics of Electron Transfer • Full electron transfer 3. Charge transfer 4. Charge translocation 15 Oxidation and Reduction Excited states of diamagnetic molecules with closed shell ground states are better oxidizing and reducing agents than their corresponding g.s. 16 Calculating G Get from cyclic voltammetry 17 Approximations and Example • Approximations coulombic energy gain ignored -e2/r E*D is an enthalpy not a Gibbs energy is solvent dielectric constant Forward e- transfer favored in the excited state and the reverse for g.s. Coulombic term 18 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 19 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) 20 Marcus Theory of electron Transfer 21 Marcus Theory of electron Transfer is the transmission coefficient N is the electronic factor is the reorganisational energy 22 Marcus Theory of electron Transfer Reference: www.chem.unc.edu/undergrads/2002fall/chem145_murray/classnotes/ETtheory.pdf 23 Marcus Theory of electron Transfer Reference: www.chem.unc.edu/undergrads/2002fall/chem145_murray/classnotes/ETtheory.pdf 24 Inverted Region 25 Chemical Spectroscopy • Determine ket from product ratios 26 7.8 Contact and Solvent Separated Radical Ion Pairs • CRIP • No solvent molecules between D+ and A- •SSRIP •Shielding effect high in polar solvents 27 7.8 Contact and Solvent Separated Radical Ion Pairs : Example • Y=H • CRIP is more Stable than SSRIP • k2 values vary with structure 28 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 29 30