2 7.1 Mechanisms for Energy and Electron Transfer By exchange mech.
3 Processes that Compete with Energy Transfer Radiative or radiationless processesEnergy transfer (ET)Energy wastedChemical reactionModes of deactivation ofD* by AEfficiency of energy transferQuantum yield of energy transfer
4 7.2 The Trivial Mechanisms for Energy Transfer There is no interaction between D* and A that triggers the transferNo encounter necessaryD* 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 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.
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 Mechanism1. No analogy with electrontransfer since no electrons aretransferred. Electrons do notchange molecules2. Electrons are transferredAs seen fig 1 energy transferis sum of electron and holetransfer
8 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 absorptionFor energy transferIf 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 solvent2 depends on orientation of dipolesk°D radiative rate constantJ overlap integral
10 Efficiency of Energy Transfer by Dipole-Dipole Mechanism R0 is distance at whichET is 50% efficient
11 7.5 Electron Exchange Process Processes that can occur by electron transfer1. Energy transfer2. Triplet-triplet annihilation3. Charge transfer4. 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 interactionsJ normalized spectral overlap(no dependence on A)rDA D_A separation relative toVan der Waals radiiL
13 2. Triplet-Triplet Annihilation by Electron Exchange 1/9 singlet encounters3/9 triplet encounters5/9 quintet encountersSince quintet encountersare dissociative, max rateis 4/9 of diffusion controlLong 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) KETR-6b) depends on the oscillator strengths of D* to D and A to A* transitionsc) Efficiency related to oscillator strength of Ato A* and of KDDexter(e- exchange)a) KETexp(-2r/L)b) independent of oscillator strengthc) ET not related to an experimental quantity
15 7.6 Types and Energetics of Electron Transfer Full electron transfer3. Charge transfer 4. Charge translocation
16 Oxidation and Reduction Excited states of diamagnetic molecules with closed shell ground statesare better oxidizing and reducing agents than their corresponding g.s.
18 Approximations and Example coulombic energy gain ignored -e2/r is solvent dielectric constantE*D is an enthalpy not a Gibbs energyForward 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 Transfer1) Trivial(e- ejection-e- capture)2) Marcus TheoryProcesses that occur by e- exchange1) Energy Transfer2) TTA3)Charge Transfer4) Charge Translocation
20 7.7 Marcus Theory of ElectronTransfer Solvent sphere needs to reorganizeFollow isotopicallyMolecular or Solvent ReorganisationLibby MarcusFollowing electron transfer Libby violates energy conservation so rearragements duringe- transferinner sphere (bond lengths and angles)outer sphere (rearrangement of solvent)
26 Chemical Spectroscopy Determine ket from product ratios
27 7.8 Contact and Solvent Separated Radical Ion Pairs SSRIPShielding effect high inpolar solventsCRIPNo solvent molecules between D+ and A-
28 7.8 Contact and Solvent Separated Radical Ion Pairs : Example Y=HCRIP is more Stable than SSRIPk2 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 completeRadiative 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