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The End

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Title: The End


1
Chapter 7
  • The End

2
7.9 Energy Transfer Equilibrium
  • Equilibrium between singlet states expected to be
    quite rare except when linked by a spacer
  • Intermolecular equilibrium for triplets readily
    achieved

Concentration ratio of A/ D is
monitored as a fct of time. A cst ratio is
indicative of energy transfer equilibration
To allow comparison of energy from
spectroscopic measurements and energy
transfer equilibrium
3
7.9 Electron Transfer Equilibrium
  • In the ground state
  • In the excited state

4
7.10 Chemiluminescent Ion Recombination
  • Ion recombination in solution can lead to an
    excited state
  • 2 factors a) Marcus inverted region where
    smaller ?G to excited state may be kinetically
    preferred
  • b) a triplet radical ion
    pair can populate an excited triplet state of D,
    but formation of ground state products is spin
    forbidden

5
7.11 Role of Molecular Diffusion in Energy and
Electron Transfer Processes in Solution
  • Delivery Processes
  • 1. Organized Proximity
  • Linked chromophores Ex, Photosynthesis
  • 2. Diffusional Processes
  • Transfer requires proximity and the medium
    allows mobility of the reaction partners
  • 3. Conducting Medium
  • Ex Trivial Mechanisms for Energy and Electron
    transfer

6
7.11 Example with Energy Transfer
  • (a) D and A diffuse through the solution until
    they meet as collision partners in an encounter
    complex DA.
  • (b) Collisions occur between D and A in the
    encounter complex DA, and one of these
    collisions eventually leads to energy transfer
    and generation of a new encounter complex DA.
    (CAGE EFFECTS)
  • (c) The encounter complex DA breaks up into free
    D A.

7
7.11 Estimation of Rate Constants for Diffusion
Controlled Processes
  • Smoluchowski eqn.
  • For large solute molecules in small solvent
    molecules
  • Debye eqn.
  • For small solute molecules in large solvent
    molecules ?2000

8
K diff as a fct of Viscosity
You can also make an Arrhenius plot since
viscosity is temperature dependent
9
Diffusion-controlled Energy Transfer
  • Experimental Criteria
  • A) k obs close to k calc
  • B) k obs is a fct of T/?
  • C) k obs is invariant for quenchers of widely
    varying structures
  • D) k obs reaches a limiting value that
    corresponds to the fastest bimolecular rate
    constant measured for that solvent
  • Totally diffusion controlled energy transfer
    processes are rare

10
Reactions that are Near Diffusion-Control
  • Group 1
  • Spin Statistical Factors come into play
  • They behave as typical diffusion controlled
    processes but rate constants are a constant
    fraction of k diff
  • Group 2
  • First step is reversible

Kobsltkdif
11
Viscosity and Diffusion-Control
  • Experimental rate constant approach diffusion
    control rate constants for ?2cP in a range
    considered viscous fluid.
  • Common situation
  • EadifgtEa ET
  • If ET is exothermic,
  • kET will be temperature indep.
  • So experimentally EaobsltEa dif
  • kdif will change more with temperature than k
    obs
  • And kobs kdif at low temperature

12
7.12 Cage Effect
  • Average frequency of collisions unchanged but
    distribution in time is changed
  • Secondary cage reeencounterdue to non-uniform
    distribution of reactants who can react together
    even when separated by at least one solvent
    molecule

13
7.13 Distance-Time Relationships for Diffusion
  • Random walk equation
  • Brownian movement

14
7.14 Diffusion Control in Systems Involving
Charged Species
  • Charge as well as viscosity, temperature and
    molecular dimensions play a role in cage effects
  • In the case of electron transfer, coulombic
    effects may affect the separation of the
    products
  • Need to add an electrostatic correction to k dif

15
7.15 Effect of Rapid Molecular Processes on the
Mechanism of Processes Approaching Diffusion
  • 1) Quenching by diffusional processes
  • Most common regime. Processes with long time
    scales
  • 2) Quenching influenced by transient effects
  • When conc. are high and lifetimes are short,
    redistribution of quenchers (A) around the probe
    molecule (D) cannot be reestablished during the
    lifetime of D
  • Time -dependent rate constant
  • 3) Static quenching
  • No transport of molecules through the solution as
    in solid matrices (Perrin Formulation)

16
Transient Effects
  • the average path traveled by a molecule with D
    10 5 cm2s 1 in 100 fs, 10 ps and 1 ns, we find
    these distances to be 0.14 Å, 1.4 Å and 14 Å,
    respectively, clearly, in at least the first two
    cases, distances that are too short for diffusion
    to be able to re-establish

17
Concentration of Quenchers with Time
10-9 s
0 s
  • Rate of quenching at short time scales is faster
    than those in the diffusional regime

18
7.16 Energy Transfer in the Absence of Diffusion
  • In rigid systems Possible mechanisms
  • 1) Physical mass transport
  • system not rigid at a molecular level
  • 2) Proximity (Perrin Formulation)
  • D and A are close enough to permit transfer
  • Competing processes are slower in rigid media
    than in fluid media
  • 3) Conducting Media
  • 4)Energy or Electron migration
  • Molecular wires
  • Hopping occurs in polymeric materials

19
Perrin Formulation (for Static Quenching)
  • No displacement of D and A during lifetime of D
  • No quenching from outside the quenching sphere
    ?0 and ?1 in the quenching sphere
  • ?º and ? are the efficiencies for donor emission
    in the absence and presence of quencher(A)
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