![Page 1: Fotoquímica - NUPEG · Fotoquímica Aula 5 e 6 Transferência de Energia e Transferência de Elétron Prof. Amilcar Machulek Junior IQ/USP - CEPEMA Caminhos de espécies fotoexcitadas](https://reader034.vdocumento.com/reader034/viewer/2022052519/5f1844e69eb1123c8d0c2fe3/html5/thumbnails/1.jpg)
Prof. Amilcar Machulek Junior (IQ/USP)
1
FotoquímicaAula 5 e 6
Transferência de Energia e Transferência de Elétron
Prof. Amilcar Machulek Junior
IQ/USP - CEPEMA
Caminhos de espécies fotoexcitadas
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Prof. Amilcar Machulek Junior (IQ/USP)
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Diagrama de Jablonski
S0
S1
S2
T1
Fluorescence(τ ~1 ns)
Phosphorescence(τ > 1 μs)
Intersystem Crossing(τ ~10 ns)
Relaxation(τ < 1ps)
Internal Conversion(Radiationless process) hν
Chemicalreactions
Chemicalreactions
Estado Fundamental: o mais baixo, não excitado, estado eletrônico de um átomo ou molécula
Transferência de energia intermolecular
D* + A → D + A*Donor Acceptor
E- Etransfer – both D* and A* are electronically excited.
Often referred to as “quenching” as it removes excess electronic energy of initially excited molecule.
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Prof. Amilcar Machulek Junior (IQ/USP)
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Transferência de energia intermolecular
Energy and electron transfer nomenclature
D+ + A → D + A+ hole transfer
D- + A → D + A- electron transfer
D* + A → D + A* energy transfer
* Electronic energy transfer
Radiative : trivial mechanism
Non-radiative
coulombic : 50 Å, long-range
electron-exchange : 10~15 Å
Trivial mechanism for energy transfer(radiative emission-absorption energy transfer)
D* → D + hνhν + A → A*
Trivial energy transfer between D* and A, with A* emitting following absorption of emission by D*. Part B emphasizes the independence of emission and absorption by notingthat D and A could be in different containers.
The "trivial" mechanism requires that D* emits photons which A is capable of absorbing. The rate or probability per unit time of energy transfer from D* to produce A* will depend on:
- High quantum yield for emission from D* φeD
- High concentration of A [A]- High extinction coefficient for A εA
- Overlap of emission from D* and absorption from A J = ∫0 ID εA dν∞ ~
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Prof. Amilcar Machulek Junior (IQ/USP)
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Radiative transfer
D* → D + hνhν + A → A*
Long rangeOverlap of absorption and emission spectra
PabsA - probability of absorption of A
FD(ν) – spectral distribution of donor emission
εA(ν) – molar absorption coefficient of acceptor
l - path length of absorption
*][DPkRate Aabs
Dfl=
ννεν dFlAP ADA
abs )()(][0∫∞
∝
Overlap of absorption spectrum of A and emission spectrum of D
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Prof. Amilcar Machulek Junior (IQ/USP)
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Non-radiative mechanism
A + D* → [AD*] → [A*D] → A* + D
Formation of collision complexIntramolecular energy transfer within complex
H’ is perturbation due to intermolecular forces (Coulombic, long range – “Forster”) or electronic orbital overlap (exchange, short range – “Dexter”)
)('2 2
*** EHk ADDA ρπ
∫ ΨΨ=h
DADA ΨΨ=Ψ ** ** DAAD ΨΨ=Ψ
Long Range (Forster) energy transfer
601 ⎟⎠⎞
⎜⎝⎛= −
rrk DT τ
There will be a critical distance r0 at which the rate of energy transfer is equal to the rate of decay of fluorescence of D (Typically r0 = 20 – 50 Å)
At this point kT = 1/τD. At any other distance,
∫∞Φ
=0
44
26
0 )()(529.0
νννεν
κ dFNn
r ADA
Df
Note ΦfD τD
-1 is equal to the fluorescence rate constant for D.
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Prof. Amilcar Machulek Junior (IQ/USP)
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Short range energy transfer (Dexter)Exchange interaction; overlap of wavefunctions of A and D
L is the sum of the van der Waals radii of donor and acceptor
Occurs over separations ≈ collision diameter
Typically occurs via exciplex formation (see below)
)/2exp()( Lrexchangek DAT −∝
Comparison of the Coulombic and Exchange mechanisms of electronic energy transfer
donor acceptor
Coulombicinteraction Coulombic
interactio
n
Coulombic
donor acceptor
Electron Exchange
Initial FinalIn Coulombic energy transfer no electrons "change molecules", but rather two transitions occur simultaneously in a process that could be described as the transfer of a "virtual photon"; Since no electrons are actually transferred in the Coulombic mechanism, it is clear that this process cannot have an analogy in the case of electron transfer.
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Prof. Amilcar Machulek Junior (IQ/USP)
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Endotermic and exotermic energy transfer
Schematic surface representation of collisional energy transfer
M* + Q → M + Q*
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Prof. Amilcar Machulek Junior (IQ/USP)
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Energy transfer
Energy transfer: triplete-triplete
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Prof. Amilcar Machulek Junior (IQ/USP)
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Energy transfer: triplete-triplete
Diffusional Encounter:D + A DA
Electron Transfer:DA → D+A-
Diffusional Separation:D+A- D+ + A-
Back Electron Transfer:D+A- → DA
Dynamics of electron transfer
The rate of electron transfer depends on:
(1) the distance between the electron donor and acceptor;
(2) standard Gibbs free energy change of the reaction;
(3) the energy associated with molecular rearrangements.
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Prof. Amilcar Machulek Junior (IQ/USP)
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OUTER SPHERE Electron Transfer
InnerSphere
Central Ion
OuterSphere
InnerSphere
Central Ion
OuterSphere
INNER SPHERE Electron Transfer
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Prof. Amilcar Machulek Junior (IQ/USP)
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Chemical Kinetics
Chemical Kinetics
Free Energy Scale: Only one unique lowest free energy pathway!
0 1Extent of Reaction
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Prof. Amilcar Machulek Junior (IQ/USP)
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Fe2+ Fe3+Fe3+ Fe2+
Self-Exchange Electron Transfer Reaction
(ΔGo = 0)
Fe2+ + Fe3+ Fe3+ + Fe2+
Fe3+Fe2+Fe3+ Fe2+
Reaction
Fe2+ + Fe3+ Fe3+ + Fe2+
Fe3+Fe2+
Forward Reaction
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Prof. Amilcar Machulek Junior (IQ/USP)
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Fe3+Fe2+Fe3+ Fe2+
Reaction
Fe2+ + Fe3+ Fe3+ + Fe2+
Fe3+Fe2+
Inverse Reaction
Principle of Microscopic ReversibilityLowest Free Energy Path in Forward Direction is Alsothe Lowest Free Energy Path in the Reverse Direction
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Prof. Amilcar Machulek Junior (IQ/USP)
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Principle of Microscopic ReversibilityLowest Free Energy Path in Forward Direction is Alsothe Lowest Free Energy Path in the Reverse Direction
Fe3+Fe2+Fe3+ Fe2+
Fe2+ + Fe3+ Fe3+ + Fe2+
Fe2.5+Fe2.5+
Reaction Profile that obeys the Hammond Postulate andMicroscopic Reversibility!
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Prof. Amilcar Machulek Junior (IQ/USP)
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Marcus Theory of Electron Transfer
Electron Transfer in a Donor-Acceptor Pair DA:
DA → D+A-
The rate of electron transfer in DA depends on:The standard Gibbs free energy change of the reaction;
The energy associated with molecular rearrangements
(reorganization of solvation shell and ligands).
λλ
0 10
The (Solvent/Ligand) Reorganization Free Energy (λ)
ΔG‡
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Prof. Amilcar Machulek Junior (IQ/USP)
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00 1
ΔGoxy
The Marcus ProcedureX + Y’ X’ + Y λxy = (λxx + λyy)/2 = 4ΔGxy
‡o
ΔGoxy < 0
G
x‡
ΔGxy‡ = (λxy/4)[1+ ΔGo
xy/λxy]2
The Free Energy of Activation is a Function of the Overall Free Energy Change for the Reaction!!
Diffusional Encounter:D* + A (DA)*
Electron Transfer:(DA)* → D+A-
Diffusional Separation:D+A- D+ + A-
Back Electron Transfer (to Ground State):D+A- → DA
Excited State Electron Transfer in Homogeneous Systems
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Prof. Amilcar Machulek Junior (IQ/USP)
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Diffusional Encounter:D* + A (DA)*
Electron Transfer:(DA)* → D+A-
Diffusional Separation:D+A- D+ + A-
Excited State Electron Transfer in Homogeneous Systems
Electrochemiluminescence
Diffusional Encounter:D* + A (DA)*
Electron Transfer:(DA)* → D+A-
Diffusional Separation:D+A- D+ + A-
Excited State Electron Transfer in Homogeneous Systems
Rehm – Weller Kinetic Model
kdif / k-dif
ket / k-et
ksep
kq = kdif[1 + (k-dif /ket) + (k-dif/ksep)(k-et/ket)]
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Prof. Amilcar Machulek Junior (IQ/USP)
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Excited State Electron Transfer in Homogeneous Systems
Rehm – Weller Kinetic Model
kq = kdif[1 + (k-dif /ket) + (k-dif/ksep)(k-et/ket)]
A exp(ΔG‡et/RT) exp(ΔGo
et/RT)
kq = 2 x 1010 M-1s-1
{1 + 0.25 [exp(ΔG‡et/RT) + exp(ΔGo
et/RT)]}
In Acetonitrile:
D*A
D + AD + ADA
D+.A-.
D+. + A-.wp
E½ox - E½
red
ΔGoet
wr
E*
ΔGoet = 23.06(E½
ox - E½red) - E* + (wp-wr)
Calculating the Free Energy Change for Electron Transfer
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Prof. Amilcar Machulek Junior (IQ/USP)
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Excited State Electron Transfer in Homogeneous Systems
Rehm – Weller Kinetic Model
kq = kdif[1 + (k-dif /ket) + (k-dif/ksep)(k-et/ket)]
A exp(ΔG‡et/RT) exp(ΔGo
et/RT)
kq = 2 x 1010 M-1s-1
{1 + 0.25 [exp(ΔG‡et/RT) + exp(ΔGo
et/RT)]}
In Acetonitrile:
Dynamics of electron transfer
The rate of electron transfer depends on:(1) the distance between the electron donor and acceptor;
(2) standard Gibbs free energy change of the reaction;
(3) the energy associated with molecular rearrangements.
Excited State Redox Potentials and Rate Constants for Outer Sphere Electron Transfer in Solution can be Estimated
via the Rehm-Weller Model when the Appropriate Redox Parameters are Available or can be Estimated.
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Prof. Amilcar Machulek Junior (IQ/USP)
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Reações Fotoquímicas
Reações Fotoquímicas
R R* R R*
R* Produto(s) R* I
I + Y Produto(s)
a) Reação Fotoquímica b) Reação Foto-iniciada
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Prof. Amilcar Machulek Junior (IQ/USP)
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Reações Fotoquímicas