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Photochemistry. Lecture 5 Intermolecular electronic energy transfer. Intermolecular Energy Transfer. D* + A  D + A* Donor Acceptor E-E transfer – both D* and A* are electronically excited.

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photochemistry

Photochemistry

Lecture 5

Intermolecular electronic energy transfer

intermolecular energy transfer
Intermolecular Energy Transfer

D* + A  D + A*

Donor Acceptor

E-E transfer – both D* and A* are electronically excited.

Often referred to as “quenching” as it removes excess electronic energy of initially excited molecule.

radiative transfer
Radiative Transfer

D*  D + h

h + A  A*

  • Long range
  • Radiative selection rules
  • Overlap of absorption and emission spectra

PabsA-probability of absorption of A

FD() – spectral distribution of donor emission

A() – molar absorption coefficient of acceptor

 - path length of absorption

non radiative mechanism
Non-radiative mechanism

A + D*  [AD*]  [A*D]  A* + D

  • Formation of collision complex
  • Intramolecular energy transfer within complex – Apply Fermi’s Golden Rule
  • H’ is perturbation due to intermolecular forces (Coulombic, long range – “Forster”) or electronic orbital overlap (exchange, short range – “Dexter”)
energy gap law
Energy Gap Law
  • Collisional energy transfer most efficient when the minimum energy taken up as translation

i.e., ED*-ED EA*-EA

  • This can be thought of arising from Franck Condon principle within collision complex
long range energy transfer
Long-range energy transfer
  • Interaction between two dipoles A, D at a separation r.
  • Insert H’ into Fermi’s Golden Rule

Dependence on transition moments for A and D

Thus transfer subject to electric dipole selection rules

r

D

A

long range energy transfer1
Long range energy transfer

Overall energy transfer rate must be summed over all possible pairs of initial and final states of D and A* subject to energy conservation

- Depends on overlap of absorption spectrum of A and emission spectrum of D

long range forster energy transfer
Long Range (Forster) energy transfer

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/D. At any other distance,

Note fD D-1is equal to the fluorescence rate constant for D.

efficiency of energy transfer
Efficiency of energy transfer

Define wT the rate of energy transfer, ET the efficiency of transfer relative to other processes

w0 is the rate of competing processes (fluorescence, ISC etc)

wT can be identified with the rate of energy transfer at the critical distance R0 (see above)

short range energy transfer dexter
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)
spin correlation
Spin Correlation
  • Resultant vector spin of collision partners must be conserved in collision complex and subsequently in products
  • D(S1) + A(S0) both spins zero, thus resultant spin SDA=0 - can only form products of same spin
  • D(T1) + A(S0) SD=1, SA=0, thus SDA=1 – must form singlet + triplet products
  • D(T1) + A(T1) SDA = 2, 1, or 0 thus can form e.g., S + S, S + T, or T + T
quenching by oxygen
Quenching by oxygen

3O2 + D(S1)  3{O2;D(S1)}  3O2 + D(T1)

S=1 S=1 S=0,1,2

Oxygen (3g-) recognised as strong inducer of intersystem crossing.

De-oxygenated solutions used where reaction from S1 state necessary.

triplet sensitization
Triplet sensitization
  • Use intermolecular energy transfer to prepare molecules in triplet state
  • e.g.,
  • benzophenone (T1) + naphthalene (S0)

benzophenone (S0) + naphthalene (T1)

Important in situations where S1 state undergoes slow ISC or reacts rapidly.

p type delayed fluorescence
P-type delayed Fluorescence

Delayed fluorescence (after extinction of light source):

  • Kinetic scheme

After initial [S1] population lost

dynamic versus static quenching
Dynamic versus static quenching
  • Dynamic quenching: in solution energy transfer processes depend on D* and A coming into contact by diffusion – very fast processes may be diffusion limited.
    • As quencher concentration increases, fluorescence decays more rapidly.
  • Static quenching – in a rigid system, energy transfer is effectively immediately if a quenching molecule is within a certain distance of D*. Thus the initial fluorescence intensity is lower.
dynamic vs static quenching effect on fluorescence decay of increasing quencher concentration
Dynamic vs static quenching- effect on fluorescence decay of increasing quencher concentration

Static quenching – no change in lifetime but initial intensity lower

Dynamic quenching – fluorescence decays more rapidly as [A]

exciplex formation
Exciplex formation

Electronically excited state of the collision complex more strongly bound than ground state

Fluorescence leads to ground state monomers

M* +M

M + M

excimer formation
Excimer formation
  • Exchange interaction stabilizes M*M (cf helium dimer)
  • Emission at longer wavelength than monomer fluorescence
  • Time dependence of excimer fluorescence
    • - builds up and decays on short time scale
  • Exciplexes are mixed complexes of the above type

M* + Q  (M*Q)

slide25

Excimer Laser

Population inversion between exciplex state and unpopulated unbound ground state