Transcript No Slide Title

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.
Often referred to as “quenching” as it
removes excess electronic energy of
initially excited molecule.
Radiative Transfer
D*  D + h
h + A  A*



Long range
Radiative selection
rules
Overlap of absorption
and emission spectra
Rate  k P [D*]
D A
fl abs

P  [ A]l  FD ( ) A ( )d
A
abs
0
PabsA - probability of absorption of A
FD() – spectral distribution of donor emission
A() – molar absorption coefficient of acceptor
 - path length of absorption
Overlap of absorption spectrum of A
and emission spectrum of D
Non-radiative mechanism
A + D*  [AD*]  [A*D]  A* + D


Formation of collision complex
Intramolecular energy transfer within complex – Apply
Fermi’s Golden Rule
2
2
*
k
A*D H ' AD*  ( E )


A*D  A*D
AD*  AD*

H’ is perturbation due to intermolecular forces
(Coulombic, long range – “Forster”) or electronic orbital
overlap (exchange, short range – “Dexter”)
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


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
H '
 AD
r
k
f ( D D A A )
3
A 2
if
D 2
if
(R ) (R )
r
6
RifA   A** A d
A
r
D

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
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,
 r0 
kT    
r
1
D
6
r 
6
0
0.529 Df 2
4
n NA

F
D
( ) A ( )
0
Note fD D-1 is equal to the fluorescence rate
constant for D.
d
4
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)
ET 
wT
w0  wT
6
R0
R6
ET  6
 6
6
R0  R
R0  R 6
wT can be identified with the
rate of energy transfer at the
critical distance R0 (see above)
Short range energy transfer (Dexter)

Exchange interaction; overlap of
wavefunctions of A and D
kT (exchange)  exp(2rDA / L)


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

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
3O
2
+ 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

Use intermolecular energy transfer to
prepare molecules in triplet state

e.g.,

benzophenone
(T1) +
(S0)
(S0) + naphthalene (T1)
naphthalene
benzophenone
Important in situations where S1 state
undergoes slow ISC or reacts rapidly.
Triplet-triplet annihilation
D (T1 )  D (T1 )  D (S1 )  D(S0 )
*
*
*
P-type delayed Fluorescence

Kinetic scheme
S 0  S1
I abs
k2
S1 
S 0  h
k3
S1 
T1
T1  S 0  h
k4
T1  T1  S1  S 0
k5
Delayed fluorescence (after
extinction of light source):
d [ S1 ]
 (k 2  k3 )[S1 ]  k5 [T1 ]2  0
dt
k5
[ S1 ] 
[T1 ]2
k 2  k3
d [T1 ]
 k 4 [T1 ]
dt
[T1 ]  [T1 ]0 exp(k 4t )
After initial [S1]
population lost
k5 k 2
2
I df 
[T1 ]0 exp(2k 4t )
k 2  k3
P-type delayed fluorescene
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 quenching –
fluorescence decays
more rapidly as [A]
Static quenching – no
change in lifetime but
initial intensity lower
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
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)
Pyrene excimer
Excimer Laser
Population
inversion between
exciplex state and
unpopulated
unbound ground
state