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Photochemistry
Lecture 5Intermolecular 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 TransferD* D + hh + A A*
Long range Radiative selection
rules Overlap of absorption
and emission spectra
*][DPkRate Aabs
Dfl
dFlAP ADAabs )()(][
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
H’ is perturbation due to intermolecular forces (Coulombic, long range – “Forster”) or electronic orbital overlap (exchange, short range – “Dexter”)
)('2 2
*** EHk ADDA
DADA ** ** DAAD
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
)('3 AADDDA f
rH
6
22 )()(
r
RRk
Dif
Aif
dR AAAif *
*
ADr
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
6
01
r
rk DT
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,
0
44
260 )()(
529.0
dF
Nnr AD
A
Df
Note fD D
-1 is equal to the fluorescence rate
constant for D.
Efficiency of energy transfer
T
TT ww
wE
0
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)
660
6
660
60
RR
R
RR
RET
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
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 oxygen3O2 + D(S1) 3{O2;D(S1)} 3O2 + D(T1)
S=1 S=1 S=0,1,2
Oxygen (3g-) 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) + naphthalene (S0)
benzophenone (S0) + naphthalene (T1)
Important in situations where S1 state undergoes slow ISC or reacts rapidly.
Triplet-triplet annihilation)()()()( 01
*1
*1
* SDSDTDTD
P-type delayed Fluorescence Kinetic scheme
0111
01
11
01
10
5
4
3
2
SSTT
hST
TS
hSS
SS
k
k
k
k
Iabs
Delayed fluorescence (after extinction of light source):
)2exp(][
)exp(][][
][][
][][
0][])[(][
4201
32
25
4011
141
21
32
51
215132
1
tkTkk
kkI
tkTT
Tkdt
Td
Tkk
kS
TkSkkdt
Sd
df
After initial [S1] population lost
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 formationElectronically 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 typeM* + Q (M*Q)
Pyrene excimer
Excimer Laser
Population inversion between exciplex state and unpopulated unbound ground state