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