Bronislaw Marciniak
Faculty of Chemistry, Adam Mickiewicz University,
Poznan, Poland
2012/2013 - lecture 8

5. Examples illustrating the investigation of photoreaction mechanisms:
photoinduced electron transfer and energy transfer processes

Kinetic of quenching
A(S
0
)

A(S
1
)
A(S
1
)

A(S
0
) + h n f
A(S
1
)

A(S
0
) + heat
A(S
1
)

A(T
1
)
A(S
1
)

B + C
A(S
1
) + Q
 quenching
A(T
1
)

A(S
0
) + h n p
A(T
1
)

A(S
0
) + heat
A(T
1
)

B' + C'
A(T
1
) + Q
 quenching rate
I a
(einstein dm -3 s -1) k f
[A(S
1
)] k
IC
[A(S
1
)] k
ISC
[A(S
1
)] k r
[A(S
1
)] k q
[A(S
1
)] [Q] k p
[A(T
1
)] k'
ISC
[A(T
1
)] k' r
[A(T
1
)] k' q
[A(T
1
)] [Q]

Kinetic of quenching
Energy transfer
A(T
1
) + Q

A + Q*
Q*

Q + h n e
Q*

Q + heat
Q*
 products rate k' q
[A(T
1
)] [Q] k” e
[Q*] k” d
[Q*] k” r
[Q*]

Stern-Volmer equation
 0 p
 p

1
 k
' q
 0
T
[Q]

'
0
R

'
R

1
 k q
'  0
T
[Q]
 0
T

T

1
 k
' q
 0
T
[Q] for T
1
1

T

1

T
0
 k q
' [Q] k obs
 k
0 + k q
' [Q]
 0
T

1 k p
+ k
'
ISC
+ k r
'

T

1 k p
+ ' k
ISC
+ k r
' + k q
' [Q]

Stern-Volmer equation
Sensitized emission of Q
1

Q

1

Q




1
 k
' q
1
 0
T
[ Q ]



 modified Stern-Volmer equation

Q
= k” e
/(k” e
+ k” d
+ k” r
)
(observation of any process from Q* gives a direct evidence for the participation of energy transfer)

Quenching of triplet states of organic compoundes by lanthanide 1,3-diketonate chelates in solutions
1. B. Marciniak, M. Elbanowski, S. Lis,
Monatsh. Chem . , 119 , 669-676 (1988)
"Quenching of Triplet State of Benzophenone by Lanthanide 1,3-
Diketonate Chelates in Solutions"
2. B. Marciniak, G. L. Hug
J. Photochem. Photobiol. A: Chemistry , 78 , 7-13 (1994)
"Energy Transfer Process in the Quenching Triplet States of Organic
Compunds by 1,3-Diketonates of Lanthanides(III) and Magnesium(II) in
Acetonitrile Solution. Laser Flash Photolysis Studies"
3. B. Marciniak, G. L. Hug
Coord. Chem. Rev.
, 159 , 55-74 (1997)
"Quenching of Triplet States of Organic Compounds by 1,3-Diketonate
Transition-Metal Chelates in Solution. Energy and/or Electron Transfer"

M = Ln (III) or Mg(II) acac hfac
R
1
= R
3
= CH
3
R
2
= H
R
1
= R
3
= CF
3
R
2
= H

Benzophenone phoshorescence in the presence of Eu(acac)
3
(
 ph
= 455 nm)

Stern-Volmer plot for quenching of BP phosphorescence by Eu(acac)
3 in benzene
1.0
0.8
0.6
0.4
0.2
0.0
0
 ph
= 455 nm
K = k q
 0
T
= (1.93 +- 0.16) x 10
3
M
-1
5 1 2 3
[Eu(acac)
3
] x 10
4
(M)
4

Modified Stern-Volmer plot for emission of
Eu(acac)
3 in benzene
0.25
0.20
0.15
 em
= 618 nm
0.10
0.05
K = k q
 0
T
= (2.3 +- 0.6) x 10
3
M
-1
)
0.00
0 2 4 6 8 10 12 14 16 18 20 22
1/[Eu(acac)
3
] x10
-3
M
-1

Results for Eu(acac)
3
: quenching: K = k q

0
T
= (1.93

0.16)

10 3 M -1 sensitization: K = k q

0
T
= (2.3

0.6)

10 3 M -1 for Tb(acac)
3
: quenching: K = k q

0
T
= (1.70

0.15)

10 3 M -1 sensitization: K = k q

0
T
=

1.4

10 3 M -1
K quenching
= K sensitization k q
(from quenching)

0
T
= constant
= k q
(from sensitized emission)

Conclusions
1. BP phosphorescence is quenched by Ln(acac)
3
(Ln= Sm,
2.
Eu, Gd, Tb, Dy) and Mg(acac) k q
2 with the rate constants

9

10 8 M -1 s -1 (in acetonitrile).
k q for quenching by Eu +3 and Tb +3 (perchlorates) are at least 5 times lower.
3. k q

4

10 9 M -1 s -1 for quenching by Eu(hfac)
3
4. Similar k q values obtained from the quenching and sensitization indicate the energy transfer process:
A(T
1
) + Q

A + Q*
5. Similar k q values for all Ln(acac)
3 and Mg(acac)
2 used indicate the energy transfer from BP tiplet state to the ligand localized triplet state.

3 D* + Q

D + 3 Q*
Energy transfer from BP tiplet state to the ligand localized triplet state
Sandros relation: k q
/k dyf
= [1 + exp -(E
T
(D) - E
T
(Q))/RT] -1

Rates of energy transfer vs donor-aceeptor energy differences k q
/k dyf
= [1 + exp
- 
E
T
/RT]
-
1

Quenching of triplet states of organic compoundes by lanthanide 1,3-diketonate chelates in solutions. Laser flash photolysis studies

Decay of BP triplet (

TT
= 530 nm) and rise of Tb(III) emission (
 e
= 550 nm)
([BP] = 1 mM, [Tbacac)3 = 0.19 mM in MeCN) k decay
=2.2

10 5 s -1 k rise
=2.7

10 5 s -1
3 D* + Q

D + Q*

Dependence of k q on E
T

sk d k en k
-d
3 D* + m Q n (D*...Q) n (D...Q*)

1 D* + n Q* k
d k
en s = n/3m (spin statistical factor)

G en
=
-
Nhc [ n
0-0
( 3 D*)
- n
0-0
( n Q*) ]


G en and

G el
- the standarg free-energy changes for energyand electron transfer processes

G
 en and

G
 el
- thre free energy of activation for energyand electron transfer processes k d
- the diffusion rate constant k
-d
- the dissociation rate constant for the encounter complex

 en and
 el
- transmission coefficients k 0 en and k 0 en
- preexponential factors
Limiting value of k q
(plateau value): k pl q
 k s k d k
0 en ( el )
0 en ( el )
 k
d

k d is the diffusion rate constant k d
= 8000RT/3

(Debye equation) k
d is the dissociation rate constant for the encounter complex k
d
= 3000k d
/4
 r 3 N
0
(Eigen equation) for CH
3
CN at room temperature: k d
=1.9

10 10 M
-
1 s
-
1 k
d
= 2.2

10 10 s
-
1 (r = 7A)

Energy transfer to ligand-localized triplet states of Tb(acac)
3’
Gd(acac)
3
, Mg(acac)
2
,and Mg(hfac)
3 taking: k q pl = (3-7)

10 9 M -1 s -1
(for energy transfer to acac or hfac triplet states) s = 1
( 1 Q and 3 Q*) k 0 en
 en

5

10 9 s -1

1

10 -3

3 taking: k q pl = 3

10 6 M -1 s -1
(for energy transfer to Tb(III) 5 D
4 level) s= 5/21
(Q and Q* are 7 F
6 and 5 D
4 level) k 0 en
 en
= 1.5

10 7 s -1
= 2.4

10 -6
(three order of magnitude lower than for energy transfer to ligand-localized triplet states)

Dependence of k q on E
T

Conclusions
1. Quenching of the triplet states of organic compounds by by lanthanide(III) and magnesium(II) 1,3-diketonates in
MeCN is adequately described by energy transfer to the excited ff states of lanthanide complexes or by energy transer to the ligand-localized triplet states.
2. The values of transmission coefficients for energy transfer to the ff* states are in the range of 10 -6 , and are three order of magnitude lower than those for energy transfer to ligand-localized triplets.
3. In the case of BP derivatives, an additional quenching process, i.e.
electron transfer from acac ligand to the BP triplet may occur.