1. No category

"Molecular Photochemistry - how to
study mechanisms of photochemical
reactions ?"
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
rate
hn
A(S0)  A(S1)
Ia (einstein dm-3 s-1)
A(S1)  A(S0) + hnf
kf [A(S1)]
A(S1)  A(S0) + heat
kIC [A(S1)]
A(S1)  A(T1)
kISC [A(S1)]
A(S1)  B + C
kr [A(S1)]
A(S1) + Q  quenching
kq [A(S1)] [Q]
A(T1)  A(S0) + hnp
kp [A(T1)]
A(T1)  A(S0) + heat
k'ISC [A(T1)]
A(T1)  B' + C'
k'r [A(T1)]
A(T1) + Q  quenching
k'q [A(T1)] [Q]
Kinetic of quenching
Energy transfer
rate
A(T1) + Q  A + Q*
k'q [A(T1)] [Q]
Q*  Q + hne
k”e [Q*]
Q*  Q + heat
k”d [Q*]
Q*  products
k”r [Q*]
Stern-Volmer equation

0
p
p
for T1
 1  kq' 0T [Q]
'0R
 1  k q' 0T [Q]
'R
0T
 1  k q' 0T [Q]
T
1
1
 0  k q' [Q]
T
T
kobs  k 0 + kq' [Q]
1
 
'
k p + k ISC
+ kr'
0
T
T 
1
'
k p + k ISC
+ kr' + kq' [Q]
Stern-Volmer equation
Sensitized emission of Q

1
1 
1


1 ' 0
 Q Q  k q 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 1,3Diketonate 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"
R2
R1
R3
O
M
O
n
M = Ln (III) or Mg(II)
acac
hfac
R1= R3= CH3
R1= R3= CF3
R2= H
R2= 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
ph = 455 nm
0
I p/Ip -1
0.8
0.6
0.4
0.2
0
3
-1
K = kq T = (1.93 +- 0.16) x 10 M
0.0
0
1
2
3
4
4
[Eu(acac)3] x 10 (M)
5
Modified Stern-Volmer plot for emission of
Eu(acac)3 in benzene
0.25
em = 618 nm
0.20
1/Iem
0.15
0.10
0
3
-1
K = kq T = (2.3 +- 0.6) x 10 M )
0.05
0.00
0
2
4
6
8
10
12
14
16
-3
-1
1/[Eu(acac)3] x10 M
18
20
22
Results
for Eu(acac)3:
quenching: K = kq 0T = (1.93  0.16)  103 M-1
sensitization: K = kq 0T = (2.3  0.6)  103 M-1
for Tb(acac)3:
quenching: K = kq0T = (1.70  0.15) 103 M-1
sensitization: K = kq 0T = 1.4 103 M-1
Kquenching = Ksensitization
0T = constant
kq (from quenching) = kq (from sensitized emission)
Conclusions
1. BP phosphorescence is quenched by Ln(acac)3 (Ln= Sm,
Eu, Gd, Tb, Dy) and Mg(acac)2 with the rate constants
kq  9  108 M-1s -1 (in acetonitrile).
2. kq for quenching by Eu+3 and Tb +3 (perchlorates) are at
least 5 times lower.
3. kq  4  109 M-1s -1 for quenching by Eu(hfac)3
4. Similar kq values obtained from the quenching and
sensitization indicate the energy transfer process:
A(T1) + Q  A + Q*
5. Similar kq 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.
3D*
+ Q  D + 3Q*
Energy transfer from BP tiplet state to the ligand
localized triplet state
Sandros relation:
kq/kdyf = [1 + exp -(ET(D) - ET(Q))/RT]-1
Rates of energy transfer vs donor-aceeptor energy
differences
kq/kdyf = [1 + exp - ET/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)
kdecay=2.2105 s-1
3D*
krise=2.7105 s-1
+ Q  D + Q*
Dependence of kq on ET
skd
3D*
+ mQ
ken
n(D*...Q)
k-d
k-d
n(D...Q*)
 1D* + nQ*
k-en
s = n/3m (spin statistical factor)
- (3D*) - -n (nQ*) ]
Gen = - Nhc [n
0-0
0-0
Gen and Gel - 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
kd - the diffusion rate constant
k-d - the dissociation rate constant for the encounter complex
en and el - transmission coefficients
k0en and k0en - preexponential factors
Limiting value of kq (plateau value):
k 
pl
q
s k d k 0en ( el )
k 0en ( el )  k -d
kd is the diffusion rate constant
kd = 8000RT/3 (Debye equation)
k-d is the dissociation rate constant for the encounter complex
k-d = 3000kd/4r3N0 (Eigen equation)
for CH3CN at room temperature:
kd =1.9  1010 M-1 s-1
k-d = 2.2  1010 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:
kqpl = (3-7)  109 M-1 s -1
(for energy transfer to acac or hfac triplet states)
s=1
(1Q and 3Q*)
k0en  5  109 s -1
en  1  10-3
Energy transfer to ff* level of Tb(acac)3
taking:
kqpl = 3  106 M-1 s -1
(for energy transfer to Tb(III) 5D4 level)
s= 5/21
(Q and Q* are 7F6 and 5D4 level)
k0en = 1.5  107 s -1
en = 2.4  10-6
(three order of magnitude lower than for energy
transfer to ligand-localized triplet states)
Dependence of kq on ET
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.