"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

A(S 0 ) A(S 1 )  A(S 1 )  A(S 1 )  A(S A(S A(S 1 0 0 ) ) + h n f ) + heat A(T 1 ) A(S 1 )  B + C A(S 1 ) + Q A(T 1 )   quenching A(S 0 ) + h n p A(T 1 )  A(T 1 )  A(S 0 ) + heat 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"

R 1 R 2 R 3 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 1 K = k q  0 T = (1.93 +- 0.16) x 10 3 M -1 2 3

[Eu(acac) 3 ]

x 10 4 (M) 4 5

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

1/[Eu(acac) 3 ] x10 -3 M -1

18 20 22

Results

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

 0.15)  10 3 M -1 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 2.

(Ln= Sm, Eu, Gd, Tb, Dy) and Mg(acac) 2 k q  9  10 8 M -1 s -1 with the rate constants (in acetonitrile).

k q for quenching by Eu +3 and Tb +3 (perchlorates) are at least 5 times lower.

3. k q  4  4. Similar k 10 q 9 M -1 s -1 for quenching by Eu(hfac) 3 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

3 D* + m Q sk d k d n k en k -d (D*...Q) n (D...Q*)  k en 1 D* + n Q* 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 energy and electron transfer processes  G  en and  G  el - thre free energy of activation for energy and 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  1  10 10 -3 9 s -1

Energy transfer to ff* level of Tb(acac)

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  = 2.4  10 10 -6 7 s -1 (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.