1. Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland 2012/2013 - lecture 8 "Molecular Photochemistry - how to study mechanisms of photochemical reactions ?" Bronislaw Marciniak Bronislaw Marciniak

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

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

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

5. Stern-Volmer equation for T 1

6. 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) Stern-Volmer equation Sensitized emission of Q

7. 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) 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 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 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"

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

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

10. Stern-Volmer plot for quenching of BP phosphorescence by Eu(acac) 3 in benzene

11. Modified Stern-Volmer plot for emission of Eu(acac) 3 in benzene

12. 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  0 T = constant k q (from quenching) = k q (from sensitized emission) Results

13. Conclusions 1.BP phosphorescence is quenched by Ln(acac) 3 (Ln= Sm, Eu, Gd, Tb, Dy) and Mg(acac) 2 with the rate constants k q  9  10 8 M -1 s -1 (in acetonitrile). 2. 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* 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.

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

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

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

19. 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) 3 D* + Q  D + Q* k decay =2.2  10 5 s -1 k rise =2.7  10 5 s -1

21. Dependence of k q on E T

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

23.  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 diffusion rate constant k -d - the dissociation rate constant for the encounter complex k -d - the dissociation rate constant for the encounter complex

24. Limiting value of k q (plateau value):  en and  el - transmission coefficients k 0 en and k 0 en - preexponential factors

25. 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)

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

27. 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 = 1.5  10 7 s -1 k 0 en = 1.5  10 7 s -1  en = 2.4  10 -6 (three order of magnitude lower than for energy transfer to ligand-localized triplet states) Energy transfer to ff* level of Tb(acac) 3

28. Dependence of k q on E T

29. 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.