Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland

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

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

3. 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]

4. 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*]

5. Stern-Volmer equation for T1

6. Stern-Volmer equation • 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) 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) • "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"

8. M = Ln (III) or Mg(II) • acac hfac • R1= R3= CH3 R1= R3= CF3 • R2= H R2= 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. 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)

13. Conclusions • 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.

15. 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

16. Rates of energy transfer vs donor-aceeptor energy differences kq/kdyf = [1 + exp - ET/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) kdecay=2.2105 s-1 krise=2.7105 s-1 3D* + Q D + Q*

21. Dependence of kq on ET

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

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

24. en and el- transmission coefficients k0en and k0en - preexponential factors Limiting value of kq (plateau value):

25. 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)

26. 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

27. 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)

28. Dependence of kq on ET

29. Conclusions • 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.