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

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

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

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

Stern-Volmer equation for T 1

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

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, (1988) Monatsh. Chem., 119, (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, (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 1 = R 3 = CF 3 R 2 = H 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

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

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

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.

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

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

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

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 k  d k  en s = n/3m (spin statistical factor)  G en =  Nhc [ 0-0 ( 3 D*)  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 diffusion rate constant k -d - the dissociation rate constant for the encounter complex k -d - the dissociation rate constant for the encounter complex

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

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  M  1 s  1 k  d = 2.2  s  1 (r = 7A)

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  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

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  (three order of magnitude lower than for energy transfer to ligand-localized triplet states) Energy transfer to ff* level of Tb(acac) 3

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.