Faculty of Chemistry, Adam Mickiewicz University, Faculty of Chemistry, Adam Mickiewicz University, Poznan, PolandPoznan, Poland

2012/2013 - lecture 82012/2013 - lecture 8

"Molecular Photochemistry - how to "Molecular Photochemistry - how to study mechanisms of photochemical study mechanisms of photochemical

reactionsreactions ? ?""

BronisBronisllaw Marciniakaw Marciniak  

  5. 5. Examples illustrating the investigation Examples illustrating the investigation of photoreaction mechanisms:of photoreaction mechanisms:

        photoinduced electron transfer and energy transfer processesphotoinduced electron transfer and energy transfer processes

Kinetic of quenchingKinetic of quenching

A(SA(S00)) A(SA(S11) ) IIaa (einstein dm (einstein dm-3 -3 ss-1)-1)

A(SA(S11) ) A(SA(S00) + h) + hff kkf f [A(S[A(S11)] )]

A(SA(S11) ) A(SA(S00) + ) + heatheat kkIC IC [A(S[A(S11)] )]

A(SA(S11) ) A(TA(T11) ) kkISC ISC [A(S[A(S11)] )]

A(SA(S11) ) B + C B + C kkrr [A(S [A(S11)] )]

A(SA(S11) + Q) + Q quenching quenching kkqq [A(S [A(S11)] [Q] )] [Q]

A(TA(T11)) A(SA(S00) + h) + hpp kkp p [A(T[A(T11)] )]

A(TA(T11) ) A(SA(S00) + ) + heatheat k'k'IISSC C [A(T[A(T11)] )]

A(TA(T11) ) B' + C' B' + C' k'k'rr [A(T [A(T11)] )]

A(TA(T11) + Q ) + Q quenching quenching k'k'qq [A(T [A(T11)] [Q] )] [Q]

rateratehh

Kinetic of quenchingKinetic of quenchingEnergy transferEnergy transfer

A(TA(T11) + Q ) + Q A + Q* A + Q* k'k'qq [A(T [A(T11)] [Q] )] [Q]

QQ* * Q + Q + h hee kk””ee [[Q*]Q*]

QQ* * Q Q + + heat heat kk””dd [[Q*]Q*]

QQ* * products products kk””rr [[Q*]Q*]

raterate

[Q] 0T

'

qp

p k10

[Q] 0T

'

qR

R k1''0

[Q] 0T

T

0T

'

qk1 [Q] TT

'qk

0

11

[Q]+ 'qobs kkk 0

'r

'ISCp kkk ++

0T

1

[Q]+++T 'q

'r

'ISCp kkkk

1

Stern-Volmer equationStern-Volmer equation

for T1

modified Stern-Volmer equation

Q = k”e/(k”e + k”d + k”r)

(observation of any process from Q* gives adirect evidence for the participation of energy transfer)

Stern-Volmer equationStern-Volmer equation Sensitized emission of Q

]Q[k11110T

'qQQ

Quenching of triplet states of organic Quenching of triplet states of organic compoundes by lanthanide 1,3-diketonate compoundes by lanthanide 1,3-diketonate

chelates in solutionschelates in solutions

1. 1. B.B. Marciniak, M. Elbanowski, S. Lis Marciniak, M. Elbanowski, S. Lis, , Monatsh. ChemMonatsh. Chem. , . , 119119, 669-676 (1988), 669-676 (1988)

"Quenching of Triplet State of Benzophenone by Lanthanide 1,3-"Quenching of Triplet State of Benzophenone by Lanthanide 1,3-Diketonate Chelates in Solutions" Diketonate Chelates in Solutions"

2. 2. B. Marciniak, G. L. HugB. Marciniak, G. L. HugJ. Photochem. Photobiol. J. Photochem. Photobiol. A: ChemistryA: Chemistry, , 7878, 7-13 (1994), 7-13 (1994)"Energy Transfer Process in the Quenching Triplet States of Organic "Energy Transfer Process in the Quenching Triplet States of Organic Compunds by 1,3‑Diketonates of Lanthanides(III) and Magnesium(II) in Compunds by 1,3‑Diketonates of Lanthanides(III) and Magnesium(II) in Acetonitrile Solution. Laser Flash Photolysis Studies" Acetonitrile Solution. Laser Flash Photolysis Studies"

3. B. 3. B. Marciniak, G. L. Hug Marciniak, G. L. Hug Coord. Chem. Rev.Coord. Chem. Rev. , , 159159, 55-74 (1997) , 55-74 (1997) "Quenching of Triplet States of Organic Compounds by 1,3-Diketonate "Quenching of Triplet States of Organic Compounds by 1,3-Diketonate Transition-Metal Chelates in Solution. Energy and/or Electron Transfer"Transition-Metal Chelates in Solution. Energy and/or Electron Transfer"

M = Ln (III) or Mg(II)M = Ln (III) or Mg(II)

acac acac hfachfac

RR11= R= R33= CH= CH33 RR11= R= R33= CF= CF33

RR22= H= H R R22= H= H

MO O

R 1

R 2

R 3

nM

O O

R 1

R 2

R 3

n

Benzophenone phoshorescence in the Benzophenone phoshorescence in the presence of Eu(acac)presence of Eu(acac)3 3 ((phph = 455 nm) = 455 nm)

Stern-Volmer plot for quenching of BP Stern-Volmer plot for quenching of BP phosphorescence by Eu(acac)phosphorescence by Eu(acac)33 in benzene in benzene

0 1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0

ph = 455 nm

K = kq0

T = (1.93 +- 0.16) x 103 M-1

I0 p/I p -1

[Eu(acac)3] x 104 (M)

Modified Stern-Volmer plot for emission of Modified Stern-Volmer plot for emission of Eu(acac)Eu(acac)33 in benzene in benzene

0 2 4 6 8 10 12 14 16 18 20 220.00

0.05

0.10

0.15

0.20

0.25

em = 618 nm

K = kq0

T = (2.3 +- 0.6) x 103 M-1)

1/I em

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

for Eu(acac)for Eu(acac)33::quenching: K = kquenching: K = kq q 00

T T = (1.93 = (1.93 0.16) 0.16) 10 1033 M M-1-1

sensitization: K = ksensitization: K = kq q 00T T = (2.3 = (2.3 0.6) 0.6) 10 1033 M M-1-1

for Tb(acac)for Tb(acac)33::quenching: K = kquenching: K = kqq00

T T = (1.70 = (1.70 0.15) 0.15) 10 1033 M M-1-1

sensitization: K = ksensitization: K = kq q 00T T = = 1.41.4 10 1033 M M-1-1

KKquenchingquenching = K = Ksensitizationsensitization

00T T = constant= constant

kkq q (from quenching)(from quenching) = k = kq q (from sensitized emission)(from sensitized emission)

ResultsResults

ConclusionsConclusions

1.1. BP phosphorescence is quenched by Ln(acac)BP phosphorescence is quenched by Ln(acac)33 (Ln= Sm, (Ln= Sm, Eu, Gd, Tb, Dy) and Mg(acac)Eu, Gd, Tb, Dy) and Mg(acac)22 with the rate constants with the rate constants kkqq 9 9 10 1088 M M-1-1s s -1-1 (in acetonitrile). (in acetonitrile).

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

3. 3. kkqq 4 4 10 1099 M M-1-1s s -1-1 for quenching by Eu(hfac) for quenching by Eu(hfac)33 4. 4. Similar kSimilar kqq values obtained from the quenching and values obtained from the quenching and

sensitization indicate the energy transfer process:sensitization indicate the energy transfer process: A(TA(T11) + Q ) + Q A + Q* A + Q*

5. 5. Similar kSimilar kqq values for all Ln(acac) values for all Ln(acac)33 and Mg(acac) and Mg(acac)22 used used indicate the energy transfer from BP tiplet state to the indicate the energy transfer from BP tiplet state to the ligand localized triplet state. ligand localized triplet state.

Energy transfer from BP tiplet state to the ligand Energy transfer from BP tiplet state to the ligand localized triplet statelocalized triplet state

33D*D* + Q + Q D + D + 33Q*Q*

Sandros relation:Sandros relation:

kkqq/k/kdyfdyf = [1 + exp -(E = [1 + exp -(ETT(D) - E(D) - ETT(Q))/RT](Q))/RT]-1-1

  Rates of energy transfer vs donor-aceeptor energy Rates of energy transfer vs donor-aceeptor energy differencesdifferences

kkqq/k/kdyfdyf = [1 + exp = [1 + exp EETT/RT]/RT]11

Quenching of triplet states of organic Quenching of triplet states of organic compoundes by lanthanide 1,3-diketonate compoundes by lanthanide 1,3-diketonate

chelates in solutions. Laser flash photolysis chelates in solutions. Laser flash photolysis studiesstudies

Decay of BP triplet (Decay of BP triplet (TTTT= 530 nm) and rise of Tb(III) = 530 nm) and rise of Tb(III) emission (emission (ee = 550 nm) = 550 nm)

([BP] = 1 mM, [Tbacac)3 = 0.19 mM in MeCN)([BP] = 1 mM, [Tbacac)3 = 0.19 mM in MeCN)

33D*D* + Q + Q D + Q* D + Q* kkdecaydecay=2.2=2.210105 5 ss-1-1 kkriserise=2.7=2.710105 5 ss-1-1

Dependence of kDependence of kqq on E on ETT

skskdd kkenen k k-d-d

33D*D* + + mmQQ nn(D*...Q) (D*...Q) nn(D...Q*) (D...Q*) 11D*D* + + nnQQ** kkdd k kenen

s = n/3m (spin statistical factor)s = n/3m (spin statistical factor)

GGenen = = Nhc [Nhc [0-00-0((33D*) D*) 0-00-0((nnQ*) ]Q*) ]

Gen and Gel - the standarg free-energy changes for energy- and electron transfer processes

Gen and G

el - thre free energy of activation for energy- and electron transfer processes

kkdd - the diffusion rate constant - the diffusion rate constant

kk-d-d - the dissociation rate constant for the encounter complex - the dissociation rate constant for the encounter complex

Limiting value of kLimiting value of kqq (plateau value): (plateau value):

d0

)el(en

0)el(endpl

q kkkks

k

en and el - transmission coefficients

k0en and k0

en - preexponential factors

kkdd is the diffusion rate constant is the diffusion rate constant

kkdd = 8000RT/3 = 8000RT/3 (Debye equation) (Debye equation)

kkdd is the dissociation rate constant for the encounter complex is the dissociation rate constant for the encounter complex

kkdd = 3000k = 3000kdd/4/4rr33NN00 (Eigen equation) (Eigen equation)

for CHfor CH33CN at room temperature:CN at room temperature:

kkdd =1.9 =1.9 10 101010 M M11 s s11

kkdd = 2.2 = 2.2 10 101010 s s11 (r = 7A) (r = 7A)

taking:taking:kkqq

pl pl = (3-7) = (3-7) 10 1099 M M-1-1 s s -1-1

(for energy transfer to acac or hfac triplet states)(for energy transfer to acac or hfac triplet states)

s = 1 s = 1 ( (11Q and Q and 33Q*)Q*)

kk00enen 5 5 10 1099 s s -1-1

enen 1 1 10 10-3-3

Energy transfer Energy transfer to ligand-localized triplet states of Tb(acac)to ligand-localized triplet states of Tb(acac)3’3’

Gd(acac)Gd(acac)33, Mg(acac), Mg(acac)22,and ,and Mg(hfac)Mg(hfac)3 3

taking:taking:kkqq

pl pl = 3 = 3 10 1066 M M-1-1 s s -1-1

(for energy transfer to Tb(III) (for energy transfer to Tb(III) 55DD44 level) level)

s= 5/21s= 5/21(Q and Q* are (Q and Q* are 77FF66 and and 55DD44 level) level)

kk00enen = 1.5 = 1.5 10 1077 s s -1-1

enen = 2.4 = 2.4 10 10-6-6 (three order of magnitude lower than for energy (three order of magnitude lower than for energy

transfer to ligand-localized triplet states)transfer to ligand-localized triplet states)

Energy transfer to ff* level of Tb(acac)Energy transfer to ff* level of Tb(acac)33

Dependence of kDependence of kqq on E on ETT

ConclusionsConclusions1.1. Quenching of the triplet states of organic compounds by Quenching of the triplet states of organic compounds by

by lanthanide(III) and magnesium(II) 1,3-diketonates in by lanthanide(III) and magnesium(II) 1,3-diketonates in MeCN is adequately described by energy transfer to the MeCN is adequately described by energy transfer to the excited ff states of lanthanide complexes or by energy excited ff states of lanthanide complexes or by energy transer to the ligand-localized triplet states.transer to the ligand-localized triplet states.

2.2. The values of transmission coefficients for energy The values of transmission coefficients for energy

transfer to the ff* states are in the range of 10transfer to the ff* states are in the range of 10-6-6, and are , and are three order of magnitude lower than those for energy three order of magnitude lower than those for energy transfer to ligand-localized triplets.transfer to ligand-localized triplets.

3. 3. In the case of BP derivatives, an additional quenching In the case of BP derivatives, an additional quenching process, process, i.e.i.e. electron transfer from acac ligand to the BP electron transfer from acac ligand to the BP triplet may occur. triplet may occur.