faculty of chemistry, adam mickiewicz university, poznan, poland
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"Molecular Photochemistry - how to study mechanisms of photochemical reactions ? ". Bronis l aw Marciniak. Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland. 2012/2013 - lecture 3. Contents. - PowerPoint PPT PresentationTRANSCRIPT
Faculty of Chemistry, Adam Mickiewicz University, Faculty of Chemistry, Adam Mickiewicz University, Poznan, PolandPoznan, Poland
2012/2013 - lecture 32012/2013 - lecture 3
"Molecular Photochemistry - how to "Molecular Photochemistry - how to study mechanisms of photochemical study mechanisms of photochemical
reactionsreactions ? ?""
BronisBronisllaw Marciniakaw Marciniak
ContentsContents
1.1. Introduction and basic principles Introduction and basic principles (physical and chemical properties of molecules in the excited states, (physical and chemical properties of molecules in the excited states, Jablonski diagram, time scale of physical and chemical events, Jablonski diagram, time scale of physical and chemical events, definition of terms used in photochemistry).definition of terms used in photochemistry).
2.2. Qualitative investigation of photoreaction mechanisms - Qualitative investigation of photoreaction mechanisms - steady-state and time resolved methodssteady-state and time resolved methods(analysis of stable products and short-lived reactive intermediates, (analysis of stable products and short-lived reactive intermediates, identification of the excited states responsible for photochemical identification of the excited states responsible for photochemical reactions).reactions).
3.3. Quantitative methodsQuantitative methods(quantum yields, rate constants, lifetimes, kinetic of quenching, (quantum yields, rate constants, lifetimes, kinetic of quenching, experimental problems, e.g. inner filter effects).experimental problems, e.g. inner filter effects).
Contents cont.Contents cont.
4. Laser flash photolysis in the study of photochemical 4. Laser flash photolysis in the study of photochemical reaction mechanisms (10reaction mechanisms (10–3–3 – 10 – 10–12–12s).s).
5. Examples illustrating the investigation of photoreaction 5. Examples illustrating the investigation of photoreaction mechanisms:mechanisms:
sensitized photooxidation of sulfur (II)-containing organic sensitized photooxidation of sulfur (II)-containing organic compounds,compounds,
photoinduced electron transfer and energy transfer processes, photoinduced electron transfer and energy transfer processes,
sensitized photoreduction of 1,3-diketonates of Cu(II),sensitized photoreduction of 1,3-diketonates of Cu(II),
photochemistry of 1,3,5,-trithianes in solution.photochemistry of 1,3,5,-trithianes in solution.
Identification of Identification of short-lived reactive intermediatesshort-lived reactive intermediates
1. Spectroscopic methods - flash photolysis1. Spectroscopic methods - flash photolysis
- UV-Vis absorption and emission - UV-Vis absorption and emission - IR - IR - NMR (CIDNP)- NMR (CIDNP)- EPR- EPR
2. Chemical methods2. Chemical methods
3. Kinetic methods3. Kinetic methods
AA A* A* I B + CI B + Chh
2. 2. QuaQuannttiittativeative methods methods
- - quantum yieldsquantum yields,,
-- rate constants, rate constants,
-- lifetimes,lifetimes,
-- kinetic of quenching,kinetic of quenching,
- - experimental problems, e.g. inner filter effectsexperimental problems, e.g. inner filter effects
ddifferential quantum yield:ifferential quantum yield:
Aa
d AdtI
[ ]
Ba
d BdtI
[ ]
ax I
dt]x[d
DDefinition of terms used in photochemistryefinition of terms used in photochemistry
Quantum yields Quantum yields
For a photochemical reaction AFor a photochemical reaction A BBhh
A(SA(S00)) A(SA(S
11) ) IIaa (einstein dm (einstein dm-3 -3 ss-1)-1)
A(SA(S11) ) A(SA(S
00) + h) + hff kkf f [A(S[A(S11)] )]
A(SA(S11) ) A(SA(S
00) + ) + heatheat kkIC IC [A(S[A(S11)] )]
A(SA(S11) ) A(TA(T
11) ) kkISC ISC [A(S[A(S11)] )]
A(SA(S11) ) B + C B + C kkrr [A(S [A(S
11)] )]
A(SA(S11) + Q) + Q quenching quenching kkqq [A(S [A(S
11)] [Q] )] [Q]
A(TA(T11)) A(SA(S
00) + h) + hpp kkp p [A(T[A(T11)] )]
A(TA(T11) ) A(SA(S
00) + ) + heatheat k'k'IISSC C [A(T[A(T
11)] )]
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 schemeKinetic scheme
Steady-state approximation :Steady-state approximation :
IIaa = (k = (kff + k + kICIC + k + kISCISC + k + krr + k + kqq[Q][Q]) [ A(S) [ A(S11)] = [A(S)] = [A(S11)]/)]/SS
Fluorescence quantum yield:Fluorescence quantum yield:
ff = k = kff [ A(S[ A(S11)] )] / / IIaa
ff = k = kf f SS ICIC = k = kIC IC SS ISCISC = k = kISC ISC SS
For photochemical reaction from SFor photochemical reaction from S11::
RR = k = krr [ A(S[ A(S11)] )] / / IIaa
AA = = BB = k = kr r SS
Phosphorescence quantum yield:Phosphorescence quantum yield:
pp = k = kpp[ A([ A(TT11)] )] / / IIaa
pp = = ISCISCkkppTT
For photochemical reaction from TFor photochemical reaction from T11::
''RR = k' = k'rr [ A([ A(TT11)] )] / / IIaa
''AA = = ''BB = = ISC ISC kk''rr TT
acetoneacetone = 0.22 (for 313 nm)= 0.22 (for 313 nm)
Quantum yield Quantum yield measurementmeasurement
Uranyl Oxalate ActinometryUranyl Oxalate Actinometry
Chemical actinometry:Chemical actinometry:
hvhvHH22CC22OO4 4 H H22O + COO + CO22 + CO + CO
UOUO22+2+2
R R = 0.602 (for 254 nm)= 0.602 (for 254 nm)
R R = 0.561 (for 313 nm)= 0.561 (for 313 nm)
Benzophenone-Benzhydrol ActinometryBenzophenone-Benzhydrol Actinometry
(C(C66HH55))22CO + (CCO + (C66HH55))22CHOH CHOH (C (C66HH55))22C(OH) C(OH) (CC(OH) C(OH) (C66HH55))22
R R = 0.68 (for 0.1M BP and 0.1M benzhydrol in benzene)= 0.68 (for 0.1M BP and 0.1M benzhydrol in benzene)
2-Hexanone Actinometry2-Hexanone Actinometry (Norrish Type II) (Norrish Type II)
Typical dependence of quantum yield Typical dependence of quantum yield vsvs I Iaatt
a
b
Ia t
A
B)
Quantum yield of intermediatesQuantum yield of intermediates
AApp and and A Astst transient absorbances for intermediate and actinometertransient absorbances for intermediate and actinometer
pp and and st st molar absorption coefficents of intermediate and actinometermolar absorption coefficents of intermediate and actinometer
stst quantum yield of actinometer (using benzophenone equal to quantum yield of actinometer (using benzophenone equal to ISCISC= 1)= 1)
Laser flash photolysis:Laser flash photolysis:
II = = st st AApp st st / / A Astst pp
A(A(exex) for irradiated solution = A() for irradiated solution = A(exex) for actinometer) for actinometer
Rate constantsRate constants
kkr r = = RR //SS from S from S11
kk''rr = = ''RR / (/ (ISC ISC TT)) from T from T11
SS and and T T from direct measurement (laser flash from direct measurement (laser flash
photolysis)photolysis)
Kinetic of quenchingKinetic of quenching
A(SA(S00)) A(SA(S
11) ) IIaa (einstein dm (einstein dm-3 -3 ss-1)-1)
A(SA(S11) ) A(SA(S
00) + h) + hff kkf f [A(S[A(S11)] )]
A(SA(S11) ) A(SA(S
00) + ) + heatheat kkIC IC [A(S[A(S11)] )]
A(SA(S11) ) A(TA(T
11) ) kkISC ISC [A(S[A(S11)] )]
A(SA(S11) ) B + C B + C kkrr [A(S [A(S
11)] )]
A(SA(S11) + Q) + Q quenching quenching kkqq [A(S [A(S
11)] [Q] )] [Q]
A(TA(T11)) A(SA(S
00) + h) + hpp kkp p [A(T[A(T11)] )]
A(TA(T11) ) A(SA(S
00) + ) + heatheat k'k'IISSC C [A(T[A(T
11)] )]
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
[Q] 0S
qf
f k10
[Q] 0S
qR
R k10
[Q] 0S
S
0S
qk1 [Q] SS
qk
0
11
[Q]+ qobs kkk 0
rICISCf kkkk +++0S
1
[Q]++++SqrICISCf kkkkk
1
for S1
Stern-Volmer equationStern-Volmer equation
[Q] 0T
'
qp
p k10
[Q] 0T
'
qR
R k1'
'0
[Q] 0T
T
0T
'
qk1 [Q] TT
'qk
0
11
[Q]k+kk 'q
0obs
'r
'ISCp kkk ++
0T
1
[Q]+++T 'q
'r
'ISCp kkkk
1
Stern-Volmer equationStern-Volmer equation
for T1
Quenching of Quenching of 33CB* CB* by Met-Gly in aqueous solutions at pH = 6.8by Met-Gly in aqueous solutions at pH = 6.8
[Q] T
'qobs kk
0
1
kq = (2.14 0.08) 109 M-1 s -1
Quenching Rate Constants (Quenching Rate Constants (101099 M M1 1 ss11)) for quenching for quenching of CB triplet stateof CB triplet state
Triplet QuenchersTriplet Quenchers pH neutralpH neutral
Thiaproline
Methionine
Alanine
S-(Carboxymethyl)cysteine
Met-Gly
L-Met-L-Met
Gly-Gly-Met
Met-Enkephalin
2.1
2.5
0.0005
0.81
2.1
2.9
1.8
1.9
pH basicpH basic
2.6
2.3
0.18
0.75
2.3
1.8
1.9
1.8
Rate constants of the order of 10Rate constants of the order of 1099 M M1 1 ss11
indicative of electron transferindicative of electron transfer
MethionineMethionine
H3N C COO
C
S
CH3
( H2)2
+
H2)2(
H2N C COO
C
S
CH3
pKa = 9.06
H3N C COO
C
S
CH3
( H2)2
+
H2)2(
H2N C COO
C
S
CH3
pKa = 9.06
+ >S
CB +
CBH
>SCB
+
+
3CB*
CH2 S CH2
>S
CB >S
CH3 S CH
kbt kCH
kesc
or
[ ]
Traditional SchemeTraditional Scheme
Term used in two different ways:Term used in two different ways:
(1) During an irradiation experiment, absorption of incident (1) During an irradiation experiment, absorption of incident radiation by a species other than the intended primary radiation by a species other than the intended primary absorber is also described as an inner-filter effect.absorber is also described as an inner-filter effect.
DDefinition of terms used in photochemistryefinition of terms used in photochemistry
2007 IUPAC, S. E. Braslavsky, 2007 IUPAC, S. E. Braslavsky, Pure and Applied Chemistry Pure and Applied Chemistry 7979, 293–465, 293–465
Inner-filter effectsInner-filter effects
(2) In an (2) In an emission emission experiment, it refers toexperiment, it refers to(a) an apparent decrease in emission (a) an apparent decrease in emission quantum yield quantum yield at at high concentration of the emitter due to strong high concentration of the emitter due to strong
absorption absorption of the excitation light of the excitation light
(b) an apparent decrease in emission quantum yield (b) an apparent decrease in emission quantum yield and/or distortion of bandshape as a result of and/or distortion of bandshape as a result of reabsorption of reabsorption of emitted radiation (particularly severe emitted radiation (particularly severe for emitters with for emitters with small small Stokes shiftStokes shift).).
DDefinition of terms used in photochemistryefinition of terms used in photochemistry
2007 IUPAC, S. E. Braslavsky, 2007 IUPAC, S. E. Braslavsky, Pure and Applied Chemistry Pure and Applied Chemistry 7979, 293–465, 293–465
Inner-filter effectsInner-filter effects
Ia [einstein dm3 s1]
A h
A + Q h
lcε0
Aa
AA101II
l)cεc(ε
QQAA
AAA(Q)a
QQAA101cεcε
cεI
0I
AAQ(A)a
A(Q)a
cε
cε
I
I
)(I
)(I
0
0lcε
l)cεc(ε
QQAA
AAAa
A(Q)a
AA
QQAA
101
101
cεcε
cε
I
I
l)cεc(ε
lcε
AA
QQAAobsA(Q),a
corrA,a QQAA
AA
101
101cε
cεcεII
l)cεc(ε
lcε
AA
QQAAobsf
corrf QQAA
AA
101
101
cε
cεcεII
l'[Q]ε
obsf
Q
obsfcorr
f Q10
I
T
II
Corrections for inner filter effect (1) Corrections for inner filter effect (1)
(for the absoprtion of incident light by Q)(for the absoprtion of incident light by Q)
Corrections for inner filter effect (2)Corrections for inner filter effect (2)(for reabsorption of fluorescence of A by Q) (for reabsorption of fluorescence of A by Q)
Changes of fluorescence spectra of benzene with various Cu(acac)Changes of fluorescence spectra of benzene with various Cu(acac)22 concentrations concentrations
Changes of fluorescence spectra of benzene with various Cu(acac)Changes of fluorescence spectra of benzene with various Cu(acac)22 concentrations concentrations
Stern-Volmer plot for the quenching of benzene fluorescence by Cu(acac)Stern-Volmer plot for the quenching of benzene fluorescence by Cu(acac)22
without correction
with correction
Experimental setups for measuring fluorescence spectra Experimental setups for measuring fluorescence spectra
Stern Volmer plot for quenching of benzene fluorescence by Cu(acac)Stern Volmer plot for quenching of benzene fluorescence by Cu(acac)22
- front-face technique (- front-face technique (exex=250 nm, =250 nm, ff=278 nm)=278 nm)