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

QQ

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)