Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland 2012/2013 - lecture 2 "Molecular...

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

2012/2013 - lecture 22012/2013 - lecture 2

"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.

2. 2. Qualitative investigation of photoreaction Qualitative investigation of photoreaction mechanisms - steady-state and time mechanisms - steady-state and time

resolved methodsresolved methods

- - analysis of stable productsanalysis of stable products

-- identification of identification of short-lived reactive short-lived reactive intermediatesintermediates

-- identification of the excited states identification of the excited states responsible for photochemical reactionsresponsible for photochemical reactions

Jablonski diagramJablonski diagram

S0

T2

T1

2S

1SIC

ISC IC

A F IC Ph ISC

+Q

R

+Q

R

Stable Stable productsproducts

Scheme of photochemical reactionScheme of photochemical reaction


-- identification of identification of short-lived reactive intermediatesshort-lived reactive intermediates

-- identification of the excited states responsible for identification of the excited states responsible for photochemical reactionsphotochemical reactions

AA A* A* I B + CI B + Chh

IntermediatesIntermediates

Norrish type II PhotoreactionNorrish type II Photoreaction

H

O

Ph

R R'

CH2=CRR' + PhCOCH3

PhCOCH2CH2CHRR'

Ph

OH

R'

R

(K) (1K) (3K)

h ISC

(1)

(2)

(3)

H

O

Ph

R R'

*

H

O

Ph

R R'

*

(D)

Ph

R R'

OH

H

O

Ph

R R'

CH2=CRR' + PhCOCH3

PhCOCH2CH2CHRR'

Ph

OH

R'

R

(K) (1K) (3K)

h ISC

(1)

(2)

(3)

H

O

Ph

R R'

*

H

O

Ph

R R'

*

(D)

Ph

R R'

OH

1. 1. Preparative irradiationsPreparative irradiations

2. 2. Product analysis: GC, HPLC, TLC, GCMS, LCMS, Product analysis: GC, HPLC, TLC, GCMS, LCMS, spectroscopic methods etc.spectroscopic methods etc.

3. 3. Separation of products from the reaction mixture:Separation of products from the reaction mixture:

- prepartative GC, HPLC, TLC,- prepartative GC, HPLC, TLC,

- column chromatography- column chromatography

- other methods- other methods

4. 4. Identification of separated products:Identification of separated products:spectroscopic methods: IR, NMR, UV-Vis, Fl, MS, spectroscopic methods: IR, NMR, UV-Vis, Fl, MS, elemental analysis etc.elemental analysis etc.

Analysis of stable productsAnalysis of stable products

Note:Note:Separated products can be used as reference samples in the Separated products can be used as reference samples in the quantitative analysisquantitative analysis

AA A* A* I I B + CB + Chh

Analysis of stable products Analysis of stable products example example

hh

Norrish type II photoreaction of valerophenone (0.1 mol/dm3) Norrish type II photoreaction of valerophenone (0.1 mol/dm3) in methanolin methanol

irrirr > 300 nm > 300 nm

CC66HH55COCHCOCH22CHCH22CHCH22CHCH33

CC66HH55COCHCOCH33 + CH + CH22=CHCH=CHCH33 + cyclobutanol derivative + cyclobutanol derivative

sygn

ał d

etek

tora

CH

=C

HC

H2

3

/ /

0 5 1 0 1 5 2 0

c z a s re te n c ji [m in ]

CH

OH

3

(1:2

00)

sta

ndar

d w

ewnę

trzn

y

(1:2

00)

PhC

OC

H3

(1:2

000)

PhC

OC

H C

H C

H C

H3

(1:2

0)

(1:2

0)

22

2

P h

O H

C H 3

P h

O HC H 3

Inte

rnal

sta

ndar

d

Retention time [min]

Det

ecto

r's s

igna

l

Norrish type II PhotoreactionNorrish type II Photoreaction

H

O

Ph

R R'

CH2=CRR' + PhCOCH3

PhCOCH2CH2CHRR'

Ph

OH

R'

R

(K) (1K) (3K)

h ISC

(1)

(2)

(3)

H

O

Ph

R R'

*

H

O

Ph

R R'

*

(D)

Ph

R R'

OH

H

O

Ph

R R'

CH2=CRR' + PhCOCH3

PhCOCH2CH2CHRR'

Ph

OH

R'

R

(K) (1K) (3K)

h ISC

(1)

(2)

(3)

H

O

Ph

R R'

*

H

O

Ph

R R'

*

(D)

Ph

R R'

OH

Photochemistry of Valerophenone in methanol Photochemistry of Valerophenone in methanol GCGC MS resultsMS results

acetoacetophphenonenonee CC66HH55C(O)CHC(O)CH33

m/e (m/e (relative intensityrelative intensity): ): 121(3,4), 121(3,4), 120(M120(M++,41),41), 106(8), 105(100), 78(9), 77(83), 51(30), 50(11), 106(8), 105(100), 78(9), 77(83), 51(30), 50(11) 1-1-phphenyl-2-metenyl-2-methhylcyclobutanol (izomer ylcyclobutanol (izomer transtrans)) m/e (m/e (relative intensityrelative intensity): ): 162(M162(M++,3),3), 135(9), 134(33), 133(25), , 135(9), 134(33), 133(25), 120(100)120(100), 105(76), 91(15), , 105(76), 91(15), 78(43), 77,(42), 51(18)78(43), 77,(42), 51(18) 1-1-phphenyl-2-metenyl-2-methhylcyclobutanolylcyclobutanol (izomer (izomer ciscis))m/e (m/e (relative intensityrelative intensity): ): 135(8), 134(7), 133(12), 135(8), 134(7), 133(12), 120(100)120(100), 105(56), 91(10), 78(40), 77(36), , 105(56), 91(10), 78(40), 77(36), 51(12).51(12).

Steady-state irradiation systemsSteady-state irradiation systems

1 2 3 4

6

7

5

1- excitation source, 2- diaphragm, 3- thermal filter (cell with H1- excitation source, 2- diaphragm, 3- thermal filter (cell with H22O), O),

4- lens, 5- light filter, 6- merry-goround system4- lens, 5- light filter, 6- merry-goround system

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* II B + C B + Chh

start

Z

K

C

R

P

M

Laser

ns laser flash photolysisns laser flash photolysis

BenzophenoneBenzophenone (Phenylthio)acetic (Phenylthio)acetic Tetrabutylammonium SaltTetrabutylammonium Salt

N

S CH2 C

O

OBP +

N

S CH2 C

O

OBP +

Sovent: CHSovent: CH33CNCN

FigFig. Transient absorption spectra of intermediates following the. Transient absorption spectra of intermediates following the quenching quenching of benzophenone triplet by Ph-S-CHof benzophenone triplet by Ph-S-CH22-COO-N-COO-N++(C(C44HH99))44 (0.01M). (0.01M).

Inset: kinetic trace at 710 nm.Inset: kinetic trace at 710 nm.

400 600 800

0.00

0.02

0.04

0.0 2.0x10-7

4.0x10-7

6.0x10-7

0.00

0.02

0.04

Abs

orba

nce

time [s]

150 s

110 s

45 s

12 s

1 sA

bsor

banc

e

wavelength [nm]

Fig. Fig. Transient absorption spectra following triplet quenching of BP (2 mM) by Transient absorption spectra following triplet quenching of BP (2 mM) by CC66HH55-S-CH-S-CH

22-COO-COO--NN++RR44 (10 mM) after 1 (10 mM) after 1 s and 150 s and 150 s delays after the flash in s delays after the flash in MeCNMeCN solution. solution. IInsetnsetss: kinetic traces on the nanosecond: kinetic traces on the nanosecond and and microsecond time scalemicrosecond time scaless

Reaction schemeReaction scheme

C OH N

RR

R

R

C OH

C O

+ H+

RN

R

R

RC O

RN

R

R

R

(Hofmann elimination)

RN

R

R

R

S

CH2

CO O

S

CH2

PTAASBP

RN

R

R

R

C OC O S

CH2

CO O

CO2

System studiedSystem studied

SensitizersSensitizers

OO

Benzophenone Benzophenone (BP)(BP)

O

COO

O

COO

4-4-CarboxybenzophenoCarboxybenzopheno

ne ne (CB)(CB)

amino acids

R S (CH2)n CHNH2

COO-

+

R S (CH2)n CHNH3

COO-

R S (CH2)n COO-carboxylic acids

amino acids

R S (CH2)n CHNH2

COO-

+

R S (CH2)n CHNH3

COO-

R S (CH2)n COO-carboxylic acids

Sulfur-Containing Organic Compounds Sulfur-Containing Organic Compounds (Quenchers):(Quenchers):

trithianesS

S

S

H R

H

RH

R

R S (CH2)n OHalcohols

R S Rthioethers

trithianesS

S

S

H R

H

RH

R

R S (CH2)n OHalcohols

R S Rthioethers

Sulfur-Containing Organic Compounds Sulfur-Containing Organic Compounds (Quenchers):(Quenchers):

methionine-containing methionine-containing di-, tripeptides and di-, tripeptides and polypeptidespolypeptides

Met-Gly, Gly-MetMet-Gly, Gly-MetMet-Met, Met-Met-Met Met-Met, Met-Met-Met Met-Met-AlaMet-Met-AlaMet-Gly-MetMet-Gly-MetMet-EnkephalinMet-Enkephalin

e.g.e.g.

methionine derivativesmethionine derivatives CH3 S (CH2)2 CH

NH

CO

R2

R1

CH3 S (CH2)2 CH

NH

CO

R2

R1

MotivationsMotivations

• Oxidative stressOxidative stress– Alzheimer’s diseaseAlzheimer’s disease

– Biological agingBiological aging

• Basic issuesBasic issues– Neighboring-group effectsNeighboring-group effects

– Details of oxidative schemeDetails of oxidative scheme

+ >S

CB +

CBH

>SCB

+

+

3CB*

CH2 S CH2

>S

CB >S

CH3 S CH

kbt kCH

kesc

or

[ ]

Our Traditional SchemeOur Traditional Scheme

0

5000

10000

15000

20000

25000

400 500 600 700

Wavelength (nm)

4-Carboxybenzophenone Transients

Ketyl radical

Radical anion

Triplet

(M

1cm

1)

Reference Spectra of CBReference Spectra of CB


(410 nm)

S S+ SS

(480 nm)

*

SSS

CBH (570 nm)

CB (660 nm)

3(CB) (540 nm)

S N (380 nm)

S O

+

+

+

+

+

(410 nm)

S S+ SS

(480 nm)

*

SSS

CBH (570 nm)

CB (660 nm)

3(CB) (540 nm)

S N (380 nm)

S O

+

+

+

+

+

FigFig. . Transient absorption spectra Transient absorption spectra followingfollowing llaser aser fflash lash pphotolysis hotolysis recorded at four recorded at four

different delay times. Benzophenone different delay times. Benzophenone ([([CBCB == 22 mMmM) ) and (phenylthio)acetic acidand (phenylthio)acetic acid ([C([C

66HH55-S-CH-S-CH22-COOH]-COOH] == 2200 mMmM)) in Ar-saturated aqueous solutions pH in Ar-saturated aqueous solutions pH == 7.57.5. .

Inset: kinetic trace at Inset: kinetic trace at = = 660660 nm nm

CB + C6H5-S-CH2-COOH in aqueous solution

Scavenger (Z) of free radicals:Scavenger (Z) of free radicals:- does not absorb excitation light- does not absorb excitation light- selectively react with R- selectively react with R with a large rate with a large rate- does not react with A, A* and RZ- does not react with A, A* and RZ- does not affect the mechanism of RZ - does not affect the mechanism of RZ

formationformation- form RZ easy to detect.- form RZ easy to detect.

Typical scavengers: OTypical scavengers: O22, alkenes, RNO, I, alkenes, RNO, I22


2. Chemical methods - chemical trapping2. Chemical methods - chemical trapping

AA A* A* RR RR ZZhh +Z+Z

stable productstable product

2. Chemical methods - example2. Chemical methods - example

Y.L. Chow, G. Buono-Core, Y.L. Chow, G. Buono-Core, J. Am. Chem. SocJ. Am. Chem. Soc. . 108108, 1234, (1986), 1234, (1986)„„Role of the Acetylacetonyl Radical in the Sensitized Photoreduction Role of the Acetylacetonyl Radical in the Sensitized Photoreduction of Bis( acetylacetonato)copper( II)”of Bis( acetylacetonato)copper( II)”

Spin trapping of acetylacetonyl radicals (acacSpin trapping of acetylacetonyl radicals (acac ):):

EPR spectrum of the benzophenone-sensitized EPR spectrum of the benzophenone-sensitized photoreduction of Cu(acac)photoreduction of Cu(acac)22, in the presence , in the presence of 2-nitroso-2-methylpropane measured after of 2-nitroso-2-methylpropane measured after two-minute irradiation of a methylene two-minute irradiation of a methylene chloride solution of Cu(acac)chloride solution of Cu(acac)22 (1mM), 2- (1mM), 2-nitrozo-2-methylpropane (2 mM), and nitrozo-2-methylpropane (2 mM), and benzophenone benzophenone (5 mM), hyperfine splitting constants:(5 mM), hyperfine splitting constants: aaNN = 1.363 mT, a= 1.363 mT, aHH = 0.315 mT = 0.315 mT and g = 2.0062.and g = 2.0062.

B [mT]

O

(CH3)3C NO + CH(COCH3)2 (CH3)3C N CH(COCH3)2

O

(CH3)3C NO + CH(COCH3)2 (CH3)3C N CH(COCH3)2

RZ product analysis: GCMS and NMR, IRRZ product analysis: GCMS and NMR, IR

2. Chemical methods - example2. Chemical methods - example

Y.L. Chow, G. Buono-Core, Y.L. Chow, G. Buono-Core, J. Am. Chem. SocJ. Am. Chem. Soc. . 108108, 1234, (1986), 1234, (1986)„„Role of the Acetylacetonyl Radical in the Sensitized Photoreduction Role of the Acetylacetonyl Radical in the Sensitized Photoreduction of Bis( acetylacetonato)copper( II)”of Bis( acetylacetonato)copper( II)”

Trapping of acacTrapping of acac with alkenes: with alkenes:

C C + CH(COCH3)2addition products C C + CH(COCH3)2addition products

Conclusion:Conclusion: acacacac was proved to be the reactive intermediate in the was proved to be the reactive intermediate in the sensitized photoreduction of Cu(acac)sensitized photoreduction of Cu(acac)22..

Different Actions of ScavengersDifferent Actions of Scavengers

• Direct capture of free radicals.Direct capture of free radicals.

• Repair of damage caused by radicals.Repair of damage caused by radicals.

• This second mechanism is important for the repair of This second mechanism is important for the repair of damage by free radicals in biological systems.damage by free radicals in biological systems.


Example (N.J. Turro, Modern Molecular Photochemistry, p. 261,Example (N.J. Turro, Modern Molecular Photochemistry, p. 261,

„ „Involvement of TInvolvement of T1 1 (n,(n,*) of benzophenone as the chemically reactive *) of benzophenone as the chemically reactive

agent in the photoreduction of benzophenone by benzydrol”agent in the photoreduction of benzophenone by benzydrol”

hPh2CO + Ph2CHOH Ph C C Ph

OHOH

Ph Ph

B BH2 BH-BH

hPh2CO + Ph2CHOH Ph C C Ph

OHOH

Ph Ph

B BH2 BH-BH

BB B*B* IIaa

B*B* BB kkdd[B*][B*]

B* + BHB* + BH22 BHBH kkrr[B*][BH[B*][BH

22]]

2BH2BH BH-BHBH-BH kkpp[BH][BH]22

B* + QB* + Q B + Q*B + Q* kkqq[B*][Q][B*][Q]

Conclusion:Conclusion: TT1 1 (n,(n,*) of benzophenone is the reactive state. *) of benzophenone is the reactive state.


Example (N.J. Turro, Modern Molecular Photochemistry, p. 261,Example (N.J. Turro, Modern Molecular Photochemistry, p. 261,

„ „Involvement of TInvolvement of T1 1 (n,(n,*) of benzophenone as the chemically reactive *) of benzophenone as the chemically reactive

agent in the photoreduction of benzophenone by benzydrol”agent in the photoreduction of benzophenone by benzydrol”

1 1 1

2B

d

r

k

k BH

[ ]

1 1

2 2B

d

r

q

r

k

k BH

k Q

k BH

[ ]

[ ]

[ ]

Experiments:Experiments:

kkdd // kkrr = 0 = 0..05 05 MM

kkqq // kkrr = 500 = 500

Taking kTaking kqq = 1x10 = 1x1099 M M-1-1s s -1-1

kkdd 10 1055 s s -1-1

10 10 ss

Kinetic methods in the study of the mechanism of photochemical reactionsKinetic methods in the study of the mechanism of photochemical reactions

Procedure:Procedure:

- assumption of the kinetic scheme- assumption of the kinetic scheme

- appropriate equations should be derived, e.g. dependence of - appropriate equations should be derived, e.g. dependence of RR vs. [A] or vs. [A] or

[Q][Q]

- experiments, rate constants determnation and the interpretation of the - experiments, rate constants determnation and the interpretation of the resultsresults

Kinetic methods are so-called Kinetic methods are so-called indirect methodsindirect methods and must confirmed by and must confirmed by direct methodsdirect methods..


Determination of the reactive state in a photoreaction:Determination of the reactive state in a photoreaction:

1. Direct methods ( A, F, P, EPR)1. Direct methods ( A, F, P, EPR)

2. Indirect methods (sensitization and quenching)2. Indirect methods (sensitization and quenching)

AA AA* * I B + CI B + Chh

If the photoreaction is wavelenght- independent, the involvement of If the photoreaction is wavelenght- independent, the involvement of upper excited states can be neglected. upper excited states can be neglected.

Question: SQuestion: S1 1 or/and Tor/and T11

triplet

hPS

PT

ISCS1

T1

S0

quenchingtriplet

sensitizationtriplet

hPS

PT

ISCS1

T1

S0

quenchingtriplet

sensitization

1. Only1. Only S S11 quenched, reaction inhibited quenched, reaction inhibited None None

2. Only2. Only TT11 quenched, reaction inhibited quenched, reaction inhibited TT11

3. Only3. Only TT11 quenched, reaction uninhibited quenched, reaction uninhibited SS11

4. Only 4. Only TT11 sensitized, sensitized, rea reaction does not occurction does not occur SS11

5. Only 5. Only TT11 sensitized, sensitized, rea reaction occursction occurs TT1 1 or or SS11 + T + T11

EExxperperiimentment (result) (result) Reactive stateReactive state ((conclusionconclusion))

Photosensitized

S0

T1S1

PT PS

QT QSDirect

Photosensitized

S0

T1S1

PT PS

QT QSDirect

Experimental Methods for Detection of Intermediates Experimental Methods for Detection of Intermediates and Excited States [Turro]and Excited States [Turro]

Reactive Reactive DirectDirect Indirect methodsIndirect methodsiintermediatentermediate methodsmethods

SS11 F, AF, A CIDNP, KINETCIDNP, KINETICSICS, PRODU, PRODUCTSCTS

TT11 P, A, EPRP, A, EPR CIDNP, KINETCIDNP, KINETICSICS, PRODU, PRODUCTSCTS

RR33CC ++ A, F, PA, F, P MI, CHEM, PRODUMI, CHEM, PRODUCTSCTS

RR33CC A, F, PA, F, P MI, CHEM, PRODUMI, CHEM, PRODUCTSCTS

RR33CC •• A, F, EPRA, F, EPR MI, CHEM, PRODUMI, CHEM, PRODUCTSCTS

BiradicalBiradical A, F, P, EPRA, F, P, EPR CIDNP, MI, CHEM, CIDNP, MI, CHEM, PRODUPRODUCTSCTS

Stable Stable productsproducts

Scheme of photochemical reactionScheme of photochemical reaction


-- identification of identification of short-lived reactive intermediatesshort-lived reactive intermediates

-- identification of the excited states responsible for identification of the excited states responsible for photochemical reactionsphotochemical reactions



Kinetic methods in the study of the mechanism of Kinetic methods in the study of the mechanism of photochemical reactionsphotochemical reactions

Procedure:Procedure:

- assumption of the kinetic scheme- assumption of the kinetic scheme

- appropriate equations should be derived, e.g. dependence of - appropriate equations should be derived, e.g. dependence of RR vs. [A] or vs. [A] or

[Q][Q]

- experiments, rate constats determnation and the interpretation of the - experiments, rate constats determnation and the interpretation of the resultsresults

Kinetic methods are so-called Kinetic methods are so-called indirect methodsindirect methods and must confirmed by and must confirmed by direct methodsdirect methods..


where: where: R = CH R = CH33, (CH, (CH

22))22CHCH33, (CH, (CH

22))33CHCH33, (CH, (CH

22))22CH(CHCH(CH33))22, (, (or or Ph)Ph)

Example:Example:

Photochemistry of Phenyl Alkyl Ketones in the Presence of PPhPhotochemistry of Phenyl Alkyl Ketones in the Presence of PPh33

OCH3 > 330 nm

hPhCOR + PPh3 + CH3OH Ph3PO + PhCHR

OCH3 > 330 nm

hPhCOR + PPh3 + CH3OH Ph3PO + PhCHR

Norrish type II photoreactionNorrish type II photoreaction

H

O

Ph

R R'

CH2=CRR' + PhCOCH3

PhCOCH2CH2CHRR'

Ph

OH

R'

R

(K) (1K) (3K)

h ISC

(1)

(2)

(3)

H

O

Ph

R R'

*

H

O

Ph

R R'

*

PPh3

PPh3

(D)

Ph

R R'

OH

H

O

Ph

R R'

CH2=CRR' + PhCOCH3

PhCOCH2CH2CHRR'

Ph

OH

R'

R

(K) (1K) (3K)

h ISC

(1)

(2)

(3)

H

O

Ph

R R'

*

H

O

Ph

R R'

*

PPh3

PPh3

(D)

Ph

R R'

OH

K K 11KK 33KK

33K K K + K + + H + H -abstraction-abstraction

33K K 33B B

33K + PPhK + PPh33 [K-PPh [K-PPh

33] ]

[K-PPh[K-PPh33]] PhCH(OCHPhCH(OCH

33)R + Ph)R + Ph33POPO

[K-PPh[K-PPh33]] K + PPhK + PPh

33

33BB K K

33BB AP + olefinaAP + olefina

33B B CBCB

33B + PPhB + PPh33 K + PPhK + PPh

33

hh ISC ISC = 1.0= 1.0

kk22

CHCH33OHOHkk33

kk44

kk55

CHCH33OHOHkk66

kk77

kk88

kk99

kk1010

Kinetic schemeKinetic scheme

TT = 1 = 1 // (k(k22 +k +k33)) BB = 1 = 1 // (k(k77 +k +k8 8 +k+k99))

= k= k55 // (k(k5 5 +k+k66) = ) = ee

maxmax

00, , quantum yields of acetopnenone (AP) in the absence and presence of PPh quantum yields of acetopnenone (AP) in the absence and presence of PPh33

((APAP , , CBCB, , KK))

ee quantum yield of ether (PhCH(OCH quantum yield of ether (PhCH(OCH33)R (or Ph)R (or Ph33PO)PO)

231043104

0

][][)(1 PPhkkPPhkk BTBT

][

111

34 PPhk Te

][)(1 3104

0

PPhkk BT

sygn

ał d

etek

tora

CH

=C

HC

H2

3

/ /

0 5 1 0 1 5 2 0

c z a s re te n c ji [m in ]

CH

OH

3

(1:2

00)

sta

ndar

d w

ewnę

trzn

y

(1:2

00)

PhC

OC

H3

(1:2

000)

PhC

OC

H C

H C

H C

H3

(1:2

0)

(1:2

0)

22

2

P h

O H

C H 3

P h

O HC H 3

Inte

rnal

sta

ndar

d

Retention time [min]

Det

ecto

r's s

igna

l

Stern-Volmer plot for the valerophenone photolysis Stern-Volmer plot for the valerophenone photolysis in the presence of PPhin the presence of PPh33

Reciprocal of Reciprocal of ee vs reciprocal of [PPh vs reciprocal of [PPh33]]

TableTable

Summary of kinetic dataSummary of kinetic data

kk44TT ++ kk1010BB kk44TT TT BB kk44101099 kk1010101088

[M[M11]] [M [M11]] [ns][ns] [ns][ns] [M [M11ss11]] [M[M11ss11]]

PhCOCHPhCOCH22CHCH22CHCH33 87871212 99 993030 7070 110110 1.41.4 << 22

PhCOCHPhCOCH22CHCH22CHCH22CHCH33 343433 303055 1616 102102 1.91.9 << 11

PhCOCHPhCOCH22CHCH22CHCH22(CH(CH33))22 9.4 9.41.61.6 101033 4.74.7 9797 2.12.1 << 0.30.3

PhCOCHPhCOCH33 919144 250250 200200 1.01.0

KetonKeton

ConclusionsConclusions

• Application of Stern-Volmer relation for Application of Stern-Volmer relation for APAP, , CBCB, ,

and and K K leads to the same values of rate constantsleads to the same values of rate constants

• kk4 4 = 2 = 2 101099 M M11s s 11

(for all aromatic ketones used)(for all aromatic ketones used)

• kk10 10 << k << k44

(reaction of biradical with PPh(reaction of biradical with PPh33 can be neglected) can be neglected)

• eemaxmax = = = = 0.08 = = 0.08

88 % % chemical quenching (reaction) chemical quenching (reaction)

9292 % % physical quenching physical quenching

kk55

kk5 5 +k+k66

Norrish type II photoreactionNorrish type II photoreaction

H

O

Ph

R R'

CH2=CRR' + PhCOCH3

PhCOCH2CH2CHRR'

Ph

OH

R'

R

(K) (1K) (3K)

h ISC

(1)

(2)

(3)

H

O

Ph

R R'

*

H

O

Ph

R R'

*

PPh3

PPh3

(D)

Ph

R R'

OH

H

O

Ph

R R'

CH2=CRR' + PhCOCH3

PhCOCH2CH2CHRR'

Ph

OH

R'

R

(K) (1K) (3K)

h ISC

(1)

(2)

(3)

H

O

Ph

R R'

*

H

O

Ph

R R'

*

PPh3

PPh3

(D)

Ph

R R'

OH

Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland 2012/2013 - lecture 2 "Molecular...

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