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

2012/2013 - lecture 62012/2013 - lecture 6

"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:

      sensitized photooxidation of sulfur (II)-containing organic compoundssensitized photooxidation of sulfur (II)-containing organic compounds example III example III

Our Traditional SchemeOur Traditional Scheme

+ >S

CB +

CBH

>SCB

+

+

3CB*

CH2 S CH2

>S

CB >S

CH3 S CH

kbt kCH

kesc

or

[ ]

Sensitized Photooxidation Sensitized Photooxidation of (Phenylthio)acetic Acidof (Phenylthio)acetic Acid

System studiedSystem studied

CO

S CH2 C OHO

+CO

S CH2 C OHO

+

Solvent: CHSolvent: CH33CNCN

FigFig. . Transient absorption spectra Transient absorption spectra followingfollowing llaser aser fflash lash pphotolysis hotolysis recorded at four different recorded at four different delays timedelays time benzophenonebenzophenone ([BP] = 2 ([BP] = 2    1010–3–3 MM) ) and (phenylthio)acetic acidand (phenylthio)acetic acid

([C([C66HH55-S-CH-S-CH22-COOH] = 2-COOH] = 2    101022 MM) ) in Ar-saturated acetonitrilein Ar-saturated acetonitrile. . Inset: kinetic trace at Inset: kinetic trace at = 540 nm = 540 nm

400 500 600 700

0.00

0.01

0.02

0.03

0.04

0.05

50 100 ns 1 1.5 s 10 12 s 140 160 s

A

Wavelength [nm]

0 50 100 150

0.00

0.01

0.02 = 540 nm

Time [s]

A

FigFig. Reference . Reference sspectra pectra of intermediatesof intermediates (BPH (BPH, , 33BP*):BP*): (i) ketyl(i) ketyl radical radical BPH BPH in acetonitrile,in acetonitrile, (ii) (ii) triplet state of triplet state of benzobenzophphenonenonee 33BP* BP* in acetonitrile,in acetonitrile, andand (iii) (iii) phenylthiyl radical phenylthiyl radical CC66HH55-S-S

in waterin water (from pulse radiolysis) (from pulse radiolysis)

350 400 450 500 550 600 650 700

0

1000

2000

3000

4000

5000

6000

7000

BPH

C6H

5-S

3BP

[dm

3 mol

1cm

1]

Wavelength [nm]

TabTablele 1 1aa.. Quenching rate constants of Quenching rate constants of benzobenzophphenonenonee triplet state triplet state byby (phenylthio)acetic acid(phenylthio)acetic acid ( (kkqq) ) andand quantum yields for formation quantum yields for formation

of intermediatesof intermediates, , disappearance ofdisappearance of benzo benzophenonephenone ( (BPBP)),, and formation ofand formation of CO CO22 ( (COCO22

))

b

b b – – results for tetrabutylammonium saltresults for tetrabutylammonium salt

~

+ >S

CB +

CBH

>SCB

+

+

3CB*

CH2 S CH2

>S

CB >S

CH3 S CH

kbt kCH

kesc

or

[ ]

Our Traditional SchemeOur Traditional Scheme

BP . . . Ph

SCH2

COOH

Ph-S

Ph

SCH2

Ph

SCH

COOHBPH

BPH

not observed

= 0ksep

Ph-S-S-Ph

Ph-SH

= 0.39 = 0.28 = 0.33

Ph-S-CH3

Ph-S-CH2-CH2-S-Ph

Ph

SCH2

COOH

Ph

SCH2

COOH

= 0.30

= 0.28

CO2

kH2kH1

BP

kbt

+3BP*

BP . . . Ph

SCH2

COOH

Ph-S

Ph

SCH2

Ph

SCH

COOHBPH

BPH

not observed

= 0ksep

Ph-S-S-Ph

Ph-SH

= 0.39 = 0.28 = 0.33

Ph-S-CH3

Ph-S-CH2-CH2-S-Ph

Ph

SCH2

COOH

Ph

SCH2

COOH

= 0.30

= 0.28

CO2

kH2kH1

BP

kbt

+3BP*

BP . . . Ph

SCH2

COOH

Ph-S

Ph

SCH2

Ph

SCH

COOHBPH

BPH

not observed

= 0ksep

Ph-S-S-Ph

Ph-SH

= 0.39 = 0.28 = 0.33

Ph-S-CH3

Ph-S-CH2-CH2-S-Ph

Ph

SCH2

COOH

Ph

SCH2

COOH

= 0.30

= 0.28

CO2

kH2kH1

BP

kbt

+3BP*

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

N

S CH2 CO

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-CH22-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

TabTablele 1 1bb.. Quenching rate constants of Quenching rate constants of benzobenzophphenonenonee triplet state triplet state byby (phenylthio)acetic acid(phenylthio)acetic acid ( (kkqq) ) andand quantum yields for formation quantum yields for formation

of intermediatesof intermediates, , disappearance ofdisappearance of benzo benzophenonephenone ( (BPBP)),, and formation ofand formation of CO CO22 ( (COCO22

))

b

b b – – results for tetrabutylammonium saltresults for tetrabutylammonium salt

~

+ >S

CB +

CBH

>SCB

+

+

3CB*

CH2 S CH2

>S

CB >S

CH3 S CH

kbt kCH

kesc

or

[ ]

Our Traditional SchemeOur Traditional Scheme

J. Am. J. Am. Chem. SocChem. Soc., ., 125, 125, 11182 (2003)11182 (2003)

C OH

C O

ASand/or

PTA AS(2b)

N C OH

(2a)(1)(E2 Hofmann elimination)

C O N

(a)

CO2C O

S

CH2

S

CH

CO O

N

N

N

S

CH2

CO O

(b)C OH

S

CH2

CO O

N

C O

System studiedSystem studied

Solvent: HSolvent: H22OO

+ S CH2 C OO

CO

+ S CH2 C OO

CO

OOCOOC

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([C66HH55-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

Spectral ResolutionsSpectral Resolutions

TabTablele 1 1cc.. Quenching rate constants of Quenching rate constants of benzobenzophphenonenonee triplet state triplet state byby (phenylthio)acetic acid(phenylthio)acetic acid ( (kkqq) ) andand quantum yields for formation quantum yields for formation

of intermediatesof intermediates, , disappearance ofdisappearance of benzo benzophenonephenone ( (BPBP)),, and formation ofand formation of CO CO22 ( (COCO22

))

b

b b – – results for tetrabutylammonium saltresults for tetrabutylammonium salt

~

+ >S

CB +

CBH

>SCB

+

+

3CB*

CH2 S CH2

>S

CB >S

CH3 S CH

kbt kCH

kesc

or

[ ]

Our Traditional SchemeOur Traditional Scheme

SchemeScheme

CB

3CB* +

CB

kbet

CB

kH

CO2

. . .

= 0.92 ~ 0.93

C6H5

SCH2

O O

C6H5

SCH2

~ 0

= 0.97 0.10

ksep

Products

++ +

+

C6H5

SCH2

O O

C6H5

SCH

O O

CBH

( C6H5SCH3, C6H5SCH2CH2SC6H5 )

C6H5

SCH2

O O

C6H5

SCH2

O O

Conclusions:Conclusions:

PPhotochemical pathways hotochemical pathways (primary and (primary and secondary reactions) for the secondary reactions) for the sensitized oxidation sensitized oxidation of phenylthioacetic acid of phenylthioacetic acid depend ondepend on

its ionization formits ionization form (solvent used) (solvent used)

and the presence of and the presence of associated associated counter cationscounter cations ((tetratetraalkylalkylammonium saltammonium salt))

Application of Photooxidation Application of Photooxidation of Sulfur-Containing Organic of Sulfur-Containing Organic Compounds in Free Radical Compounds in Free Radical

PolymerizationPolymerization

Reaction schemeReaction scheme

+ >S

CB +

CBH

>SCB

+

+

3CB*

CH2 S CH2

>S

CB >S

CH3 S CH

kbt kCH

kesc

or

[ ]

- CO2

R'R"• •

CH2 C C2H5

CH2

CH2

O

O

O

C

C

C

O

O

OCH

CH

HCH2C

CH2

CH2

Systems studiedSystems studied

BP + CBP + C66HH55-S-CH-S-CH22-COO-COO––NN++RR44

(R = (R = n-butyl, n-propyl, etyln-butyl, n-propyl, etyl, metyl), metyl)

BP + BP + CC66HH55-S-CH-S-CH22-COO-COOHH

monomer:

Solvent: CHSolvent: CH33CNCN

2-Ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate 2-Ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate (TMPTA)(TMPTA)

CH2 C C2H5

CH2

CH2

O

O

O

C

C

C

O

O

OCH

CH

HCH2C

CH2

CH2

Reaction schemeReaction scheme

CO2SCH2

CO O

C O C O

+ monomer

Polymerization

RN

R

R

R

BP PTAAS

SCH2

RN

R

R

R

SCH2

CO O

(Hofmann elimination)

R

N

R

R

R

C O

RN

R

R

R

+ H+

C O

C OH

RN

RR

R

C OH

BP

BP + C6H5-S-CH2-COOH, [0,1M]

BP + C6H5-S-CH2-COO–N+(C4H9)4

BP + C6H5-S-CH2-COO–N+(C3H7)4

BP + C6H5-S-CH2-COO–N+(C2H5)4

BP + C6H5-S-CH2-COO–N+(CH3)4

BP + C6H5-S-CH2-COOH

Photopolymerization kinetic traces

2CO

S CH2 COOHBP +

S CH2 COOBP + N(CH3)4

N(C2H5)4 +BP S CH2 COO

S CH2 COOBP + N(C3H7)4

N(C4H9)4 +BP S CH2 COO

Układy fotoinicjujące Rp [mol/s] p

BP 23,4 – 400

52,9a

28.4b 0,53 910a

480b

50,1 0,42 850

76,0 0,65 1300

74,5 0,62 1270

73,8 0,67 1260

Polymerization ratesPolymerization rates ( (RRpp), ), quantum yield of polymerizationquantum yield of polymerization ( (pp) ) and and quantum yield ofquantum yield of CO CO22 ( (COCO22

))

aa concentration of acidconcentration of acid 0 0..1 1 mol/dmmol/dm33

bb concntration of acidconcntration of acid 0 0.0.01 1 mol/dmmol/dm33

Plot of polymerization rate (RPlot of polymerization rate (Rpp) vs. square root of the CO) vs. square root of the CO22 quantum quantum yieldyield

t

ARpp ]M[

kI

kR

ConclusionsConclusions

• BP + CBP + C66HH55-S-CH-S-CH22-COO-COO––NN++RR44 (R = (R = n-butyl, n-propyl, n-butyl, n-propyl, and etand ethhylyl) ) were shown to be effective co-initiators of were shown to be effective co-initiators of free-radical photopolymerizationsfree-radical photopolymerizations. .

• A linear correlation was found for the polymerization A linear correlation was found for the polymerization rates vs. the square root of the COrates vs. the square root of the CO22 quantum yields, quantum yields, and this indicates that the Cand this indicates that the C66HH55SCHSCH22

radicals are radicals are responsible for the initiation step of the responsible for the initiation step of the polymerizations. polymerizations.

• Application of the laser flash photolysis and steady-Application of the laser flash photolysis and steady-state photochemical methods allowed led to state photochemical methods allowed led to description of the mechanism of free radical description of the mechanism of free radical polymerization.polymerization.