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

2012/2013 - lecture 72012/2013 - lecture 7

"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:

      photochemistry of 1,3,5,-trithianes in solutionphotochemistry of 1,3,5,-trithianes in solution

S

S

S

HH

HH

HH

TT (1,3,5-trithiane)TT (1,3,5-trithiane)

S

S

S

CH3H

HCH3

HH3C

TMT (2,4,6-trimethyl-1,3,5-trithiane) TMT (2,4,6-trimethyl-1,3,5-trithiane) ISOMER ISOMER (cis-cis), ISOMER (cis-cis), ISOMER (cis–trans) (cis–trans)

H5C6

S

S

S

C6H5H

HC6H5

H

TPT (2,4,6-triphenyl-1,3,5-trithiane) TPT (2,4,6-triphenyl-1,3,5-trithiane) ISOMER ISOMER (cis–cis), ISOMER (cis–cis), ISOMER (cis–trans) (cis–trans)

Trithiane structuresTrithiane structures

SS

SC

CCS

SS

SS

S

TMTTMT

TPTTPT

TTTT

Isomers of the trithianesIsomers of the trithianes

R

S

SS R

RH

HH

-form (cis-trans)-form (cis-trans) -form (cis-cis)-form (cis-cis)

RS

SS R

RH H

H

R = CHR = CH33, C, C66HH55

Ground-state absorptions of trithianes in Ground-state absorptions of trithianes in MeCNMeCN

0

1

2

3

4

220 240 260 280 300 320 [nm]

A

TT

TMT

TMT

0.0

0.5

1.0

1.5

2.0

2.5

220 260 300 340 380 420 [nm]

A

t = 0 t = 1 min t = 2 min t = 3 min t = 4 min t = 5 min

254 nm photolysis of TT in MeCN254 nm photolysis of TT in MeCN

Stable productsStable products(GC, GCMS, HPLC, UV)(GC, GCMS, HPLC, UV)

For TT:For TT:

primary productprimary product

secondary productsecondary product

H CS

S CH3H CS

S CH3

H CS

S CH2 S CH3H CS

S CH2 S CH3

HPLC following 254 nm photolysisHPLC following 254 nm photolysisof TT in MeCNof TT in MeCN

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0 2 4 6 8 10 12 14 16 18

Retention time [min]

A

238 nm308 nm

substrate

CH(S)SCH3

CH(S)SCH2SCH3

254 nm photolysis of TT in MeCN254 nm photolysis of TT in MeCN

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 2 4 6 8 10 12Irradiation time [min]

Con

cent

ratio

ns [m

M]

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

Con

cent

ratio

ns [m

M]substrate

HC(S)SCH2SCH3

HC(S)SCH3

313 nm photolysis of TT in MeCN313 nm photolysis of TT in MeCNpreirradiated at 254 nm for 12 minutespreirradiated at 254 nm for 12 minutes

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 5 10 15 20 25Irradiation time [min]

Con

cent

ratio

ns [m

M]

0.00

0.01

0.02

0.03

0.04

0.05

0.06

Con

cent

ratio

ns [m

M]

substrate

HC(S)SCH2SCH3

HC(S)SCH3

254 nm photolysis of 254 nm photolysis of -TM-TMT in MeCNT in MeCN

0.0

0.5

1.0

1.5

2.0

2.5

220 260 300 340 380 420 [nm]

A

t = 0 t = 1 min t = 2 min t = 3 min t = 4 min t = 5 min

For For -TMT:-TMT:

-TMT-TMT

primary productprimary product primary productprimary product

secondary productsecondary product

Stable productsStable products

(GC, GCMS, HPLC, UV)(GC, GCMS, HPLC, UV)

CH3 CS

S CH S CH2

CH3

CH3CH3 CS

S CH S CH2

CH3

CH3

CH3 CS

S CH2 CH3CH3 CS

S CH2 CH3

HPLC following 254 nm photolysisHPLC following 254 nm photolysisof of -TM-TMT in MeCNT in MeCN

0.00

0.05

0.10

0.15

0.20

0 2 4 6 8 10 12 14 16 18 20Retention time [min]

A

238 nm308 nm

-TMT

-TMTCH3C(S)SC2H5

CH3C(S)SCH(CH3)SC2H5

Extrapolation of Extrapolation of to zero timeto zero time

0 3 6 9 120.30

0.35

0.40

0.45

0.50

Time [min]

Steady-state photolysis at 254 nmSteady-state photolysis at 254 nmLaser flash photolysis at 266 nmLaser flash photolysis at 266 nm

0.520.520.170.170.320.320.250.250.520.52Thioester formation Thioester formation from laser flash from laser flash photolysisphotolysis

0.010.01 0.010.010.100.100.010.01––Isomer formationIsomer formation

0.440.440.140.140.320.320.220.220.490.49Thioester formationThioester formation

0.480.480.190.190.430.430.380.380.540.54Trithiane Trithiane disappearancedisappearance

-TPT-TPT-TPT-TPT-TMT-TMT-TMT-TMTTTTT

Quantum yields Quantum yields

266 nm laser flash of TT in MeCN266 nm laser flash of TT in MeCN

300 350 400 450 5000.00

0.02

0.04

0.06

0.08

0.10

0.12

Transient product Stable product (thioester)

Abso

rban

ce

Wavelength [nm]

-40 0 40 80 120 1600.00

0.02

0.04

0.06

= 300 nm

Time [s]

A

-40 0 40 80 120 1600.00

0.01

0.02

0.03 = 360 nm

Time [s]

A

0

0.01

0.02

0.03

0.04

0.05

0.06

250 300 350 400 450 500 [nm]

A

Transient product Stable product (thioester)

0.00

0.02

0.04

0.06

-20 0 20 40 60 80Time [s]

A

= 360

0.000.010.020.030.040.05

-20 0 20 40 60 80Time [s]

A = 310

a)

b)

266 nm laser flash of 266 nm laser flash of -TM-TMTT in MeCN in MeCN

Mechanism forMechanism forTrithiane = TT, Trithiane = TT, -TMT, or -TMT, or -TMT-TMT

Trithianeh I Dithioester

(plus isomers for - and -TMT)

266 nm laser photolysis of 266 nm laser photolysis of -TPT in MeCN-TPT in MeCN

300 350 400 450 5000.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Transient product Stable product (thioester)

Abso

rban

ce

Wavelength [nm]

-40 0 40 80 120 1600.00

0.04

0.08

= 310 nm

Time [s]

A

-40 0 40 80 120 1600.00

0.02

0.04

0.06

= 410 nm

Time [s]

A

Laser-intensity dependenceLaser-intensity dependence-TPT in MeCN-TPT in MeCN

0 2 4 6 8 10 120.00

0.02

0.04

0.06

0.08

Linear Fit

A

Laser intensity [mJ]0 2 4 6 8 10 12

0.00

0.04

0.08

0.12

0.16

monitor

= 410 nm monitor

= 310 nm

Quadratic Fit

A

Laser intensity [mJ]

Mechanism forMechanism forTrithane = Trithane = -TPT or -TPT or -TPT-TPT

Trithiane

2h

I

Dithioester

h

  

CS

CS

CS

R1

R2 R2

R1

R1 R2

CS

CS

CS

R1

R2 R2

R1

R1 R2

substrate

isomer

R1 = H, CH3, C6H5R2 = H, CH3 R1C(S)SCR2(R1)SC(R2)2R1

h 254 or 313 nm

R1C(S)SC(R2)R1

h254 nm

Solvent effectSolvent effect

TableTable: Quantum yields: Quantum yieldsaa of trithiane disappearance ( of trithiane disappearance (disdis) ) andand dithioester formation (dithioester formation (prodprod) in various solvents) in various solvents

TrithianesTrithianes SolventSolvent disdis prodprod

TTTT CHCH33CNCN 0.540.54 0.510.51

CHCH33OHOH 0.510.51 0.0850.085

-TMT-TMT CHCH33CNCN 0.390.39 0.220.22

CHCH33OHOH 0.260.26 0.010.01

-TMT-TMT CHCH33CNCN 0.430.43 0.320.32

CHCH33OHOH 0.200.20 0.040.04

-T-TPPTT CHCH33CNCN 0.480.48 0.460.46bb

CHCH33OHOH 0.240.24 0.030.03a All quantum yields were extrapolated to zero irradiation times; estimated error is

equal to 10 %.b Sum of 0.34 + 0.12 for RC(=S)SCH(R)SCH2R and RC(=S)SCH2R, respectively.

TrithianeTrithiane SolventSolvent decaydecay ( (s)s) growthgrowth ( (s)s) kkIIII (M (M11 s s11))

TTTT CHCH33CNCN 2828 3131aa

CHCH33OHOH 3.83.8 bb

EtOEEtOEtt 5757 6161

1-BuOH1-BuOH 2020 1313cc

-TMT-TMT CHCH33CNCN 1414dd 99dd 8.4 × 108.4 × 1044

CHCH33OHOH 0.130.13 bb

CyclohexaneCyclohexane 1717 1313

-TMT-TMT CHCH33CNCN 1313ee 1313ee

CHCH33OHOH -- --

-TPT-TPT CHCH33CNCN 2929aa 3030aa 7.8 × 107.8 × 1033

CHCH33OHOH 1.31.3 aa

CyclohexaneCyclohexane 2020 2323

a Previously measured [9].

b No growth observed.

c Determined from a growth/decay fitting function

d The decay lifetime of the shorter component of a biexponential decay.

e Previously measured [6].

Decay time (decay) of intermediate I, growth time (growth) of the dithioesters absorbing at 310 nm, and rate constant (kII) of I with CH3OH

266 nm laser flash of 266 nm laser flash of -TPT in Me in MeOHOH

60100 ns

600700 ns

1.41.6 s

68 s

= 1.3 s

266 nm laser flash of 266 nm laser flash of -TPT in Me in MeOHOH

Initial spectra of 266-nm photolysis of -TPT in various solventsOpen circles: CH3CN, filled circles: CH3OH, squares: cyclohexane

Quenching of intermediate, I, by methanol, following 266-nm laser excitation of -TPT in acetonitrile

kqII= 7.8 × 103 M1 s1

R = H, CH3, C6H5

h

I

Scheme 1

C

SC

S

CS

HR

HR

HR

C

SC

S

CS

HR

HR

HR

C

SC

S

CS

HR

HR

RH

S

RC

R

SCHSCH2Rsubstrate

C

SC

S

CS

HR

HR

HR

*

isomersubstrate

I

Scheme 2

C

SC

S

CS

HCH3

HCH3

HCH3

C

SC

S

CS

HCH3

HCH3

CH3H

substrate

CH3 C S C

CH3

H

S CH2 CH3

S

+CH3OH

products

S S

S

HCH3

CH3H

Chart 1

S S

S

CH3

HCH3H