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  1. "Molecular Photochemistry - how to study mechanisms of photochemical reactions ?" Bronislaw Marciniak Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland 2012/2013 - lecture 6

  2. 5. Examples illustrating the investigation • of photoreaction mechanisms: • -   sensitized photooxidation of sulfur (II)-containing organic compounds example III

  3. Our Traditional Scheme

  4. Sensitized Photooxidation of (Phenylthio)acetic Acid

  5. System studied Solvent: CH3CN

  6. Fig. Transient absorption spectra followinglaser flash photolysis recorded at four different delays timebenzophenone ([BP] = 210–3M) and (phenylthio)acetic acid([C6H5-S-CH2-COOH] = 210-2M) in Ar-saturated acetonitrile. Inset: kinetic trace at  = 540 nm

  7. Fig. Reference spectra of intermediates (BPH, 3BP*):(i) ketyl radical BPHin acetonitrile,(ii) triplet state of benzophenone3BP* in acetonitrile,and (iii) phenylthiyl radical C6H5-Sin water(from pulse radiolysis)

  8. Table 1a.Quenching rate constants of benzophenonetriplet state by(phenylthio)acetic acid (kq) andquantum yields for formation of intermediates, disappearance of benzophenone (BP),and formation of CO2 (CO2) ~ b b – results for tetrabutylammonium salt

  9. Our Traditional Scheme

  10. Benzophenone-(Phenylthio)acetic Tetrabutylammonium Salt Sovent: CH3CN

  11. Fig. Transient absorption spectra of intermediates following thequenching of benzophenone triplet by Ph-S-CH2-COO-N+(C4H9)4 (0.01M). Inset: kinetic trace at 710 nm.

  12. Fig. Transient absorption spectra following triplet quenching of BP (2 mM) by C6H5-S-CH2-COO-N+R4(10 mM) after 1 s and 150 s delays after the flash in MeCN solution. Insets: kinetic traces on the nanosecond and microsecond time scales

  13. Table 1b.Quenching rate constants of benzophenonetriplet state by(phenylthio)acetic acid (kq) andquantum yields for formation of intermediates, disappearance of benzophenone (BP),and formation of CO2 (CO2) ~ b b – results for tetrabutylammonium salt

  14. Our Traditional Scheme

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

  16. System studied Solvent: H2O

  17. CB + C6H5-S-CH2-COOH in aqueous solution Fig. Transient absorption spectra followinglaser flash photolysis recorded at four different delay times. Benzophenone ([CB=2mM) and (phenylthio)acetic acid([C6H5-S-CH2-COOH]=20mM) in Ar-saturated aqueous solutions pH=7.5. Inset: kinetic trace at  = 660 nm

  18. Spectral Resolutions

  19. Table 1c.Quenching rate constants of benzophenonetriplet state by(phenylthio)acetic acid (kq) andquantum yields for formation of intermediates, disappearance of benzophenone (BP),and formation of CO2 (CO2) ~ b b – results for tetrabutylammonium salt

  20. Our Traditional Scheme

  21. Scheme

  22. Conclusions: Photochemical pathways (primary and secondary reactions) for the sensitized oxidation of phenylthioacetic acid depend on its ionization form (solvent used) and the presence of associated counter cations (tetraalkylammonium salt)

  23. Application of Photooxidation of Sulfur-Containing Organic Compounds in Free Radical Polymerization

  24. Reaction scheme

  25. Systems studied BP + C6H5-S-CH2-COO–N+R4 (R = n-butyl, n-propyl, etyl, metyl) BP + C6H5-S-CH2-COOH monomer: 2-Ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate (TMPTA) Solvent: CH3CN

  26. Reaction scheme

  27. 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

  28. Układy fotoinicjujące Rp [mol/s] Fp 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 rates (Rp), quantum yield of polymerization (Fp) and quantum yield of CO2 (CO2) aconcentration of acid 0.1 mol/dm3 bconcntration of acid 0.01 mol/dm3

  29. Plot of polymerization rate (Rp) vs. square root of the CO2 quantum yield

  30. Conclusions • BP + C6H5-S-CH2-COO–N+R4 (R = n-butyl, n-propyl, and ethyl) were shown to be effective co-initiators of free-radical photopolymerizations. • A linear correlation was found for the polymerization rates vs. the square root of the CO2 quantum yields, and this indicates that the C6H5SCH2 radicals are responsible for the initiation step of the polymerizations. • Application of the laser flash photolysis and steady-state photochemical methods allowed led to description of the mechanism of free radical polymerization.