1. Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland 2012/2013 - lecture 7 "Molecular Photochemistry - how to study mechanisms of photochemical reactions ?" Bronislaw Marciniak Bronislaw Marciniak

2. 5. Examples illustrating the investigation 5. Examples illustrating the investigation of photoreaction mechanisms:  photochemistry of 1,3,5,-trithianes in solution

3. TT (1,3,5-trithiane) TMT (2,4,6-trimethyl-1,3,5-trithiane) ISOMER  (cis-cis), ISOMER  (cis–trans) TPT (2,4,6-triphenyl-1,3,5-trithiane) ISOMER  (cis–cis), ISOMER  (cis–trans)

4. Trithiane structures TMT TPT TT

5. Isomers of the trithianes  -form (cis-trans)  -form (cis-cis) R = CH 3, C 6 H 5

6. Ground-state absorptions of trithianes in MeCN

7. 254 nm photolysis of TT in MeCN 

8. Stable products (GC, GCMS, HPLC, UV) For TT: primary product secondary product

9. HPLC following 254 nm photolysis of TT in MeCN

10. 254 nm photolysis of TT in MeCN

11. 313 nm photolysis of TT in MeCN preirradiated at 254 nm for 12 minutes

12. 254 nm photolysis of  -TMT in MeCN 

13. For  -TMT:  -TMT primary product secondary product Stable products (GC, GCMS, HPLC, UV)

14. HPLC following 254 nm photolysis of  -TMT in MeCN

15. Extrapolation of  to zero time

16. Steady-state photolysis at 254 nm Laser flash photolysis at 266 nm 0.520.170.320.250.52 Thioester formation from laser flash photolysis  0.01 0.100.01– Isomer formation 0.440.140.320.220.49 Thioester formation 0.480.190.430.380.54 Trithiane disappearance  -TPT  -TPT  -TMT  -TMT TT Quantum yields 

17. 266 nm laser flash of TT in MeCN

18. 266 nm laser flash of  -TMT in MeCN

19. Mechanism for Trithiane = TT,  -TMT, or  -TMT

20. 266 nm laser photolysis of  -TPT in MeCN

21. Laser-intensity dependence  -TPT in MeCN

22. Mechanism for Trithane =  -TPT or  -TPT

25. Solvent effect

26. Table: Quantum yields a of trithiane disappearance (  dis ) and dithioester formation (  prod ) in various solvents TrithianesSolvent  dis  prod TT CH 3 CN 0.540.51 CH 3 OH 0.510.085  -TMT CH 3 CN 0.390.22 CH 3 OH 0.260.01  -TMT CH 3 CN 0.430.32 CH 3 OH 0.200.04  -TPT CH 3 CN 0.48 0.46 b CH 3 OH 0.240.03 a 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)SCH 2 R and RC(=S)SCH 2 R, respectively.

27. TrithianeSolvent  decay (  s)  growth (  s) k II (M  1 s  1 ) TT CH 3 CN 28 31 a CH 3 OH 3.8  b EtOEt 5761 1-BuOH20 13 c  -TMT CH 3 CN 14 d 9d9d9d9d 8.4 × 10 4 CH 3 OH 0.13  b Cyclohexane1713  -TMT CH 3 CN 13 e CH 3 OH --  -TPT CH 3 CN 29 a 30 a 7.8 × 10 3 CH 3 OH 1.3  a Cyclohexane2023 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 (k II ) of I with CH 3 OH

28. 266 nm laser flash of in MeOH 266 nm laser flash of  -TPT in MeOH 60  100 ns 600  700 ns 1.4  1.6  s 6  8  s 

29.  = 1.3  s 266 nm laser flash of in MeOH 266 nm laser flash of  -TPT in MeOH

30. Initial spectra of 266-nm photolysis of  -TPT in various solvents Open circles: CH 3 CN, filled circles: CH 3 OH, squares: cyclohexane

31. Quenching of intermediate, I, by methanol, following 266-nm laser excitation of  -TPT in acetonitrile k q II = 7.8 × 10 3 M  1 s  1