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

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

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)

Trithiane structures TMT TPT TT

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

Ground-state absorptions of trithianes in MeCN

254 nm photolysis of TT in MeCN 

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

HPLC following 254 nm photolysis of TT in MeCN

254 nm photolysis of TT in MeCN

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

254 nm photolysis of  -TMT in MeCN 

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

HPLC following 254 nm photolysis of  -TMT in MeCN

Extrapolation of  to zero time

Steady-state photolysis at 254 nm Laser flash photolysis at 266 nm Thioester formation from laser flash photolysis  – Isomer formation Thioester formation Trithiane disappearance  -TPT  -TPT  -TMT  -TMT TT Quantum yields 

266 nm laser flash of TT in MeCN

266 nm laser flash of  -TMT in MeCN

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

266 nm laser photolysis of  -TPT in MeCN

Laser-intensity dependence  -TPT in MeCN

Mechanism for Trithane =  -TPT or  -TPT

Solvent effect

Table: Quantum yields a of trithiane disappearance (  dis ) and dithioester formation (  prod ) in various solvents TrithianesSolvent  dis  prod TT CH 3 CN CH 3 OH  -TMT CH 3 CN CH 3 OH  -TMT CH 3 CN CH 3 OH  -TPT CH 3 CN b CH 3 OH a All quantum yields were extrapolated to zero irradiation times; estimated error is equal to 10 %. b Sum of for RC(=S)SCH(R)SCH 2 R and RC(=S)SCH 2 R, respectively.

TrithianeSolvent  decay (  s)  growth (  s) k II (M  1 s  1 ) TT CH 3 CN a CH 3 OH 3.8  b EtOEt 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

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 

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

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

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