Femtosecond to microsecond observation of the photochemical reaction of 1,2-di(quinolin-2-yl)disulfide with methyl methacrylate

The mechanism of the thiol–ene reaction induced by 330 nm ultraviolet excitation of 1,2-di(quinolin-2-yl)disulfide (QSSQ) in the presence of methyl methacrylate (MMA) is investigated by sub-picosecond to microsecond transient absorption spectroscopy. The measurements, spanning more than seven orders of magnitude of time, directly reveal multiple radical reaction steps. The ground state quinoliene-2-thiyl radical (QS) is formed with a time constant of ∼200 fs by photolysis of QSSQ, followed by (64 ± 1)% decay of the initially formed QS radical because of solvent cage induced geminate recombination and QS dimer formation with a rate coefficient of (3.4 ± 0.2) × 1010 M−1 s−1 in methanol solution. In MMA solution, the carbon centered radical QS-MMA forms with a bimolecular reaction rate coefficient of (2.8 ± 0.2) × 107 M−1 s−1. The distinct infrared band at 1653 cm−1 assigned to the CO stretch mode of the QS-MMA radical decays rapidly in aerated solution, in contrast to observations in a solution purged of O2 by N2 bubbling. This decay is attributed to reaction of the QS-MMA radicals with molecular oxygen, producing peroxy radicals. Kinetic analysis of the intensity of the band at 1653 cm−1 reveals a bimolecular reaction rate coefficient of (3.3 ± 0.3) × 109 M−1 s−1 for the reaction of the QS-MMA radicals with molecular oxygen, and indicates that this reaction step is reversible.