Photochemical reaction dynamics of 2,2'-dithiobis(benzothiazole): direct observation of the addition product of an aromatic thiyl radical to an alkene with time-resolved vibrational and electronic absorption spectroscopy

The photochemical reaction dynamics of the benzothiazole-2-thiyl (BS) radical, produced by 330 nm ultraviolet photolysis of 2,2′-dithiobis(benzothiazole) (BSSB), are examined on the picosecond time scale. The initial addition product of a thiol–ene reaction between the BS radical and styrene is directly observed by transient vibrational absorption spectroscopy (TVAS). Transient electronic absorption spectroscopy (TEAS) in the ultraviolet and visible spectral regions reveals rapid formation of the ground state BS radical with a time constant of ∼200 fs. The photolytically generated BS radical decays through geminate recombination to the parent molecule BSSB and competitive formation of a BS radical dimer with a rate coefficient of (3.7 ± 0.2) × 10¹⁰ M⁻¹ s⁻¹ in methanol, and thereafter (36 ± 1)% of the initially formed BS radicals survive at the longest time delay (1.3 ns). In styrene solution, in contrast to methanol and toluene solutions, kinetic traces of the BS radical show an additional decay with a time constant of 305 ± 13 ps, and a broad band at 345–500 nm grows with the same time constant, suggesting a bimolecular reaction of the BS radical with styrene. The TVAS measurements reveal an absorption band of the ground state BS radical at 1301 cm⁻¹ in toluene solution, and the band decays with a time constant of 294 ± 32 ps in styrene solution. Two product bands grow at 1239 cm⁻¹ and 1429 cm⁻¹ with respective time constants of 312 ± 68 ps and 325 ± 33 ps, and are attributed to the addition product BS–St radical formed from the BS radical and styrene. A bimolecular reaction rate coefficient of k_react = (3.8 ± 0.2) × 10⁸ M⁻¹ s⁻¹ is deduced and 22 ± 1% of the initially formed BS radicals are converted to the BS–St radical in neat styrene solution.