Ultrafast Photochemical Reaction Dynamics of a Cyclic (Alkyl)(Amino)Carbene-Carbon Disulfide Dimer Probed by Femtosecond Infrared Spectroscopy.

The ultrafast photochemical reaction dynamics of a cyclic(alkyl)(amino)carbene-carbon disulfide (CAAC-CS2) dimer containing two adjacent S-S bonds were investigated using femtosecond time-resolved infrared spectroscopy in combination with multireference electronic structure calculations. Time-resolved vibrational spectra and global kinetic analysis reveal that photoexcitation of the S-S n → σ* transition at 375 nm induces subpicosecond (<0.3 ps) homolytic cleavage of one S-S bond, generating a bis-thiyl diradical intermediate. This intermediate undergoes two competing pathways: recombination to regenerate the parent dimer with a time constant of 5.7-8.5 ps, or secondary cleavage of the remaining S-S bond to yield two CAAC-CS2 monomers with a time constant of 30-35 ps. Wavelength- and temperature-dependent kinetic measurements demonstrate that the branching between these pathways is governed by excess excitation energy and thermally driven radical-pair fluctuations. Multireference electronic structure calculations support a sequential S-S bond cleavage mechanism, in good agreement with the experimental observations. These findings provide direct spectroscopic evidence for a bis-thiyl diradical intermediate and offer new mechanistic insight into the ultrafast photochemistry of adjacent S-S bonds.