Ultrafast dynamics of aromatic disulfides

Ultrafast photodissociation and geminate recombination reactions of aromatic disulfides and thiols are studied in the condensed phase. Femtosecond laser pump-probe methods are used to investigate the earliest stages of the dissociation. Both magic angle and anisotropy data are obtained. Substantial recombination of the geminate radical pair occurs on the subpicosecond timescale in both thiols and disulfides. The dynamics are strongly influenced at early times by the presence of bulky substituents on the aromatic rings, which affect the equilibrium conformations, electronic structures and orientational motion of the parent molecules and their radical photoproducts. The most highly-substituted disulfide shows oscillations in both the magic angle and anisotropy signals. The oscillations have a common phase and frequency but decrease in amplitude at shorter probe wavelengths. Thiols show simple, monotonic anisotropy dynamics while disulfides show dynamics that increase in complexity with the degree of substitution. Electronic and conformational structures of the target molecules are calculated using ab initio and semiempirical methods and compared to literature results. Significant systematic differences are observed between both the conformations and the electronic structures of the different disulfides and thiyl radicals. Several possible explanations of the dynamics are proposed and critically examined. The observations may be explained by the creation of a coherent superposition state in the photodissociation process. Another hypothesis is that coherent motion occurs between the members of the geminate pair and the solvent cage.