Disulfide bond photochemistry: the effects of higher excited states and different molecular geometries on disulfide bond cleavage

Abstract

The disulfide bond is prone to ultraviolet light-induced cleavage, but the microscopic details of the light-activated bond breakage remain elusive. Here, we carry out quantum chemical calculations and the first TSH simulation of the excited state dynamics of disulfides at the MS-CASPT2 level. We demonstrate that during relaxation of the S₁ state, IC to the S₂ state is the predominant relaxation pathway and efficient ISC to the T₂ state is geometry-dependent. Moreover, the bond cleavage leads to a strong coupling region of singlet–triplet quasidegeneracy and enlarged SOC, from which both returning to the S₀ state and effective triplet formation happen. On the basis of the simulation results, the proposed electronic relaxation mechanism of light-activated disulfides is S₁ → S₂(T₂) → region of singlet–triplet quasidegeneracy → S₀, which emphasizes the competitive participation of the triplet states in the relaxation dynamics of disulfides. This theoretical work provides insights into the intrinsic excited-state properties of disulfide molecules.