Triplets in the cradle: ultrafast dynamics in a cyclic disulfide

TL;DR

This study probes how spin-orbit coupling modulates ultrafast photodynamics of the cyclic disulfide 1,2-dithiane after UV excitation, by comparing singlet-only and singlet-plus-triplet nonadiabatic dynamics using SA$(4|4)$-CASSCF$(6,4)$ on a truncated basis. Surface-hopping trajectories reveal that triplet states rapidly siphon population from the singlets and suppress intramolecular S–S recombination, while the characteristic Newton's cradle oscillations of the S–S bond persist with a period of about 350 fs in both cases. Despite SOC, the mean nuclear dynamics remain similar, with triplet inclusion driving the system toward a statistical singlet/triplet distribution by ~1 ps, yet measurable differences in ultrafast X-ray scattering signals may still be detectable at XFEL facilities. The results highlight the importance of including triplet states in modeling disulfide photochemistry and provide a framework for connecting ultrafast dynamics to experimental observables such as UXS and potentially time-resolved photoelectron signals.

Abstract

The effect of spin-orbit coupling on the "Newton's cradle"-type photodynamics in the cyclic disulfide 1,2-dithiane (C4H8S2) is investigated theoretically. We consider excitation by a 290 nm laser pulse and simulate the subsequent ultrafast nonadiabatic dynamics by propagating surface-hopping trajectories using SA(4|4)-CASSCF(6,4)-level electronic structure calculations with a modified ANO-R1 basis set. Two simulations are run: one with singlet states only, and one with both singlet and triplet states. All trajectories are propagated for 1 ps with a 0.5 fs timestep. Comparison of the simulations suggests that the presence of triplet states depletes the singlet state population, with the net singlet and triplet populations at long times tending towards their statistical limit. Crucially, the triplet states also hinder the intramolecular thiyl radical recombination pathway via the efficient intersystem crossing between the singlet and triplet state manifolds.

Figures (8)

• Figure 1: A schematic representation of the "Newton's cradle" motion in photoexcited 1,2-dithiane, whereby the molecule repeatedly rotates about the carbon framework to bring the two sulfur atoms back into close proximity before the molecule springs open to reform the linear biradical.
• Figure 2: The orbitals used in the active spaces for the electronic structure calculations in 1,2-dithiane. The complete set used in the larger (10,8) active space corresponds to all orbitals shown. The four orbitals included in the smaller (6,4) active space are the subset in the grey-dashed box on the right.
• Figure 3: A summary of static results in 1,2-dithiane. (a) Potential energy cuts for the four lowest-energy singlet states S_0-S_3 and triplet states T_1-T_4 along the LIIC pathway connecting the S_0 minimum S$_{0}$(min) to the S_1 minimum S$_{1}$(min), and then onwards to the minimum energy conical intersection between the S$_0$ and S$_1$ states, MECI(S_0,S_1). (b) The dominant character and corresponding CI-coefficient for each electronic state at the S$_0$(min) molecular geometry. (c) The optimised molecular geometries S$_{0}$(min) (left), S$_{1}$(min) (centre), and MECI(S_0,S_1) (right).
• Figure 4: Experimentalwreo20420Bergson1962UltravioletAS (dotted line) and computational UV absorption spectra (shifted by -$0.21$ eV) for 1,2-dithiane calculated at the SA(4$\vert$4)-CASSCF(6,4)/ANO-R1(t) (top) and the SA(4$\vert$4)-CASSCF(10,8)/ANO-R1(t) (bottom) levels of theory, respectively. The proposed excitation window, $4.25$--$4.31$ eV is indicated by the shaded light-grey region on both plots. For a discussion of the strong absorption at energies $>4.5$ eV in the experimental spectrum, see the text.
• Figure 5: Populations and S-S distances as a function of time (fs) for the triplet-inclusive (left column) and the singlet-only (right column) simulations. (Top row) The classical molecular Coulomb Hamiltonian (MCH) state representation populations. The left panel shows the ground state population S_0, the net singlet excited-state population S_n (summing the populations for all singlet states with $n>0$), and the net triplet population T (summing over all triplet states). The right panel is the same, but excluding the net triplet population. (Bottom row) The S-S bond length, $r_{\mathrm{S-S}}$, for each trajectory with the mean bond length fitted to an exponentially damped sine function and shown as a thick black curve, analogously to the fitting in Rankine et al.rankine_theoretical_2016 For reference, the grey dot-dashed line indicates the S-S distance at the S_0 equilibrium geometry, 2.13 Å.