Ring-to-Chain Structural Relaxation of Elemental Sulfur upon Photoexcitation

The emergence of high sulfur content polymeric materials and their applications in a number of important areas call for a comprehensive understanding of the chemical and physical properties of elemental sulfur. In spite of many earlier efforts, a thorough description of the electronic structure and photorelaxation pathways of the S8 ring, the thermodynamically stable structure of elemental sulfur, remains largely unexplored. It can be expected that the photoinduced homolytic processes of S8 and intermediates are analogous to those in the species observed via thermally induced processes. Here, we combine time-dependent density functional theory (TD-DFT) and highly correlated wave function STEOM-CCSD calculations to describe the photoinduced homolytic pathways in both singlet and triplet excited-state manifolds and explore the ring-to-chain relaxations. The results of our calculations indicate that, upon photoexcitation, the S8 ring undergoes a fast intersystem crossing (ISC) from the S1 state to the triplet manifold, with estimated ISC rates of over 1011 s-1 as a result of significant spin-orbit couplings. The TD-DFT geometry relaxations in the singlet and triplet excited states underline that, upon photoexcitation, an S-S bond within the S8 ring readily undergoes homolytic fragmentation, a feature that is thus also expected upon thermal excitation. Importantly, this represents the first theoretical demonstration that the molecular structure in the triplet state evolves from a ring configuration to an open chain configuration that carries a diradical character, which is consistent with the reported photoinduced polymerization processes in elemental sulfur.