Light-Induced SO Extrusion from Tribenzothiepine S-oxides: A Precursor Approach to the Triphenylene Core.

The precursor approach is a powerful strategy for accessing insoluble or unstable polycyclic aromatic hydrocarbons (PAHs) through the synthesis and solution-processing of molecular precursors, ultimately converted in situ into functional materials in response to specific stimuli. Developments have mostly focused on retrocycloaddition and decarbonylation as key processes, but the wide structural diversity of PAHs cannot be reached with these sole transformations. It thus appears crucial to develop new reactivity schemes for the precursor approach. Here we investigated the tribenzo[b,d,f]thiepine S-oxide pattern as a photoactivatable precursor for the triphenylene core upon SO extrusion. Synthesis of the tribenzothiepine S-oxide model system was achieved chemo- and stereoselectively, exploiting mCPBA and a boron-based Lewis acid to give rise to unprecedented exo vs endo stable isomers. Spectroscopic analyses complemented by DFT-based mechanistic studies revealed that these isomers display stereodependent photochemical behavior. Upon UV light irradiation, the exo tribenzothiepine S-oxide is converted into triphenylene via a cheletropic photoreaction in yields up to 90% in solution, while the endo isomer undergoes first a photoisomerization into the more stable exo counterpart. Eventually, light-induced SO extrusion from exo-tribenzothiepine S-oxide was performed in the solid state, thus highlighting the synthetic potential of cyclic sulfoxides for post-processing photoconversion as a green route toward triphenylene-based materials.