Three regioisomeric bis[1]benzothieno[1,4]thiazine-S-oxides were synthesized by mild oxidation. Crystal structures reveal S-extra-oxide, N-intra conformers with paired-like substructures, and intermolecular distances as low as 3.30 Å. UV–vis absorption and emission spectroscopy identifies the anti-anti-regioisomer as a strong blue light emitter. Supporting (TD)DFT calculations additionally corroborate the electronic structures.

Abstract

Three regioisomeric N-para-fluorophenyl bis[1]benzothieno[1,4]thiazines (BBTT) are selectively oxidized under mild conditions to give the corresponding BBTT-S-oxides, which were fully characterized by single crystal structural analysis. The newly formed S-oxide is uniformly oriented in a pseudo-axial S-extra position, whilst the N-aryl substituent adopts an N-intra conformation. In comparison to the diminished planarization of the BBTT scaffold according to DFT calculations, the structures are considerably planarized in the crystal. Only the anti-anti-BBTT-S-oxide emits intensively and the electronic structure of the absorption and emission spectra can be elucidated by TDDFT calculations. This blue emission, both in solution (Φ_f = 17%) and in the solid state (Φ_f = 66%), is significant and qualifies this isomer as an attractive emitter.