Bis[1]benzothieno[1,4]thiazines: Planarity,Enhanced Redox Activity and Luminescence by Thieno-Expansion of Phenothiazine

Twofold Buchwald–Hartwig aminations selectively furnish three regioisomers of bis[1]benzothieno[1,4]thiazines; X-ray structure analyses and DFT calculations were corroborated for correlation of their electronic properties. All regioisomers outscore the parent compound phenothiazine with respect to a low-lying oxidation potential and reversible redox activity. The anti-anti bis[1]benzothieno[3,2-b:2′,3′-e][1,4]thiazines possess the lowest oxidation potentials in this series and displayed pronounced green luminescence in solution (Φ_F≈20 %) and in the solid state. Syn-anti regioisomers were only weakly luminescent in solution, but showed aggregation-induced emission enhancement and solid-state luminescence. Most interestingly, X-ray structure analyses revealed that anti-anti derivatives have an amazingly coplanar structure of the pentacyclic anellated 1,4-thiazine system, emphasizing a structural similarity to heteroacenes. The calculated theoretical nucleus-independent chemical shifts additionally suggested that these 8π-electron core systems can be considered as the first electronically unbiased anellated 1,4-thiazines with antiaromatic character.