Conformationally enforced planarization of bis[1]benzothieno[1,4]thiazines: a rational design of redox-active fluorophores with increased radical cation stability

Abstract

A DFT-based approach guides the rational design of the substance class of anti–anti-bis[1]benzothieno[1,4]thiazines (BBTTs) bearing N-aryl substituents, previously generated as enhanced phenothiazine congeners, based upon the assumption that a minimization of the intrinsic butterfly folding in favor of a more planarized BBTT structure allows for better electronic tuning. Therefore, an intramolecular conformational restriction imposed by a sterically demanding ortho(,ortho′)-substitution pattern on N-aryl moieties is envisioned to provide this planarization of the BBTT's backbone, in solution and in the crystal solid state. Calculations of BBTT's minimum structures with variable N-aryl substitution as well as relaxed geometry PES scans assessing the energy barriers of the intramolecular restriction were carried out, identifying anti–anti-N-ortho,ortho′-disubstituted-phenyl-BBTTs as promising target structures. Synthetically, cyclizing Buchwald–Hartwig aminations starting from brominated sulfanes followed by subsequent functionalizations through organometallic catalysis and reagents pave the way to a substance library of the proposed BBTTs. Extensive structural analysis via X-ray diffraction reveals the molecular structures as well as the superordinate crystal packing patterns. NICS (nucleus-independent chemical shift) calculations indicate that strongly planarized BBTTs reveal a paratropic ring current and, therefore, an anti-aromatic character of their central 1,4-thiazine core. Comprehensive investigations of their optoelectronic properties by cyclic voltammetry, spectroelectrochemistry, and UV/vis absorption and emission spectroscopy shed light on the electronic structure, supported by aid of (TD)-DFT calculations. A broader range of their first redox potentials E^0/+1₀, larger semiquinone formation constants in comparison to para-N-aryl substituted BBTTs causing an increased radical cation stability, and a distinct tunability of their luminescence characteristics render ortho(,ortho′)-substituted BBTTs as highly functional strong donor moieties for future application in various fields.