Synthesis and structures of N-alkyl-1,13-dimethoxychromeno- [2,3,4-kl]acridinium salts: the missing azaoxa[4]helicenium.

Helical structures are interesting due to their inherent chirality. Helicenium ions are triarylmethylium structures twisted into configurationally stable helicenes through the introduction of two heteroatom bridges between the three aryl substituents. Of the configurationally stable [4]helicenium ions, derivatives with sulfur, oxygen and nitrogen bridges have already been synthesised. However, one [4]helicenium ion has proven elusive, until now. We present herein the first synthesis of the 1,13-dimethoxychromeno[2,3,4-kl]acridinium (DMCA(+)) [4]helicenium ion. A series of six differently N-substituted DMCA(+) ions as their hexafluorophosphate salts are reported. Their cation stability was evaluated and it was found that DMCA(+) is ideally suited as a phase-transfer catalyst with a pKR+ of 13.0. The selectivity of nucleophilic addition to the central carbon atom of DMCA(+) has been demonstrated with diastereotopic ratios of up to 1:10. The single-crystal structures of several of the DMCA(+) salts were determined, and structural differences between N-aryl- and N-alkyl-substituted cations were observed. The results of a comparative study of the photophysics of the [4]helicenium ions are presented. DMCA(+) is found to be a potent red-emitting dye with a fluorescence quantum yield of 20 % in apolar solvents and a fluorescence lifetime of 12 ns. [4]Helicenium ions, including DMCA(+), all suffer from solvent-induced quenching, which reduces the fluorescence quantum yields significantly (ϕfl < 5 %) in polar solvents. A difference in photophysical properties is observed between N-aryl- and N-alkyl-substituted DMCA(+), which has tentatively been attributed to a difference in molecular conformation.