Synthesis, twofold oxidative cyclization and dual emission of diaryl-substituted benzodithieno[5.5.5.6]fenestranes

Abstract

The syntheses of three 3,8-diaryl-substituted benzodithieno[5.5.5.6]fenestranes (BDTFs) and two doubly o-phenylene-bridged BDTFs are presented. These syntheses require careful consideration of the steric and electronic factors of the thiophene-fused polycyclic system. The electron-rich bis(3,4-dimethoxyphenyl)-BDTF displays a dual fluorescence profile where the long-wavelength emission is attributed to exciplex emission, as supported by concentration- and solvent-dependent fluorescence experiments as well as fluorescence lifetime studies. In contrast, the less electron-rich diphenyl-BDTF shows a much weaker exciplex emission, whereas the two cyclized analogues show only monomer emission. Additional emission measurements of model compounds reveal that the through-space interaction between the electron-rich 3,4-dimethoxyphenyl groups and the benzo unit may be crucial to the exciplex formation. Single-crystal X-ray diffraction of one of the twofold oxidatively cyclized BDTFs shows the highly distorted thiophene–benzene moieties and also multiple short contacts that involve the sulfur atoms and the bridgehead methine groups of the fenestrane core. This work demonstrates that there still exist vastly unexploited properties of (hetero)arene-fused fenestranes due to their unique three-dimensionally arranged aromatic units.