A fully conjugated meso-boron-substituted porphyrinoid combining Lewis acidity with redox-switchable aromaticity

The structural motif of porphyrin is relevant for many essential biological processes and has emerged as a versatile component of functional materials. Here, we introduce a thiophene-based porphyrinogen having electron-deficient boron atoms in all four meso-positions. Its fully π-conjugated backbone exhibits effectively concealed antiaromaticity, with locally confined aromaticity to the thiophene units. The macrocycle readily binds fluoride ions, signaled by changes in its photophysical characteristics. Global aromaticity is switched on via facile consecutive (electro)chemical one-electron reductions to give the radical anion and the dianion, the potassium salts both of which were isolated and characterized by single-crystal X-ray diffraction. The bowl-shaped dianion constitutes a tetrathiophene-based porphyrinoid with a metal cation in its coordination sphere. This 18-π-electron macrocycle shows absorption features typical of porphyrins, while its low-energy Q bands are unusually intense in relation to the Soret bands. In addition, it displays fluorescence emission in the near-infrared (NIR) spectral region.