Boron-Doping of a Macrocyclic Tetrafuran Framework Unveils an Expanded Porphyrinoid With Switchable (Anti)Aromaticity...

Expanded porphyrinoids are an emerging class of organic chromophores with often exceptional optoelectronic properties such as broad absorption extending into the near-infrared (NIR) spectral region. To optimize porphyrinoid-based materials for specific purposes, it is essential to specifically tailor their electronic properties. This can be achieved by incorporation of heteroatoms into their π-conjugated framework. Replacing C atoms of their backbone with trivalent boron centers yields electroneutral boraporphyrinoids that feature a reduced π-electron count compared to their carbon analogs. While some thiophene-based boraporphyrinoids have been reported, boron macrocycles based solely on the renewable furan building block have remained elusive so far. Herein, we present the unprecedented diboratetraoxaporphyrin(2.1.2.1) 1E and the potassium salt of its dianion K₂[1E], as well as open-chain reference compound 6. Investigation of the electronic structure of 1E, [1E]^2-, and the radical anion [1E]^•- revealed that 1E exhibits magnetically antiaromatic features that are more pronounced compared to its thiophene analog diboratetrathiaporphyrin(2.1.2.1), while upon transition to [1E]^2- it becomes strongly aromatic. Interestingly, neutral porphyrinoid 1E is conformationally flexible, while the macrocyclic backbone becomes rigid upon reduction to K₂[1E]. The aromatic porphyrinoid K₂[1E] has absorption bands in the blue, red, and NIR spectral regions and exhibits photoluminescence at 1032 nm.