Expanded Boraporphyrinoids: Boron Macrocycles with Redox Switchable Magnetic (Anti)aromaticity and Strong Near-Infrared Absorption

Synthetic porphyrinoids are an intensely researched class of organic chromophores that can exhibit exceptional optoelectronic properties such as efficient light-harvesting capabilities due to panchromatic absorption and high extinction coefficients as well as emission in the long-wavelength electromagnetic spectral region. To harness these features for specific applications, it is pivotal to effectively tune their electronic properties, which can be achieved, for example, by incorporating heteroatoms into their π-system. Although the introduction of three-coordinate boron is particularly interesting due to the effect of its vacant p-orbital, boraporphyrinoids have hardly been explored so far. Herein, we report the synthesis of electron-deficient diboratetrathiaporphyrin(2.1.2.1) 1 and its doubly reduced form K 2 [1], as well as their partially saturated analogs 2 and K 2 [2], respectively. Experimental and computational results show that neutral 1 is globally antiaromatic in the context of the magnetic criterion, while twofold reduction to K 2 [1] leads to a switch to global aromaticity. In contrast, reduction of 2 to K 2 [2] results in the attenuation of local aromaticity in the thiophene units. These findings are corroborated with a detailed comparative computational study on the aromatic properties of simplified model systems. The observed redox switching between global antiaromaticity and aromaticity is unprecedented in organoboron macrocycles. Porphyrinoid K 2 [1] exhibits strong absorption ranging from the UV to the NIR spectral region, as well as photoluminescence in the NIR region (985 nm).