Switching of aromaticity in fluorene-embedded telluraporphyrins by protonation.

We report the synthesis of rare nonaromatic fluorene-embedded telluraporphyrins, in which one pyrrole ring of 21-telluraporphyrin is replaced by a fluorene moiety. These macrocycles were synthesized via the condensation of differently meso-substituted fluorene diols with 16-telluratripyrrane under acid-catalyzed conditions, followed by open-air 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) oxidation. The tellurophene ring plays a crucial role in macrocycle formation, as analogous systems containing furan, thiophene or selenophene afforded unstable products. Spectroscopic and theoretical studies reveal the nonaromatic nature of the neutral macrocycles. Remarkably, protonation induces a switch to aromatic character in the corresponding diprotonated species through enhanced π-delocalization, as supported by nuclear magnetic resonance (NMR) spectroscopy and theoretical calculations. These findings demonstrate protonation-controlled modulation of aromaticity and electronic structure in fluorene-embedded porphyrinoids.