Protonation-Triggered Hückel and Möbius Aromatic Transformations in Nonaromatic Core-Modified [30]Hexaphyrin(2.1.1.2.1.1) and Annulated [28]Hexaphyrin(2.1.1.0.1.1)

Abstract

Two 1,2-diphenyl-1,2-dithienylethene-embedded hexaphyrins 6 and 8 containing 30π and 28π electrons, respectively, in conjugation exhibit nonaromatic characteristics. Compound 6 shows an unusual bond length equalization in its electronic structure. However, on protonation, 6 exhibits Huckel aromatic characteristics while 8 shows Mobius aromatic characteristics. The changes in aromaticity are accompanied by change in conformation from a figure-eight/twisted structure to an open extended structure.

Redox Assisted Reversible Aromaticity Transition Between 30π Hückel and 28π Möbius Dication of a Core‐Modified Isophlorinoid

“…This observation is in stark contrast to the earlier reports on transition between Hückel non-aromatic or anti-aromatic and Möbius aromatic topologies. [15][16][17][18] Also, the observed electronic absorption spectrum of 10 and [10] 2 + was in sound agreement with the calculated UV spectra obtained from TD-DFT calculations (Supporting Information Figure S11, S20). Prior to our findings, computational calculations had predicted higher stability for a 9π monocation [31] and 14π dication species in unsubstituted Möbius annulenes.…”

“…Such a small difference in the energy gap value suggests that HOMO‐LUMO energy may not vary significantly during transition between Möbius and Hückel aromaticity. This observation is in stark contrast to the earlier reports on transition between Hückel non‐aromatic or anti‐aromatic and Möbius aromatic topologies [15–18] . Also, the observed electronic absorption spectrum of 10 and [10] 2+ was in sound agreement with the calculated UV spectra obtained from TD‐DFT calculations (Supporting Information Figure S11, S20).…”

“…Alternatively, pH dependent switching was observed when a Hückel non‐antiaromatic 36π octaphyrin, 3 , transformed to a mono anionic 36π Möbius aromatic species in a basic medium [17] . Later, switching of a 28π Hückel non‐aromatic core‐modified hexaphyrin to 28π Möbius aromatic species 4 was observed under acidic conditions [18] . Invariably, either Hückel non‐aromatic or anti‐aromatic expanded porphyrinoids have pronouncedly exhibited potential to reversibly switch to Mobius aromaticity.…”

“…[17] Later, switching of a 28π Hückel non-aromatic core-modified hexaphyrin to 28π Möbius aromatic species 4 was observed under acidic conditions. [18] Invariably, either Hückel non-aromatic or anti-aromatic expanded porphyrinoids have pronouncedly exhibited potential to reversibly switch to Mobius aromaticity. One-step reversible switching between Huckel and Mobius aromaticity is very rare and is still a formidable challenge to synthetic chemists.…”

Corroles and Hexaphyrins: Synthesis and Application in Cancer Photodynamic Therapy

“…The change in the polarization results in electronic perturbations of the π conjugation. In this regard, chalcogen-substituted hexaphyrins have been synthesized [ 103 , 104 , 105 , 106 , 107 , 108 ], some of them having been explored as NIR photodynamic therapy agents [ 109 ], and others such as dithiabronzaphyrin 130 [ 108 ] show intense absorption and fluorescence in the NIR region, opening the way for their use in biological applications.…”