Synthesis and properties of p-benzithiahexaphyrin(1.1.1.1.1.1)s

Abstract

Novel 28π p-benzithiahexaphyrin(1.1.1.1.1.1)s with Möbius aromaticity were synthesised by the [3 + 3] condensation of p-benzitripyrrane dicarbinol with 16-thiatripyrranes under acid-catalysed conditions, followed by DDQ oxidation in open air at room temperature. DFT studies revealed that the macrocycles adopted a figure-eight structure, wherein the phenylene and thiophene rings were present at the intersection. A significant twisting was observed at the intersection, along with a major deviation of heterocyclic rings from the mean plane. The high negative NICS(0) values ∼ −8.0 were observed at the centre of the macrocycle and at all other points along the conjugation, indicating the Möbius aromaticity of the macrocycle. The ¹H-NMR studies at room temperature indicated a fairly Möbius aromatic nature of the macrocycles, showing significant upfield shifts of the inverted thiophene ring protons and slight downfield shifts of β-pyrrole protons due to the ring current effect. The ¹H NMR studies at 223 K suggested the absence of structural change and intact Möbius aromatic character of the macrocycles. The absorption spectra of the macrocycles exhibited typical aromatic features with one intense Soret band accompanied by Q-bands of lower intensity. The electrochemical studies showed that the macrocycles were relatively electron-rich and underwent easier oxidations. TD-DFT studies were in accordance with our experimental findings. Thus, the 1D and 2D NMR, absorption and theoretical studies reflected the Möbius aromatic features of p-benzithiahexaphyrins.