Modified 26 and 28π hexaphyrins with five meso-links: optical, redox, and structural properties

Abstract

Redox active hexaphyrins: modified hexaphyrins with five meso links have been synthesized and characterized. These hexaphyrins not only exhibit conformational flexibility but also undergo facile redox interconversion between aromatic 26π and 28π non-aromatic systems in accordance with (4n+2) Hckel's rule. The syntheses of new aromatic 26π and non-aromatic 28π hexaphyrins through a [4+2] acid catalyzed condensation of easily available and air-stable precursors are reported. Both 26π and 28π hexaphyrins are reversibly transformed into one another by two-electron reduction with NaBH₄ or two-electron oxidation with dichlorodicyanobenzoquinone (DDQ) respectively. Detailed optical and NMR spectral studies suggests that the [26]hexaphyrin is aromatic and possesses a diatropic ring current, while [28]hexaphyrin exhibits non-aromatic characteristics. The structural characterization has been done with extensive ¹H and 2D NMR studies. Theoretical calculations performed with various conformational possibilities tested for the unsubstituted hexaphyrin, at semiempirical level, suggest that the most stable conformation takes an inverted structure with one pyrrole ring inversion. The energy optimization for the final geometry of the unsubstituted hexaphyrin, performed at the B3LYP/6-31G^∗ level of DFT, shows excellent agreement with the structure derived from the solution NMR data. Electrochemical data reveals HOMO destabilization with increasing π -electron conjugation consistent with the large red shifts of the absorption bands.