Modified 26 and 28 π Hexaphyrins with Five meso-Links: Optical, Redox, and Structural Properties

Abstract

The syntheses of new aromatic 26p and non-aromatic 28p hexaphyrins through a [4+2] acid catalyzed condensation of easily available and air-stable precursors are reported. Both 26p and 28p hexaphyrins are reversibly transformed into one another by twoelectron reduction with NaBH4 or twoelectron 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 p-electron conjugation consistent with the large red shifts of the absorption bands.