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

The syntheses of new aromatic 26pi and non-aromatic 28pi hexaphyrins through a [4+2] acid catalyzed condensation of easily available and air-stable precursors are reported. Both 26pi and 28pi hexaphyrins are reversibly transformed into one another by two-electron reduction with NaBH(4) 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 (1)H 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 pi-electron conjugation consistent with the large red shifts of the absorption bands.