Impact of phenothiazine-to-phenoxazine replacement in hexaphyrin frameworks on structural, spectroscopic, and redox properties

Three phenoxazine-embedded hexaphyrins were synthesized from commercially available phenoxazine in a few simple steps and compared with our previously reported phenothiazine-embedded hexaphyrins. A striking structural distinction was observed between these two series. The phenoxazine-embedded hexaphyrins showed a more planar and symmetric structure, where the oxygen atom of phenoxazine faces outward and the nitrogen atom points inward toward the macrocyclic core. In contrast, the phenothiazine-embedded hexaphyrins were more distorted, with the sulfur atom facing inward and the nitrogen atom outward. Thus, replacing phenothiazine with phenoxazine caused a "<i>flip</i>" of the heterocyclic unit and changed the overall electronic structure of the macrocycle. The phenoxazine-embedded hexaphyrins showed similar NMR, absorption, and fluorescence patterns to the phenothiazine analogues but with noticeable shifts in peak positions. Electrochemical studies indicated that the phenoxazine-based systems are more electron-rich, which was further supported by DFT and TD-DFT calculations.