Solution Versus On-Surface Synthesis of Peripherally Oxygen-Annulated Porphyrins through C−O Bond Formation

This study investigates the synthesis of tetra- and octa-O-fused porphyrinoids employing an oxidative O-annulation approach through C−H activation. Despite encountering challenges such as overoxidation and instability in conventional solution protocols, successful synthesis was achieved on Au(111) surfaces under ultra-high vacuum (UHV) conditions. X-ray photoelectron spectroscopy, scanning tunneling microscopy, and non-contact atomic force microscopy elucidated the preferential formation of pyran moieties via C−O bond formation and subsequent self-assembly driven by C−H⋅⋅⋅O interactions. Furthermore, the O-annulation process was found to reduce the HOMO–LUMO gap by lifting the HOMO energy level, with the effect rising upon increasing the number of embedded O-atoms.