Reversible On-Surface Metalation and Demetalation of Porphyrins in a Solvent-Free Ultrahigh-Vacuum Environment

Controlling molecular interconversions reversibly is of great interest in chemistry. Reversible molecular interconversions have been extensively realized on solid surfaces by the regulation of intermolecular noncovalent interactions and intramolecular covalent bonds. Among others, molecular reactions involving the reversible tuning of intramolecular covalent bonds have typically been induced by local manipulations on individual molecules using a scanning probe microscopy (SPM) tip. However, reversible control of on-surface molecular reactions in a global manner remains challenging. In this work, by a combination of scanning tunneling microscopy (STM) imaging, X-ray photoelectron spectroscopy (XPS) measurements, and density functional theory (DFT) calculations, we report the reversible metalation and demetalation of porphyrin molecules on Au(111) in a controllable and global manner in a solvent-free ultrahigh-vacuum (UHV) environment by providing different atmospheres. The universality of such reactions involving the alkali metal Na was demonstrated by two types of porphyrins with different molecular backbones, while the on-surface metalation with the transition metal Co forming Co-porphyrin was found to be unidirectional. DFT calculations revealed the driving force for their different demetalation behaviors and further illustrated the critical role of extrinsic H atoms in the demetalation pathways. Our results represent a general strategy to reversibly steer the on-surface molecular metalation and demetalation, which should provide fundamental understanding of reversible molecular interconversions.