Direct Imaging of Precursor Adcomplex States during Cryogenic-Temperature on-Surface Metalation: Scanning Tunneling Microscopy Study on Porphyrin Array with Fe Adsorption at 78.5 K

On-surface metalation of metal-free pi-conjugated planar molecules, such as metal-free tetraphenylporphyrin (2HTPP), using 3d transition metals prepared in ultrahigh vacuum (UHV), attracted significant attention as a newly developed bottom-up method to create a building block for 1-nm-size nano devices, one- or two-dimensional nano networks, as a precursor of further on-surface synthesis or a magnetic atom array aligned on a well-ordered molecular template with an extreme low impurity concentration. Experimental and theoretical studies for this type of on-surface metalation via dehydrogenation (e.g., 2HTPP + Fe -> H-2 + FeTPP) have been successfully demonstrated by depositing 3d metal atoms on metal-free pi-conjugated planar molecular array at 300 K with subsequent annealing of 350-700 K in UHV. Significantly, photoemission spectroscopy study for the on-surface metalation process at cryogenic temperatures suggested a precursor adcomplex state different from the normal metalation state. In this study, we demonstrate that on-surface metalation at cryogenic temperature reduces thermal diffusion of both metal atoms and molecules, which allows gentle adsorption of the 3d metal atoms inside the molecule, providing precursor adcomplex states while preserving the original well-ordering molecular array structure. Using scanning tunneling microscopy (STM) we investigated Fe deposition on a well-ordered metal-free tetraphenylporphyrin (2HTPP) monolayer array islands with an Au(111) substrate temperature maintained at 78.5 K. Through direct STM topographic and spectroscopy imaging supported by density functional theory calculations, we identify three types of TPP appearance: (1) original 2HTPP, (2) 2HTPP with an Fe atom on top (precursor adcomplex "alpha" state), and (3) 2HTPP with an Fe atom inside (precursor adcomplex "gamma" state, where a clear LUMO peak change was observed).