On-Surface Synthesis of Porphyrin Nanoribbons via Cycloaromatization of Isopropyl Substituents on Au(111)

We report the surface-mediated synthesis of quasi-one-dimensional (quasi-1D) porphyrin nanoribbons (PNRs) via a hierarchical process involving: (i) [3+3] cycloaromatization of isopropyl substituents, (ii) intramolecular cyclodehydrogenation, and (iii) in situ metalation on Au(111). The 5,10,15,20-Tetrakis[4-(1-methylethyl) phenyl]-21H,23H-porphine (TIPP) was selected as precursor which has an isopropyl group on each phenyl. Combining scanning tunneling microscopy, and density functional theory (DFT) calculations, we systematically elucidate the hierarchical transformation of TIPP molecules into quasi-1D PNRs on Au(111). Scanning tunneling spectroscopy measurements demonstrate that pristine PNRs exhibit a HOMO-LUMO gap of ~0.77 eV, in good agreement with theoretical predictions. Upon in situ metalation with Au atoms, the HOMO-LUMO gap is reduced to ~0.24 eV, which is attributed to n-type doping from Au atoms. Charge density difference analysis reveals a significant electron transfer (~1.04 e⁻ per Au) from the Au atoms to the porphyrin π-system, supporting the observed n-type doping behavior.