Exploring the effect of porphin tautomerism on the regioselectivity of its derivatives is a big challenge, which is significant for the development and application of porphyrin drugs. In this work, we demonstrate the regioselectivity of 2-diphenylporphyrin (H-DPP) in the planarization reaction on Au(111) and Ag(111) substrates. H-DPP monomer forms two configurations ( and ) via a dehydrogenation coupling, between which the yield of the -configuration exceeds 90%. Using high-resolution scanning tunneling microscopy, we visualize the reaction processes from the H-DPP monomer to the final two planar products. Combined with DFT calculations of the potential reaction pathway and comparative experiments on Au(111) and Ag(111) substrates. Using M-DPP (M = Cu and Fe), we confirm that the regioselectivity of H-DPP is derived from the reaction energy barrier during the cyclodehydrogenation reaction of different tautomers. This work reveals the regioselectivity mechanism of H-DPP on the atomic scale, which holds great significance for understanding the chemical conversion process of organic macrocyclic molecules.
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