Single-Molecule Investigations of Conformation Adaptation of Porphyrins on Surfaces

The porphyrin macrocyclic core features dynamic conformational transformations in free space because of its structural flexibility. Once attached to a substrate, the molecule-substrate interaction often restricts this flexibility and stabilizes the porphyrin in a specific conformation. Here using molecular dynamic and density-functional theory simulations and scanning tunneling microscopy and spectroscopy, we investigated the conformation relaxation and stabilization processes of two porphyrin derivatives (5,15-dibromophenyl-10,20-diphenylporphyrin, Br₂TPP, and 5,15-diphenylporphyrin, DPP) adsorbed on Au(111) and Pb(111) surfaces. We found that Br₂TPP adopts either dome or saddle conformations on Au(111) but only the saddle conformation on Pb(111), whereas DPP deforms to a ruffled conformation on Au(111). We also resolved the structural transformation pathway of Br₂TPP from the free-space conformations to the surface-anchored conformations. These findings provide unprecedented insights revealing the conformation adaptation process. We anticipate that our results may be useful for controlling the conformation of surface-anchored porphyrin molecules.