Electronic energy levels of porphyrins are influenced by the local chemical environment.

Self-assembled islands of 5,10,15,20-tetrakis(pentafluoro-phenyl)porphyrin (2HTFPP) on Au(111) contain two bistable molecular species that differ by shifted electronic energy levels. Interactions with the underlying gold herringbone reconstruction and neighboring 2HTFPP molecules cause approximately 60% of molecules to have shifted electronic energy levels. We observed the packing density decrease from 0.64 ± 0.04 molecules per nm 2 to 0.38 ± 0.03 molecules per nm 2 after annealing to 200 °C. The molecules with shifted electronic energy levels show longer-range hexagonal packing or are adjacent to molecular vacancies, indicating that molecule-molecule and molecule-substrate interactions contribute to the shifted energies. Multilayers of porphyrins do not exhibit the same shifting of electronic energy levels which strongly suggests that molecule-substrate interactions play a critical role in stabilization of two electronic species of 2HTFPP on Au(111).