Experimental Demonstration of a Spectral Fingerprint for the Saddle and Inverted Conformations of Porphyrins on Copper.

Combined density functional theory (DFT) and X-ray standing wave (XSW) studies have previously provided evidence for the preferential adoption of an inverted conformation of 2H-TPP on Cu(111) in contrast to the saddle conformation usually favored by porphyrin molecules adsorbed on metals. We experimentally demonstrate, via X-ray photoelectron spectroscopy (XPS) analysis, that the binding energies of the aminic and iminic nitrogen species provide a spectral fingerprint for both inverted and saddle conformations, as predicted by DFT studies. Our complementary scanning tunneling microscopy (STM) characterization also reveals conversion from the saddle to inverted conformation at an elevated temperature for an analogous porphyrin species (BrTPP).